Effect of raw clay type, fineness, water-to-cement ratio and fly ash addition on workability and strength performance of calcined clay – Limestone Portland cements

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Effect of raw clay type, fineness, water-to-cement ratio and fly ash addition on workability and strength performance of calcined clay – Limestone Portland cements S. Ferreiro⁎, D. Herfort, J.S. Damtoft Aalborg Portland A/S, Cementir Holding S.p.A., 9100 Aalborg, Denmark

A R T I C L E I N F O

A B S T R A C T

Keywords: Metakaolin (D) Calcined smectite clay Fly ash (D) Blended cement (D) Workability (A)

Workability and strength performance of blended cements with different replacement levels have been investigated for two calcined clay mineral additions (1:1 and 2:1 type) and mixtures with limestone. The effect of fly ash addition and different water-to-cementitious ratios was also evaluated. The results show that the SO3 content of the blended cements, and the ratio of calcined clay to limestone, can be optimized for maximum strength depending on the content of early soluble and total reactive alumina in the calcined clay. Workability is strongly affected by the calcined clay content, particularly for the 1:1 clay which greatly reduces the efficiency of superplasticizer (SP) required for achieving acceptable flow even at higher w/c ratios. On the other hand, delayed addition of the SP and/or the addition of fly ash significantly improve rheology of any binder containing calcined clay and maximize strength for same workability at a given clinker content in the paste.

1. Introduction One of the key levers in achieving CO2 emissions from cement and concrete production has been the increased use of SCMs such as fly ash over recent years. However, with the increasing commitments to reduce the reliance on coal burning, most recently with the “Paris agreement on climate change”, fly ash will become less abundant. This can, for a time at least, be mitigated by more efficient distribution and/or transport of fly ash over longer distances, but other alternative SCMs will be needed to meet this shortfall. Calcined clays, particularly in combination with limestone are emerging as one of the most promising solutions for this, both with regard to performance and the abundance of adequate reserves [1]. Fly ash has been extensively used in blended cement and concrete production for several reasons: Chemically, when mixed with Portland cement and water, fly ash reacts with calcium hydroxide released by hydration of cement to produce more calcium silicate hydrates and calcium aluminate hydrates, which increase the final strength and enhance the durability of the concrete. Physically, fly ash also improves workability and pumpability of the wet concrete. Consequently, it permits production at lower water contents needed for high strength performance concrete. Historically, fly ash has been used since the mid-1900s at levels ranging from 15 to 60% with good mechanical properties [2], but

⁎

definitively, the experience has clearly shown there is not just one replacement level of fly ash best suited for all applications, as might be expected for calcined clays and combination with limestone. Numerous investigations [3–6] have demonstrated the excellent chemical properties of calcined clays as SCM, in particular metakaolin showing earlier and greater pozzolanic properties than fly ash, with regard to strength performance and durability [7]. However just a few focused on the undesirable rheological problems found with these mineral additions and compatibility of chemical admixtures [8–11], or compared both strength and workability performance between different calcined clay types [12,13]. Most of published research on the use of chemical admixtures agree that polycarboxylate ether (PCE) based superplasticizers are the most effective to disperse the particles of calcined clay blended cements [11,14]. The maximum dosage specified for this SP type generally fluctuates between 1.5 and 2% of binder content depending on SP type and dosage. Antoni et al. [14] found that the amount of SP required to maintain constant flow varied linearly with the metakaolin content, and reported 2% of PCE was needed for 50% substitution at a w/c ratio of 0.5 to maintain an acceptable mortar flow measured according to ASTM C-1437. Water-reducing agents or plasticizers, and high-range water-reducing admixtures or superplasticizers (SP) are extensively used in modern concrete production since they are more cost-effective than

Corresponding author at: Research and Development Center, Aalborg Portland, 9220 Aalborg, Denmark. E-mail address: [email protected] (S. Ferreiro).

http://dx.doi.org/10.1016/j.cemconres.2017.08.003 Received 3 March 2017; Received in revised form 30 June 2017; Accepted 3 August 2017 0008-8846/ © 2017 Elsevier Ltd. All rights reserved.

Please cite this article as: Ferreiro, S., Cement and Concrete Research (2017), http://dx.doi.org/10.1016/j.cemconres.2017.08.003

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for each blended cement was selected depending on the purpose of the test. OPC1 was specifically used to evaluate the performance of blended cements for a complete range of pozzolana-to-pozzolana plus limestone ratios (P/(P + L)) from zero to one, whilst OPC2 was chosen to evaluate blended cement performance for intermediate P/(P + L) ratios at lower Blaine fineness typical for CEM I 52,5N type cements. In addition, compositions of blended cements containing calcined 1:1 clay were SO3-optimized with regard to strength by adding finely ground calcium sulfate hemihydrate (hemihydrate or H, hereinafter) as a function of calcined clay content in blended cements. The SO3 content and density of the hemihydrate sample was 55.35% and 2610 kg/m3, respectively. OPC, SCMs and extra hemihydrate (if added) powders were always properly homogenized prior mixing with sand and water. Upon mixing the mortar constituents, CEN standard sand as described in EN 196-1 and polycarboxylate ether (PCE) based superplasticizer (SP) manufactured by BASF were employed to achieve measurable flow of all the mortars. Bulk mixing water contents were adjusted by including the water content of the SP as a part of mixing water in order to ensure equal w/c ratio of all mortar compositions, whilst the 24.5% solid content of the SP was considered negligible. The maximum dosage recommended by admixture supplier is 2.0% by wt. of cement and pozzolan. The w/c ratio and sand-to-cementitious ratio were kept constant to 0.5 and 3.0, respectively, for all mortars with the exception of the tests described in the last section of this paper, where the w/c was modified but paste volume was maintained as control cement for specific SP quantities regarding clinker replacement. Compositions of these special mortars are given in Table 3.

adding more cement that would otherwise be needed to compensate for high water contents. However, dosage of these chemical admixtures is constrained due to other undesired effects, such as retardation of the hydration, excessive entrainment of air, bleeding and increased plastic shrinkage [15]. According to EN 206-1, concrete recipes should not exceed the maximum dosage specified by admixture producer, normally as % by wt. cement or cement and pozzolan content, but efficiency of the SP is not linearly related to dosage and moreover above the saturation point, where no larger slump or flow can be observed for increased dosage [16]. Quick and convenient measurements such as mortar flow can be successfully used to screen the effect of fineness, replacement level and combinations with limestone of different calcined clay mineral additions on workability and the efficiency of chemical admixtures. Thus, the convenience of increasing the SP up to the maximum dosage recommended by the supplier can be evaluated as an alternative to the higher water content needed to achieve acceptable workability and increased cement content to maintain the strength. 2. Materials The composition of raw materials and other physical characterization of blended cement constituents are shown in Tables 1 and 2. Two OPC CEM I types (strength class 52,5R and N, abbreviated as OPC1 and OPC2) ground in an industrial scale cement mill to different fineness were employed in this study as control cements. Note that OPC2 contains a significant amount of limestone filler on its own, and although both OPCs were produced with same grey clinker type, the clinker mineralogy may be slightly different between cement samples. A total of six Supplementary Cementitious Materials (SCMs) were used to substitute the OPCs. Three fillers, 2 limestone-based, L1 is a high purity Maastrichtian chalk from Rørdal, Northern Denmark, L2 is a raw meal dust produced at Aalborg Portland cement plant and I is an inert quartz and feldspar filler. As pozzolanic materials, two raw clays, one containing smectite and illite 2:1 clay minerals and the other, consisting mainly of kaolinite were flash calcined at different temperatures and ground to several fineness (C2:1 and C1:1). Finally, a siliceous fly ash (FA) obtained from the combustion of pulverized coal was also employed in this study. Inert and limestone fillers (I and L1) and calcined clay samples were ground in a laboratory ball mill with steel “mini peps” as grinding media, whilst fly ash and limestone filler L2 were used as received. Particle size distributions of all SCMs are shown in Fig. 1. Four batches of calcined 2:1 clay were ground to produce different particle size distributions, but 45 μm residues and Blaine fineness were also determined in some of these samples to monitor the grinding process. Limestone filler L1 and one of the batches of calcined 2:1 clay were ground as fine as physically possible (e.g. up to flake formation in the mill), whilst the inert filler was ground up to typical Blaine fineness for CEM I 52,5N type cement (approximately 400 ± 20 m2/kg). The calcined 1:1 clay sample was ground to reach a 45 μm residue of ca. 30%. Blended cements were prepared substituting either 30wt% of OPC1 by these SCMs or several clinker replacements, such as 10, 15, 20, 30, 35, 45 and 55% in case of OPC2 as according to EN 197-1, i.e. taking into account the limestone filler content (L2) on its own. The OPC used

3. Methods Mineralogical composition of raw clays was determined semiquantitatively using PANalytical CubiX PRO X-ray diffractometer and Rietveld analysis of randomly oriented clay powders using the 2010 ICSD database. Semi-quantification of phases was carried out by the comparison of the mass fraction of each component with a well-known standard. Thus, 10% by wt. of highly crystalline anatase (TiO2) was added to samples and taken into account during refinements and quantification. The chemical composition of the main blended cement constituents was determined by XRF. Particle size distributions were determined in a CILAS 920 laser diffractometer by measuring the angular variation in intensity of light scattered when laser passes through sample suspended in a cell with isopropanol. Blaine measurements were carried out following the air permeability method and 45 and 20 μm residues were determined by jet air sieving in accordance with EN 196-6. All mortars were prepared in a Hobart mixer, but mostly following different mortar mixing procedures (Table 4) rather than specified by EN 196-1, to evaluate the efficiency of admixtures on workability with different delayed addition times whilst mixing always at low speed. Prior to casting, mortar flow was determined in accordance with EN 1015-3, but 20 jolts were applied and the diameter was measured in four directions, instead of two. Compressive strengths were determined at specific terms as specified in EN 196-1, but all reported results have been normalized to 2% air content. Heat of hydration measurements of the above mortars were carried out using an eight channel I-Cal 8000 Isothermal Calorimeter at 20 °C for 24 h.

Table 1 Mineralogical composition of raw clays.

Raw clays, wt%

Clay type

Smectite, C2:1

Kaolinitic, C1:1

Montmorillonite Kaolinite Illite Quartz Muscovite Anorthite

68 – 8 9 9 6

– 92 – 8 – –

4. Results and discussion 4.1. Workability of calcined clay blended cements and SO3 optimization Fresh properties of concrete like workability can be dramatically affected by the total SO3 content and solubility of the sulfate bearing phases, which may result in early stiffening (flash or false set) due to 2

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Table 2 Chemical composition of blended cement constituents. Inert filler

Constituent, wt%

Cements

Limestone

Specifications

CEM I 52,5R

CEM I 52,5N

Chalk

Limestone filler

Abbreviation

OPC1

OPC2

L1

L2

SiO2 Al2O3 Fe2O3 CaO MgO K2O Na2O SO3 TiO2 Cl P2O5 Cr2O3 L.O.I. L2 content (%) Density (kg/m3) Blaine fineness (m2/kg) 45 μm residue (%) 20 μm residue (%)

19.01 5.51 3.81 64.49 0.99 0.43 0.28 3.69 0.27 0.01 0.35 0.01 1.10 – 3180 593 0.5 –

19.06 5.30 3.88 62.96 0.85 0.45 0.27 3.36 0.28 0.03 0.28 0.01 2.34 4.6 3120 418 7.5 –

3.92 0.33 0.14 53.73 0.35 0.05 0.08 0.05 0.02 0.01 0.10 0.00 41.80 – 2700 1211 – –

10.20 2.70 1.44 40.35 0.41 0.37 0.12 0.30 0.15 0.12 0.17 0.01 34.84 – 2710 1262 – –

Calcined clays

Fly ash

Calcined 2:1 clay

Calcined 1:1 clay

Siliceous

I

C2:1

C1:1

FA

82.40 9.82 0.72 0.67 0.08 3.54 2.99 0.20 0.04 0.00 0.02 0.01 0.21 – 2680 422 – –

61.99 17.02 9.55 1.55 2.97 2.91 1.28 0.15 0.89 0.02 0.22 0.02 1.67 – 2430 354 89 –

54.53 41.11 0.85 0.05 0.24 0.12 0.00 0.23 1.47 0.00 0,13 0,01 1.45 – 2570 – 30.5 –

55.04 19.92 5.53 4.48 1.81 2.16 1.12 0.38 0.90 0.004 0.53 0.02 8.40 – 2210 352 – –

675 45.1 –

803 18.9 –

– 0.8 10.5

1:1 clay require around 3 times more SP than finely ground calcined 2:1 clay to show comparable mortar flows for any addition time at this specific w/c ratio. Mortar flows of both the ultrafine calcined 2:1 clay, and finely ground calcined 1:1 clay, are comparable to control cement without any SP when addition of admixture is delayed 30s after mixing water (Fig. 2), whilst longer times for delayed addition clearly improve flow above control cement. It was also found that 90 s is the optimum time for delay for the calcined 2:1 clays since any further delay is not statistically improving workability above the uncertainty of the measurement. On the other hand, mortar flow of the calcined 1:1 blended cements seems to be more enhanced if the SP is added after 3 min, but this improvement would not allow a reduction of the SP dosage to be within the typical range for concrete production or achieve similar workability as calcined 2:1 clays. Furthermore, such long addition times of SP can significantly reduce the capacity of a typical concrete batching plant. Based on these preliminary results, blended cements with several clinker replacements according to EN 197-1, such as 15, 25 and 35%, were mixed with either 1 or 3% of SP with respect to the clay content following mixing procedure 4 (Table 4) in order to assess the SO3 balance by isothermal calorimetry and evaluate the potential effect of the SP on early hydration for comparable mortar flow. Heat development measurements shown in Fig. 3 revealed the four typical stages of cement hydration during the first 24 h [17], starting with the initial heat evolution from the initial dissolution and heat of wetting and hydration. Some early ettringite (Eq. (1)) is also formed during this period, followed by the dormant period and then, acceleration of C3S hydration associated with setting, and finally the deceleration of alite hydration and sulfate depletion that occurs when gypsum is used up and subsequently, AFm phases are formed such as monosulfate (Eq. (2)) and hydrated calcium aluminates (Eq. (3)).

Fig. 1. Particle size distribution of SCMs determined by laser diffraction.

too little or too much early available sulfate, respectively. Additionally, the types and dosages of the chemical admixtures which are added to achieve the desired initial flow in self compacting concrete (SCC), and slump in conventional concretes, should be compatible with the solubility of the sulfate bearing phases in cement. In order to evaluate the compatibility of SP type with calcined clays, preliminary mortar trials (Fig. 2) were carried out on OPC2 based cements with 35% of clinker substitution according EN 197-1, but taking into account the limestone filler content (L2) in OPC2 on its own, with a constant dosage of SP with respect to the calcined clay content in cement at w/c ratio equal to 0.5. The addition time of SP regarding bulk mixing water was modified according to mixing procedures 1, 2 and 3 described in Table 4. Calcined 2:1 clay sample selected for these tests was ultra finely ground (45 μm residue = 0.8%) in order to assess the negative effect of ultrafine particles on mortar workability. Although mortar flow of the calcined 1:1 blended cements follow similar trends as the calcined 2:1 (Fig. 2), it was found out that calcined

C3 A + 3CSH2 + 26H → C6 AS 3H32

(1)

2C3 A + C6 AS 3H32 + 4H → 3C4 ASH12

(2)

C3 A + CH + (x − 1)H → C4 AHx

(3)

Although blended cements demonstrate the same sequence of reactions, it the total amount of heat released is clearly seen to decrease with the increase in any SCM content (both calcined clay types and L2) due to dilution effect and initial setting is delayed more due to the higher content of SP in the mortar. Calcined 2:1 clay blended cements 3

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Table 3 Mortar and binder compositions tested in diverse water-to-cementitious ratios, but constant SP quantities regarding clinker replacement. Binder

Clinker replacement EN 197-1 (%)

Binder composition OPC2 (g)

OPC2 OPC2 + y% C2:1 / (C2:1 + L2) = 0.75 cements

OPC2 + y% C1:1 / (C1:1 + L2) = 0.67 + x% H cements

OPC2 + 35% C2:1 / (C2:1 + L2) = 0.5 cements + z% FA

4.7 20 30 35 45 55 20 30 35 45 55 35 45 50 55

450 373.9 325.2 301.1 246.1 194.8 340.8 283.9 258.9 208.2 162.9 302.4 270.9 254.0 235.0

C2:1 (g)

C1:1 (g)

64.3 95.8 111.5 138.4 163.7 52.1 74.4 85.2 104.1 121.7 74.6 66.9 62.4 58.0

L2 (g)

4.3 17.1 23.4 34.9 45.7 10.5 24.2 30.8 42.5 53.4 60.8 54.5 50.9 47.3

H (g)

Water (g)

SP (g)

225 220.8 218.3 217.2 219.2 221.0 234.6 238.5 239.6 243.7 246.8 218.1 211.8 208.5 205.0

0.64 0.96 1.12 1.38 1.64 0.64 0.96 1.12 1.38 1.64 1.12 1.38 1.51 1.64

Sand (g)

FA (g)

1.7 2.5 2.9 3.5 4.1 48.0 73.9 101.7

1350

Table 4 Mortar mixing procedures. Time

Mixing procedure 1

Mixing procedure 2

Mixing procedure 3

Mixing procedure 4

From 0 to 30 s After 30 s

Mixing of cement and sand Addition of Addition of water and SP water

Addition of water

Addition of water Addition of SP

After 1 min After 2 min

Addition of SP

After 3:30 min At 5 min

Addition of SP Stop

Fig. 3. Isothermal heat development of calcined clay blended cements.

It can therefore be deduced that a considerable fraction of reactive Al2O3 in the calcined 1:1 clay is quickly dissolved during the acceleration period of alite reaction, and reacts with gypsum to form ettringite (Eq. (4)), and subsequently reacts to form AFm phases (Eqs. (5) and (6)) when gypsum is used up, analogous to the C3A reactions (2) and (3). On contrary, Al2O3 in the calcined 2:1 clay is practically insoluble up to 1 day and additional monosulfate formation could be considered as insignificant. These results are in agreement with the observations by Garg et al. [18] for kaolinite and montmorillonite standard clays calcined at different temperatures in a preheated laboratory scale oven. They found Si and Al are dissolved from both calcined clays following a linear relationship over time for the first 24 h, but optimum calcined kaolinite shows a maximum rate of dissolution of 23 μM of Al/h whilst calcined montmorillonite at 800 °C only shows a maximum rate of dissolution 1 μM of Al/h. Hence, in order to achieve best practice with calcined 1:1 clay, the SO3 content of the blended cements must be correctly balanced [19] with a surplus of a quickly soluble calcium sulfate bearing phase to form more ettringite, delaying and reducing monosulfate formation.

Fig. 2. Effect of time addition of SP on mortar flow of blended cements.

show analogous heat development profiles to the control cement, whilst the increase of the calcined 1:1 clay content causes a faster sulfate depletion and greater formation of monosulfate, as shown by the time difference between initial sulfate depletion and maximum peak corresponding to alite hydration in Fig. 4. 4

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Fig. 6. Effect of extra hemihydrate content on time difference between sulfate depletion and maximum of C3S hydration and mortar flow.

Fig. 4. Effect of calcined clay type and content in cement on time difference between sulfate depletion and maximum peak of C3S hydration.

Along with heat development, flow measurements (Fig. 6) revealed that either 1 or 2% extra hemihydrate do not statistically influence workability, however a higher content decreased the mortar flow beyond standard deviation of the measurement clearly showing early stiffening due to false set.

2Al(OH)4− + 3Ca2 + + 4OH− + 3CaSO4 . 2H2 O + 20H2 O → C6 AS 3H32 (4)

4Al(OH)4− + 6Ca2 + + 8OH− + C6 AS 3H32 → 3C4 ASH12 + 8H2 O

(5)

2Al(OH)4− + 4Ca2 + + 6OH− + (x − 7)H2 O → C4 AHx

(6)

4.2. Workability and strength development of calcined clay-limestone blended cements

Fig. 5 illustrates how the sulfate depletion and aluminate reaction peak can be successfully delayed and abated for increased amounts of hemihydrate (H) added to mortars containing calcined 1:1 clay, whilst Fig. 6 demonstrates that 1% of extra hemihydrate by wt. is needed in the blended cement with 35% of clinker replacement to obtain a similar degree of sulfatisation as the control cement. The good linearity found in Figs. 4 and 6 indicates the correct extra dosage of hemihydrate is proportional to the calcined 1:1 clay content in the blended cement.

Mortar flow and compressive strength were determined in OPC1based blended cements for a constant cement substitution of 30% by wt. but varying the calcined clay to limestone ratio, expressed as C/ (C + L1), where C and L1 stands for calcined clay and limestone contents, respectively. Based on preliminary results, SP was added to mortar with 30s of delay as described in mixing procedure 4 in Table 4, in order to reach a similar flow as the control cement. As in the previous tests, the amount of SP was calculated as a percent of calcined clay content in the cement, but SP was added to the mortar in different dosages to reveal the real efficiency of SP for a specific w/c ratio. 4.2.1. Mortar flow Figs. 7 and 8 show the mortar flow of the control OPC1 without any addition of SP, and the OPC1 based blended cements with 30% of calcined 2:1 or 1:1 clay and limestone filler L1 with an addition of 0, 1 and 2 or 3% of SP. Although no SP was added to the plain limestone blended cement because it does not contain any calcined clay (i.e. C/(C + L1) = 0), it showed practically same flow as control cement. On the other hand, the mortar flow is noticeably lowered with increasing calcined 2:1 clay content without any SP, and drastically drops for calcined 1:1 clays, even with up to 3% SP (Fig. 8). OPC1 despite having a higher fineness than OPC2, shows similar mortar flow compared to OPC2 (Fig. 2). For the blended cements, on the other hand, the higher fineness of the OPC1 is observed to affect workability of blended cements, lowering mortar flow below the control cement with either 1 or 3% SP (for calcined 2:1 and 1:1 clay, respectively) added 30s after the water (Figs. 7 and 8). For this reason, OPC2 was preferred to OPC1 to evaluate the influence of fly ash or variable water-to-cement ratio on calcined clay–limestone blended cements. Both calcined clay types (Figs. 7 and 8) give mortar flows linearly

Fig. 5. Isothermal heat development of calcined 1:1 clay blended cements with extra dosage of hemihydrate.

5

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may only retain some longer-range order due to stacking of its hexagonal layers [20]. Thus, the SP is mainly adsorbed and trapped at the open and amorphous structure of dehydroxylated 1:1 clay particles, and are rendered unavailable for providing a good dispersing effect, whilst just a fraction of total SP added to mortar containing calcined 2:1 clay is adsorbed on grain boundaries blocking the interlayer of pseudo-laminar structure which may persist after calcination [11], greatly increasing the effectiveness for any additional SP. This hypothesis also explains why mortar flow is practically independent of fineness for the calcined 2:1 clay (Fig. 7). Most of water demand related to calcined 2:1 clay is due to water uptake by interlayer which can be effectively blocked with SP. Unfortunately, workability of the blended cements is intensely and directly affected by calcined 1:1 clay content since water uptake cannot be lowered by an effective adsorption of the chemical admixture. Therefore, strength performance could be maximized within 28 days of hydration by finer grinding of the calcined 2:1 clay without adversely affecting the workability properties of the blended cements. Furthermore, it provides the option of intergrinding calcined 2:1 clay, limestone and clinker, simplifying production at large scale, and/or reducing extra investments in mills for separate grindings of cement constituents, since calcined clays are actually much easier to grind than clinker.

Fig. 7. Influence of calcined 2:1 clay to limestone ratio and SP dosage on mortar flow of blended cements with 30% of replacement.

4.2.2. Compressive strength Relative compressive strengths of blended cements determined at 1, 2, 28 and 90 days compared to the control OPC1 are shown in Figs. 9 and 11. Early strengths of calcined 2:1 clay blended cements appear to be largely independent of particle fineness which indicates this calcined clay type does not significantly react at 1 day or accelerate hydration. On the other hand, long term performance can be strongly improved by increasing the calcined 2:1 clay fineness and optimizing the C2:1/ (C2:1 + L1) fraction to 0.75 (Fig. 9). The calcined 1:1 clay shows much faster and higher strength development than the calcined 2:1 clay at any C/(C + L1) ratio, which is optimized for the plain calcined 1:1 clay blended cement at 1 day in accordance with Antoni et al. [19]. From 2 days, the peak performance is shifted to a C1:1/(C1:1 + L1) fraction of 0.67 with a greatly enhanced strength at 90 days (Fig. 11).

Fig. 8. Influence of calcined 1:1 clay to limestone ratio and SP dosage on mortar flow of blended cements with 30% of replacement.

related to the C/(C + L1) ratios for any dosage of SP. However, the dispersion effectiveness is noticeably different between calcined clay types at this range of SP dosages. Furthermore, for any given C/ (C + L1) ratio, mortar flow of calcined 1:1 clay increases linearly with SP dosage, whilst the C2:1 shows a non-linear trend. I.e. the variation of flow versus C2:1/(C2:1 + L1) is certainly greater with 1% to 2% SP compared to SP dosages ranging from 0 to 1%. This fact clearly indicates the efficiency of certain dosages of SP depends on the type of calcined clay and may be attributed to structure of raw clay. The multilayer initial structure of 2:1 clays, with an octahedral sheet of alumina sandwiched between two tetrahedral sheets of silica is the key to maintaining a quasi-layered conformation after thermal treatment to a higher extent than the 1:1 clay, with one tetrahedral sheet of silica bonded to the octahedral sheet of alumina, that

Fig. 9. Relative compressive strength at 1 day (dashed lines) and 28 days (solid lines) of blended cements containing 30% of several combinations of finely ground limestone and calcined 2:1 clays or inert filler. Grey dotted line shows the strength improvement due to synergy between C3A and limestone.

6

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products. Therefore, the semi-quantification carried out based on strength performance of blended cement compositions shown in Table 5 only refers to the contribution of each factor to the final performance of the hydrate phase assemblage at 90 days: 1. Nucleation effect. Finely ground materials act as nucleation sites which accelerate the hydration of the cement, accelerating setting times and increasing early strengths. The noticeable higher relative strength for C/(C + L1) = 0 at early term demonstrates that the finer particles of limestone (Fig. 10) have a greater nucleation density than the inert filler or any of calcined clays, which have comparable particle size distributions (Fig. 1). The horizontal and dotted tie line at the bottom of the Figs. 10 and 12 represents the dilution effect of an inert filler, which was estimated by extrapolation up to setting time of the logarithmical fitting of relative strengths at 1, 2, 28 and 90 days for the inert filler blended cement. In comparison with the inert filler blended cement, it is seen that most of the strength improvement from setting to 90 days given by the limestone filler is caused by nucleation effect, but there is a peak of performance between these fillers due to the next factor. Fig. 10. Illustration of separate contribution of 4 factors involved on compressive strength development of calcined 2:1 clay-limestone blended cements up to 90 days of hydration (solid lines).

2. Synergetic reaction of limestone and AFm phases from hydration of the clinker aluminates, mainly C3A. When carbonate is present, AFm phases (monosulfate and hydrated calcium aluminates) generated from clinker hydration slowly react with CaCO3 to form carboaluminate and ettringite (Eq. (7)). This synergetic effect contributes to strength development due to reduced porosity caused by the increased volume of new hydrated phases.

The enhanced performance is attributed to the contribution of several factors listed below, which are applicable to any blended cement with a replacement of combinations of both reactive aluminosilicate pozzolan (in these particular cases, calcined 2:1 or 1:1 clay, as illustrated in Figs. 10 and 12) and other main cement constituents containing calcium carbonate. Note that the main purpose of the division into factors is merely to illustrate the constituents and reactions to explain differences in strength performance among blended cement compositions. Discrete tie lines plotted in Figs. 10 and 12 for each factor do not necessarily fit with strength developed at any specific hydration time in between setting and 90 days, since some of reactions occur simultaneously and are not completed at specific terms and additionally, there are intermediate phases formed from different initial reagents and/or are involved in more than one reaction to generate final

3C4 ASH12 + 2CC + 18H → C6 AS 3H32 + 2C4 ACH11

(7)

C4 AHx + CC → C4 ACH11 + CH + (x − 12)H

(8)

The effect of these reactions can be easily appreciated by the improved performance at both 28 and 90 days for I/(I + L1) fractions ranging from 0.67 to 0.75 comparted to the dotted tie line (Fig. 9) which corresponds to the relative strengths of blended cements containing only either inert filler or limestone (i.e. at I/(I + L1) = 0 and 1), or the relative strengths predicted without any synergetic effect. Accordingly to Matschei et al. [21], the amount of reacted

Table 5 Semi-quantification of strength improvement due to discrete factors. Additional hydrated phases formed due to presence of calcined clay and/or limestone in blended cement as a function of y = C / (C + L1) ratio

Cement type

Limestone blended cement

Calcined clay-limestone blended cements

Calcined clay blended cements

Inert blended cement

SCM type

L1

C2:1 + L1

C1:1 + L1

C2:1

C1:1

I

SCM/(SCM + L1)

0

0.75

0.67

1

1

1

Gain of relative strength up to 90 days regarding performance at setting of inert filler

0.27

0.64

0.67

0.52

0.55

0.15

75 25

25 11

25 10

28 –

27 –

100 –

All 0 ≤ y ≤ 0.9 Ett, Mc

–

– 36 9 3 16

2 30 6 2 25

– 72

17 56

–

0 < y≤1

–

–

–

0 < y ≤ 0.9 Ett, Mc

Strength improvement normalized to 100% of discrete factors

1 2

3* 3a 3b 3c 4

Nucleation effect Synergetic reaction of limestone and AFm phases from clinker aluminate hydration Pozzolanic reaction

Synergetic reaction of limestone with AFm phases formed from reaction of calcined clay

–

7

0.9 < y < 1 Ett, Hc, Mc

Ett, Ms C4AHx C-S-H C-(A-)S-H 0.9 < y < 1 Ett, Hc, Mc

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If replacement is relatively high in the blended cements, Ca(OH)2 is typically the limiting component in (Eqs. (6) and (11)), since it is an intermediate product of preceding reactions: 3a. Hydration of alite and belite (Eqs. (12) and (13)):

3CaO·SiO2 + zH2 O → x(CaO)·Si(OH) y ·(z + x − 3 − y 2)H2 O + (3 − x) Ca (OH) 2

(12)

2CaO·SiO2 + zH2 O → x(CaO)·Si(OH) y ·(z + x − 2 − y 2)H2 O + (2 − x) Ca (OH) 2

3b. Synergetic reaction of limestone with AFm phases (Eq. (8)) formed from the hydration of clinker aluminate phases, (Eqs. (2) and (3)). As described above in factor 2, Ca(OH)2 is released when carboaluminates are formed. Hence, this factor 3b is intrinsically coupled to factor 2. Since aluminate content in the clinker does not change for selected cement replacement, factors 2 and 3b have a similar impact on relative strength gain independently of calcined clay type or C/(C + L1) ratio. 3c. Synergetic reaction of limestone with AFm phases (Eq. (8)) generated from the pozzolanic reaction of calcined clay (Eqs. (5) and (6)) (explained below at factor 4). Similarly to previous factor, factor 3c is also intrinsically connected to factor 4.

Fig. 11. Relative compressive strength at 1 (dashed lines), 2 (dotted lines) and 28 days (solid lines) of blended cements containing 30% of several combinations of finely ground limestone and calcined 1:1 clays or inert filler.

Additionally, and prior to these reactions, quickly soluble alumina in the calcined 1:1 clay reacts with calcium sulfate to form additional AFt phases in a primary reaction (4), and a secondary reaction (7) from monosulfate formed at (Eq. (5)). This factor has been labelled 3* in Fig. 12 and used only for the calcined 1:1 clay. Since the SO3 content of the blended cements was adjusted in line with the calcined clay content, greater amounts of ettringite and monosulfate (Eqs. (4) and (5)) are formed within 1 day of hydration as the C1:1/ (C1:1 + L1) ratio increases. The heat flow for 24 h (Fig. 13) and additional heat of hydration of the blended cements containing any calcined 1:1 clay compared to the limestone cement without clay released during initial hydration (between 0 and 2 h), and between sulfate depletion and 16 h (Fig. 14) is in agreement with the trend for 1 day strengths versus C1:1/(C1:1 + L1) fraction (Fig. 11). Based on enthalpy of formation from the oxides (ΔH = − 452 KJ/mol of AFt and ΔH = − 357.6 KJ/ mol of monosulfate, respectively) [22], and stoichiometry of reactions (4) and (5), it can be estimated that 2.9% of both AFt and monosulfate by wt. anhydrous blended cement have been formed in the plain calcined 1:1 clay blended cement up to 1 day of hydration. In the particular case of calcined 1:1 clays limestone blended cements, the gain in relative strength due to this particular factor 3* (Fig. 12) was estimated by subtracting 1 day strength of plain calcined 1:1 clay cement from baseline of plain limestone cement. The upper tie lines correspond to the additional formation of C4AHx phases, C-S-H and C-(A-)S-H gels due to pozzolanic reaction of calcined clay and portlandite, which comes from: - Clinker hydration (factor 3a) was simply drawn by connecting the 90 day strength of the limestone and the calcined clay blended cements. - Synergetic reaction of limestone with AFm phases formed from the hydration of clinker aluminate phases (factor 3b) was plotted considering the interpolated performance at 90 days of calcined clay-limestone blended cement for P/(P + L) ratio = 0.9. - Synergetic reaction of limestone with AFm phases generated from the pozzolanic reaction of calcined clay (factor 3c) is estimated by default after quantification of factor 4.

CaCO3 calculated by thermodynamic modeling as a function of Al2O3 contents and SO3 to Al2O3 mass ratio in any of these inertlimestone fillers blended cements is 2.7%. Thus, considering L1 contains 95% of CaCO3, this synergic effect is theoretically maximized for any I/(I + L1) ≤ 0.90 in any blended cements with a cement substitution of 30% by wt. The relative gain in performance in the range of 1 > I/(I + L1) ≥ 0.9 is attributed to reactions (9) and (10) when carbonate is the limiting reagent and therefore, hemicarboaluminate is formed instead of monocarboaluminate.

3C4 ASH12 + CC + CH + 19H → C6 AS 3H32 + 2C4 AC 0.5H12 C4 AHx + 0.5CC → C4 AC 0.5H12 + 0.5CH + (x − 12.5)H

(9) (10)

Ideally, blended cements without limestone (i.e. C/(C + L1) = 1) should not form any carboaluminate and therefore, monosulfate and C4AHx are preserved as long as the hydrated cement is not exposed to atmospheric carbonation. The second and third tie line from the bottom in Figs. 10 and 12 illustrate the strength improvement due to the factor 2. The intercept of the second tie line was estimated from the strength difference between limestone and inert filler blended cements at 90 days and therefore, the slope of the second tie line was calculated from this intercept and 90 day strength of inert filler blended cement. The third tie line was plotted based on the minimum CaCO3 content that results in complete monocarboaluminate formation for inert filler – limestone blended cements. So, its slope is the same as the second tie line up to P/(P + L) ratio ≤ 0.9 and then drops to the 90 day strength of the inert filler blended cement. 3. Pozzolanic reaction. It is well known that the reactive silica and alumina in pozzolans reacts with Ca(OH)2 to form longer and less dense chains of calcium alumino-silicates hydrates (C-(A-)S-H gels) with lower Ca/Si ratio than C-S-H gels (Eq. (11)) and enhanced uptake of Al. Moreover, the reactive alumina of calcined clay can also reacts separately with Ca(OH)2 and further extends the formation of AFm (Eq. (6)).

H 4 SiO4 + xCa (OH) 2 + (y − x − 2)H2 O → (CaO) x ·SiO2 ·(H2 O) y

(13)

4. Synergetic reaction of limestone with AFm phases formed in pozzolanic reaction of calcined clay. As with the reaction with C3A in the clinker, AFm phases generated from the pozzolanic reaction of Al2O3 in

(11) 8

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Fig. 12. Illustration of separate contribution of 4 factors involved on compressive strength development of calcined 1:1 clay-limestone blended cements up to 90 days of hydration (solid lines).

Fig. 14. Additional heat of hydration of blended cements compared to plain limestone cement.

Fig. 15. Relative compressive strength at 2, 28 and 90 days of blended cements with 30% of cement replacement and C2:1/(C2:1 + L1) = 0.75.

Fig. 13. Isothermal heat development of mortars with 70% OPC1 + 30% calcined 1:1 clay with extra dosage of hemihydrate.

actually corresponds to a C/(C + L1) fraction of 0.76 and 0.64 which are practically the optimum fractions found experimentally for 30% replacement. Therefore, the positions of the maximum of the tie lines for factor 3b are defined by the minimum CaCO3 content that results in complete monocarboaluminate formation of AFm formed in pozzolanic reaction, e.g. x-axis values for C/(C + L1) of 0.76 and 0.64, respectively. The y-axis values are defined by the difference between the interpolated performance at 90 day of calcined clay – limestone blended cements at these specific C/(C + L1) ratios, and the strength improvement from the content of CaCO3 that has reacted. These are given at the strength-to-CaCO3 ratio estimated in factor 2.

calcined clay (Eqs. (5) and (6)) may also react with CaCO3 to form carboaluminates (Eqs. (7)–(10)). The relative improvement of strength was attributed to the additional amount of reacted CaCO3 and strength gain described in factor 2, because of identical stoichiometry of reactions. As with the reactions in factor 2, the amount of reacted CaCO3 was consequently calculated according to Matschei et al. [21]. The higher contents of Al2O3 content in the calcined clay and the lower SO3 to Al2O3 ratio in blended cement with respect to the calcined clay content, leads to high degrees of reaction of CaCO3, i.e. 6.7 and 10.2% for 22.9% and 19.1% contents of calcined 2:1 and 1:1 clay, respectively. Thus, for L1 which contains 95% of CaCO3, this 9

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Fig. 16. a (left). Mortar flow and b (right) compressive strength of calcined clay – limestone blended cements for clinker replacements from 10 to 55% (solid lines and filled markers) and with an addition of fly ash on top of blended cements with 35% clinker replacement (dashed lines and not filled markers).

compared with the pozzolanic factor 3a, they actually define optimum C/(C + L1) ratios at 0.64 and 0.76. These values should not be considered as constant since they depend on the content of reactive Al2O3 in the calcined clay, followed by the C3A and SO3 contents of cement. Therefore, optima C/(C + L) ratios could be slightly shifted to higher values if grade of limestone or reactivity of calcined clay is reduced, a clinker with a lower Al2O3 content is used and/or the SO3 content is not correctly balanced. In fact, carboaluminates formation (Mc and Hc) has a coupled positive contribution to strength due to the recovery of Ca(OH)2 into solution (factors 3b and 3c), which is the key factor to enable further pozzolanic reaction at lower clinker contents. The balance of consumption of Ca(OH)2 to form C4AHx and recovery during transformation to carboaluminates greatly differ depending on the source of reactive alumina. Al2O3 in calcined clay demands 3 mol of Ca(OH)2 more than C3A to form 1 mol of C4AHx, whilst a maximum of 1 mol of Ca (OH)2 is released when carboaluminates are generated. Because of this, the gain in performance from the extended pozzolanic reaction of factor

The gain in the relative strength of the factors has been semiquantified separately and normalized to 100% (Table 5) for the plain limestone or calcined clay blended cements and tested for the clay/ (clay + limestone) closest to optimum for these SCMs. It can be clearly seen that pure pozzolanic reaction (factor 3a) of calcined clays is always the major contributor to strength development mainly due to enhanced formation of C4AHx phases, calcium silicates hydrates (C-S-H gels) with lower Ca/Si ratio and enhanced uptake of Al, i.e. C-(A-)S-H gels. The higher impact on strength of factor 3a found for the calcined 2:1 clay is due to the higher SiO2 content (Table 2), potentially all reactive, resulting in greater C-(A-)S-H gel formation by consumption of portlandite. On the other hand, the higher and early reactive Al2O3 content of calcined 1:1 clay improves performance at around 17% [19] with additional hemihydrate, enabling maximum synergy with limestone (factor 4) at lower C/(C + L1) ratios. Although formation of carboaluminates (factor 2 and 4), and additional ettringite (Ett) and monosulfate (Ms) of quickly soluble Al2O3 in calcined 1:1 clay (factor 3*), provides relative lower strength gain

Fig. 17. a (left). Mortar flow of calcined 1:1 clay – limestone blended cements with adjusted w/c accordingly to clinker replacement to achieve same flow as control cement and b (right) compressive strength of blended cements for same flow.

10

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On the other hand, the addition of fly ash on top of blended cements with low C2:1/(C2:1 + L2) ratio maximizes flow at high clinker replacements, so the w/c ratio may be potentially reduced to get even better strength performance.

3b is higher than factor 3c, since synergetic reaction of CaCO3 with AFm from hydration of the clinker aluminates (factor 2) releases more Ca (OH)2 than AFm from reaction of calcined clay (factor 4). Fig. 15 shows the influence of fineness of calcined 2:1 clay on strength development of blended cement for the optimum calcined clay to limestone ratio. Three of the factors described above (nucleation, pozzolanic and synergetic reaction of limestone with AFm phases formed from calcined clay) are responsible of around 90% of strength developed up to 90 days, which are clearly enhanced with the increase of fineness of calcined 2:1 clay. Therefore, Ca(OH)2 is not limiting reagent up to 28 days and kinetics of both pozzolanic and synergetic reactions might be accelerated by finely grinding of calcined clay without detrimental of workability as demonstrated above.

4.4. Influence of w/c on workability and strength of calcined clay – limestone blended cements Additionally to clinker reduction in cement by SCMs, clinker content in mortar or concrete can be eventually lowered by increasing the water content (and water/cementitious ratio) to achieve the desired workability if strengths are higher than required and excessive bleeding is observed. Mortar compositions (Table 3) were designed to maintain constant the cement paste volume or aggregates spacing, so aggregateaggregate interaction is not affected by lubricant effect of paste [24]. SP was added to blended cements taking as a baseline the quantities used on corresponding C2:1-limestone blended cements (1% of calcined clay content) for each clinker replacement. Mixing procedure 2 (Table 4) was selected since maximizes and provides similar effectiveness of SP to disperse both clay types for a given delay addition time. Fig. 17a displays the efficiency of SP for larger w/c ratios on calcined 1:1 clay blended cements needed to achieve the same flow as control cement. Equivalent mortars tested at w/c = 0.5 or larger values without SP reveal that the gain of flow of calcined 1:1 clay blended cements due to SP with an increased w/c is not much better than with a fixed w/c as shown in Fig. 8. Therefore, SP efficiency is not improved for larger quantities of mixing water, but 28 day strength is diminished with increasing w/c (Fig. 17b) below the performance of the calcined 2:1 clay. The addition of fly ash allows lower water to cement ratios for same flow, which slightly increases strength of the calcined 2:1 clay and limestone blended cements at lower clinker contents in paste.

4.3. Workability and strength of fly ash-calcined clay – limestone blended cements Fly ash or ground granulated blast-furnace slag can also provide comparable quantities of reactive silica and alumina needed to form additional AFm and C-(A-)S-H in the blended cements, but with completely different rheological performance. It is well known that the incorporation of fly ash particles to OPC improves the workability of concrete with low w/c ratio [2] and therefore a similar trend can be expected when added to calcined clay-limestone blended cement. Thus, mortar tests of this section were carried out without SP to exclude any other external factors and following the procedure in EN 196-1, for several clinker replacements and C/(C + L2) ratios slightly lower than optimum found for maximum strength. Calcined 2:1 clay sample selected for these tests was ultra finely ground (45 μm residue = 0.8%) showed comparable workability to the control cement for replacements up to 30% (Fig. 16a), whilst it workability is reduced for higher or any replacement with the coarser calcined 1:1 clay (Fig. 1). In addition, it can be seen the addition of fly ash on top of cements with 35% clinker replacement has a positive effect, with same trend independently of calcined clay type or C/(C + L2) ratio, mainly due to dilution of calcined clay content in binder by the fly ash particles. It is remarkable that blended cements with 35% calcined 2:1 claylimestone with an addition of fly ash on top for a total 50% of clinker replacement can achieve the same flow as control cement without any admixture. In contrast, calcined 1:1 clay would demand much larger fly ash addition than investigated here or the initial clinker replacement of blended cement must be restricted to around to 15–20% in order to obtain similar flow performance [23]. 28 day strength measurements (Fig. 16b) confirmed that calcined 1:1 clay provides much higher strength than calcined 2:1 clay, showing the same performance with this coarser control cement for 55% of clinker replacement. Higher strengths are found a C/(C + L2) ratio close to the optima determined in the previous Section 4.2.2, but performance drops more sharply at replacements levels higher than 30% due to lack of portlandite. The influence of fly ash addition on strength of blended cements with 35% clinker replacement depends on the clay type and C/(C + L2) ratio, which in turn governs the availability of portlandite needed for the pozzolanic reaction of fly ash and the availability of both portlandite and limestone for the subsequent synergetic carbo-aluminate reaction. The addition of fly ash on top of C2:1/(C2:1 + L2) = 0.5 forms an assemblage of hydrated phases which provides comparable performance to the corresponding plain calcined clay-limestone blended cement up to 50% clinker replacement, whilst performance is lowered in blended cements with calcined 1:1 clay or C2:1/(C2:1 + L2) = 0.7 when fly ash is added. Fig. 16b reveals that the remaining portlandite is more efficiently used up forming more hydrated phases with the combination of calcined 1:1 clay and limestone rather than those from fly ash due to higher reactive Al2O3/SiO2 ratio of calcined clay, whilst improvement on flow provided by fly ash cannot compensate for this loss of strength.

5. Conclusions - SO3 content of blended cements and the mixture of calcined clay and limestone can be optimized for maximum strength depending on the content of early soluble and total reactive alumina content from calcined clay. - Calcined 1:1 clay reacts faster resulting in higher strength than calcined 2:1 clay, enabling higher limestone content to achieve best performance for the same water to cement ratio at any replacement and/or period of hydration. - On the other hand, blended cement containing calcined 1:1 clay requires much higher water contents compared to calcined 2:1 clay in order to achieve an acceptable workability using typical dosages of SP. Therefore, calcined 2:1 clay result in the most convenient choice to maximize 28 day strength for low clinker content and SP dosage in concrete with similar workability. - Both delayed addition of SP, and fly ash on top of blended cements, can significantly improve the rheology of any binder containing calcined clay. Blended cements with a combination of calcined 2:1 clay and limestone mixed in a ratio slightly below optimum gave the best strength development with a further addition of fly ash on top. This is the best overall solutions for sustainable concrete production, since lowers the contribution of CO2 emissions from the clinker reduction, and from the calcination of the clay, whilst providing acceptable workability for the low w/c ratios specified in today's concrete standards especially for the more aggressive exposure classes. Acknowledgements The Danish National Advanced Technology Foundation is acknowledged for the financial support to the SCM project. FLSmidth R & D Centre Dania, Mariager, Denmark is acknowledged for determination of mineralogical composition and flash calcination of raw clay 11

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samples. We also thank to staff at Aalborg Portland A/S, especially Finn Bendixen and Jens Jespersen for their contribution to experimental mortar testing.

[12]

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