Thermal Methods

Thermal Methods L. Ben-Dor Department of Inorganic and Analytical Chemistry, The Hebrew University of Jerusalem, Jerusalem, Israel

CONTENTS

1 2

Introduction Behaviour of Construction Materials 2.1 Clinkerization and Clinker Composition 2.2 Calcium Silicates 2.3 Calcium Aluminates 2.4 Cements, Mortars, Concrete and Carbonation Reaction 2.5 Slags, PIC, FRC, Fly Ash and Pozzolanas 2.6 Plasters Acknowledgement References

673 674 674 675 681 690 694 701 703 ^03

1 INTRODUCTION Thermal analysis techniques have been in existence for many years, dating as far back as Le Chatelier [1], for the analysis of fundamental properties of many materials. Lately, several studies of a more practical nature have been reported, showing the use of thermal analysis technique in the evaluation of construction materials amongst others. A combination of DTA and TG have been used to study the behaviour of cements as a result of hydration, corrosion, durability, etc., and their relation to structure [2,3]. Attention should be drawn to the chapter dealing with theory, apparatus, technique and applications of DTA by Mackenzie in Taylor's book [4]. Weight-loss curves normally give the sharply defined decomposition temperatures. When used in dynamic techniques they are more rapid and can be compared directly with results obtained by DTA. When using DTA a note of caution should be added: identification of synthetic cement components by comparison with authentic samples should be checked by some other method. In fact one should endeavour to use as many techniques as are available. Besides identification, other applications of DTA are: thermal stability, phase changes and solid phase reactions. DTA was found to be much faster and its accuracy only subject to the degree of completed hydrate formation. The accuracy is sufficient to detect gross shortages of cement content leading to unacceptably low strengths of concrete [5]. In conjunction with TG, it is possible to evaluate the extent of prehydration and precarbonation of the clinker. The information regarding temperatures associated with these two reactions is important since it affects later strength development. DTA curves of hydrated cement in general show endothermic peaks corresponding to loss of water or carbon

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L. Ben-Dor

dioxide; high-temperature additional peaks correspond to the appearance of new phases. Several reviews of the developments in thermal analytical studies of cements are available (see, for example, refs. 6 and 7). Kalousek and co-workers [8-12] were the first who studied the phases formed in neat cement pastes at temperatures around 100°C. Ettringite first formed is replaced by a solid solution with the monosulphate which in turn disappears passing into the gel. They also studied by DTA the phases produced in autoclaved cement-silica pastes [8,9]. Biffen [13] measured the free lime and carbonate contents of calcium silicate hydrates. The thermogram'showed a gradual slope for a straight line between 375 and 650 C and the break in the range 700-900°C was due to the decomposition of carbonate content in the sample. The break in the curve at about 500°C was an indication of the calcium hydroxide content. Ramachandran [14] also determined the calcium hydroxide content in calcium silicate mixtures by the water loss between 450 and 550°C. Glasser [15] reviewed the polymorphism and thermal behaviour of di- and tri-calcium silicates. The effect of added impurity is also very important in considering the interpretation of DTA traces. Calcium silicate hydrates occur both in nature (cf. réf. 16) and in the hydration products of Portland cements. For most of them two events appear on the DTA pattern: a low-temperature endotherm (100-300°C, loss of water) and a high-temperature (usually) exotherm (^800°C, recrystallization of amorphous material). Midgley and Rosaman [17] were able to interpret DTA curves of Portland cement pastes. Other important phases studied were reviewed by Gutt and Majumder [18]. The general application of DTA to building research had been reviewed by Webb [19]. The review by Ramachandran [6] and that of Webb and Krüger [20] cover a wide field of cements studied by DTA between the early 1950s and late 1960s. Special attention was given to hydration of cement minerals in the absence or presence of accelerators and retarders. Cements are an excellent subject for the application of DTA since large energy changes occur both in manufacture and during use 121]. DTA has proved useful for checking raw mixes, "synthesis" of cement, hydration and the effect of additives; also for analysis of Portland cement, slag cements, pozzolana cements, aluminous cements, glasses, asbestos cements and polymer cements. •2 BEHAVIOUR OF CONSTRUCTION MATERIALS 2.1

Clinkerization and Clinker Composition

The formation of silicate minerals and the extent to which the reactions are carried substantially to completion are of key importance in the clinkering process. The sequence of exothermic and endothermic reactions which occur as raw feed is heated and clinker minerals are formed have been characterized by Courtault [22] and by Moore [23] through DTA. Fierens and Picquet [24], using DTA-TG and high-temperature XRD, studied the thermal synthesis of C2S at temperatures above 1400°C. An initial solid-state reaction between CaC03 and ground quartz (870-1418°C) to form af-C2S was suggested. At 1427°C the material undergoes af-a endothermal transition and the reactive product reacts with additional Si02 to form a material with the approximate composition C3S2. Above 1443 C this phase liquefies and reacts with residual free CaO to form the remaining 01-C2S. The reaction mechanism controlling C2S formation

Thermal Methods

675

in a dynamic process is a significant improvement over Jander and Hoffman's classical model of 1934. The extent of 2% magnesium or calcium silicofluorides as mineralizers in the formation of cement clinker components was monitored by the free lime content and correlated with DTA-TG (and XRD) . Alite and C3S formation were markedly influenced by the use of these mineralizers [25] (Figs. 1-3). DTA was used to study sintering processes in cement raw mixes and to determine the heat of formation [26] and study reactivity of raw meal [26a]. Also, the composition and the physico-chemical properties of cement were discussed [27]. Angelov and co-workers [28] examined samples taken from a cement kiln. The different minerals formed during the step-wise heating of the kiln up to 1450°C were identified. 2.2

Calcium Silicates

DTA-TG indicated that synthetic C2S.CaCl2 decomposed at 1084°C to solid cx'-C2S and liquid CaCl2. These results are important in the study of structural relics [29]. Mitsuda [30] obtained for 11 Λ tobermorite a broad endotherm between 100 and 350°C associated with a sharp weight loss, a gradual weight loss between 300 and 650°C and a sharp exotherm between 820 and 850°C corresponding to the recrystallization to wollastonite. The talclike hydrated magnesium silicate gave a broad endotherm below 300 C, another very weak one between 300 and 650 C and a sharp one at 800°C. These peaks are all associated with weight loss events. The effect of calcium lignosulphonate on C3S, hydrated C3S and CH was studied [31 ]. At low admixture concentrations, C-S-H adsorbs the lignosulphonate. A more recent study [32] also dealt with the hydration of C3S in presence of lignosulphonates and sugars. Delay in hydration was found which was discussed in terms of poisoning of nucleation sites. The DTA trace with 0.5% additions showed no endotherms due to hydration products. When 0.1% was added some hydration products appeared after 7 days but the CH peak at ca. 500 C scarcely grew with time. The water sorption of natural cement gel was studied between 40 and 100°C with a vacuum TG. The isosteric heat of adsorption for water vapour was independent of the amount adsorbed and was found to be 11.3 kcal/mol [33]. Regourd and Guinier [34] studied the polymorphism of C3S as a function of temperature by DTA. Chiocchio and Collepardi [35] studied the influence of CaCl2 on the autoclave hydration of C3S and found an exothermic peak in the thermograms, which was attributed to the chemisorption of Cl" on the C-S-H surface or its penetration between molecular layers of C-S-H. This theory had been introduced earlier by Ramachandran [36]. The former team [37] examined also the hydration products and anhydrated residues of ten mixtures of C3S with other cement minerals with the aim of evaluating the effect of the various minerals on the autoclaved hydration of C3S. Veprek and coworkers [38] also studied by DTA the hydration reaction occurring under steam pressure. Feldman and Ramachandran [39] used DTA-TG to determine the hydration states of C-S-H gel for different relative humidities. Larionova and Garashin [40] investigated the room-temperature hydration and autoclave-cured, alite and bellte samples and found that the hydration products for both minerals are the same depending only on the curing conditions: C-S-H (II) in the former case and Y-C2SH in the latter. Another study dealt with the hydration of various doped alites. Based on DTA results the conclusion was reached that the structure and composition of C-S-H formed differed from those of the dopant [41].

676

L. Ben-Dor Dusting //-C2S

l "^"^" \ _

E ω

/ 505°C

—^_

\ '—>\ a-^S-SiOaVv 572°C

*

\ \

c 2 s- -2 wt% MgSiF6 \ 1 IO°C/min \J

\

a -*~a C2S Ι.440 °C

Heating Cooling

x/

^

a -*-a'C2SJ 1430 °C 1 ^

CaC0 3 decomposition o

Ό

c LU

0

L

200

1

1

400

1

600

800

1

1000

Temperature, Fig.

1.

1

1200

1

\l \\ \\

1400

1

1600

°C

T y p i c a l DTA t r a c e of C2S s y n t h e s i s .

C2S no mineralizer

0.15 (010

Sample wt. = 20 mg

0.05

400 350

300

250

200

150

Temperature,

Fig. 2.

100

Low-temperature thermogram of C2S with and without 2 wt% additions of MgSiF6.6H20 and CaSiF6.2H20 depicting nonstoichiometric weight loss. Underlined values are the expected SiFif+H20 wt losses. Heating rate, 6 C/min.

Thermal Methods 1.2 1.0 Γ 0.8

0.6 0.4 0.2 0 1.2 "α> 1.01U Ο.βΙ

(Q)

I

Sample wt. = 20 mg SiF 4 =0.250mg

C2S 2wt.% MgSiF6

Wt. loss « 0 . 1 ma

I

L_

C3S 2wt.%MgSiF 6

(b)

0.61

0.4] 0.2 0 1.2 1.0 0.8 0.6 0.4 0.2 0 1.2 1.0 L

677

Wt. loss« 0.07mg

(C)

C2S 2wt.%CaSiF 6 Wt. loss« 0.06 mg

Sample wt = 20 mg SiF« =0.228 mg

c3s

(d)

2wt.%CaSiF 6

0.8

0.6 0.4 0.2

Wt. loss «0.05 mg I

1450

1400

1350

I

1300

L 1250

1200

Temperature, Fig. 3.

High-temperature thermogram of C2S and C3S with 2 wt% additions of MgSiF6 and CaSiFß depicting SiFi+ volatilization. Heating rate, 6 C/min.

Valenti and Sobatelli [42] had used TG to determine the stoichiometry of C3S hydration. This technique makes it possible to estimate rather accurately the water content of calcium silicate hydrates and calcium hydroxide. The utility and convenience of TG and DTA were emphasized by Ben-Dor and coworkers [43] who studied the influence of several additives: CaCl2, CrCL3, Cdl2, on C3S paste hydration. A shift of the CH dehydration temperature with age was observed, this shift being slightly different when the additives were present (Figs. 4-7). Rio and Biagini [44] studied by thermal methods (and XRD) the hydration of C3S pastes with and without the addition of quartz. A similar study was carried out by Berardi and co-workers [45]. Mitsuda and Taylor [46] studied the conversion of silicate hydrate, gels to tobermorite by DTA as well as other methods. Bensted and co-workers [47-47a] illustrated the benefit of combining IR information on cement hydration products with DTA study of the same specimens. This combination of methods was also used by Ben-Dor and Perez [48] in a study of the hydration of C3S paste with various metal halides. The effect of phosphate on C3S hydration [48a] was studied thermally and compared to mechanical properties.

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L. Ben-Dor

Endo

Fig. 4.

1

Thermogram of neat C3S paste.

2

3

4

5

6

~T~

T> °Cx!00 Fig. 5.

Thermogram of C3S paste with 2 wt% CaCl2.

Thermal Methods

679

Time in days Fig. 6.

Weight loss for C3S neat and with 2 wt% additions of (a) Cdl2, (b) CrCl3, (c) CaCl2, (d) neat.

Singh [49] investigated the effect of doping C3S with NiO. Shifts in polymorph-transition temperatures were observed by DTA. Synthetic ot-C2S has been examined by Bensted [50]. Pastes with W/S=l were hydrated for a period of 3 years and studied by IR and DTA. The results showed that a-C2S is hydraulic but its rate of hydration is slower than that of $-C2S. A compilation relating to comparisons between DTA and other methods of studying cement hydration compounds has appeared [51]. Some differences in DTA characteristics between CSH phases were reported, but unfortunately they do not appear to be consistent. Thermogravimetry was compared with other methods to determine chemical composition of CSH from calcium silicate pastes and extraction was considered to be the most reliable method [51a], Kalousek and Greene [52] indicated that the endothermic dehydration response of CSH gel produced by paste hydration of alite is relatively sharp and occurs at 130°C as it does with gel formed from cement hydration. In contrast, the endotherm from the dehydration of gel from 3-C2S occurs at 150°C. Another difference is: alite derived CSH is said not to produce an exotherm in the 600-900°C region; 3-C2S gel does show a small exotherm at 860°C. However, studies on the hydration products of synthetic doped alites [53] indicated that all such materials showed an exotherm between 850 and 900 C, the differences in the peak temperature probably reflecting differences in the underlying C-S-H gel produced. Gard and Taylor [54] presented extensive structural data, including thermal data, pertaining to the structure of C-S-H (II). C2SH (o-rh) was identified by DTA on the basis of multiple endotherms at 400-440°C, 460-480°C, 520°C, 720-800°C and 880-925°C [55]. Mohan and Glasser [56] investigated the thermal decomposition of C3S at temperatures below 1250°C. It was found that pure C3S decomposed very slowly at first, but as nuclei of C2S and CaO form, the rate of decomposition was accelerated. When C3S and CaO were added at the start, they acted as nuclei and decomposition began immediately at an approximately linear rate. C3S solid solutions with Al,Mg,Fe or Na were used for a further study. It was found that Na had a slight, Mg a strongly retarding effect, and Fe a strongly accelerating effect on the decomposition. Water vapour also appreciably accelerated the decomposition.

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12

16

20

Time in days Fig. 7.

(a) Shift of Ca(0H)2 DTA peak; (b) percent reaction for C3S neat and with 2 wt% additives.

DTA and TG techniques were used in the investigations of autoclaved Y-C2S quartz mixtures [57], In the products examined, the coexistence of gyrolite and tobermorite was suggested; heating converted these products into pseudowollastonite and wollastonite, respectively. A further report from the same laboratory [58] dealt with the thermal decomposition of 01-C2SH to give C1-C2S, then to otf and ultimately its transition in cooling to 3-C2S. Kalousek and co-workers [59] conducted an extensive study characterizing xonotlite using TG and other methods. It was concluded that the material is deficient in Ca and charge balance is achieved by H. In a study devoted mainly to the microstructure of 3-year-old C3S pastes, DTA results were also given [60]. Maki and Chromy [61] showed that phase transitions in C3S can be easily observed by DTA and not always by optical methods. Dent-Glasser and co-workers [62] and Mohan [63] proposed a novel hypothesis for the mechanisms of hydration of C3S using multitechniques including TG. They differentiated between "surface" and "precipitated" C-S-H. Lukas and co-workers [64] used thermo-

Thermal Methods

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analytical techniques to study the forms of water in the calcium silicate hydrates. A similar study was carried out by Kolbosov and co-workers [65] who found several structural types of water to be present in C-S-H. Perullo and co-workers [66] have determined the CH in C3S pastes or mixtures of CH and C-S-H, by TG, and found that it gives low values. It was believed that CH can partially react with C-S-H by the heat generated during the thermogravimetric analysis. The system CaO-Si02-NaOH-H20, a source of building materials, was studied by thermoanalytical and other methods [67] and the calcium silicate phases formed were identified. The thermal decomposition of the hydration products of C3S had been studied [68]. The characteristics of the C-S-H phase changed with the time of hydration. The energy of dehydration of CH formed in the process was found to be lower than that of a pure Ca(0H)2 crystalline material. Ramachandran [69] used DTA for estimating CH in cementitious phases from peak areas of the dehyrdation of CH. DTA was also used for monitoring the preparation of CH-free product from hydrated Portland cement or C3S. The method permitted the determination of the rate of formation of CH in Portland cement hydrated in the presence of up to 3£% CaCl2 (Fig. 8). A similar study was conducted by Midgley [70]. DTA-TG was found to give the most reliable results concerning the estimation of CH in Portland cement. The relationship between the peak area of the CH endotherm and its mass present in the mixture was independent of grain size. For TG, a relationship was presented between the weight loss between 410 and 560 C and the amount of CH in the sample. Menetrier and co-workers [71] studied the effect* of gypsum on the early stages of hydration of C3S Éy DTA and other means. A period of the first 96 hr of hydration was explored. A sharp doublet, characteristic of gypsum, appeared at M30°C. With the decrease of intensity of these endotherms a sharp endotherm started to grow at 475°C (decomposition of CH). With the disappearance of gypsum a wide endotherm grew at 130° and shifted to 180°C characteristic of C-S-H; probably the sulphate substituted for the silicon in this C-S-H. The CH endotherm, after attaining a maximum in intensity, decreased in turn (Fig. 9). 2.3

Calcium Aluminates

Boros and Balazs studied the system CsA-CaSO^-^O under various curing procedures [72-74]. In the first period of hydration monosulphate (CaAjCaSOi+.Hio) and ettringite are both present (C3A.3CaSOj4.H32). Curing at 70°C accelerated the formation of C3AH6 and retarded the formation of ettringite. Low temperature decelerated the hydration. Steam curing decreased the amount and stability of ettringite and increased the monosulphate and C3AH6. In a similar study [75] the reactivity and expandability of ettringite, formed from the system CA-CaSOi+-CH, were reported. Diamond [76] studied interactions between cement minerals and hydroxylic acid retarders. Thermograms of CeA paste with and without salicylic acid showed a peak at 230°C (Ο+ΑΗ^) and another at 340°C (C3AH6) ; the latter was present only for the pure material. When adding salicylic acid there were also some high temperature features which reflected the presence of the acid in the material.

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L. Ben-Dor

350

400

450

500

Temperature,

Fig. 8.

550

600

°C

Endothermal effect due to removal, by extraction for different periods, of free Ca(0H>2 in hydrated C 3 S.

683

Thermal Methods Hydration time (hr)

100

Fig. 9.

200 300 Temperature,

400

Thermograms in the system C3S - gypsum - H2O.

Ramachandran and Feldman [77-8] studied hydration characteristics of CA by DSC and also physicomechanical characteristics of C3A hydrates. CA hydrated at 20°C loses water from alumina gel at M O O C and various endotherms appeared ascribed to calcium aluminate hydrates. The thermal behaviour at 80 C was significantly different. The endotherms appeared after a short hydration time and the one at 100 C disappeared after 2 days while the others increased in intensity (Figs. 10-11). The conversion of alumina gel and the hexagonal phases to gibbsite and the cubic phases, respectively, was enhanced with temperature. TGA showed inflections corresponding to the DSC curves but the temperatures were displaced. The 20°C hydration of C3A produced the hexagonal aluminate phase initially. At 80°C there was direct formation of C3AH5. Ramachandran [79] also studied the action of triethanolamine on the hydration of C3A. Pure C3A exhibits endotherms at M50°C (C2AH8) and 250°C (CitAHi3). With time, endotherms developed at ^300°C and between 400-500°C typical of C3AH6. Triethanolamine showed acceleration connected with the greater intensity of the peaks and the effect increased with increasing amount of the additive. When gypsum was added, the formation of ettringite was accelerated: an endotherm at 170 C and a doublet between 250-300 C representing hexagonal aluminate hydrate and its solid solution with the monosulphoaluminate (Fig. 12).

684

L.

Ben-Dor

Ihr 5hrs I Ohrs I day 2 days 4 days

5 days

10 days j -

<3

21 days

60 days

100

200

300

400

500

600

Temperature, °C Fig. 10. Thermograms of CA hydrated at 20°C at w/c * 0.15. Revay studied the hydration of aluminous cements [80-1 ]. DTA/TG curves before and after autoclave treatment can indicate the constitution of the hardened product. Loss of strength of hydrated aluminous cements is due to the hexagonal ·*- cubic transformation which has a lower molar volume and increased porosity. DTA was used to determine the composition of the materials. It was difficult to interpret the 280-350°C endotherm since it appeared also for calcium aluminate hydrates and for gibbsite. Uchikawa and Tsukigama [82] investigated two types of jet cement with different retarders ; to the one were added CaCÛ3 and citric acid and to the second hemihydrate, and both contained sodium sulphate. DTA gave the following features: endotherms at 130 and 280°C corresponding to ettringite, another at M90°C (monosulphate hydrate) and another at 120 C (calcium aluminate hydrate). The endotherm at ^500°C, for CH, was remarkably strong after one day of hydration indicating the considerable hydration of alite. Uchikawa and Uchida [83] studied the

685

Thermal Methods

hydration of CnA7.CaF2 at 20 C. The hydration products were identified by DTA and the degree of hydration was determined by weight loss. Marchese and co-workers [84] prepared low silica hydrogarnets by hydrothermal treatment. Thermograms of these materials between 300 and 400 C showed shift of the endotherm with an increase of the silica content: C 3 AS 0#

316

C3AS0.25 - 321

C3AS0.35 - 333

C3AS0.49 - 343 C

The hydration, reactivity and expansibility of calcium sulphoaluminate clinkers were studied by various means including electron microscopy [85]. Singh [86] showed by DTA and other methods that, with respect to very early hydration reactions, additions of CaCl2 and sucrose accelerate the reaction of C3A with gypsum to form ettringite. Calcium gluconate retards this reaction, which is further retarded when sucrose is added to the gluconate. Traetteberg and Grattan-Bellew [87] studied C3A hydration by DTA (XRD and SEM). The hydroaluminates formed are poorly crystalline and their formation was studied also in the presence of CaCl2 and/or gypsum.

20 days

60 days

100200300400500600

Temperature, Fig. 11.

°C

Thermograms of CA hydrated at 80°C at w/c = 0.15.

686

L. Ben-Dor Temperature, °C 0

0 min I min

i

100

300

1—i—rn

500

0

i

100

r

3 6 0 5 0 0

ι—rr

i min-

2 min

5 min

10 min

30 min

60 min

0% Triethanolamine Fig. 12.

1% Triethanolamine

Thermograms of C3A + 5% gypsum without and with 1% triethanolamine.

Satava and Veprek [88] used DTA to follow the hydrothermal reactions occurring between cement minerals and water vapour. The thermal changes occurring in the system ettringite-water at different temperatures were also studied. The same group [89] reported on the hydrothermal reactions of calcium aluminates with gypsum and investigated thermal decomposition of ettringite under hydrothermal conditions [90]. A DTA apparatus for continuous recording of processes occurring in liquid water at saturated vapour pressure during heating was used. Above 110°C hydrothermal decomposition started in step-wise reactions. Vazquez and co-workers [91] observed that in a high'alumina cement-water system, hydration of cement surfaces is affected by CO2 even in small amounts. Thermal analysis was used in this study amongst other methods. The same group studied the mechanism of carbonation of cubic C3AH6 [92] by a variety of methods including thermal analysis. Formation of intermediate "C02 hydrogamets" and, finally, CaCÜ3 in a matrix of AI2O3 were observed.

Thermal Methods

687

Following the collapse in England of structures made with high alumina cement, the BRE had conducted field and laboratory studies [93]. A DTA technique was described for determining the degree of conversion of CAH}o to C3AH6 which was used to appraise the condition of hydrated high alumina cement paste in existing structures. The thermal decomposition of C3AH5 and Ci+AHis [94] and the thermal stability of CAHJO [95] was also studied in the Soviet Union. The stability of ettringite to dehydrati on was studied [96—7 ] and it was put forward that only 30 of the 32 water molecules are structural in character, dehydration below 18 water molecules causes the crystals to become amorphous and below 6 water molecules the crystals disintegrate. The thermal decomposition of C3AH6 was reinvestigated and an intermediate phase of composition C12A7H was confirmed [98], The formation and dehydration of a number of AFm phases of hydrated calcium aluminates resembling Ci+ΑΗχ 3 were investigated. The lattice of these phases contracted and expanded reversibly with removal and re-entry of water [99]. Bradbury and co-workers [100] examined the transformations occurring in calcium aluminate cements by DTA (and SEM). It was claimed that the major variations in local extent of conversion were influenced by local inhomogeneities in the w/c ratios of large concrete members. Other reports on the conversion of high alumina cements were presented at the 1st European Symposium on Thermal Analysis (1976) [101-103]. Other studies on such cements were devoted to dehydration kinetics [103a], to performance at elevated temperatures [103b] and to the determination of expected decrease in strength [103c]. Another study presented at the 1976 Symposium [104] dealt with the formation of ettringite during cement hydration. An increase in the content of water-soluble alkalies or gypsum contributed most to ettringite formation. Organic admixtures serve as water reducers and retarder s in the hydration of tricalcium aluminate hydrates [105-105a]. Hovrath and co-workers [106] determined with activation energy for the thermal decomposition of C3AH5 and its deuterium analogue. Details of the decomposition mechanism of this material were also reported [107]. Krzywoblocka-Laurow and Zielinska [108] discussed the influence of i^CrO^ on the hydration of C3A. Thermoanalytical and other means were used to show retardation of early hydration. Another report [109] followed the low temperature hydration of Ci+AF, in the absence and presence of NaCl and CaCl2 by DTA-TG and other methods. Different hydration products were obtained under the different conditions. Derivative thermogravimetry [110] and semi-isothermal thermogravimetry [111] were used to evaluate the degree of conversion of high alumina cement. DTG is reproducible with a given set of experimental conditions, and if those are carefully controlled, it is possible to identify CAH10 and AH3 even if ettringite is present. The semi-isothermal technique, in contrast to the dynamic techniques, provides a better possibility of separation between overlapping components and also a better identification of the individual phases, that are associated with weight losses during the thermal treatment. This was demonstrated by the separation of C2AH8 or Ο+ΑΗ^ from the main components of high alumina cement, and also by the separation of C3AH6 from AH3. Two samples of HAC were studied: for one the degree of conversion was

688

L. Ben-Dor

28% and for the second 66% - according to BRE standards. For the first sample the following data were obtained: 101 C isotherm corresponding to CAH^Q 171°C " " " C2AH8 or &+ΑΗ13 (corresponding to known M AH 3 endotherm at 175°C) 264°C " " 363°C dynamic " " C3AH6 The data for the second sample: 123 C isotherm corresponding to ΟΑΗχο M 271°C " " AH3 350°C dynamic " " C3AH6 Midgley [112] studied the same problem and also the use of thermoanalytical techniques for detecting chemical attack on such concretes [112a], Besides phase transformation of calcium aluminate hydrates, the dehydration was also studied [113]. Strobel [114] studied the hydration of Portland cement and the decomposition of calcium aluminate decahydrate by DTA. The dehydration behaviour of CAHIQ and its conversion products [114a] were studied. The hydration of CA in presence of quartz and CaC03 was discussed [114b]. Daerr and co-workers [115] studied the stability of ettringite at various temperatures; above 130.5°C the material was unstable and decomposed to gypsum, hemihydrate and C3AHg. DTG was used to determine hydration kinetics of expansive cement and its products [116]. Ettringite was the stable phase formed after hydration of 7 days. The presence of excess CaO (above stoichiometry) favoured the conversion of some ettringite to monosulphate hydrate. When the cement was prepared from the monosiilphate, instead of the trisulphate, with Portland cement, the hydration products were ettringite and monosulphate, the former being formed initially and then transforming to the latter. Ramchandran and Beaudoin [117] hydrated C^AF with different w/c ratios at 23° and 80 C. DTA gave evidence of hexagonal phases (XRD did not). Collepardi and co-workers studied the hydration of Ct+AF extensively [118]. The weight-loss peaks at 120, 190 and 270°C are very similar for Ci+AF and C3A. It was shown that DTG was more sensitive than XRD for identifying the hexagonal phases (Fig. 13). The hydration of both the aluminate [119] and the ferrite phases was retarded by gypsum and CH and not by sodium sulphate. Sodium lignosulphate and sodium carbonate were found to block completely the hydration of Ci+AF. However, following the longer induction period, the hydration was strongly accelerated by this additive system. The decomposition of C3AH6 and studied to a temperature of 1400°C [120], The material was found to lose 21.7% at 300°C and 6.0% at 500°C. The reactions were: 7C3AH6 + C 12 A 7 H + 9CH + 32H + C 12 A 7 H + 9C + 41H. DTA results gave for the first reaction 30.6 kcal/mol and for the second 31.4 kcal/mol as the apparent activation energy. TGA was used [121 ] to study a complex mixture of reaction products from hydrothermally treated refractory cement and castables. The results of calcium aluminate cement and a 94% A1 2 0 3 castable after exposure to pressurized steam were analysed and the results agreed with the manufacturer's data. The method also gave quantitative phase analyses for the reaction CA and C 3 AH 6 with pressurized steam. CA appeared to react quantitatively to form C3AHg and CH in 6 days. C3AHg under high steam pressure decomposed to ^ Α Η 3 and CH. Kozlova and Poluektova [122] investigated the composition and

689

Thermal Methods

p r o p e r t i e s of hydration products in the system C-A-H in the absence and presence of admixtures. After autoclaving, calcium sulphoaluminate hydrate and calcium nitroaluminate hydrates were detected by DTA and other a n a l y s e s . 0 hr{—

0 hr

10 hr

I7dhr

=J|7d

28dfcr

-J28d

60dkr-

100200 300400 500

100 200 300 400 500

100 200 300400500

100 200 300400500

Fig. 13. DTG of C^AF (A) neat, (B) with lime, (C) with gypsum, (D) with lime and gypsum. Note changes in CH dehydration at approx. 500 C. The hydrated phases of the system CaO-Al2Û3 - SO3-S1O2 were studied and the effect of the presence of varying amounts of CaO on the hydrated phases was also investigated [123]. These phases were assessed by the aid of DTA and XRD. One mole of CaO in excess reacted with available S1O2 to give sulphoaluminate hydrates. With increase of CaO there was an increase of hydraulic phases; addition of 2 moles CaO formed ettringite among the sulphoaluminate hydrates but some anhydrous forms still existed. Four moles CaO gave better formation of hydraulic phases and some excess CH, some of which reacted to give CaCÛ3. CH delayed early hydration.

690

L. Ben-Dor 2.4

Cements, Mortar, Concrete and Carbonization Reactions

DTA-TG enabled the study of the transformation of vaterite into calcite (exotherm at ^450°C), the decomposition of the latter (endotherm at ^700°C) and the decomposition of the long-term effect of CO2 on tobermorite or C-S-H gel (endotherm at 900°C) [124] (Fig. 14). The long-term carbonation of C3AH5 and C3AH1+ was followed by DTA [125]. The proposed reactions were: 1.

C 3 AH 6 + 3C02 + H 2 0 -»» C3A.CaC03.H20 + A1(0H) 3 i H20 (Vaterite ■> Calcite) CaC03 + A1(0H) 3 CO

2,

03ΑΗι+ (hydrogamet)

Zj—

CaC03 (Calcite)

Free lime contained in high alite Portland cement moved to the surface and was carbonated by atmospheric C0 2 . This was proved by DTA-TG examination of the layers of the cured samples [126]. The same laboratory reported on long term effect of phenol solution on Portland cements [127]. Phenol reacts with calcium hydroxide producing calcium phenolate and this could be tested by DTA-TG. Phenol was shown to be corrosive when in contact with cement for long periods. Reactions between methanol and Portland cement paste were investigated by TG [127a]. Feldman and Ramachandran used static thermobalance and DTA to differentiate between interlayer and physically adsorbed water in hydrated Portland cement [128]. A similar study was carried out by Englert and co-workers [129]. · Formation of lumps in cement was due to the formation of syngenite CaK2(S0i+)2.2H20, as was shown by thermograms [130], Lehmann and co-workers showed that weight-loss measurements and XRD were in good agreement in determining CH in hydrated cement while extraction method gave variable results [131 ]. Pihlajavaara and Pihlman [132] used TG to examine the effect of carbonation on the porosity and bound water contents of hardened cements. Carbonation of hydrated calcium silicates was also studied by Maycock and Skalny [133]. A structured evolution of water during decomposition of the hydrates was shown. Most.of the C0 2 comes from CaCÛ3, but some is evolved at lower temperature and may be attributed to surface desorption of C0 2 and water, to the decomposition of the mineral scawtite CaySißO^He .CO3. Ruiz and co-workers [134] investigated the mechanism of carbonation in hydrated high alumina cement pastes by thermal analysis and other methods. A gradual substitution of water molecules by C0 2 molecules was found. Chekhovsky and Berlin [135] studied the kinetics of pore structure formation in cement stone by means of mercury porosity and thermal analysis. Magnan and co-workers [136], using DTA and other techniques, had shown that primary hydrates are formed by adsorption of water in unhydrated lattice. These hydrates then initiated the formation of the final hydration products. Jambor [137] examined phase compositions of hydrated pastes of Portland cement and some of its minerals by thermogravimetry. Theisen and Johansen [138] investigated the effect of prehydration of cement on compresive strength. The degree of absorbed water was obtained from TG curves, and DTA showed the distinct possibility of the presence of ettringite when the cement was exposed to low-temperature humid atmospheres. Prehydration was found to reduce strength at all ages. Litvan [139] presented

691

Thermal Methods

DTA data for the freezing of cement pastes which had been soaked in NaCl and urea solutions. Further results [140] were reported from DTA curves during freezing of cement pastes impregnated with NaCl. Two exothermic peaks were observed during cooling: one at -8 C due to ice formation; the other at ca. 22 C due to the freezing of the solution of eutectic composition. Gebaur and Harnik [141 ] examined the cement paste in 84-year-old concrete by SEM, thermal analysis (and other means).

1

I

I

I

I

1 2 3 · +

i

5

1

6

7

I

1

8

9

2

I0 CTD

Fig. 14. DTA of carbonated 11 Â-tobermorite sample, storage period 1 year, showing phase change vaterite -*- calcite with increase in r.h. 1: 1% C0 2> 50% r.h. 2: 1% C02, 75% r.h. 3: 1% C0 2 , 100% r.h. 4: 10% C0 2 , 50% r.h. 5: 10% C0 2 , 75% r.h. 6: 10% C0 2 , 100% r.h. 7: 30% C0 2 , 50% r.h. 8: 30% C0 2 , 75% r.h. 9: 30% C0 2 , 100% r.h.

692

L. Ben-Dor

Ledneva and co-workers [142] contributed to the understanding of the dehydration of cement paste at high temperatures. Gouda and Roy [143] studied the effects of heat and pressure on hydrating cement paste by various methods including DTA. In addition to gel C-S-H, well-crystallized hydrates were detected in the hot-pressed pastes. A comprehensive monograph by Ramachandran [144] on the use of CaCl2 in concrete, reviews some of the more important research techniques that are used in the study of cementitious systems including thermal methods. Occasionally addition of admixtures to concrete causes a quick-setting phenomenon. This was demonstrated by Greene, who used DTA in the case of white cement in the presence of a water reducer [145]. Bensted [145a] studied early hydration of Portland cement in accelerating media and discussed the CH double peaks in the thermograms and also some thermogravimetric applications of Portland cement [145b-145c]. El-Jazairi and Illston [146] used TG for the mineralogical examination of cement pastes. The method used was semi-isothermal, as opposed to the dynamic one, and was more rapid in measuring weight changes and in separating overlapping DTA peaks. El-Jazairi and Illston [163] presented further results of measurements by the semi-isothermal method of DTG of the decomposition of hardened cement pastes, of various w/c ratios, and for a considerable range of ages. Three distinct hydration events were associated with calcium silicate hydrates between 105-440 C; a fourth event occurring between 440 and 520 C is caused by dehydroxylation of CH and the fifth, beyond 520 and below 1007 C was caused by decarbonation of CaC03. Lagvida and co-workers [147] investigated the hydration of Portland cement at negative temperatures. The hydration products were flaky and finely divided, and first appeared those of C3A and only after several months appeared those of C3S. The appearance of a weak endotherm at ^330°C, presumably silica gel, preceded the appearance of C-S-H gel and CH. The effect of pre-drying temperature on the CH content of hydrated cement paste was determined by TG [148]. Furthermore, the effects produced on the shapes of the thermal curves by different pre-drying procedures were also studied. Mikhaelyan and co-workers [149] published a review dealing with a DTA study of the hydration of cement with admixtures. A durability recognition system using DTA and strength of hardened cements exposed to cycles of wetting and drying was described by Mchedlov-Petrosyan and co-workers [150]. Hlozek and co-workers [151] used TG to determine the free CaO formed during the heating of refractory Portland cements between 600 and 1250 C. Carbonation of the CH occurred during the first phase of rehydration. It was shown [152] that a white cement-gypsum mixture lost strength on carbonation. Sudoh and co-workers [153] had studied the freezing of water in hardened cement paste by DTA. Supercooling of the water lead to irreproducibility of the exothermal peak, the endothermal peak during thawing was used to estimate the amount of frozen water. Accurate determinations of the phase composition of autoclaved cellular concrete were made by Tobak [154]. The thermal decomposition of concrete was related to its strength at high temperatures by Schneider and Weiss [155]. A "kinetic constitutive model" was developed and theory and experiment were found to agree over a wide temperature range. Wesolowski [156] examined concretes from two damaged power-station chimneys using thermal analysis and other methods. In addition to sulphate attack there was evidence of

Thermal Methods

693

incomplete hydration of the cement due to heating during the initial period of the operation. The corrosion resistance of cement mortar to various organic materials was studied by Medgyesi [157] and co-workers using thermal analysis. By applying DTA and IR Ol'ginskii and Doroshenko [158] found that in the presence of CaCl2 and Ni (^3)2, the induction period of the hydration of Portland cement decreased and larger amounts of low basic calcium silicate hydrates were formed. Singh [159] studied the hydration of cements in the presence of sugars, applying various techniques including DTA. The retardation occurred by the adsorption of the admixtures on the C3A grains which hindered the approach of gypsum and water molecules. Marchese and co-workers [160] studied mature Portland cement pastes which were cleaned by removing the old patina. The fresh surfaces were etched by tap water, sea water, and solution of NaCl and MgS0i+ and the extent of attack was confirmed by TG on ground portions of the specimens. Rosova and Tichomirov [161] studied the phase composition of Portland cement and compared it with slag-Portland cement. Thermal analysis, SEM and other techniques were used. Stockhausen and co-workers [162] investigated freezing phenomena in hardened cement phase by DTA. Two-phase transitions were observed: one between -10 and -25 C and a second near -43 C. The phase transitions were correlated to the radius of the water filled pores and also to the relative humidity and water content. DTA curves showed the behaviour of cement pastes hydrated with and without some liquefying agencs [164]. The additives enhanced the formation of ettringite and the disappearance of gypsum, but retarded the hydration of pure C3S as well as that component in Portland cement. Jawed and co-workers [165] studied by DTA and other means the effect of w/c ratio, alkali carbonate and lignosulphonate on the early hydration of Portland cement. The lignosulphonate retarded the conversion of the hexagonal to the cubic hydroaluminate phases and the Na2CÛ3 affected the reaction even more. A probable synergistic interaction exists between the two additives through an ionic complex involving lignosulphonate and C0 3 ~. No direct evidence was found for the formation of a carboaluminate hyarate {cf. Odler Cem, Concr. Res. _8, 525 (1978)). Bukowsky and Berger [166] pre-dried carbonated calcium silicate powder and ignited it in a furnace at temperatures between 400 and 1000 C for 6 hr each. The loss in weight was measured after each ignition period and studied for strength development. Damaging changes in carbonated cement binders of concrete were observed by DTA on the basis of the different decomposition regions of the two types of CaCÛ3 formed [167]. Humidity caused the formation of coarse, stable CaC03· Transformation and recrystallization of fine-grained CaCÛ3 was associated with strength decrease. Deheul and Sinsoillier [168] combined DTA and TG at 1550 C to study carbonates and cement raw materials. The heat of reaction and decarbonation of magnesite and dolomite were determined and the distribution of the exothermic heat from clinker phase formation was related to temperature. At 700-950 C the percent of heat phase formation was related to the percent of raw material calcination. The effects of secondary constituents on phase formation were determined at medium and high temperatures. In samples of autoclaved aerated concrete, Alexanderson [169] obtained an exotherm above 800 C in the thermograms, likely due to the formation of 3-wollastonite. The height of the exotherm was related to the content of C-S-H(I) and evidence was given that this hydrate was the cause of the

694

L. Ben-Dor

exotherm. Al-substituted tobermorite could also be the cause of the exotherm. Calcium silicate hydrate produced from desilication of siliceous geothermal water and calcium hydroxide was dried with CaO and a white hydraulic cement was produced [170], The material was studied by physical and mechanical methods (including DTA). The dried gel showed an endotherm at 597°C (decarbonation of CaCÛ3) and an exotherm at 789°C (crystallization of wollastonite). The cement showed endotherms at 500 C (dehydration of CH) and 660°C (decarbonation of CaC03), and an exotherm at 758°C (crystallization of wollastonite). Emanation thermal analysis, using 2 2 8 Th and 22l+Ra, was used by Balek and co-workers [171] for cement stone at temperatures <1000°C. Between 100 and 200 C an endothermic reaction took place, corresponding to loss of free water; between 200 and 400 C chemically bonded water was lost, especially from C-S-H gels; above 400 C the dehydroxylation of CH appeared, and between 550 and 600 C there were microstructural changes in the material. Between 600 and 800 C the carbonates decomposed and above 860°C sintering took place. 2.5

Slags, PIC, FRC, Fly Ash and Pozzolanas

Garg and Rai [172] studied bricks made by autoclaving sand-lime, fly ash-lime, calcined clay-lime and blast-furnace slag-lime. The phases present were studied by DTA. Glasses similar in composition to blast-furnace slags were activated with saturated lime and studied by Mascolo [173]. The end member hydration products were G+A.aq and M+A.aq. The DTA gave the following data: the area of the endotherm at M 8 0 C, characteristics of gehlenite, decreased with increasing Mg content in the original glass. -This decrease was associated with an increase in the area of the endotherm at 230 C, which, together with that at 410 C, was characteristic of the hydrated magnésium phase. The solid solutions (C,M)i+AHx of Portland blast-furnace cement are responsible for the volume stability of the cement in contrast to the unsoundness of Portland cement in which Mg(0H)2 is formed. Mixtures corresponding in composition to Portland cements and granulated blast-furnace slags were autoclaved and the results reported [174], A variety of structural data were accumulated including thermal analysis data. Hardened products of various slags were investigated by Dasgupta [175] and by Vyrodov and co-workers [176] using thermal methods. Using quantitative DTA between -60 and +20 C the hydration of ordinary Portland cement and slag cement was studied at ages between 1 and 28 days [177], During hydration the volume of the bigger pores decreased and that of the medium pores increased. The technique could be an improved measuring one for evaluating the degree of hydration and for elucidating the kind of hydration products. The same laboratory [178] reported a DTA and interferometric study on cement pipes of Portland and furnace slag cement. Expansion anomlaies increased with moisture content and at -43 C an exothermic phase conversion was observed. The freezing of free water in the large pores provoked an expansion, while the ice in the micropores caused contraction, thus creating internal stresses. The hydration of compacts of blast-furnace slag in the presence of gypsum was investigated using various methods including thermal analysis [179]. An exothermic peak at 850 C was caused by slag hydration. Another exotherm at 920 C was ascribed to the vitreous phase of anhydrous slag (Fig. 15).

695

Thermal Methods DTG

DTG

(b)

(α)

±

ι

20

250

500

750

1000

20

250

500

750

1000

840

^ÏÏV/—-

^JW l>

Vv

(0 20

250

500

Fig. 15.

750

1000

1 20

120

!

250

XL.*. (d)

1

500

1 750

1

1000

TG, DTG and DTA of slag: (a) not hydrated, (b) 15 hr at 70°C, (c) 15 hr at 180°C, (d) 72 hr at 180°C.

Polymer-cement and polymer-mortar components had been investigated by DTA-TG and other methods by Gebauer and Coughlin [180], The characteristic endotherm at 530 C for CH was smaller as compared to pure cement paste showing a remarkable decrease in quantity of CH in the polymer-cement due to a possible reaction between methyl methacrylate (MMA) and the CH. Koos [18] investigated by DTA-TG the possibility that organic compounds of calcium were produced in polymer-impregnated cements. Auskern and co-workers [182] also observed that a calcium salt of methacrylic acid is formed when MMA is polymerized in the presence of bulk CH. However, no such reaction was observed during DTA studies of poly-MMA (PMMA) impregnated cement paste. Further DTA studies [183-4] indicated that the thermal degradation of PMMA commenced before the CH endotherm was reached (^530°C), and that the decomposition products of PMMA reacted with CH (Figs. 16 and 17).

APC - W

L. Ben-Dor

696

Temperature, 0

200

400

600

800

900 0

°C 200

600

800 900 0

900

Temperature, Fig. 16. Thermograms of A - Portland cement paste, B - PMMA impregnated cement paste, new thermal effects not due to components nor to interaction between them, C - PMMA - cement mixture, similar to B, few differences due to DTA heating. Cook and co-workers [185] examined the rate of hydration of several premixed polymer modified cements and, using DTA, found evidence for possible interactions between the cement and polymer. A discussion [186] was offered on this paper and in response [187] further evidence, based on DTA, for the interaction of PMMA with a hydrating cement was presented. A mixture of PMMA and cement showed an endotherm at 300°C which was practically absent in a hydrated PMMA-cement mixture, indicating a possible interaction between methacrylate and hydrating cement.

697

Thermal Methods Temperature, 200

400

600

800

D-l

Fig. 17. Thermogram of PMMA {cf.

Fig. 16 A,B,C).

Additions of polymers to concretes and autoclaved cement, and examination by DTA-TG and other methods, revealed that microstructural modification occurred [188-9]. The influence of low molecular weight monomers and polymers on the hydration and hardening characteristics of cement and its minerals was followed by Cherkinskii and Slipchenko [190] who used various methods including DTA. Hydration was affected differently by the various polymers, but all of them retarded the hardening process. Moser and co-workers [191] had examined several polycarboxylate cements by thermogravimetry. Sugaraa and Kukacka [192] showed from DSC that C2S and C3S had a significant effect on the thermal stability of PMMA and polystyrene. An interaction probably occurred between the CaO of the silicates and the -CH 2 - group of the vinyl type polymer. A group from the same lab. [193] developed hydraulic cement fillers which can prevent the hydrothermal decomposition of vinyl-type polymer concrete. Co-polymer was used in combination with an aggregate containing Portland cement-slag cement and oil-well cement. Here, too, a strong interaction occurred between the cement grains and the -CH2- groups in the main chain of the co-polymer. The test methods included DTA-TG and samples were exposed to hot brine. Weight losses occurring at 400 C were in the range of 10%, at 450 and 500°C the losses were about 27% and 75% respectively. The DTA showed exotherms complimentary to the weight loss. Two further studies were recently published [193a-193b] which dealt with hydrothermally developed polymer concrete and its durability.

698

L. Ben-Dor

Recently more and more thought has been devoted to fibre-reinforced cement. One of the fibres used is asbestos. The long-time performance of asbestos cement was studied by a variety of methods including DTA-TG [194]. Another important field of study deals with fly ash both in view of pollution hazards and the conservation of energy. Kovacs [195] reported the results of an investigation on the hydration products and properties of Portland fly ash cements. Thermal analysis confirmed that the composition and the character of the hydration products in control Portland and Portland fly ash cements were similar while their relative proportions were different (Fig. 18). A group from the same lab. [196] presented a further report on the hydration and properties of cement containing various amounts and types of fly ash. Fly ash showed the initial hydration but the rate increased and the amount of hydrate formation was nearly the same after 3 months as shown by derivatography. With increased fly ash content the amount of gel-like C-S-H increased and that of CH decreased, while the amount of calcite changed according to a maximum-type curve. Sauman [197] investigated reactions between fly ash and calcium oxide under hydrothermal conditions. The products were examined by DTA and by other methods. The C-S-H (I) formed initially changes to 11 A tobermorite. Stebnicka [198] used thermal analysis to investigate the degree of carbonation of hydrated Portland fly ash cements. Specimens were cured in air, water or hydrothermally. The highest degree of carbonation was found in autoclaved mortars and the formation of CaC03, through the decomposition of C-S-H, was associated with increased porosity. Casselouri and Parissakis [199] reported DTA and other data to confirm the effectiveness of siliceous fly ash and metallurgical slags in stabilizing high-magnesia cement. An extensive study of soil lime stabilization was presented by the Portland Cement Association [200]. This report involves the application of various methods including thermal analysis. DTA was used to assess the increased reactivity of an interground lime-quartz mix compared to a blend of the separately ground components [201] . Thermogravimetric studies [202] of alkali-treated flint at temperatures up to 900ÖC showed that water is absorbed strongly at different rates. This absorption caused expansion and occurred only in the presence of OH". Flint in neutral NaCl/KCl solutions showed no increase in activity. The same author [203] stated that only amorphous and micro-crystalline silica will lead to an alkali-silica reaction and the expansion associated with it. It was found by TG that crystalline silica with lattice defects, such as flint for example, will absorb water and thus cause an increase in volume. Thermogravimetry was used by El-Hemaly and co-workers [204] in their study of autoclaved limequartz materials. For C-S-H of C:S«0.8, the silica occurred wholly as polysilicate and the TG curve resembled that of tobermorite. As the C:S ratio increased, the fractions of silica present as mono- and di-silicate increased but always polysilicate predominated. For any given temperature on the thermogram, H:S tended to rise with C:S. Several physical methods have been used to assess pozzolanic activity including DTA [6,205-6]. However, these methods were not found reliable to predict reactivity of fly ashes.

699

Thermal Methods

VO 28 days 200

-j

200

400

i

:

400

V2 28 days

600

i

i

600

1000oC

800

i

i

i ,,)

800

200

i

IOOO°C

i

400

i

200

600

i

400

L

600

800

L _ J

800

IOOO°C

i

ι

IOOO°C

V4 28 days 200

J

400

600

800

IOOO°C

! i I i I i I i I 200 400 600 800 IOOOeC

Fig. 18. TG DTG and DTA of cement (VO) - neat, (V2) - with 25% fly ash, (V4) - with 40% fly ash.

700

L. Ben-Dor

Various inorganic cements had been studied. Beaudoin and Ramachandran [207] studied Sorel cement treated in water at 85 C. XRD and DTA showed that the cement which had a glassy compact appearance consisted of Mg(0H>2 with most of the oxychloride complex removed (Fig. 19). Ved and co-workers [208] have shown that the addition of Al- and Fe-phosphates to Sorel improved its water resistance. The disintegration of the cement was followed by XRD and DTA. A further study on the structure of the hydration products of Sorel, magnesia cement, was reported by the same group [209] using TG and other means. Magnesium phosphate cements were studied by Krylov and co-workers [210] who observed the presence of different hydrates by DTA (and IR). The amount of water liberated at 100°C and 280°C was studied. System Π DTA trace

System Π

/....

System HI

^-Mg(OH)2 ■T Δ Τ = 2 mV (DTA)

— ±

\ Δ Τ = 5 meal /sec ( DSC) \ 150 sec (DSC) 100

Fig. 19.

J L 200 300 400 Temperature, °C

500

600

DTA and DSC of Sorel cement. System _II. " cement paste prepared at MgCl2«6H20/MgO solution: solid = 0.59, hydrated 10 months, ground and fabricated into compacts. System III - system ΓΙ compacts with chloride and oxychloride leached out.

Cements were formed from powdered metals and mineral acids and studied by DTA [211]. Thermogravimetric studies dealt with the dehydration process in the system Al203-Cr203-P205-H20 [212-13]. Milestone [214] compared several techniques for the identification of water reducers and found DTA to be a more rapid method. Biddle and co-workers [215] investigated the chemistry of ethyl silicate binders. The mechanism of gelation was studied using thermal analysis and IR. Courtault [216] coupled microthermogravimetry and mass spectrometry for the identification and determination of additives in cement and concrete. Quantitative microdeterminations were given for NaHC03, a synthetic mixture of gypsum CH-CaC03, PVC and polyvinyl alcohol.

Thermal Methods 2.6

701

Plasters

Khalil and co-workers [217-19] assessed gypsum plasters. It was found that below 100 C the hemihydrate was formed, and above that temperature γ-anhydrite was obtained. When gypsum slabs were fired [220] the temperature increased rapidly in the first 20 min, then a plateau was obtained, characteristic of the endothermal dehydration of the dihydrate to the hemihydrate, and lastly a rapid rise and failure of the specimen. This sudden increase might be due to the exothermal hexagonal to orthorhombic transformation of anhydrite. The dependence of the dehydration of gypsum on temperature and moisture was investigated by Hansen and Clausen [221]. Below 110 C the reaction proceeded slowly and no dehydration occurred within 30 min. Between 110-130 C the moisture content determined whether the final product would be hemihydrate or soluble anhydrite; above 130 C the latter was the final product irrespective of water-vapour pressure. Negro and Stafferi [222] studied the dehydration of four types of gypsum dihydrate. Murat and co-workers [223] used DTA to study the conversion to dihydrate of hemihydrates. In a further study [224] the conversion of soluble anhydrite to insoluble anhydrite was followed by DTA (and XRD). The conversion temperature was affected by impurities and working conditions. Lehman and Mehta [225] prepared hemihydrate under various conditions and obtained different forms. It was shown that grinding converted a- into B-hemihydrate. In another report [226J the dehydration of gypsum was studied using TG. The main purpose was to establish whether the conversion of the dihydrate to soluble anhydrite does or does not involve the hemihydrate stage. The conclusion arrived at earlier was reaffirmed, viz. a TG curve without a hemihydrate step was proof for the absence of this species at any stage of the dehydration process. This is an erroneous conclusion since the feature of the curve is governed by particle size. In a further article [227] there was a reconfirmation of an earlier erroneous conclusion. The dehydration of gypsum was described on the basis of DTA studies. Under low water vapour pressure the dihydrate converted to soluble anhydrite without an intermediate step. There is a well-established fact that under these conditions the dehydration is rather fast and the two dehydration steps overlap. Eipeltauer and Banik [228] examined the adsorption of water on plaster. Using DTA-TG the existence of a monohydrate was stipulated and the existence of amorphous or "gel" phases of both hemihydrate and gypsum at an intermediate stage was considered highly likely. Murat [229] used thermal and other methods to describe changes of the physical properties of dihydrate on dehydration. Two steps, corresponding to the dehydration of dihydrate and hemihydrate, were observed. DTA and TG were used for kinetic studies and for demonstrating that dihydrate from different sources dehydrated differently. The characteristic thermogram of a-hemihydrate was obtained at increased pressures

(cf.

Lehman et al.t

221-3).

A thermodynamic analysis of gypsum was carried out by Uvaliev and Anezev [230]. Isobaric specific heat of various modifications of gypsum during dehydration was obtained, and it was shown that the isobaric potential decreased with temperature between 25 and 400°C. Gardet and co-workers [231] studied the rate of dehydration of gypsum. The kinetic curves obtained were sigmoidal and could be described beyond the point of inflection by a process of interface advancement. A group from the same lab. [232] studied the rehydration of hemihydrate using TG and microcalorimetry.

702

L. Ben-Dor

The properties of gypsum obtained from the desulphurization of stack gases were described by Chiba [233] and results of DTA and DSC were given. DTA was used [234] for the determination of kinetic data of gypsum dehydration. Nucleation of hemihydrate was found to be the rate determining process up to α*0.3, and the growth of this species became important for a>0.6. Quantitative analyses of gypsum-hemihydrate systems were obtained by DTA [235] from room temperature to 200°C and the chemically bound water was determined gravimetrically. Jeandot and Barriac [236] compared the setting of gypsum by various methods including DTA. A theoretical study of the thermal dissociation of lumpy natural gypsum under water-vapour pressure was carried out by Pievskii and Virozub [237]. The thermogram of the dehydration was used for constructing a model; the experimental data and the model agreed to within 5%. DTA (and XRD) examination of the transition zone between anhydrite III and II were carried out by Bachiorrini and co-workers [238] and the effect of various chemical additives on the transition temperature was studied. In another report [239] the analyses of a- and 3-hemihydrate using both DTA and IR was performed. Kazimir [240] used DTA to confirm that the dehydration of gypsum in water suspension under hydrothermal conditions proceeded in the same manner as in air. Endothermic peaks at 130 and 235 C indicated the formation of α-hemihydrate and α-anhydrite (III), respectively. A further small endotherm indicated the formation of anhydrite (II) . Natural gypsum gave a similar thermogram. Bertoldi [241] measured the dehydration of gypsum by thermogravimetry: in the first step, gypsum was converted to the hemihydrate, at 600° CaO, CaS and anhydrite II were formed. Abramyan and co-workers [242] prepared the hemihydrate by various methods and examined the materials by DTA. It was stated that the presence or absence of an exotherm at 255°C was related to the method of preparation of the material. Haubert and Krönert [243] studied the course of hydration of hemihydrate by DTA among other methods. The thermal transformation of hemihydrate formed during the reaction of apatite by I^SOi*. had been studied derivatographically [244], Jahan and co-workers [245] described a method for determining the gypsum content of gypsum-rich rocks by DTA, using Ca(0H)2 as an internal standard. The effect of natural impurities was discussed. Lyashkevich and co-workers [246] studied the dehydration of natural gypsum at elevated temperatures and pressures. Another report [247] dealt with the dehydration of gypsum between 25 and 200°C by DTA and TG. The dehydration of gypsum for dental use [248] was studied using micro-DTA. The separation between the two endothermic peaks was shown to be dependent on particle size. Soustelle and co-workers [249] studied the effect of water-vapour pressure and temperature at constant pressure, on the hydration properties of pure hemihydrate. Schlickenmaier [250] used differential calorimetric analysis to detect and determine residual gypsum in hemihydrate. Ramachandran and Polomark [251 ] discussed the use of combined DSC-DTA to estimate various constituents in white coat plasters (Fig. 20). Ludwig and Khan [252] used DTA and other methods to provide a detailed evaluation of various methods for the analysis of different phases in the system CaSO^-^O present in blended plasters. Phase analyses by thermal methods present difficulties since only soluble materials can be determined quantitatively.

Thermal Methods

703

Plaster A

Plaster B

Plaster A (autoclaved) Plaster B (autoclaved)

500 600 700 800900 Temperature, °C Fig. 20. Thermograms of plasters. Kondrashenkov and co-workers [253] studied the dehydration of gypsum in the range 120-200 C. The dehydration was dependent on temperature and particle size. The exothermic effect between 200 and 220 C depended on the crystal habit of the hemihydrate. Dunn and co-workers [254] determined gypsum and lime in set plaster, rapidly and accurately, in small samples by a DSCcomputer method. The samples were heated at 20 and 50 /min, the latter giving improved peak definition and shorter analysis time (15-20 min). ACKNOWLEDGEMENT The author is grateful to Elsevier Scientific Publishing Company for permission to reproduce figures from Thermochim. Acta 5 (1973) and 25 (1978). The author also thanks the American Ceramic Society for permission to reproduce material from the American Ceramic Society Bulletin 52 (1973) and the Journal of the American Ceramic Society 58(1975). REFERENCES 1. 2. 3. 4.

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L. Ben-Dor A. Ruiz de Gauna, F. Trivino and T. Vazquez, Cuad. Invest.

Torroi a Constr. Bldg. V. A. Chem. A. A. N. N. N. N.

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