Calcium Aluminate Cements

12

Calcium Aluminate Cements
Fryda and Bruno Touzo Jason H. Ideker, Karen L. Scrivener, Herve

12.1 INTRODUCTION In terms of the length of time it has been produced, the volume produced and the breadth of applications, calcium aluminate cements (CACs) are by far the most important class of non-Portland cements. Nevertheless, the volume used per year is still more than three orders of magnitude less than Portland cement. Calcium aluminate cements are a relatively large family with a range of compositions which varies much more widely than Portland cement as shown in Fig. 12.1. All CACs contain reactive calcium aluminate phases, predominantly monocalcium aluminate. To obtain these calcium aluminate phases it is necessary to use raw materials which contain a much higher proportion of alumina relative to silica, such as bauxite in addition to limestone. This, along with the small scale of production and the range of technical application means that they are more than four times as expensive as Portland cement so, clearly it makes no sense to use them as a simple substitute for Portland cement. Today, the two major markets for calcium aluminate cement are in castable refractories and in dry mix mortars for special construction applications, which together account for around 80% of consumption. The use in technical concretes, for example, sewer linings, rapid repair, etc. is rather small. This chapter discusses the chemistry of calcium aluminate cements in the different applications found today.

12.2 HISTORICAL NOTE Calcium aluminate cements were developed around the end of the 19th century and beginning of the 20th, initially as an answer to the poor durability of Portland cements in calcium sulfate environments. It should be borne in mind that concrete mix design was not well developed at this time and with no superplasticisers even the best concretes had relatively high water to cement ratios of around 0.60 or higher. It is perhaps not surprising that the researchers at this time hit upon the calcium aluminate phases as alternative hydraulic phases to the calcium silicates (C3S, C2S) found in Portland cement. Just eight elements—oxygen, silicon, aluminium, iron, calcium, sodium, potassium and magnesium—make up over 98% of the earth’s crust. The hydrates based on sodium and potassium are too soluble to be of much use in hydraulic systems. Iron and magnesium have very low solubility in alkaline solutions, so do not contribute well to bridging the spaces between grains. This leaves CaO–SiO2–Al2O3 as the dominant system in which to find hydraulic compounds, which can be produced in sufficient abundance to make cements. In this system, the only other rapidly reactive phases apart from the calcium silicates are the calcium aluminates, and by extension the calcium sulfoaluminate phases (ye’elimite, C4A3$
3CA + C$x1) (Fig. 12.1). Work was done on calcium aluminate cements and patents were taken out in several countries—France, Germany, United Kingdom, France and United States.1–7 However, the industrial development of calcium aluminate cement is mainly associated with the work of Jules Bied in the laboratories of the J. & A. Pavin de Lafarge Company at Le Teil, France.8 In 1908 this work resulted in the patenting of a manufacturing process in which bauxite, or other aluminous and ferruginous material of low silica content, was fused together with limestone—‘ciment fondu’ (literally ‘melted cement’ in French).9 Although the origin of the work had been to find a cement more resistant in calcium sulfate containing environments, it was quickly realised that the new cement hardened much more rapidly than Portland cement, which led to its use in gun emplacements in the first world war, before eventual commercialisation in 1918 under the name Ciment Fondu Lafarge. Between the wars, the use of calcium aluminate cements developed gradually, both for applications where rapid hardening was needed, such as road repair and in applications where durability, particularly to sea water, was desired. In the latter category two notable uses of calcium aluminate cement were for the pilings of the Ford Motor Factory on the Thames Estuary, now closed and demolished, and some of the quays in Halifax harbour still in use today. Both structures showed excellent durability.10–12 Concrete from these structures has been studied in recent years and is discussed later. After World War II there was an urgent need for reconstruction which led to the widespread use of CAC in prestressed beams in the United Kingdom, and also in Spain. Following the collapse of three buildings containing CAC prestressed beams in the United Kingdom, in 1973 and 1974, there was a period of great uncertainty regarding the use of CAC in structural x1

C$ ¼ calcium sulfate.

Lea’s Chemistry of Cement and Concrete. https://doi.org/10.1016/B978-0-08-100773-0.00012-5 © 2019 Elsevier Ltd. All rights reserved.

537

538 Lea’s Chemistry of Cement and Concrete

SiO2

Portland cement Calcium aluminate / calcium sulfoaluminate

CaO

Al2O3

FIG. 12.1 Zones of cementitious materials in the CaO–SiO2–Al2O3 system.

applications. The general perception, that remains widespread in the construction community, is that these collapses were related to loss of strength due to the conversion process which is discussed later. These collapses were carefully investigated, and for two of them it was concluded that the primary cause of the collapse was poor construction detailing with inadequate linkage between the beams and the supporting nibs.13 In the third case, the investigation found the cause to be loss of strength from conversion aggravated by subsequent chemical attack from sulfates. In the light of this uncertainty, the application of CAC in concrete was limited to non-structural application in many countries. Today, as already mentioned use in concrete is limited to specialist applications where the advantages of CAC outweigh its much higher cost. From the 1980s the application of CAC in refractory castables started to increase. Here CACs have a unique advantage of retaining strengths at high temperatures due to the formation of ceramic bonding and being able to be heated and cooled repeatedly (this cannot be done in Portland based concretes due to the formation of free lime, which then rehydrates with deleterious expansion). Around the same time there was the first development of ready to use mortars (known as dry mix mortars) for specialist application such as repair and self-levelling floor screeds (building chemistry marketx2). These systems contain calcium sulfate hemi-hydrate, anhydrite or gypsum and PC in addition to CAC and the main hydrate phases are ettringite and alumina hydroxide. Today these two applications dominate the sales of calcium aluminate cement.

12.3 PRODUCTION AND MINERALOGY 12.3.1 A Wide Range of Compositions—Overview As mentioned in the introduction, the range of compositions encompassed by calcium aluminate cements is much wider than for Portland cement. Table 12.1 shows the main chemical composition range of CACs available today. The main factor underlying this wide range of compositions has been the demands of the refractory market for products which can withstand higher temperature. This has led to the development of cements with higher alumina contents and lower contents of impurities, notably silica and iron oxide. The light or white colour of these calcium aluminate cements was then taken up by the Building Chemistry market for aesthetic reasons. The reactivity of CACs depends on the amounts of the different reactive phases. In general, monocalcium aluminate, CA, is the main reactive phase. Small amounts of the phase C12A7 are also common.

12.3.2 Manufacture 12.3.2.1 Raw Materials The basic raw materials for the manufacture of calcium aluminate cement are limestone and bauxite. Although alumina is widely distributed in nature is usually occurs with silica (as in clays), bauxite is the only suitable mineral available commercially, on a scale adequate for cement production. The standard grades of CAC (38%–40% Al2O3) are made with ferruginous bauxites and contain up to 20% iron oxide. The silica content must be fairly low (<6%), but several per cent titanium dioxide x2

The term ‘Building Chemistry’ refers to the use of formulated cementitious systems in applications such as tile adhesives, specialty grouts, and self-levelling floor screeds. These are complicated mixtures but often contain CAC + PC + C$ as well as a wide variety of chemical admixtures in powder form.

Calcium Aluminate Cements

TABLE 12.1

539

Main Variants of CAC Clinkers

Grade

Colour

Main Use

Al2O3

CaO

SiO2

Fe2O3

TiO2

MgO

Na2O

K2O

Standard

36–42

3–8

12–20

<3

<1

0.1

0.1

48–60

36–42

3–8

1–3

<3

<1

0.1

0.1

65–75

25–35

<0.5

<0.5

<0.1

0.1

<0.3

0.05

 High 80

White

Concrete and formulated mortars Peri-refractory Refractory concrete and formulated mortars Refractory concrete and formulated mortars Refractory

36–42

 High 70

Grey or buff to black Light buff or light grey White

>79

<20

<0.5

<0.5

<0.1

0.1

<0.3

0.05

 Medium

is not a problem. During World War II, when bauxite was difficult to obtain, use was made of aluminium dross and the red mud from the Bayer process for the production of alumina.14 For the grades of calcium alumina cements with higher alumina contents, purer raw materials are used, for example, low-iron bauxites, alumina, lime, etc. Further details on raw materials can be found in work by Bolger.15

12.3.2.2 Process A variety of processes have been used for the manufacture of calcium aluminate cements. The alumina content is the main factor which determines the manufacturing method. Clinkers with up to 60% Al2O3 can be manufactured by the complete fusion process. Originally, they were made in a water-cooled vertical furnaces,16 lined with refractory material, rather similar to, but much smaller than, a blastfurnace. For the production of the standard grades, this type of furnace has been superseded by the reverberatory open-hearth furnace, which is the most common method of manufacture today (Fig. 12.2). This openhearth furnace is arranged with a tall vertical stack into which the bauxite and limestone, or chalk, are charged. It is fired with pulverised coal or oil with a hot-air blast. The furnace gases pass through the charge of raw materials, driving off water, decomposing the limestone into CaO and driving off carbon dioxide. Melting occurs at the point where the charge drops from the vertical stack into the hearth of the furnace. The cement is maintained in a liquid state in the hearth by heat radiated from the arched roof. The molten cement pours out continuously from a tap hole, runs into moulds and is cooled. The temperature reached is 1450°C–1550°C depending on the chemical composition. Electric arc furnaces are also used in some plants. These furnaces are tapped intermittently and so have a fairly low output. After cooling, the fused cement clinker Weigh feeders

Burden

Reverberatory furnace

Combustion Fusion Solidification

Liquid

Pour tube Test sampler for control

FIG. 12.2 Reverberatory furnace.

540 Lea’s Chemistry of Cement and Concrete

resembles a dark, fine-grained compact rock such as basalt. The clinker ingots are crushed and then ground in a ball mill. The clinker is quite strong and thus requires more energy than Portland cements to grind. In contrast to Portland cements, where gypsum is added to regulate setting, no mineral additions are made during grinding. Grades of higher aluminate content (60%–80% Al2O3) are usually made by sintering in rotary kilns similar to Portland cement although, given the much lower volumes produced, the kilns are much smaller with outputs in the 100 s of tonne per day range. As metallurgical alumina (refined from bauxite) is used instead of bauxite, they contain practically no silica or iron oxide and are white in colour. These grades are predominantly used as binding agents in castable refractories (Section 12.5.9) although they are also used in some building chemistry formulations (Section 12.5.1). Calcium aluminate cements are manufactured in France, United Kingdom, Spain, United States, Japan, Croatia, China, Poland, Turkey, Brazil, India, and on a small scale in Russia.

12.3.3 Physical Characteristics of Calcium Aluminate Cements The specific gravity of calcium aluminate cement increase with iron content. For the standard grades, it is around 3200–3250 kg/m3, which is somewhat higher than that of Portland cements. The loose bulk density depends on packing, but is typically around 1100–1400 kg/m3, rising to 1850–1950 kg/m3 on consolidation. The standard grades of calcium aluminate cement on the market have a specific surface area (Blaine) of 250–400 m2/kg, with a typical residue of 5% on a 100 mm sieve. CACs with higher fineness are available, especially for grades with a higher Al2O3 content, which may have fineness above 800 m2/kg. Since CACs contain no free lime, calcium sulfate or periclase, tests for soundness are not relevant. Expansions in the autoclave test are below 1 mm.

12.3.4 Main Phases and Phase Equilibria Related to CACs 12.3.4.1 C–A Binary System Fig. 12.3 shows the CaO–Al2O3 phase diagram which is the most important binary system relevant to calcium aluminate cements. There are five intermediate calcium aluminate phases between lime and alumina: C3A, C12A7, CA, CA2 and CA6. The reactivity of these phases decreases as the content of calcium oxide decreases: C3A, the phase found in Portland cements, reacts very rapidly with water and produces flash set in the absence of added calcium sulfate. C12A7 is slightly less reactive, but much more reactive than CA. Pure CA can have setting times of tens of hours. The small proportion of C12A7 in 2300

2100 Liquid 1900 CA2 + liq.

⬚C

CA6 + liq.

1700 C + liq. C12A7 + liq.

1500 C + C3A 1300 30

Ca + liq.

C 3A + liq. C3A + CA 40

CaO

50

CA + C12A7 60

CA + CA2

70

%

FIG. 12.3 CaO–Al2O3 phase diagram.17

CA2 + CA6

80 AI2O3

90

Calcium Aluminate Cements

541

common CACs is critical to their typical setting times of a few hours, similar to Portland cement. CA2 reacts at a much slower speed than CA, but reacts significantly at the higher temperatures (>100°C) experienced during the manufacture of refractory castables. CA6 can be considered inert. This phase diagram shows how the melting temperature increases with the Al2O3 content from the eutectic between C12A7, CA and liquid at 1390°C to the melting point of pure alumina at 2050°C. The cements produced by fusion lie in the range of C/A ratios where C12A7 and CA can coexist, while the refractory products produced by sintering lie in the range of coexistence of CA and CA2.

12.3.4.2 Calcium Aluminate with Silica and Iron Oxide The phase diagram for the CaO–Al2O3–SiO2 system is shown in Fig. 12.4.24 There are several solid solutions in this system, the most important for CACs being the melilite group around C2AS (gehlenite).18 The incorporation of silica into the calcium aluminate phases is negligible, so the presence of silica results in the formation of silicates, or aluminosilicates. The main silicate phases found in CACs are C2S and C2AS. C2S has several polymorphs, but mainly the intermediate temperature b-C2S is present.

SiO2 1723°

Crystalline phases

1707°

Notation Oxide formula Cristobalite SiO2 Tridymite Pseudowollastonite CaO.SiO2 3CaO.2SiO2 Rankinite Lime CaO Corundum Al2O3 Mullite 3Al2O3.2SiO2 Anorthite CaO.Al2O3.2SiO2 Gehlenite 2CaO.Al2O3.SiO2

}

1590° ∼1590° Two Liquids 1470° Cristobalite Cnstobalite

00 146

1707°

Temperatures up to approximately 1550°C are on the Geophysical Laboratory Scale; those above 1550°C are on the 1948 International Scale.

1368° 1345° Tridymite

1470° 1436°

0

00 14

1170°

Mullite

Pseudowollastonite Anorthite

Rankinite 1460° 1464° 3CaO·2SiO2 3CaO·2SiO

1512°

1307°

0 40

1 1318°

1265° 1265°CaO·Al 2O3·2SiO2 1553° ∼1310° 1310° 1315° 1400 ∼1315°

2CaO·SiO 2CaO·SiO2 ∼2130° 2130°

1547°

1495° 1385° 1380° Ca3SiO4

1545° Gehlenite .Al O .SiO 2CaO 2CaO.Al 2CaO. 2 33. 2 1593°

CaAl12O19 CaAl

Corundum

3Al2O3·2SiO2 ∼1850° 1850° ∼1840° 1840°

1800

1475°

1600

O5 Si a3 C

2050° ∼2050° 2070° ∼2070° 3CaO·2SiO 3CaO 3CaO·SiO ·2SiO2

800 16

CaO·SiO2 CaO·SiO ∼1544° 1544° 1544°

1600

140

1552° Lime

∼1535° 1535° 1535° 3Cao·Al 3Cao 3CaO·Al Al22O O33

∼1395° 1395° ∼1400° 1400° 1395° 1400° 12CaO·7Al 12Cao·7 12Cao ·7Al Al22O O33 1455°

∼1595° 1595° CaO·Al Cao·Al Cao Al22O O33 ∼1605° 1605°

2000

0

0 160

180

0 200

0 220

0 240

CaO ∼2570° 2570°

1500° 1455° 1512° 1350°1380° 1470° 1335° CaAl4O7 CaAl 1335° Ca3Al2O6 CaAl2O4 CaAl 1730° 1850° ∼1730° ∼1850° Cao·6 Cao ·6Al Al22O O33 CaO·2Al Cao·Al Cao Al22OO33 CaO·6Al ∼1750° 1750°

Al2O3 ∼2020° 2020°

Ca12Al14O33

FIG. 12.4 CaO–Al2O3–SiO2 phase diagram. (Reprinted from Osborn EF, Muan A. Phase equilibrium diagrams of oxide systems. The System CaO-Al2O3SiO2. In: Phase diagrams for ceramists, vol. 1; 1960, with the permission of The American Ceramic Society.)

542 Lea’s Chemistry of Cement and Concrete AI2O3 CA6 CA3 CA2

CA

CA + CA2

C12A7 + CA C12A7 C12A7 + C3A + C12A7 CA + C12A7 C2F C 3A + C2F + C3A C3A + C2F

C + C3A

CA6 + A

CA2 + CA6 CA + CA2 + T

CA + C 2F

CA6 + A+F

CA6 + F CA2 + CA6 + T

FA CA6 + T

CA + CF C2F

CA CA + + T T + CF

T

C + C3 A + C2F

+ CA6

+

F

T+F

C + C2F CF + C2F CaO

C2F

CF CF CF2 + + + T

T + CF2 T + F

CF

CF2 CF3

Fe2O3

FIG. 12.5 CaO–Al2O3–Fe2O3 phase diagram. (From Lister DH, Glasser FP. Phase relations in the system CaO–Al2O3–Fe2O3. Trans Brit Ceram Soc 1967;66(7):293–305. Copyright © Institute of Materials, Minerals and Mining, reprinted by permission of Taylor & Francis Ltd, http://www.tandfonline.com on behalf of Institute of Materials, Minerals and Mining.)

The sub-solidus CaO–Al2O3–Fe2O3 system is shown in Fig. 12.5.25 Here there are several extensive solid solutions. The most extensive is brown millerite, usually described by the formula C2A1 xFx with x taking value between 1 and 0.3.19,20 This ferrite phase is the main iron containing phase found in CACs. The x value is usually close to 0.5,21,22 similar to Portland cements (C4AF). Ferrite is not found in CACs with low iron content as F can replace A in the calcium aluminate phases. In CA, a substitution level of up to 5 wt% has been reported.23 No quaternary phases containing all four components (C, A, S, F) have been detected in calcium aluminate cements. Burning in an industrial kiln is usually optimised so that all the oxygen of the combustion air is used in the combustion reaction. This leads to somewhat reducing conditions. In addition, the stability of Fe2+ increases more with temperature than Fe3+. This results in the occurrence of Fe2+ as well as Fe3+ in CACs, and in the formation of phases able to accommodate such ferrous iron. Phases with the spinel structure can be found, such as magnetite (Fe3O4). More complex chemical compositions have also been reported where other species (Al, Cr, Mn) can substitute partly for Fe2+ or Fe3+ in the spinel structure. For cements made in extremely reduced burning conditions, wustite (FeO) can also be detected.

12.3.4.3 Modifications Due to Other Chemical Components, and Their Combinations (TiO, MgO) Titanium dioxide is often present in significant amounts in bauxite, so CACs made with bauxite contain around 3% TiO2. Titanium does not enter into solid solution in the calcium aluminate phases and is found in phases with the perovskite (CT) structure. Magnesium oxide is an impurity of limestone; it is present in low amounts (<1%–2%) and is usually present in solid solution in other phases rather than specific minerals. It is important to control the amount of MgO due to the possible formation of a quaternary phase (pleochlorite or Q-phase), (Ca20Al26Mg3Si3O68). This phase has poor hydraulic properties and can strongly reduce the reactivity of the cement as it ties up a lot of calcium and alumina in a non-reactive phase. The conditions of its formation have been studied,26,27 and its stability is extended when Fe2O3 is present but the full extent of its stability domain is still not established. Chemical components other than CaO, Al2O3, SiO2, Fe2O3, TiO2, MgO are usually present in very low amounts and do not generate any other phases in detectable quantities.

Calcium Aluminate Cements

543

12.3.5 Mineralogy of CACs There is no equivalent to the Bogue calculation to estimate the phase composition of CACs.28 However, in the past two decades there has been considerable progress in the use of Rietveld method for the deconvolution of X-ray diffraction patterns.23,29–31 With a well calibrated and controlled Rietveld calculation it is now possible to determine the quantities of the mineral phases to within 1%–2%. Table 12.2 summarises the phases found in the three main types of CAC. For the refractory grade, high Al2O3 content calcium aluminate cement types, it is important to minimise the content of components other than CaO and Al2O3 for good refractory performance. These are made from lime and metallurgical grade alumina. They contain only 2–3 main calcium aluminate phases with typically >50% CA. CACs with 70% Al2O3 also contain a large amount of CA2, and minor amounts of C12A7. For cements with 80% Al2O3, pure a-Al2O3 is usually also present. In the medium alumina range, natural raw materials are used, so that other phases are formed. CA is typically around 65%, and C2AS around 25%. The remainder is mainly CT and small amounts of C12A7. The 1%–3% of Fe2O3 is included in solid solution (mainly in CA). Low alumina CACs, because of the much higher Fe2O3 content, have a much more complex mineralogy. In addition, due to the rapid solidification from the melt, the product is not at equilibrium, which results in the formation of even more phases. Low alumina CACs may contain up to 8 or 9 phases (w€ustite is only present under reducing conditions). Their crystallo-chemistry is also more complex, with extensive solid solution. The texture is composed of some intergrown crystals which led in the past to confusion on the phase composition. This is the case for pleochroite and gehlenite27,32 and as detailed below, for the ‘ferrite’ domains.33 During the cooling process, and most probably after the liquid has solidified, the partial migration of some elements leads to the formation of two phases, which are organised in alternative layers with a thickness of a few nanometres. Fig. 12.6 illustrates the microstructure of a fused 40% alumina cement. CA can be seen as 2–20 mm dark areas. CA grains are well developed as it is one of the first phases to precipitate from the melt. The areas are probably linked in three dimensions rather than being individual grains. Between the CA regions, a number of other phases can be identified. These are, in order of increasing brightness, gehlinite, pleochroı¨te, dicalcium silicate (belite), ‘ferrite’ and spinel. No distinct areas of C12A7 are shown on the picture, as its concentration is low it cannot easily be found in the microstructure. The areas identified as ‘ferrite’ seem homogeneous in the range of SEM magnification. But it has been shown using TEM examination that these areas contain two intergrown phases.33 One of them has the crystal structure and chemistry of brown millerite, while the other one is closer to a perovskite structure, but with a large proportion of F. The intergrown texture is shown in Fig. 12.7.

12.3.6 Recent Developments As explained in the previous section, the composition of CACs is usually chosen so that the amount of CA is maximised. In most CACs this is the main reactive phase giving the cementitious properties. The hydration kinetics of CA (in the presence of small amounts of C12A7) give a setting time of a few hours followed by rapid hardening, which allows time to prepare and

TABLE 12.2

General Composition of High, Medium and Low Alumina Content CACs

Alumina Calcium di-aluminate Calcium monoaluminate Mayenite Gehlenite Larnite Pleochroite Perovskite Ferro-Perovskite Ferrite Spinel Wustite

a-Al2O3 CaAl4O7 CaAl2 xFexO4 x < 0.2 Ca12Al14O33 Ca2Al2xFexSiO7 x  0.4 b-Ca2SiO4 Ca20Al26Mg3Si3O68 2.5 < x < 3.5 CaTiO3 Ca3FeTiO8 Ca2AlxFe1 xO5 x  0.7 (Fe, Mg)(Fe, Al)2O4 FeO

Short Name

ICsD

Ref

High

Med.

Low

A CA2 CA

52,025 16,191 260

34 35 36

X X X

X

X

C12A7 C2AS

2,593 27,427

37 38

X

X X

X X

b-C2S Q-phase

963 26,353

39 40

X X

X X

CT C3FT C4AF

71,916 203,100 9,197

41 42 43

X

Ff f

30,860 60,683

44 45

X X X X

544 Lea’s Chemistry of Cement and Concrete

F E C A B

D

FIG. 12.6 BSE micrograph of a polished section of CAC clinker. A: CA; B: Ferrite; C: gehlenite; D: pleochroı¨te; E: C2S; F: spinel.21

Perovskite

Brownmillerite

FIG. 12.7 Microstructure of ciment fondu brown millerite-perovskite intergrowth.33

place mortars and castables, and let then harden in a short time. However, for some applications other calcium aluminates as the main reacting phases may be of interest. C12A7 tends to accelerate the hydration of CAC and for CACs used with water alone it is preferable to keep the amount of this phase low. But for CACs used in formulations with calcium sulfate, cements containing a majority of C12A7 have reasonable open times and can be useful to make rapid hardening mortars.46–48 In Japan, amorphous aluminates have also been developed49,50 to give even faster reacting products, mostly used in shotcreting for tunnelling. These are also used in combination with calcium sulfate. In the refractory field, a high purity cement containing spinel (MgAl2O4) is now available.51,52 Spinel aggregate and powder are frequently used with tabular alumina in refractory castables for steel ladles as they help reduce slag corrosion and penetration. In this new cement spinel is not added as a mix, but forms with the calcium aluminate phases during the production of the clinker. The spinel grains are much smaller than those from simple addition of aggregates, which makes the castables more resistant to corrosion by slag.

Calcium Aluminate Cements

TABLE 12.3

545

Overview of Main CAC-Based Binders, Main Hydrated Phases and Applications

Binder System

CAC

CAC + C$ + PC

Hydrates Related properties

CAH10, C2AH8, C3AH6, AH3 Rapid hardening Rapid drying Acid resistance Abrasion resistance High temperature resistance No free lime Rapid and repair concrete Industrial flooring Fire protection Refractory Protection liner for sewers

AFt, AFm, CH, C-S-H, AH3 Rapid setting Rapid hardening Rapid drying

Applications

Expansion/shrinkage compensation Rapid mortars and concrete Flooring Tile adhesives Grouts Fire protection

12.4 HYDRATION Calcium aluminate cements are used in a wide variety of applications. This versatility comes from different hydration reactions, either alone with water or as part of a formulation with other binders such as calcium sulfate and Portland cement. These two main categories of reaction are covered in this section: 100% CAC (Section 12.4.1) and blended systems comprising CAC + C$ + PC (Section 12.4.3). Table 12.3 gives an overview of the hydration products, related properties and applications of these two binder types.

12.4.1 Hydration of CAC with Water Alone Fig. 12.8 shows the solubility curves for hydrates in the lime alumina system at 20°C. The hydration behaviour of CACs can be understood, by referring to the solubility curves of the different hydrates, which are shown in Fig. 12.8 for the system at 20°C. Upon contact with water CA develops a hydroxylated surface layer Ca[Al(OH4)]2 which dissolves congruently and is continuously replenished. As dissolution proceeds, rapidly at first, the concentration of AlðOHÞ4  and Ca2+ in solution will move along the C/A ¼ 1 line. This line intersects the solubility curve of CAH10, but nucleation of this phase is difficult due to the need to form Al–O–Al bonds, so the concentrations of AlðOHÞ4  and Ca2+ may remain at high levels for some time before nucleation occurs as shown in Fig. 12.9. Once nucleation occurs the hydrates grow rapidly. In contrast to Portland cement where the hydrates precipitate mainly on the surface of the grains, the hydrates of calcium aluminate precipitate throughout the solution. In the case of pure CA the induction period can be quite long (e.g. 6–12 h).

FIG. 12.8 Solubility curves for hydrates in lime alumina system at 20°C. The dotted lines show the evolution of the solution for different ratios of C/A (CaO/Al2O3).

546 Lea’s Chemistry of Cement and Concrete

FIG. 12.9 Evolution of the concentration of C and A ions during hydration of CA in a solution of water:solid ¼ 10.79.89

The presence of a small amount of C12A7, changes the ratio of AlðOHÞ4  and Ca2+ in solution. As the ratio of C/A increases, the composition of the solution now intersects the solubility curves for C3AH6 and most importantly C2AH8.x3 The nucleation of C3AH6 is also difficult due to the high 3-dimensional symmetry, but C2AH8 nucleates much more easily due to its layer structure, and thus decreases setting time considerably. In the presence of significant quantities of C12A7 the setting time may reduced to tens of minutes. However, as can be seen from the solubility curves, C2AH8 is metastable with respect to C3AH6 (the solubility of C3AH6 is lower, so it is more stable). This means that once C3AH6 precipitates C2AH8 will redissolve. The solubility of all hydrates is influenced by temperature with CAH10 being most affected. The hydrated phase formed below 15°C is CAH10.53–57 Between 15°C and 30°C both CAH10, C2AH8 and AH3 can form.57–59 Though the formation of CAH10 is increasingly difficult as temperatures approach 30°C. At temperatures above 20°C both CAH10 and C2AH8 are metastable and over time these will convert to C3AH6 and AH3.57 Hydration and conversion of metastable hydrates are strongly influenced by temperature. For temperatures above 30°C, C3AH6 and AH3 gel become the favoured hydration products.28 Usually C2AH8 will form initially along with some AH3. This phase may rapidly dissolve and C3AH6 + AH3 will precipitate. As the temperature increases above 30°C, the kinetics of this reaction increase dramatically.61 It is the temperature history of the material, and specifically the amount of time spent at or above certain temperatures that determines the relative amounts of the different hydrates formed during initial hydration when CAC is used as the only binder. The metastable hydrates will eventually convert to stable phases, provided the relative humidity is high enough (as it is a through solution reaction) and the temperature of the material exceeds 20°C during at least some of its lifespan. Generally, the hydration reactions of CA (monocalcium aluminate) are considered as shown in Eqs (12.1)–(12.5). CA + 10H ! CAH10

(12.1)

2CA + 11H ! C2 AH8 + AH3

(12.2)

3CA + 12H ! C3 AH6 + 2AH3

(12.3)

3C2 AH8 ! 2C3 AH6 + AH3 + 9H

(12.4)

3CAH10 ! C3 AH6 + 2AH3 + 18H

(12.5)

The hydration reactions between CA and water are shown in Eqs (12.1)–(12.3). The ‘conversion’ of metastable C2AH8 and CAH10 (above 20°C) to the stable phases C3AH6 and AH3 (generally amorphous) is shown in Eqs (12.4), (12.5). Both CAH10 and C2AH8 have a low density and fill space rapidly. Table 12.4 shows the density and combined water of the metastable and stable hydrates. This provides the rapid strength gain of CACs.28,60 Figs 12.10 and 12.11 show the different hydrates in cement pastes cured at 20°C and 38°C, respectively.61 During conversion, the lower density hydrates convert to denser C3AH6. At a constant degree of hydration this conversion process will lead to an increase in porosity (and decrease in strength). However, if unhydrated material remains, the water released by the conversion reactions (Eqs 12.4, 12.5) can lead to further hydration, which will produce more space filling hydrates. The consequences of conversion on strength development are a function of the water to cement ratio and the time–temperature history and are discussed in detail in Section 12.6.

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TABLE 12.4 Density of Metastable and Stable Hydrates62 Phase

Density (kg/m3)

Combined Water (%)

CAH10 C2AH8 C3AH6 AH3

1720 1750 2520 2400

53 40 28 35

FIG. 12.10 CAC paste hydrated at 20°C for 24 h. The microstructure contains mainly CAH10 with some C2AH8 and partially reacted CAC grains.

FIG. 12.11 CAC paste cured at 38°C at 24 h, C2AH8 dominates, but some C3AH6 is observed near the cement grains. AH3 can be seen in the darker grey areas between C2AH8.

548 Lea’s Chemistry of Cement and Concrete

FIG. 12.12 Cumulative heat of CAC hydrated at 20°C for different w/c, without and with 0.012% wt Li2SO4.101

Space is the major factor limiting the degree of hydration. Fig. 12.12 shows how the amount of heat evolved (proportional to the amount of reaction) increases with the water to cement ratio.

12.4.2 Supplementary Cementitious Materials (SCMs) and Fillers Nowadays SCMs and limestone filler are widely used with Portland cements, either as part of blended cements or added at the concrete stage. These can also be used with CACs. With CAC alone, the lack of portlandite will mean that the pozzolanic reaction cannot occur. However, silica in slag and calcium carbonate can lead to the formation of stable AFm phases, stratlingite or mono-carboaluminate. The formation of such phases may reduce the amount of C3AH6 in the stable phase assemblage and so the decrease in volume of hydrates (and strength) on conversion.63–66 Solutions containing nitrate or chloride may also form stable AFm phases and have a similar effect. Many authors claim that these additions ‘inhibit’ conversion, but this is incorrect. The formation of the dense C3AH6 hydrate is not inhibited, but may no longer be part of the thermodynamically favoured phase assemblage.67,68

12.4.3 Blends of CAC, PC and C$ 12.4.3.1 Description of Ternary Diagram CAC can be used in combined systems, where it is mixed with other binders such as Portland cement and/or calcium sulfates. In such systems hydration is different from pure CAC systems. In these systems, the components can be tailored to give specific properties such as rapid setting or shrinkage compensation. This section details the hydration mechanisms and related properties of such systems. Table 12.5 shows the different hydrates which can be formed depending on the binder phases and the resulting ions in solutions. CAC supplies aluminate and calcium ions to the solution. The presence of calcium sulfates brings sulfate ions as a third component. Portland cement will contribute to the calcium ions in solution and also bring some silicates ions. Compared to pure CAC, the metastable hydrates CAH10 and C2AH8 no longer form in significant amounts. Instead new hydrates can form C3A(CaSO4)3H32, (ettringite), C3A(CaSO4)H12, C4AH13, and C2ASH8. AH3 also often present. Ettringite (Fig. 12.13) is in most cases the most important hydrate largely responsible for the specific properties of these systems: quick setting, rapid hardening, rapid humidity reduction (drying) and shrinkage compensation. Depending on the relative proportion of CAC, PC and C$, different hydrates and properties are obtained. Fig. 12.14 illustrates the three most important composition zones from an application standpoint. The main hydrates which form are shown in Fig. 12.15. Zone 1 covers binary systems of PC and CAC. Such systems give quick or even flash setting of only a few minutes which can be useful for small repair works. The advantage of such systems is their simplicity with only two components, but only moderate strengths are achieved (lower than PC) and the absence of shrinkage compensation limits their use to less

Calcium Aluminate Cements

TABLE 12.5

Main Binder Phases in Blended Systems and Resulting Hydrates

Binder Phase

Ions

CA (35% CaO_65% Al2O3) CaSO4  0.5 H2O C3S (74% CaO + 26% SiO2) Combined system CA + C$ + C3 S

C3A

(A)

549

Hydrates 

AlðOHÞ4 Ca2+ Ca2+ SO4 2 Ca2+ SiOðOHÞ3  AlðOHÞ4  SO4 2 Ca2+ SiOðOHÞ3  Ca2+ Al(OH)4

CAH10, C2AH8, C3AH6 AH3 CaSO4H2O CSH ‘gel’ Ca(OH)2 ‘portlandite’ C3A(CaSO4)3H32 ¼ ettringite (Aft) C3A(CaSO4)H19 ¼ monosulfo (Afm) C4AH13 C2ASH8 AH3 C3A(CaSO4)3H32 ¼ ettringite C3A(CaSO4)H19 ¼ monosulfo

(B) FIG. 12.13 Example of ettringite crystals morphology (A) and crystallographic hexagonal cell (B).

demanding applications. The proportion of CAC needed for quick setting varies between 5% and 50% (mostly 10%–30%) depending on the required setting time and the PC composition. To understand the mechanism behind this quick setting we need to consider the C3A content of the PC. Normally the fast reaction of C3A is controlled by the addition of calcium sulfate during grinding. The sulfate ions adsorb on the reactive sites of this phase and slow down its dissolution. The dosage of calcium sulfate (typically around 5% added during clinker milling) is optimised depending on the C3A content and the nature of the available calcium sulfate (gypsum, hemi-hydrate or anhydrite). When CAC is added to PC, there is a global excess of calcium aluminate phases: C3A from PC and now also CA from CAC; compared the initially optimised calcium sulfate. These aluminate phases rapidly react with the available sulfate to form ettringite, so the sulfate is no longer sufficient to inhibit the reaction of C3A. C4AH13 may also form once the sulfate is exhausted, but it seems ettringite is the main hydrate giving the initial stiffening of the paste. The major drawback is that the concentration of alumina in solution increase and this in turn inhibits the C3S hydration, by adsorption on reactive sites, leading to a lower long-term strength compared to pure PC. The final hydrate assemblage will be mainly composed of ettringite and C-S-H with some C4AH13. Fig. 12.16 illustrates an acceleration curve obtained by adding CAC to PC. The best dosage of CAC will depend on the desired setting time, but also on the nature of the PC and in particular the reactivity of its calcium sulfate (gypsum, hemihydrate or anhydrite) and the content of C3A. If small amounts of PC are added to CAC you will get a similar effect.

550 Lea’s Chemistry of Cement and Concrete

C$

3 2 1 CAC

PC

FIG. 12.14 Three different possible zones of composition of blended systems.

C$Hx

C-S-H CH AFm Strätlingite Hydrogarnet Ettringite + AH3 + monosulfate

CAC

PC FIG. 12.15 Ternary diagram showing the possible hydrates in the CAC–PC–C$ system.69

In systems blended with calcium sulfate (usually as anhydrite), PC (majority) and CAC the calcium sulfate increases the amount of ettringite formed at early ages and keeps the aluminate ion concentration in solution low. This thereby minimises its effect on slowing down C3S hydration. Such systems enable high early strength to be obtained (e.g. 20 MPa at 3 h) without compromising long term strength as shown in Fig. 12.17. It is also possible to adjust the shrinkage compensation to cast large horizontal surfaces with limited risk of cracks. These properties are brought by early formation of ettringite, whose intrinsic properties are discussed in Section 12.4.4. The strength development of systems in zone 2 is characterised by two steps as illustrated in Fig. 12.17. The first step, occurring during the first 24 h, is related to the massive precipitation of ettringite, resulting mainly from the hydration of CAC and C$, but also with a contribution from the PC phases, which can contribute calcium ions. Ettringite is formed according to the following equation:

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350 Portland cement A Portland cement B

Initial set (min)

300 250 200 150 100 50 0 0

10

20

30

40

50

% CAC addition to OPC

Compressive strength (MPa)

FIG. 12.16 Example of setting time acceleration by adding CAC to PC.62

Hydrates

C-S-H

PC

Ettringite C$ CAC

(A)

80

7

60 40

3

y

d

d

h

20

70% OPC + 30% [CAC+C$] 100% OPC

0 1

Time

1

28

(B)

10

100

1,000

10,000

Time in hours (log scale)

FIG. 12.17 (A) Schematic of the contribution of CAC and PC to hydrate development in blended systems. (B) Strength evolution from a mix in zone 2 (w/b ¼ 0.35) compared to a 100% PC reference (w/b ¼ 0.40).96

(3–n) CA + 3C$ + nC + (32+3n) H from CAC and

‡

C3 A(C$)3 H32 + (2–n) AH3

from PC

It can be seen that the ratio of ettringite to hydrated alumina is determined by the supply of lime (C). The dissolution of the PC phases can contribute calcium ions to the system, expressed by the ‘nC’ part of the equation. The second step, occurring between 1 and 28 days, results from hydration of PC phases, mainly C3S. The final phase assemblage is close to that from PC with C-S-H as the main hydrate, together with ettringite, and AFm phases (monosulfoaluminate, hemi- and mono-carboaluminates). Either CH or AH3 will also be present according to the amount of lime in the system. Fig. 12.18 illustrates how the degree of hydration of these two components: CAC + C$ (ettringite precursor part) and the PC varies over time. The evolution from zone 2 to zone 3 corresponds to a reduction of PC content and an increase of [CAC + C$], which means that ettringite and hydrated alumina become the major hydrate phase as shown in Fig. 12.13. In these systems calcium sulfate is usually added as hemi-hydrate (plaster) or gypsum. By comparison with zone 2, the properties of systems in zone 3 related to ettringite formation are significantly improved: higher early strength, better shrinkage compensation and quicker humidity reduction (drying), as illustrated in Fig. 12.19. The higher cost of such systems (due to the higher content of CAC) is justified in highly demanding applications such as flooring (see Section 12.5.2). The link between ettringite and such properties is explained in the next section.

552 Lea’s Chemistry of Cement and Concrete 100%

80%

60%

1 day 28 days

40%

180 days

20%

0% DH [CAC+C$]

DH OPC

FIG. 12.18 Degree of hydration of anhydrous phases from a mix in zone 2 (70 wt% PC + 30% (CAC + C$), w/b ¼ 0.35).70

16 Formulation zone 2, binder wt% = 25 wt% CAC 9 wt% C$ 66 wt% OPC

Performance measured at 6 h

14 12

Formulation zone 3, binder wt% = 66 wt% CAC 20 wt% C$ 14 wt% OPC

10 8 6 4 2 0 Compressive strength (MPa)

Flexural strength (MPa)

Residual humidity (wt%)

FIG. 12.19 Comparison of properties delivered at 6 h by system in zone 2 and zone 3 (both formulations contain 30 wt% of binder with a w/b of 0.77). Samples ¼ prisms 20  20  160 mm, unmoulded at setting time (1 h in both cases) and cured at 23°C/50% RH.

12.4.4 Intrinsic Properties Brought by Ettringite Formation There have been many investigations on ettringite as a minor phase of Portland cement, but few studies published on systems where ettringite is the major hydrate. This section proposes some mechanisms and hypothesis to explain the special properties brought by ettringite binders.

12.4.4.1 Rapid Setting and Hardening Rapid setting and hardening properties are due to the rapid precipitation of ettringite, which is not the case for all hydrates. For example, CAH10, AH3 or C3AH6 have long nucleation times even when highly supersaturated, while C2AH8, like ettringite, nucleates easily and shows quick growth, even at low degrees of supersaturation. The reasons for these differences have not really been studied in detail; the difference may be related to the need to form Al–O–Al bonds in the former case, while the rapidly nucleating phases contain isolated Al atoms linked by oxygen to calcium or hydrogen. Despite the ease of precipitation of ettringite, its formation can be controlled with suitable admixtures to get optimised workability and setting time depending on the application (see Section 12.4.6). 12.4.4.2 Rapid Drying Moisture reduction is a key function of self-levelling compounds used for flooring applications, such as self-levelling underlay (SLU) (Section 12.5.2). This is usually termed drying even when it is more a case of water combination in hydrates

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TABLE 12.6 Drying Capacity (wt% of Crystallised Water Over wt% of Anhydrous Phases) of Ettringite Compared to Other Hydrates Drying Capacity in wt% Ettringite C3A 3(CaSO4)32H CAH10 C2AH7.5 C3A(CaSO4)12H AH3 microcrystalline AH3 gibbsite C2/3SH1.5 C5/6S2/3H1.83 C1.33SH2.17 C1.5S0.67H2.55 CaSO4.2H

85 113 67 54 54 54 32 32 32 32 27

rather than loss of water to the atmosphere. The ‘drying time’ is the time at for the residual moisture (free water) to fall below a given threshold which depends on the application (typically 3 wt%). Moisture reduction is driven by two mechanisms (i) internal drying by binding free water within hydrates and (ii) external drying by water evaporation at the upper surface or diffusion from the lower surface into the substrate when it is porous (e.g. concrete). Ettringite binders allows a rapid internal drying for two main reasons:  For a given quantity of cement, the quantity of water combined in ettringite crystals is higher than in other hydrates. This is shown in Table 12.6 which compares the internal drying capacity of different hydrates. The capacity of ettringite is 85%, meaning that 100 g of anhydrous phases can combine 85 g of water within the crystallised network of ettringite. As a comparison, the capacity of gypsum is only 27%, whereas it is around 30% for C-S-H.  Ettringite crystals typically form more quickly than other hydrates.

12.4.4.3 Expansion/Shrinkage Compensation Shrinkage compensation is another key function of horizontal thin compounds to prevent cracking or curling. Under certain conditions ettringite formation can generate expansion. If this expansion occurs during hardening, it can compensate later shrinkage due to external drying, chemical or autogenous shrinkage, this is referred to as ‘shrinkage compensation’. However, if this expansion occurs in the long-term within a hardened material, it can generate disorder and cracks (see Section 12.4.5). The mechanism behind expansion is based on the pressure generated by hydrate crystals on the pores surfaces while they are growing.71–73 The relatively high solubility of gypsum enables pore solutions supersaturated with respect to ettringite to be produced as long as gypsum is present. This allows the growth of ettringite to generate expansion.

12.4.5 Some Formulation Guidelines 12.4.5.1 Impact of Calcium Sulfate Types In all the zones 1, 2 or 3, ettringite formation is responsible for early age properties. In zone 1, it is the calcium sulfate from PC which reacts with CAC to form ettringite. In zone 3 the main contributor to ettringite is the reaction of calcium sulfate with CAC. In zone 2 both calcium sulfate from PC and the added calcium sulfate contribute. The nature of the added calcium sulfate has a strong influence on the early age properties. Calcium sulfate exist with three types of mineralogy: anhydrite (CaSO4), hemi-hydrate (CaSO4  0.5H2O), and di-hydrate (CaSO4  2H2O). Although their dissolution produces the same ions in solution, Ca2+ and SO4 2 , their solubility and rate of dissolution are different, as shown in figure below. This has a significant impact on ettringite formation and its related properties. In zone 3, the preferred type is hemi-hydrate or gypsum, whereas in zone 2 the preferred type is anhydrite. The use of the inappropriate type may lead to incomplete reaction of the calcium sulfate source, a lower amount of ettringite formed initially leading to poor early age performance and risk of damaging expansion at later ages (see section on control of expansion). If the content of PC is increased, which is the case from zone 3 to zone 2; the optimum type of calcium sulfate changes from hemi-hydrate or gypsum to anhydrite. The use of hemi-hydrate or gypsum leads to a strong delay of hydration.74

554 Lea’s Chemistry of Cement and Concrete

12.4.5.2 Impact of PC The composition of the PC can affect the hydration and properties of formulated systems in several ways. C3S contributes calcium ions to the solution, C3A calcium and aluminium ions, calcium sulfates calcium and sulfate ions and alkali sulfates bring sulfate and affect the pH. All of these can impact the formation of ettringite. In zones 1 and 2, where PC is the major phase it is variations in the calcium sulfate and type of alkali sulfates in the PC which have most impact. For example, in zone 1 the % of CAC to obtain a setting time of 10 min can vary from 14% where anhydrite is the main type of calcium sulfate in the PC to 27% for gypsum and up to 37% for hemi-hydrate.75 In zone 3, where the content of PC is small, the amount of sulfate phases from the PC are small and the contribution of PC will mainly come from C3S as a calcium source. In addition to these effects on ettringite formation, the hydration of PC will lead to the formation of C-S-H in zone 1 and 2, but not in zone 3 where all the calcium will be consumed to form ettringite. 12.4.5.3 Impact of CAC Mineralogy Historically, the formulated systems discussed here have been designed around fused CAC clinker with CA as the main phase. The secondary phases result in colours varying from light grey to dark. CAC produced by sintering are generally richer in Al2O3 and contains much lower amounts of secondary phases, with colour near to white. Fused CAC is most widely used in combined system, but sintered CAC can also be used for some application, for example, if colour is one of the key functions. Recently new fused CACs with C12A7 as the main phase have been developed to be used specifically in combined systems in zone 3. With a molar ratio of C/A of 1.7 compared to 1 for the phase CA, it brings more calcium ions into solution, so that ettringite forms easily, without the need for addition of PC, removing this source of variability. This is illustrated in Fig. 12.20. The use of the binary system using C12A7 based CAC (F2) leads to a single strong peak in the calorimetry (left), reflecting an enhanced formation of ettringite. This enhanced formation of ettringite has a positive impact on strength development, especially at 5°C (right). 12.4.5.4 Control of Expansion Early expansion by ettringite formation is desired in thin layer applications like flooring. It can compensate latter drying shrinkage and allow large joint less surfaces to be produced without cracking.73 However, long-term ettringite formation in an already hardened material has to be avoided because it can cause disorder and cracking.60 This can happen if some unreacted calcium sulfate remains in the mortar. The first cause can be too high calcium sulfate dosage in the formulation. A general rule of thumb to avoid this is to keep a ratio CAC/calcium sulfate >2. A second reason could be an inappropriate calcium sulfate source, for example, using anhydrite in zone 3 or hemi-hydrate/gypsum in zone 2. In this case, the initial reaction of calcium sulfate may be incomplete, leaving some unreacted phase.

12.4.6 Impact of Admixtures and Organic Additives (See Also Chapter 14) As for ordinary Portland cement, the workability of calcium aluminate cement based concrete and mortar can be optimised with the use of chemical admixtures. However, due to very different hydration mechanisms, formulation rules can vary significantly from the usual PC rules. As an example, CaCl2 is well known as setting accelerator for PC whereas it is a retarder for CAC. As the major use of CAC is as part of formulations sold as dry mix, most of the admixtures used are in the powder form. Admixture and formulation rules depend on the type of binder system used, either pure CAC or blends of CAC, PC and C$, as the hydration is very different. It also depends on the final application as refractory concrete or self-levelling mortar requires very different properties. Despite these differences, the first step is often the same, use of a retarder and set accelerator to regulate hydration kinetics and thereby setting time and hardening rate. A combination of retarder and accelerator might seem a contradiction, but it is a way to obtain a more robust behaviour for example, combining fluidity for placement with rapid setting and strength gain at the desired point in time. Further formulation steps are the use of superplasticiser to fine tune the water demand, the use of thickening agent, air management (entrainment or defoamers) or even polymer addition such as latex in order to obtain specific consistency, rheological behaviour, improve flexibility and adherence. These secondary steps can vary considerably depending on the system and final application.

12.4.7 Retarders The more universal retarders either for pure CAC system or blended binders are hydroxycarboxylic acids—citric, tartaric or gluconic as well as the salts of these acids.

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FIG. 12.20 Impact of CAC mineralogy on hydration (A) and strength (B) of formulation in zone 3, Mortar ¼ 30% of binder, water/binder ¼ 0.80, Binder F1-w PC ¼ 65% of CA based CAC + 23% of hemi-hydrate + 13% of PC, Binder F1-wo PC ¼ 67% of CA based CAC + 33% of hemi-hydrate, Binder F2 ¼ 67% of C12A7 based CAC + 33% of hemi-hydrate.76

Some of these salts, notably sodium citrate, also have marked water reducing properties in some systems and can increase the fluidity of the cement paste at low water/cement ratios. Sugars are also efficient but not very often used due to too strong a retardation effect (Table 12.7).

12.4.8 Acceleration In pure CAC system, accelerator is needed to reduce setting times. In blended systems, the setting time is usually rapid via early ettringite precipitation. However, as a retarder is often used in these systems to manage the workability, an accelerator may also be needed to counter balance the effect of the retarder and obtain a more robust system that balances workability with setting time. The most common accelerators for CAC hydration are lithium based salts, the most widely used being Li2CO3 salts. Gosselin and co-workers showed that the microstructural development of a commercial (Al2O3  50%) CAC was not only dependent on the curing temperature, but also on the use of accelerating admixtures, in this case Li2SO4. Li2SO4 provided

556 Lea’s Chemistry of Cement and Concrete

TABLE 12.7 Main Setting Regulators Used with CAC Accelerators

Retarders

Lithium carbonate Lithium sulfate Lithium hydroxide Sodium carbonate Calcium hydroxide Aluminium sulfate Sodium aluminate

Sodium gluconate Sodium citrate Citric acid Tartaric acid Sugar

better nucleation of hydrate products and thus a more even microstructure.101 The mechanism of action of Li2SO4 in combination with tartaric acid in a blended system was studied by Pommay and co-workers. They showed that the main impact of Li2SO4 was to accelerate the rate of ettringite formation and hardening, but not necessarily to reduce setting time. In blended systems, the need for lithium salts decreases with the content of CAC. In formulations dominated by PC, an accelerator may still be needed. This is often Na2CO3, although a combination of Na2CO3 and a lithium salt can bring better early performance for certain cases.

12.4.9 Water Reducers, Fluidifiers and Superplasticisers Superplasticiser based on polycarboxylic ether (PCE) technology will work well to give fluid calcium aluminate cement mortars or concrete. It is also possible to obtain very long setting times and high fluidity with C12A7 based CAC, especially in blended systems. Sulfonated melamine formaldehyde (SMF), is also known to be a good water reducer for calcium aluminate cement systems, but its use tends to be limited for safety reasons due to undesirable volatilisation of formaldehyde. Sodium tri-polyphosphate or hexa-metaphosphate are also widely used for refractory concrete containing silica fume. In these specific formulations they play the role of superplasticiser and retarder by deflocculating the silica fume and retarding the hydration of CAC. These are generally added as part of a complete dry-mixed product sold to the consumer.

12.4.10 Air Content Management Air entraining agents or defoamers can be used with CAC systems depending on the objectives (improve durability the freeze–thaw, compensate air entraining of some superplasticisers, etc.). In general the molecules used for PC in these domains also work with CAC with some slight differences. For example, proprietary air-entraining agents mainly based on neutralised vinsol resins do not entrain as much air in CAC as with Portland cement.90

12.4.11 Thickener Thickening agents are commonly used in dry-mix mortars for building chemistry to fine tune rheological properties like anti bleeding in self-levelling compounds or pasty behaviour in tile adhesive. Common thickening agent used for Portland cement can generally be used with CAC, the most widely used product being cellulose ether.

12.4.12 Latex and Polymer Resins—Formulated Products Both natural and synthetic latex have been used successfully in CAC in order to produce proprietary mortars, screeds and surface coatings. These are often used to produce chemically resistant floor toppings, where CAC performs well compared to Portland cement.90 Styrene butadiene latex is used to improve bonding to many substrates90 as are many other polymer resins, such as acrylic polymers, polyvinyl acetate) and epoxy resins. In addition to improving adhesion and flexural strengths, the very wide range of polymer admixtures available today is used to enhance the physical properties of many formulated products. These include repair mortars, tile adhesives, grouts and self-levelling floor screeds. CAC is used in such products either as the main cementitious binder or, more often, mixed with other hydraulic binders such as Portland cement, plaster, gypsum and lime. Great skill is required when formulating such products since the incorrect use of mixtures of these materials can lead to undesirable effects such as flash setting, cracking and uncontrolled expansion (see also Section 13.10.7).

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12.5 APPLICATIONS 12.5.1 Building Chemistry The term ‘Building Chemistry’ is often used by the dry-mix mortar industry referring to finishing or repair mortars. The use of calcium aluminates in these applications has grown continuously during the last 30 years corresponding to a need of increased productivity and aesthetic considerations. In most of these applications, combined systems containing CAC, with calcium sulfate and or PC (as described in Section 12.4.3) and the calcium aluminates cements are a precursor to ettringite formation. Early ettringite formation gives rapid setting or hardening, rapid reduction of internal humidity (drying) and shrinkage compensation.

12.5.2 Flooring In modern flooring systems formulations based on early ettringite formation can allow high productivity. Products can be walked on typically after 3 h and a top coating (carpet, wood flooring, tile, etc.) can be applied after 24 h. Mortars used in these applications are typically self-levelling thanks to high water content (typically w/b  0.8) and use of superplasticiser. Despite this high water content, there is a quick reduction in relative humidity (commonly known as drying) due to the water of crystallisation in ettringite, which also gives rapid hardening. Shrinkage compensation is another key characteristic allowing crack free surfaces despite large and thin layers with no joints. Typical flooring applications are (see Fig. 12.21):     

Self-levelling underlayment for floor-sealing and smoothing Self-levelling screeds Rapid drying screeds Industrial floors Decorative screeds

12.5.3 Adhesive and Grouts Cement based adhesives are used to achieve good adhesion between ceramic, natural stone, marble and glass tiles and a wide variety of supports (concrete slab, cement or anhydrite screed, wood, ceramic tiles, PVC). Calcium aluminates can be used when rapid adhesion is needed for rapid return to service, such as commercial or industrial areas. White calcium aluminates are sometimes used for aesthetic considerations (colour grouts, transparent marble, etc.). Typical applications are (Fig. 12.22):    

Rapid set tile adhesives Adhesives for natural stone and marble Fluid adhesives Grouts

FIG. 12.21 Examples of self-levelling underlay.

558 Lea’s Chemistry of Cement and Concrete

FIG. 12.22 Examples of tile adhesives and grouts.

12.5.4 Technical Mortars Technical mortars are used for repair of precast concrete or on road surfaces, for waterproofing, anchoring. Rapid hardening and shrinkage compensation are key properties for rapid return to service and durability with no cracks. Typical applications are (Fig. 12.23):    

Masonry and waterproofing Repairs to road surfaces and drains Concrete repairs Sealing and anchoring mortars

(A)

(B)

(C) FIG. 12.23 (A) Anchoring mortar, (B) repair of precast element and (C) fixing mortar on road surface.

Calcium Aluminate Cements

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FIG. 12.24 Cement-based paint.

12.5.5 Priming and Finishing Walls and Facades In walls and facades, calcium aluminates systems can be used for productivity reasons, but also for aesthetic reasons, rapid drying and the absence of free lime leading to smother and more homogeneous coatings compared to Portland cement systems. Typical applications are shown in Fig. 12.24:  Paint coatings, cement-based paint  Decorative renders  External insulation finishing systems

12.5.6 Rapid Repair/Construction Calcium aluminates cement can be used as plain binder to make technical concrete with specific properties not achievable using ordinary Portland cement: rapid hardening, resistance to mechanical and thermal shock, resistance to aggressive environments. One distinct advantage of CACs, compared to PC, is that the strength gain is also rapid at low temperatures. This can make CACs an attractive option for rapid repair in cold-weather climates and at seasonally cooler times of year. Fig. 12.17 in Section 12.4.3 shows strength gain of CAC-based compared to PC-based rapid setting concretes. Fig. 12.25 shows a rapid repair done with CAC concrete to PC reinforced concrete bridge deck in Chicago, Illinois in 2010 and the use of CAC for control of abrasion in a dam spillway. Typical applications of rapid concrete are:      

Overnight repair work on highway slabs with a turnaround time of as little as 3 h following the placing of the concrete Repairs of airport runways and ramps Industrial infrastructure repairs with minimum disruption to operations Repairs of bridge joint closures Works in tunnels and mines with rapid turnaround Cold-weather repairs

12.5.7 Abrasion Resistance Calcium aluminates can also be used as aggregates in combination with calcium aluminate binder. The surfaces of calcium aluminate aggregates hydrate and react with the binder, leading to continuity of phases and properties (thermal dilation

560 Lea’s Chemistry of Cement and Concrete

(A)

(B)

FIG. 12.25 (A) Rapid repair of reinforced concrete bridge deck in Chicago, Illinois, 2010 and (B) dam spillway with strong abrasion.

FIG. 12.26 Interfacial transition zone (ITZ) between CAC and alag aggregate.77

coefficient), with no weakness at the paste/aggregate transition zone (see Fig. 12.26). This specific interaction leads to exceptional resistance to mechanical, thermal and chemical aggressions. CACs are in particular known for their high resistance to abrasion. Typical application of such concretes are:    

Places subject to high temperature and thermal shock (e.g. foundry floors) Places subject to mechanical wear caused by abrasion and mechanical impact (ex ore passes in mining) Hydraulic structures subject to abrasion, erosion and impact (e.g. dam spillway) Places subject to corrosion caused by diluted acids (pH in the range 4–7) in industrial environments (e.g. milk industry)

Calcium Aluminate Cements

561

12.5.8 Heat-Resistant and Refractory Concretes Although calcium aluminate cements were not originally developed for their heat-resistant properties, this application has become one of the most important today. The high lime and silica contents of Portland cement render it unsuitable to produce refractory concrete for use at high service temperatures, due to the formation of low melting point eutectics. The refractoriness of calcium aluminate cements increases in with the alumina lime ratio; thus, the higher alumina content white CACs may be used at higher temperatures than the basic grades made by fusion. The capacity to resist high temperatures, and cycling between high and ambient temperatures, are associated with the absence of hydrated lime (portlandite, Ca(OH)2) in the hardened cement. At temperatures above 450°C the hydrated lime in Portland cement is dehydrated to quicklime (CaO). This is a reversible reaction78 and subsequent cooling and exposure to moisture will lead to disruption of the concrete. Hydrated lime is not formed during the hydration of calcium aluminate cement and, although the hydrates that are formed (Section 12.4.1) are dehydrated at high temperature, the decomposition products are stable in themselves or form stable compounds with the aggregates used. Fig. 12.27 shows the relationship between the Al2O3/CaO ratio and refractoriness. However, this is not the upper limit of the service temperature of concretes made with these cements. The aggregates used in refractory concretes may extend the service temperatures 100°C–200°C beyond the fusion point of the pure cement by formation of higher melting point eutectics.79 Thus combinations of the commercially available CACs having alumina contents of 40%–80% with the very extensive range of heat-resisting, insulating and refractory aggregates has led to the development of hundreds of proprietary refractory concretes, known in the industry as ‘castables’. Table 12.8 gives an indication of the composition of some of these concretes. The high-temperature performance of modern refractory concretes is now equivalent to or better than some refractory bricks (see below). The major thermal industries, including steel, non-ferrous metals, ceramics, potteries and petrochemicals, could not function today without refractory calcium aluminate cements.

12.5.9 Heat-Resistant Concretes Concretes resistant, up to 900°C–1000°C are generally referred to as heat resistant rather than refractory. These concretes are based on the standard CAC (about 40% Al2O3) together with aggregates such as granite, basalt, whinstone and traprock. Such concretes are dense and abrasion resistant and are used for floors in industries such as steel, foundries and aluminium, where hot metal spillage or splashing may occur (Section 12.5.7). A more specific use for such concretes is in the construction of fire training areas and buildings. In these structures fires may be set and extinguished many times during a day and a typical structure is likely to have a lifetime of several years. The structural integrity of Portland cement concretes would be rapidly destroyed in these conditions and only CACs are suitable.

12.5.10 Conventional Dense Refractory Castables Castable refractories tend now to be classified according to their cement contents, conventional castables is the term used for the traditional type of refractory concrete, having 15%–30% CAC. The refractory and thermal properties of these castables are

Pyrometric cone temperature/C

1800 1700 1600 1500 1400

Type of CAC Dark brown Grey White White

1300

Al2O3/CaO 1.15 1.4 2.5 4.7

PCE/C 1270–1290 1430–1450 1590–1620 1770–1810

1200 0.5

1.5

2.5 3.5 4.5 Weight Al2O3/CaO ratio

5.5

FIG. 12.27 Relationship between Al2O3/CaO ratio and refractoriness.

562 Lea’s Chemistry of Cement and Concrete

TABLE 12.8 CAC Composition and Refractory Temperatures Cement Type Heat-resistant concretes Dark brown CAC Dark brown CAC Dark brown CAC Grey CAC Dense refractory concrete Dark brown CAC Grey CAC White Dark brown CAC Grey CAC White CAC Dark brown CAC Grey White White White White White White Thermally insulating concretes Grey Grey Grey Brown White White

Al2O3 (%)

Aggregate Type

Approximate Temperature Limit (°C)

40 40 40 50

Granite/whinstone/basalt Emery AIag Olivine

700–800 1000 1100 1200

40 50–55 70 40 50–55 70 40 50–55 70 80 70 80 70 80

Chamotte (42%–44% Al2O3) Molochite (kaolin)

1300 1400 1450 1350 1450 1550 1400 1550 1650 1750 1800 1850 1800 1900

40 40 40 50 70 80

Sillimanite, gibbsite

Brown fused alumina

White fused alumina Tabular alumina

Pumice, diatomite Vermiculite, perlite Lytag, Lecat Expanded chamotte Bubble alumina

900 1000 1100 1300 1700 1800

governed by the type of cement used (40%–80% alumina) and the aggregates (Table 12.4). These products have been in use for many decades and were, until the 1980s, the only type of hydraulically bound refractory castable in use. While these concretes are still very widely used, their mechanical performance at high temperatures is limited by the relatively high cement content. Since the refractoriness of a castable is strongly related to the Al2O3/CaO ratio, the drive to produce higher alumina content cements has led to the development of CACs with 80% alumina. These cements, with aggregates such as fused or tabular alumina, can produce castables with a temperature resistance up to 1800°C–1900°C. However, their mechanical performance at temperature, although very good, is limited by the lime (CaO) content of the cement.78 In order to push the performance even higher, cements with 90% alumina were tried, but these were never widely used.80 Low-cement castables (see below) were developed in order to improve further the mechanical performance of refractory concretes at high temperatures.

12.5.11 Insulating Concretes Typical dense refractory concrete has a relatively high thermal conductivity, and thus in any heat-retaining vessel, furnace, kiln, etc., the heat losses would be high unless thermally insulated. CACs with suitable lightweight and heat resistant aggregates (Table 12.4) may be used to produce insulating concretes with thermal conductivities in the range 0.5–0.5 W/(m K). The hot-face temperature limit of these concretes is governed by the performance of the aggregates and these should be selected on the basis of temperature and thermal performance required. It is common practice to combine a dense hot-face refractory concrete with an insulating backing concrete in order to control the thermal gradient in the furnace wall and limit heat losses. In extreme cases, different grades of CAC (40%–70% Al2O3) may be used in the cold and hot faces. Since these cements are all compatible with each other, such concretes can be cast monolithically.

12.5.12 Low-Cement Castables In the mid 1980s, refractory castables with cement contents of 5%–8% were developed. These became known as low-cement castables (LCC) or in the United States, as low-water castables. The technology which allowed such concretes to be developed, was dependent on two main factors:

Calcium Aluminate Cements

563

 Theories of particle packing and optimised granulometry81,82  The availability of ultrafine silica (silica fume) and alumina. In a conventional castable, the cement would normally be the finest ingredient. Good-quality silica fume has a mean particle size in order of magnitude lower than cement and thus allowed the formulation of continuous grading curves down to submicrometre levels. The water demand of such systems is optimised at its lowest level by the filling all interstitial voids with these very fine particles (hence the term low-water castables). The binder in these systems consists of roughly equal quantities of CAC, silica fume and reactive alumina, hence the cement content is reduced to about 5%.83 The simultaneous reduction of the cement content of the castable, together with refractory properties of the alumina and silica at high temperatures, led to a significant improvement in the refractory performance of these materials.84 During the 1980s and 1990s, the technology which led to the development of LCCs was improved by the use of additives and superior grades of fume silica and alumina, and thus cement contents were pushed even lower to 3%–5%. In some specific cases, the cement content, usually white, 70% cement can go down to 1% or 2%. These products became known as ultra-low cement castables (ULCC) and refractory performance was again improved. The latest technology has concentrated on two areas. The rheology has been optimised such that castables can now be placed without mechanical vibration, these are known as self-flow castables (SFC)85 and ultrafine refractory alumina have been used to replace the fume silica with the objective of further increasing the temperature resistance.86 High-performance refractory concretes have in many cases now replaced refractory bricks, bringing improved performance, life and ease of installation and repair.

12.5.13 Refractory Concretes for Reducing Atmospheres In some industries, the furnace atmosphere in certain processes is severely reducing, leading to high carbon monoxide (CO) levels in the gases. Refractories containing iron or iron oxide compounds are attacked by carbon monoxide87 leading to disruption of the concrete or bricks. The so-called mid-range CACs having alumina contents of 50%–60% will normally have iron oxide contents of about 3% or less. These cements, and the higher range cements, having virtually no iron compounds, are suitable for concretes used in furnaces with reducing atmospheres.

12.5.14 Pipes and Wastewater One very specific property of calcium aluminates cement is their resistance to biogenic corrosion in sewer and wastewater systems. The mechanisms determining this are described in more detail in Section 7.7. A major factor is that alumina can directly suppress the action of bacteria. A material made with calcium aluminate aggregates and calcium aluminate cement, 100% calcium aluminate material, will last longer as the aggregates can also limit microorganisms’ growth and inhibit the acid generation. Typical applications are (see Fig. 12.28):  Ductile iron pipe for waste water have an internal lining made of calcium aluminate cement mortar,  Concrete pipes for sewerage are made either with full mass calcium aluminate cement concrete or with an internal liner of calcium aluminate cement mortar,  Rehabilitation of man-accessible sewer infrastructures with 100% calcium aluminate mortar using one of the following installation methods: low pressure wet spray, spinning head wet spray or high pressure dry spray (gunite).

12.6 STRENGTH AND CONVERSION IN CAC CONCRETES The primary property of concrete for any design is the long-term strength. For Portland cement concrete this is typically measured at 28 days with the assumption that the long-term strength will remain somewhat higher than this. Conversion in calcium aluminate cement concretes has led to concern over their use, as strength measured at 28 days may not be a good indication of the long-term strength if the concrete has been maintained at low temperatures during the first 28 days so that conversion has not occurred. It is important to understand that strength of CAC concrete (as any concrete) is related to the porosity and so to the volume of hydrates. Converted CAC concretes can have good strength provided the water to cement ratio is kept low so that the originally water filled space can be adequately filled by the converted hydrates. Fig. 12.29 shows schematically the volume partition in CAC pastes with high and low water to cement ratios in the unconverted and converted state. For the high water to cement ratio, most of the hydratable material can react to give metastable hydrates which fill most of the volume and thus give

564 Lea’s Chemistry of Cement and Concrete

FIG. 12.28 (A) Ductile iron pipes internally coated with CAC lining and (B) repair of manhole with spayed CAC mortar.

100%

100%

75%

75%

50%

50%

25%

25%

0%

0% Initial Unreactive anhydrous

Non converted Reactive anhydrous

Hydrates

Initial

Converted Water

Pores

Unreactive anhydrous

Non converted Reactive anhydrous

Hydrates

Converted Water

Pores

FIG. 12.29 Volume partition of CAC paste with low w/c (0.40), left, and high w/c (0.70), right, in the unconverted and converted state.

a high strength, but when conversion occurs the volume of the hydrates decreases and so does the strength. For a low water to cement ratio, because hydration is limited by lack of space, only part of the hydratable materials will react to give metastable hydrates. In this case, when conversion occurs, water is released and can react with the remaining hydratable material. Consequently, the porosity remains low, and the strength is good even after conversion. Fig. 12.30 shows the microstructure of CAC mortar (0.40 w/c) cured isothermally at 20°C after 24 h representing a system with very low degree of conversion, and thus a high strength. The space filling nature of the metastable hydrates leads to a dense microstructure even though much unreacted cement remains. Fig. 12.31 shows the microstructure of a CAC mortar (0.40 w/c) cured isothermally at 38°C for 8 days representing a system that is fully converted. A high degree of reaction can be seen with many partially reacted cement grains in contrast to the low level of reaction in the 20°C system.

12.6.1 Influence of Water to Cement Ratio on Conversion Work by several authors has demonstrated the effect of w/c on conversion. Fig. 12.32 shows data from four authors on converted and unconverted CAC mortar cube strength as a function of increasing w/c. This figure shows a progressive decrease in strength with increasing water to cement ratio, similar to that seen for Portland cement. To ensure a high quality concrete with a converted strength around 40 MPa a water to cement ratio below 0.40 and a

Calcium Aluminate Cements

565

FIG. 12.30 CAC mortar, 0.40 w/c cured at 20°C 1 day.88

FIG. 12.31 CAC mortar, 0.40 w/c cured at 38°C 8 days.88

minimum cement content of 400 kg/m3 is recommended. For concrete without superplasticiser (as in the examples shown in the figure) a cement content above 400 kg/m3 would usually be necessary to obtain this w/c.

12.6.2 Influence of Temperature on Kinetics of Conversion The most important factor determining how quickly conversion occurs is temperature. Fig. 12.33 shows the influence of isothermal curing at temperatures from 25°C to 80°C on the strength development of CAC mortars. Fig. 12.33 shows that at a curing temperature of 80°C, the conversion process occurs rapidly and a monotonic gain in strength is observed throughout the evaluation period. When the curing temperature was reduced to 50°C a higher initial

566 Lea’s Chemistry of Cement and Concrete [GEO 90] - Unconverted [GEO 90] - Converted [NEV 81] - Unconverted [NEV 81] - Converted [ROB 82] - Unconverted [ROB 82] - Converted [COL 88] - Unconverted [COL 88] - Converted

Compressive strength of cubes (MPa)

100 90 80 70 60 50 40 30 20 10 0 0.25

0.35

0.45

0.55

0.65

0.75

0.85

Water to cement ratio (w/cm) FIG. 12.32 Influence of w/c on converted strength of CAC cubes.89

FIG. 12.33 Schematic of CAC hydration and conversion based on temperature.90

strength of approximately 50 MPa occurs, followed by a slight reduction in strength to around 40 MPa followed by further strength gain. Curing at 30°C produced an initial high strength of approximately 85 MPa followed by a reduction in strength to near 55 MPa at around 10–20 days. Samples cured at 25°C isothermally only began to show strength reduction after 90 days of testing. In reality concrete will not experience isothermal curing, but undergo a temperature profile determined by the size of the specimen and the environment. The hydration of CAC occurs rapidly, once started so the heat release is concentrated over a short time period and high temperatures can develop due to self heating in elements of any significant size. The microstructure that develops as a result of the time–temperature history will be a mixture of varying proportions of CAH10, C2AH8, C3AH6 and AH3 gel, unhydrated cement and porosity. The longer the material is at or above 30°C, the more converted the concrete will be that is, higher amounts of stable hydration products compared to metastable ones. Conversely if the material is kept below 30°C, the microstructure will be dominated by metastable hydration products. Fig. 12.34 shows

Calcium Aluminate Cements

CAC w/c ~ 0.4, ~20°C, no self heating

567

Portland w/c ~ 0.4

Strength

CAC w/c ~ 0.4, with self heating

CAC, w/c >~ 0.7, ~20°C, no self heating

CAC, w/c >~ 0.7, with self heating

hours days months

years

Time

FIG. 12.34 Schematic strength development of CAC and PC concretes of varying w/c comparing an isothermal cure at 20°C with a sample undergoing high self-heating (e.g. full conversion).

schematically the strength development of concretes comparing an isothermal cure at around 20°C with a sample undergoing self heating.

12.6.3 Influence of Aggregate Mineralogy In addition to w/c and the time–temperature history, the aggregate mineralogy also has a significant impact on long term strength and the process of conversion. Several researchers have shown that CAC concretes with limestone aggregates are less affected by conversion (e.g. minor or no strength loss) than CAC made with siliceous or aluminosiliceous aggregates.91–93 Cussino and Negro, Fentimen and L’Amourt showed that this may be due to the formation of mono-carboaluminate.93–95 Recent research by Adams and Ideker also showed that CAC concrete made with limestone and granite aggregate resulted in significantly higher converted strengths compared to the siliceous aggregate (following the 50°C accelerated curing regime: Section 12.6.4). A higher porosity was measured by both mercury intrusion porosimetry (MIP) and image analysis in the system with siliceous aggregate compared to limestone aggregate. Aggregates in the siliceous system showed less well developed interfacial transition zones compared to those in the limestone system. Their research showed that the addition of finely ground limestone powder (8% by mass) had the potential to significantly mitigate the strength loss due to conversion in siliceous aggregate systems with CAC.

12.6.4 Accelerated Conversion Testing To obtain the stable long term strength of a concrete for design or quality control (QC) purposes it is of interest to accelerate the conversion by placing the concrete at a higher temperature. As shown in Fig. 12.34 the concrete strength generally goes through a minimum and then increases again as water released by the metastable hydrates leads to further reaction of the anhydrous calcium aluminates. Accelerating the conversion process in the laboratory generally involves casting the concrete sample and either curing it in a mould with some additional insulation to promote self-heating or immersion of the concrete into a temperature regulated water bath. This is usually done on similar sized samples to those used for quality assurance on Portland cement concrete. The immersion of samples is done either immediately after casting (moulds are sealed) or 24 h after initial curing in ambient conditions. Strength measurements are made regularly during the curing regime. Much research was done using direct immersion at 38°C13,38: Samples were cast in water tight moulds and placed directly into a 38° C water bath. After 24 h samples are unmoulded, a subset is tested for compressive strength and the remainder are placed back into the water bath in direct contact with the hot water (tap water with no additives). Conversion typically occurs 4– 6 days after casting. This is the method can be applied for the design of concrete mixes in the lab, but is not practical for specimens cast on site. Work by George highlighted the importance of the length of ‘pre-curing’ before beginning conversion experiments with respect to the time it took to reach converted strengths. If 24 h passed before immersion the time to complete conversion was dramatically increased. In the case of a specimens cast and cured at 38°C the time to conversion was approximately 5 days for

568 Lea’s Chemistry of Cement and Concrete 90

Compressive Strength (MPa)

80 70 60 50 40

CAC SADB

30

CAC AMB 50C @ 1D

20

CAC AMB 50C @ 3D

10

CAC AMB 50C @ 6hr

0 0

5

10

15

20

25

30

Age (days) FIG. 12.35 Comparison of different curing regimes on the converted strength of CAC concrete.100

immediate immersion. However, if concrete from that same mixture was cured at 20°C for 24 h and then placed at 38°C, the time to conversion was 100 days.96,99 More recent work has focused on a practical test which can be applied to QC specimens cast on site. Two methods have been investigated:  Delayed immersion at 50°C60,97: Samples are cast in the field as is normally done with PC. After 24 h samples are taken to a laboratory and unmoulded. A subset is tested for compressive strength and the remainder are placed into a water bath at 50°C in direct contact with tap water (no additives). Conversion typically occurs within 2–3 days after casting.  High self-heating in an insulated box98,99: High levels of self-heating were obtained by placing concrete, (e.g. 16 cylinders measuring 100  200 mm) and with a very dense insulation (250 mm thick) on all sides. The maximum temperatures obtained may be from 60°C to 90°C. Other configurations may be desirable to simulate anticipated field conditions (and maximum exotherms). The converted strength is reached 24 h after casting. Fig. 12.35 shows how these two methods give similar measures for the minimum and long term strengths. The designation of ‘SADB’ indicates curing in the highly insulated (usually with Styrofoam) box outlined above, or semi-adiabatic conditions. The designation of AMB 50°C means an initial curing in ambient conditions followed by submersion, at the indicated age, in 50°C water. In developing approaches for accelerated curing, it is important to realise that the microstructure obtained by immediate isothermal curing at high temperatures (e.g. 70°C) is not the same as the microstructure for samples cured at lower temperatures (e.g. 38°C) or in realistic field conditions. Gosselin et al. investigated the microstructural development of CAC pastes cured immediately after casting at isothermal temperatures (20°C, 38°C and 70°C),101 self-heating (up to 50°C) and increase (ramp) from 20°C to 70°C. Data is shown in Fig. 12.36. CAC pastes (0.40 w/c) cured immediately at 70°C had a higher porosity by MIP (as well as a significant increase in large pores, than samples initially cured under all other conditions: 70°C isothermal ≫ 20°C–70°C (10°C/h) > self heating (max 50°C) >38°C > 20°C. Thus immediate curing at very high temperatures does not promote formation of a microstructure that is representative of field concrete.

12.7 Durability In this section two types of CAC systems are covered: (i) systems where CAC is 100% of the binder or where CAC is blended with supplementary cementing materials and (ii) systems where CAC is blended, typically with PC + C$, to produce early strength gain from ettringite (and other aluminate phases) with later strength gain from the hydration of the calcium silicates. A general overview of durability in each system type is first presented. Then the main types of premature concrete deterioration are presented in subsequent sections. Within each of these sections the 100% CAC system is discussed first followed by the blends. Comparisons between the systems are made where appropriate.

Calcium Aluminate Cements

1d 20°C 38°C Simulated self heating Ramp to 70°C

40 35

Pore volume (%)

Maximum standard deviation on total porosity

35 30

25

25

20

20

15

15

10

10

5

5

0 10–3

90d

40

Direct 70°C

30

569

0 10–2

10–1

100

101

Pore diameter (mm)

102

10–3

10–2 10–1 100 101 Pore diameter (mm)

102

FIG. 12.36 Porosity by MIP on CAC paste samples undergoing different curing temperature regimes.61

12.7.1 General Overview of CAC Durability Calcium aluminate cements were developed for resistance to calcium sulfate (gypsum and anhydrite) and the initial use was for the reconstruction of a railway tunnel passing through a solid mass of anhydrite along the Paris-Lyon-Marseille (PLM) route. These cements have also shown good performance in seawater and excellent performance in sewer linings (see Section 12.5.14). They are inherently resistant to alkali–silica reaction and have superior abrasion resistance compared to ordinary Portland cements, especially when combined with a synthetic CAC-based aggregate (see Section 12.5.7). In cementitious systems, where CAC is the only binder, conversion must be taken into account for its impact on strength and the durability. If the initial water to cement ratio is too high, an increase in porosity during the conversion process may result in an increase in transport properties and this must be considered when doing standard performance testing.

12.7.2 General Overview of Blended Systems (CAC + PC + C$) Durability To date, most use of blended systems, e.g. CAC + PC + C$, has been for indoor applications (e.g. self-levelling floor screeds, tile adhesives and grouts). Therefore durability concerns due to environmental exposure have not been an issue and have not been extensively studied. However, there is interest in using these formulations in outdoor applications, especially for rapid repair and construction, due to their advantageous properties (e.g. set control, workability, rapid strength gain). A wide variety of hydrate assemblages may occur in these systems (Fig. 4.8). The hydration products are richer in C-S-H and CH when closer the proportion of PC is high or richer in ettringite when the proportion of PC is low. As such the durability of these systems may vary widely. It is important to emphasise that the presence of ettringite, per se, is not a durability concern; like any hydrated phase, it can be durable or not durable depending on its exposure conditions. Ettringite has similar stability, with respect to temperature and relative humidity to calcium silicate hydrate. Baquerizo and co-workers showed that at 25°C ettringite is stable down to a relative humidity of 18%. At a temperature of 50°C the limit of stability was around 30% and at 80°C that increased to about 45% RH.102

12.7.3 Producing Durable Concrete For any concrete to be durable, it must first be produced as a high quality concrete. For CAC-based concrete this includes:  A low w/c appropriate for the strength requirements and anticipated exposure conditions  Sufficient binder content to ensure strength development and long-term performance

570 Lea’s Chemistry of Cement and Concrete

 For 100% CAC mixtures w/c  0.40 and cement content 400 kg/m3  Proper aggregate gradation for optimum workability and placement  Selection of admixtures to ensure desired fresh and hardened properties (e.g. water reducers, plasticisers, set modifiers and air entraining agents)  Appropriate proportioning of supplementary cementitious materials  Proper onsite mixing and placement procedures (e.g. good consolidation, not overworking the concrete, not adding additional water on site, etc.)  Adequate curing to protect the concrete from early age damage while promoting long-term performance  Maintenance throughout its lifespan to protect and remedy the concrete from any unforeseen issues, especially contact with excess moisture or aggressive chemicals. If any of the conditions above are not met, any concrete may suffer premature degradation and the same is certainly true for calcium aluminate cement concrete. Since many mechanisms of premature deterioration are dominated by the pore structure and thereby the permeability, having a low w/c and low permeability are generally the main keys to producing durable concrete. For concrete produced with 100% CAC as the only binder, it is recommended that the w/c be kept at or below 0.40.103 The following sections outline the main types of deterioration and the factors that promote or limit durability specific to calcium aluminate cement systems.

12.7.4 Corrosion of Steel Corrosion of reinforcing steel in all concrete is the main cause of premature deterioration and is generally implicated in 90%–95% of cases of poor reinforced concrete durability. Corrosion resistance is primarily provided by two parameters in reinforced concrete: (i) high pH of the pore solution to promote formation of the passive layer on the steel and (ii) adequate cover distance and low permeability of the concrete between the exposed surface and the reinforcing steel. The passive layer may be broken down by: (i) decrease in the pH of the internal pore solution due to carbonation or (ii) the ingress of chlorides above a critical threshold at the surface of the steel, leading to localised corrosion initiation.

12.7.4.1 Pore Solution pH—CAC The equilibrium pH for the CAC metastable phase assemblage is 12.13 while that for the stable phase assemblage is only slightly different at 11.97.104 In real concretes the pH levels may be higher (12.5), due to the small amounts of alkali metal oxides impurities present, which remain in the pore solution.105 The pH of unconverted CAC paste was reported between 12.5 and 12.7 over the first 30 days of hydration.106 Recent work by Adams and Ideker confirmed that pH values of paste pore solution were generally higher for unconverted (12.6) compared to converted (12.0) CAC concrete samples.97 Such levels of pH are sufficient to ensure that the steel is passivated. 12.7.4.2 Pore Solution pH—Blended Systems There are very few reports of pH in blended systems. It is expected to be in the range 12.5–14 according to the proportions of CAC and PC in the formulation and the alkali content of the PC. Table 7.1 shows some early age unpublished data for systems with PC as the main component (>70 wt%), pH levels are high (between 13 and 14) after 1 day of hydration (Table 12.9).107 12.7.4.3 Laboratory Exposure to Chlorides—CAC Work by Moffattt and Thomas and Yi and Thomas evaluated the rate of chloride ingress into CAC in laboratory conditions.109,112 Fig. 12.37 provides a summary of chloride diffusion data for a range of CAC-based and PC-based concretes after ponding in a solution of 165 g/L NaCl. Mixtures tested include 100% CAC (41% Al2O3) as the binder and 100% CAC

TABLE 12.9

Pore Solution pH of Blended PC-CAC-C$

Paste

5H

24H

73% PC/13.5% CAC (41% Al2O3)/13.5% C$ 71% PC/13% CAC (41% Al2O3)/16% C$ 71% PC/13% CAC (45% Al2O3)/16% C$

12.4 13.2 13.5

13.8 13.3 13.7

Calcium Aluminate Cements

OPC (0.45) CAC 41% Al2O3 CAC 51% Al2O3 PC-CAC-C$ OPC (0.40) HEPC

1.2E–11 Diffusion coefficient (m2/s)

571

1E–11 8E–12 6E–12 4E–12 2E–12 0 0

100

200 Age (days)

300

400

FIG. 12.37 Chloride diffusion coefficients for CAC and PC-based concrete. (From Moffattt EG. Durability of rapid-set (ettringite-based) concrete [Doctoral dissertation]. Canada: The University of New Brunswick; 2016, 237; Yi H, Thomas MDA. The performance of CAC concrete in an aggressive marine environment. In: Fentiman C, Mangabhai RJ, Scrivener KL, editors. Calcium aluminates: proceedings of the international conference. Avignon, France: IHS; 2014. p. 556–69.)

(51% Al2O3). These mixtures were cured at 23°C and thus the level of conversion is expected to be low. The CAC concrete and a high early strength Portland cement (HEPC) concrete had lower diffusion coefficients than plain PC at 91 days. At 365 days, the diffusion coefficient was lowest in the CAC 41% Al2O3 and HEPC concretes. Other work by Andio´n and Garc
es used an accelerated anodic current density to initiate corrosion in CAC and PC mortars. They showed that a higher amount of corrosion products were needed to crack CAC specimens compared to PC specimens.108

12.7.4.4 Laboratory Exposure to Chlorides—Blended Systems Work by Moffatt and Thomas compared Portland cement concrete, high early strength Portland cement concrete (HEPC) and concrete with a blended binder of PC + CAC + C$ after 180 days of exposure to 165 mL/g NaCl (Fig. 12.37). The blended system showed improved resistance to chloride ion penetration compared to plain PC. Further investigations by Moffatt and Thomas showed that blended systems containing PC + CAC + C$ were better able to bind chlorides as Friedel’s salt after 93 days of exposure to a range of 0.1–3 M NaCl. They also showed that for both systems the amount of Friedel’s salt increased as the concentration of NaCl increased.109 12.7.4.5 Exposure to Sea Water—CAC One of the largest structures built with CAC was part of an ocean harbour at Halifax, NS Canada in 1930–1932. Over 6000 t of CAC were used in the construction. Cores were taken from the structure in the early 1990s to investigate the condition of the concrete after 60 years of exposure to seawater. The cores showed that the penetration of chlorides and sulfate from the seawater was relatively low and that the reinforcing steel was in good condition. In part, the good performance of this concrete was attributed to a dense surface layer (50 mm). At the time of construction, the recommended practice was to spray the surface of the concrete liberally and continuously with cold water to help regulate the high heat release associated with the curing of CAC concrete. This cooling of the surface layer has since been shown to provide a very dense microstructure that greatly slowed the ingress of chloride ions into the structure. The Ford Dagenham Factory in the United Kingdom, also showed good performance after >60 years of seawater exposure. Cores from this structure were also inspected and again a dense surface layer (50 mm) was observed. For both structures the surface layer was very similar and in these regions concentrations of sulfate and chloride were quite significant, mainly in the form of AFm phases (Freidel’s salt, C3ACaCl2H12 and monosulfate, interlayered with C2AH8 and C2ASH8), but also in an indeterminate form within the amorphous hydrous alumina gel. Beyond the dense surface layers the concentrations of these ions were low. The Halifax Harbour is still in service today (Fig. 12.38), while the Ford Dagenham factor was demolished after the factory was closed (not as a result of deterioration of the concrete).10–12,110 Recent work by Yi and Thomas investigated the durability of various CAC systems in laboratory (Section 12.7.4) and field exposure. Fig. 12.39 shows a comparison of the chloride profiles after 1 and 4 years of exposure to seawater in an

572 Lea’s Chemistry of Cement and Concrete

FIG. 12.38 CAC concrete cribs used in the construction of a pier at Halifax, Canada (constructed in 1930), condition in 2001.111

0.8

Chloride (% by mass of concrete)

0.7

CAC_23C

0.6 CAC_50C 0.5 OPC 0.4 0.3 0.2 0.1 0.0 0

5

10

15

20

25

Depth (mm) FIG. 12.39 Chloride profiles for CAC mixtures and PC reference exposed to seawater for four years. Note: w/c ¼ 0.40.112

aggressive exposure site in the Bay of Fundy located off the coast of Maine, United States. This location is known to have the largest tidal variation in the world and in the winter goes through approximately 100 freeze–thaw cycles. Mixtures noted as ‘CAC’ are 100% CAC (41% Al2O3) as the binder. The total cement content was 425 kg/m3 and a w/c of 0.40 was used for all mixtures. The CAC samples were not cooled during curing and so would not be expected to have a dense outer layer, as was the case in the field concrete in Halifax. Even samples cured nominally at 23°C had some degree of conversion initially, while samples cured at 50°C were fully converted.112 After 4 years of exposure these results show that for the same w/c the CAC concrete performed similarly to PC concrete in regard to chloride ingress. However, based on the corrosion potential measurements (Icorr and Ecorr) both unconverted and converted CAC_50C mixtures were shown to have a high risk of corrosion of the reinforcing steel. After 7 years of exposure both the CAC_23C and CAC_50C samples showed a main longitudinal crack with minor ancillary cracking in the same area as well as staining on the surface indicative of corrosion of the reinforcing steel. Qualitatively the CAC_23C sample was in better condition than the CAC_50C sample, indicating the corrosion may have initiated sooner in the fully converted specimen. These laboratory

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samples were not sprayed with water during curing and thus the concrete. After 5 years of exposure the PC samples were performing well.112 This is very useful data, but it is still limited compared to the vast amount of work done on PC-based systems. More work is needed to investigate the influence of w/c, aggregate type, cementitious content, etc. on chloride ingress and corrosion risk of CAC based concrete and on blended systems.

12.7.5 Corrosion of Steel Summary From the studies on corrosion of CAC concrete several overall trends can be drawn:  100% CAC concrete that is fully converted (e.g. no artificial cooling of the skin) has a greater risk of corrosion of reinforcing steel than unconverted CAC concrete, thus maintaining a low w/c (<0.40) and a minimum cement content of 400 kg/m3 is key to the performance of CAC concrete.  Concrete that is unconverted, or has a dense outer layer due to contact with sea water or from artificial cooling during hydration appears to have greater resistance to the ingress of chlorides due the formation of a dense outer layer rich typically comprised of a mixture of Friedel’s salt and metastable CAC phases.  Initial work on blended systems (e.g. PC + CAC + C$) show similar behaviour to high early-strength Portland cement concrete. However, more research and field observation is needed in this case.

12.7.6 Carbonation 12.7.6.1 Carbonation—CAC Systems The reaction of atmospheric CO2 with CACs will lead eventually to the formation of CaCO3 and AH3, irrespective of the nature of the hydrates present, corresponding to the following reactions: CAH10 + CO2 ! CaCO3 + AH3 + 7H2 O C3 AH6 + 3CO2 ! 3CaCO3 + AH3 + 3H2 O Mono-carboaluminate may be formed as a transient reaction product. The carbonation of converted CAC leads to an increase in strength,112a as for Portland cement concrete. The main concern with carbonation is the possibility of corrosion of the reinforcing steel. As for Portland concrete, active corrosion requires both moisture and oxygen in addition to the loss of steel passivity; thus, even when the concrete is carbonated, the rate of corrosion of reinforcing steel in interior concrete is likely to be low. Studies of CAC concrete beams in buildings in the United Kingdom, indicate that after 20–30 years most of these have carbonated to the depth of the reinforcement, a rate comparable to Portland cement concrete.112b Nevertheless, as expected for interior concrete, only a very small minority show signs of corrosion. Work done at the BRE, starting in 1993 and studied the carbonation of CAC concrete of different qualities and in different exposure conditions. Fig. 12.40 shows the data from the best (H400: w/c ¼ 0.4 and 400 kg/m3 cement) and poorest (H206: w/c ¼ 0.86 and 206 kg/m3 cement) concrete. The carbonation depth evolved linearly with the square root of time as for Portland concrete and depends on the exposure condition also in a similar manner (indoor < outdoor sheltered < outdoor exposed). While high carbonation depths are seen in the poor-quality concrete, those in the good quality concrete are low and comparable with those of Portland concrete: 0.5 mm in outdoor exposed, 9.3 mm in outdoor sheltered and 21 mm in indoor conditions. When calcium aluminate cement concrete was carbonated and the buffering capacity of the pore solution was diminished, general corrosion of the reinforcing steel occurred similar to that expected in ordinary Portland cement concrete of the same or similar mixture design (see Fig. 12.40).113,114

12.7.6.2 Carbonation—Blended Systems In blended systems, all hydrates may carbonate eventually to give calcium carbonate silica gel, gypsum and alumina gel according to the overall composition.115 As ettringite has a high water content and low density, there is a large volume decrease on carbonation (DV ¼ 44%). This can cause some loss of strength according to the amount of ettringite in the formulation. A formulation with a very low amount of PC (x%) was studied by Rettel and Scrivener, after complete carbonation the strength was equivalent to that of the uncarbonated system at 7 days. This is an extreme case and systems for outdoor application generally have much higher PC content and higher strengths after carbonation.

574 Lea’s Chemistry of Cement and Concrete Carbonation data for H206, (air cured) against square root of time to 10 years 40 Carbonation depth (phenolphthalein): mm

indoor

30

outdoor (s) outdoor (e)

20

10 5 years 0 0

5

10 15 Square root time (weeks)

20

25

Carbonation data for H400, (air cured) against square root of time to 10 years

Carbonation depth : mm

40 indoor

30

outdoor (s) outdoor (e)

20

10 5 years 0 0

5

10 15 Square root time (weeks)

20

25

FIG. 12.40 Relationship between carbonation depth and time for a high quality CAC concrete (H400: w/c ¼ 0.4 and 400 kg/m3 cement) and low quality CAC concrete (H206: w/c ¼ 0.86 and 206 kg/m3 cement).

TABLE 12.10 Carbonation Depth (mm), Asahikawa Outdoor Exposure Site (Japan)117 Mixture

7-Year Depth (mm)

75% CAC/25% C$ 70% CAC/30% C$ 60% CAC/30% C$/10% PC 50% CAC/30% C$/20% PC 7% CAC/7% C$/86% PC 100% PC

4 6 3 3 2 1

The carbonation depths will tend to increase as the proportion of PC in the formulation goes down as there is less Portlandite to react with CO2. Table 12.10 shows the carbonation depth of different formulations after 7 years from the work of Lamberet. Data from Moffatt and Thomas also showed that blended systems of CAC + PC + C$, showed moderately higher carbonation depths after 1.5 years of exposure compared to a high early strength Portland cement (HEPC) concrete and plain Portland cement (PC) concrete (2.6 mm). In PC-rich systems portlandite acts as the main buffer to slow the carbonation process. Further investigations on carbonation depth in blended systems is merited, especially for outdoor applications (Fig. 12.41).

Calcium Aluminate Cements

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70

Strength, Mpa

60 50 45 40 30

35

RH = 30% RH = 60% RH = 60%, 100% CO2 RH = 100% RH = 60%, 0.3% CO2 30%-->100%

20 10 0 0

20

40 60 Time, days

80

100

FIG. 12.41 Influence of relative humidity and CO2 on blended cement samples.116

12.7.7 Biogenic Deterioration A particular case of acid attack, relevant to the use of CACCs as linings for sewage pipes, is that of bacteriological corrosion. Sulfates in the sewage are converted to sulfide by anaerobic bacteria (found in the thin slime layer which develops on the sewer walls in direct contact with the effluent flow). The sulfide is released in the form of dissolved hydrogen sulfide into the effluent stream. The rate of production of H2S is governed by a number of parameters, including the amount of sulfate in the effluent stream, ambient temperature, flow speed and effluent level. When the concentration is high enough, or due to turbulence, H2S volatilises from the effluent and, carried by convection currents, it accumulates at the crown of the pipe, where it oxidises to elemental sulfur. A second set of aerobic bacteria (Thiobacillus thiooxidans) then uses the sulfur as food and excretes sulfuric acid as a waste product. This sulfuric acid goes on to attack the concrete. Two aspects of CAC are linked to their good performance. First, CAC has a larger neutralisation capacity compared to PC. One gram of CAC can neutralise around 40% more acid than a gram of PC. This means that for the same production of biogenic acid by bacteria, CAC will last longer. When the surface pH drops below 10, most of the hydrates apart from AH3 start to dissociate to form gypsum (recombination of calcium with sulfate) and more alumina gel (AH3) covering the surface of the exposed material. This AH3 layer is stable and cohesive down to pH 3.5–4 and contributes to protection against biogenic acid attack. Compared to PC solutions, no ettringite is formed during the biogenic attack, hence there is no expansion. Second, the reduction of pH on the surface of CAC concrete is much less than on PC concrete. When the pH reach 3.5–4 AH3 starts to dissolve and the aluminate ions suppress the activity of the bacteria. So, while surface pH may reach as low as 1 for Portland concrete they tend to stay in the range above 3 for CAC concrete. Comparison of the performance of cements is complicated by interactions between bacteria development and chemical composition of the cement, and thus simple testing with sulfuric acid is not representative of the practical situation. Tests in a special chamber built to simulate corrosion in sewers117a,117b indicate a good performance of CACs compared to Portland or blended blastfurnace cements. The most important evidence for the performance of CA comes from a 12-year long field study in the Virginia test sewer in South Africa (Fig. 12.42). At the end of 12 years the full section of the PC concrete pipe had been eroded in many places, while the maximum erosion of the CAC concrete section was only 10 mm.

12.7.8 Sulfate Attack Sulfate attack is the name given to degradation in ground water containing sulfate. Laboratory testing usually focuses on expansion due to ettringite formation, although in the field degradation is usually dominated by loss of surface. The formation of ettringite is only expansive if it forms from supersaturated solution in confined pores. In Portland cement materials, this occurs when ettringite forms from calcium aluminate monosulfate embedded in C-S-H. In CACs, the detailed mechanisms of degradation due to sulfate attack have been less extensively studied. Some authors have ascribed the good performance of CACs to the absence of calcium hydroxide, but this is too simplistic. Undoubtedly, the calcium aluminate hydrates can react

576 Lea’s Chemistry of Cement and Concrete

FIG. 12.42 State of concrete pipe section in test sewer after 12 years. Portland/siliceous section (left) is heavily corroded, in parts the earth behind the pipe is visible. CAC/ siliceous section (right) is relatively unaffected; only black part around the water line shows up to 10 mm of erosion. (Photographs courtesy of A. Goyns.)

to form ettringite even at comparatively low concentrations of SO4 2 . However, it is not clear under what circumstances the formation of ettringite is destructive. Clearly the penetration of sulfate into concretes with a dense surface layer is slow, and this may be in part due to the absorption of sulfate by amorphous phases, in addition to the low porosity.

12.7.8.1 Sulfate Attack—CAC Between 1916 and 1923 over 7000 t of CAC were used in the construction of the P.L.M. Railway in France, which runs through regions containing anhydrite and gypsum. No problems were experienced with this construction and test specimens immersed in water saturated with calcium sulfate showed no signs of attack even when anhydrite was used as an aggregate.118 Miller and Manson119 studied the performance of concretes in the highly-sulfated waters of Medicine Lake, South Dakota, for 20 years. The condition of the CAC concretes after this time was excellent, with a gain in strength in many cases. The Portland cement concretes failed within 5 years, while sulfate-resisting Portland concretes lost 20% of their strength in 5 years and nearly half in 10 years. Midgley120 studied an example of a foundation in soil containing Epsom salts magnesium sulfate). After 18 years only the immediate surface of the concrete had been attacked and the maximum depth of penetration was 3 mm. Bate121 reviewed long-term sulfate resistance tests and concluded that good quality CACC not subjected to elevated temperatures performs well in sulfate-rich environments. In 1970, a long-term field and laboratory investigation into sulfate resistance of concrete, including CACCs, was started by the UK Building Research Establishment (BRE). The field study involved exposure in the sulfate-rich soils of Northwick Park, London (0.26% SO3). The results up to 15 years have been reported by Harrison122,123 and Crammond.124 The CAC concretes in this study had total water/cement ratios varying from 0.47 to 0.6. All samples were noted to form a dense surface layer, which had a low content of the cubic (converted) hydrates. After 15 years, the physical condition of all the CACCs exposed in the field was good. The overall sulfate content of the highly-converted cylinders was not found to be significantly greater than that of the slightly converted piles, and there was negligible penetration of sulfate into the fully compacted cylinders. The main sulfate mineral present was ettringite, although substantial amounts of sulfate were present within amorphous phases. Cubes were also exposed to sulfate solutions in the laboratory—1.5% SO3 as Na2SO4, 1.5% SO3 as MgSO4 and 0.35% SO3 as MgSO4. After 15 years, there was significant deterioration of the cubes at the higher water/cement ratios (0.53–0.6). In many cases this deterioration had taken the form of expansion within the cube, causing surface cracking. The suggested mechanism was that the sulfates eventually penetrated the dense surface layer of the concrete and formed ettringite in the porous interior, which led to expansion. At the lowest water/cement ratio (0.47) there were some signs of deterioration in one cube in the 0.35% SO3 as MgSO4 solutions, but three cubes in this, and all the cubes in the higher concentration solutions were in excellent condition. Recent work by Lute showed that in higher concentration sodium sulfate exposure (5% Na2SO4) that

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CAC mortar bars that were converted began showing expansion and deterioration after about 3 months of exposure. This was attributed to a lower strength and higher porosity in the converted CAC concrete that was not able to resist expansion of ettringite in the CAC converted matrix.130

12.7.8.2 Sulfate Attack—Blended Systems Additions of CAC to PC generally lower the sulfate resistance relative to pure PC concrete. Small additions of PC to CAC have little effect relative to the performance of pure CAC concrete. Mixtures of CAC–slag show good durability in sulfate solutions.134 Recent work by Lute showed that blends of CAC + PC + C$ had poor resistance to sulfate attack when exposed to a 5% Na2SO4 solution (ASTM C1012 test method). This was attributed to the formation of large amounts of monosulfoaluminate at later ages which was then able to react with the source of external sulfate and produce ettringite.130

12.7.9 Freeze–Thaw Attack Some studies have been done to assess the freeze–thaw susceptibility of calcium aluminate cement concrete. Like all cementbased materials subjected to moisture and the actions of freeze–thaw cycling the concrete should be protected with an adequate air entrainment of the paste that includes proper entrained air void size range with and spacing factor.125

12.7.9.1 Freeze–Thaw Attack—CAC Converted and unconverted CAC concrete that was air entrained was tested according to ASTM C666 for 300 freeze– thaw cycles. While the unconverted samples showed slightly better performance, both sets of concrete did well in the test and both showed a slight increase in the relative dynamic modulus over the course of the test.89 CAC concrete beams (100  100  500 mm) were placed on an exposure site on Treat Island, near Eastport, Maine United States, in the Bay of Fundy and experienced about 100 freeze–thaw cycles per year. After 6 years of exposure the beams had gone through approximately 600 freeze–thaw cycles and were in sound condition; particularly those made 100% CAC (50% Al2O3). The only samples undergoing damage were the result of corrosion of the reinforcing steel (see Section 12.7.4 for detail).112 Field experience with CAC concretes has shown that they can resist freeze–thaw damage in extreme environments for long periods. For example, CAC used to build the pier in the port of Halifax, South Scotia, Canada (discussed in Section 7.4.5) is still in service after undergoing about 1000 freezing cycles over 60 years.126 Laboratory experiments and in-service monitoring indicate that for CAC concretes to have good resistance to freeze–thaw cycles, they must be of low porosity (below 13%). This is similar to the conclusions for Portland cement concretes. The limited data regarding the freeze–thaw performance of air-entrained CAC concrete indicate that it is comparable to air-entrained Portland concrete.127 However, goodquality CAC concrete with w/c < 0.4 is likely to be freeze–thaw resistant even without air entrainment. 12.7.9.2 Freeze–Thaw Attack—Blended Systems Concrete beams (100  100  500 mm) made with a blended binder (CAC (40% Al2O3 + 21% slag and 4% silica fume) were placed on the exposure site at Treat Island Maine, United States (100 freeze–thaw cycles per year). After 6 years of exposure the beams had gone through approximately 600 freeze–thaw cycles and were in sound condition.112

12.7.10 De-icing Chemicals Little work has been done on exposure of CAC to de-icing or anti-icing chemicals. Preliminary testing done by Hayman and Thomas in an accelerated exposure of potassium acetate at 80°C showed that mortar bars of CAC (51% Al2O3) outperformed both PC and calcium sulfoaluminate (CSA) mortars. The CAC samples remained intact with little to no expansion after 28 days of exposure. Whereas, the CSA had lost cohesion and disintegrated into an unmeasurable state. The PC expanded significantly and began to disintegrate after 14 days in the test solution.128 This is an area for further in-depth study.

12.7.11 Alkali–Silica Reaction Alkali–silica reaction (ASR) should generally not be a concern in 100% CAC concrete or mortar for several reasons, (i) there is no calcium hydroxide present and thus (ii) the pore solution pH, from existing literature, appears to be near or below 12.50, which is low enough to provide ASR resistance for most aggregates and (iii) the presence of alumina in the pore solution can suppress the dissolution of silica.129

578 Lea’s Chemistry of Cement and Concrete

12.7.11.1 Alkali–Silica Reaction—CAC Work by Lute and Folliard showed that when a very highly alkali–silica reactive aggregate was combined with 100% calcium aluminate cement essentially no expansion was observed in the concrete prism test (1-year data) or in outdoor exposure site testing (up to 2.5 years) in Austin, Texas United States.130 12.7.11.2 Alkali–Silica Reaction—Blended Systems Further work by Lute and Folliard showed that in a system with 70% high alkali PC and 30% CAC:C$ (2.2:1 ratio) a moderate level of ASR was observed in both expansion measurements and in optical and scanning electron microstructural analysis. This is likely due to the presence of PC hydrates and the high alkali content of the PC.130 Therefore, in blended systems, caution should be taken for the potential for alkali–silica reaction which would be favoured when the PC content is high, the alkali content is high and/or when the reactivity of the aggregate is high.130

12.7.12 Alkaline Hydrolysis—CAC Alkaline hydrolysis is the name given to a very rare form of degradation specific to plain CACs. In poor-quality porous concrete this may result in a dramatic loss of strength. Macroscopically it appears to be characterised by a white dusty precipitation on the surface of the concrete. The phenomenon was first reported in 1936131 but the precise mechanism of this ‘destruction’ are not well understood. Carbonation was thought to be an important criterion, but now this is known not to be important. Dunster et al.,132 have reported a microstructural study of a field case of alkaline hydrolysis in which large areas of paste appeared to have been leached away and large crystals of norstrandite (a polymorph of AH3) were observed. For alkaline hydrolysis to occur with detrimental effects, three conditions are necessary:  high humidity  high porosity  presence of free alkali. Alkaline hydrolysis has proved difficult to reproduce in the laboratory. Further a water/cement ratio of at least 0.7 is needed. There also seem to be some linked to the type of sand used.133 The low levels of alkali in the CAC itself do not induce alkaline hydrolysis. Neither is alkaline hydrolysis induced when good-quality CAC concrete is placed adjacent to hardened Portland concrete. Examination by microscopy of cores taken through the boundary between Portland and Fondu concrete, from a 14-year-old viaduct, showed no evidence of interaction between the two cements beyond a few hundred micrometers and no evidence of any degradation.

12.7.13 Volume Change 12.7.13.1 Shrinkage Compared to Portland cement, the rapid hydration of CAC affects volumetric changes and the development of associated stresses in several ways:  Rapid hydration leads to a rapid filling of the porosity and a decrease in residual moisture due to consumption of water by hydrates.  Higher self-heating compared to an PC system can favour the formation of thermal gradients;  The increased hydration kinetics and thus strength gain gives less time for the relaxation of part of the creep stresses.  On the other hand the dissolution and precipitation of new hydrates during the conversion process, leads to a short term burst in relaxation as seen in Fig. 12.43. The rapid hydration also results in the need for earlier baseline measurements when doing volume change measurements. Compares the volumetric changes measured on two sets of prisms of the same concrete, unmoulded at 3 h and 24 h, respectively after mixing. The prisms unmoulded at 3 h show a slight swelling due to the heat generation during hydration followed by a shrinkage of 300 mm during the first 24 h. After 10 months, the difference between the two series of measurements was only about 100 mm.89

12.7.13.2 Practical Implications To avoid cracking due to the earlier onset of shrinkage of CACCs, the bay size of unreinforced slabs is recommended to be limited to about 3 m. The rapid consumption of water is advantageous for applications where rapid floor covering is desired.89

Calcium Aluminate Cements

579

100 Start of measurement 3 h after contact of water and cement

0

Start of measurement 24 h after contact of water and cement

Shrinkage (microstrain)

–100 –200 –300 –400 –500 –600 –700 –800 –900 0.1

1

10

100

1000

Time (days) FIG. 12.43 Volumetric change as a function of time of initial measurements, stored at 20°C and 50% relative humidity.

In applications where the early strength of CAC is not essential (e.g. pipe linings) shrinkage cracks may be avoided by heating the concrete during curing to promote the formation of the stable hydrates as well as a relaxation during conversion, which result in a concrete which is less susceptible to cracking.88

12.7.13.3 Creep The creep of CAC concretes has not been extensively studied. Earlier work, discussed by Robson,134 indicates that there is no clear difference between the creep of unconverted CAC concrete and that of PC concrete. The relative magnitudes change with time, probably due to the different strength development curves. Work by Neville and Kenington found that the creep decreased considerably as the time of loading initiation was extended from 18 h to 11 days after casting. When the concrete was >2 days old the age at loading had a relatively small impact on the creep measured up to 100 days.135 Some data do exist for converted concrete made according to current guidance. During the construction of the Frangey bridge, the opportunity was taken to check the creep of the fully converted CAC concrete. A relatively high load of 15 MPa was applied to 70  70  280 mm prisms after curing for 24 h at 80°C. The results, summarised in Table 12.11, indicate satisfactory performance. The long-term creep of CACCs does not appear to have been studied. While early work showed that the creep behaviour of Portland cement concrete generally extended to CAC cement, more recent work shows that at the time of the conversion, an increased relaxation of the stresses is possible. Fig. 12.44 illustrates a series of creep tests where the conversion was induced by a continuous exposure at 40°C. A high degree of deformation during conversion is can be seen. This could be explained by the dissolution/re-precipitation of the hydrates, which is accompanied by the release of water, allows a redistribution of the stresses of the material under load. Measurements of acoustic emissions during conversion have shown that no microcracks are formed, confirming the hypothesis of a redistribution of stresses while

TABLE 12.11 Deformation and Creep of Converted CAC Deformation (mm/m) Age (Days)

Shrinkage

Creep

1 2 6 8 13 30

21 86 160 139 192 128

192 150 342 364 289 482

580 Lea’s Chemistry of Cement and Concrete

FIG. 12.44 Impact of conversion on creep rate of concrete cylinders subjected to four different loading rates, constant exposure at 40°C.89

the matter is being reorganised. In the case where the initial self-heating is sufficiently high ( 70°C, case of thick sections), the conversion can occur as early as the first hours of hydration. This may explain why, despite high thermal stresses, alumina cement concrete does not appear to be particularly sensitive to thermal cracking. Further the behaviour of concrete in the field, under loading, may be quite different from creep under laboratory-induced conversion.77

12.7.14 Thermal Properties The coefficient of thermal expansion of concretes depends mainly on the aggregates. However, the thermal expansion of CAC paste is lower than that of PC paste. With careful selection of aggregates, CAC concretes can be made which can withstand repeated exposure to fire (n.b. ‘fireproof’ PC concrete is designed to withstand a single exposure). In addition to the refractory uses discussed in Section 12.5.8, CAC concretes are used, for example, as chimney linings and for fire training buildings.28

12.8 Conclusions Calcium aluminate cements have been in continuous production for over 100 years. These cements are produced under highly controlled processes and thus are regarded as a high-quality product whose performance in specific applications are robust. Due to the composition of the earth’s crust only CAC and CSA cements can be made as alternatives to Portland cement. However, CACs are produced at about 1/1000th the volume of PC due to the availability of the main raw material—bauxite. Calcium aluminate cements are known for their rapid strength gain, especially at low temperatures, superior durability across several such categories and high temperature resistance. Their ability to consume water rapidly during hydration makes them a preferred component in building chemistry applications as this contributes to construction expediency. CACs are highly versatile materials that can be used as the full binding material, or as is more common, a component of a blended system where the contribution is based on the final desired properties. Such systems can become quite complex and thus having sound technical expertise from CAC manufacturers has been one of the key’s to their longterm success.

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emy E. Comptes Rendus Hebdomadaire des S
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FURTHER READING 136. Pommay J, Fryda H, Bordet F, Tourlakis D. Action mechanism of admixtures in ettringite systems. In: Fentiman C, Mangabhai, R, Scrivener K, editors. Calcium aluminates: proceedings of the international conference, Avignon, France; 2014. p. 371–82. 137. E.C.F. Standardization. EN 14647: Calcium aluminate cement—composition, specifications and conformity requirements; 2005. p. 1–33.