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Please cite this article in press as:J. Szczerba, et al., The application of DTAand TG methods to investigate the non-crystalline hydrationproductsof CaAl2 O4 and Ca 7 ZrAl6 O18 compounds, Thermochim. Acta (2013), http://dx.doi.org/10.1016/j.tca.2013.01.031

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Thermochimica Acta

j o u rn a l h o m ep ag e : www.e l sev i e r. co m/ l o ca t e / t ca

The application of DTA and TG methods to investigate the non-crystallinehydration products of CaAl2O4 and Ca7ZrAl6O18 compounds

Jacek Szczerba , Dominika Madej, Edyta Snie zek, Ryszard Prorok AGH University of Science and Technology, Faculty of Materials Science and Ceramics, al. A. Mickiewicza 30, 30-059 Krakow, Poland

a r t i c l e i n f o

Article history:Available online xxx

Keywords:HydrationDTATGAEGA methodsCaAl2 O4Ca7 ZrAl6 O18Calcium aluminate hydratesMicrostructure

a b s t r a c t

The hydration products and thermal decomposition mechanism of hydrated CaAl2 O4 and Ca7 ZrAl6 O18

compounds were investigated by X-ray diffraction, SEM/EDS and thermal analysis. The processes of crys-tal hydrate nucleation and precipitation were preceded by the evolution of the X-ray amorphous phaseduring the rst 24 h of hydration. DTATGAEGA techniques allowed the study of the detailed decom-position and identication of intermediate and stable to be performed. The differential thermal analysis(DTA) curves of hydrated CaAl2 O4 andCa 7 ZrAl6 O18 compounds show ve similar endothermic peaks dueto crystal water desorption. According to the quantitative TGAEGA analyses performed on hydratedCaAl2 O4 and Ca 7 ZrAl6 O18 compounds, it was found that C2 AH8 , C3 AH6 and Al(OH)3 phases are the mainhydration products of CaAl2 O4 . Under the same laboratory conditions, the hydration of Ca7 ZrAl6 O18 pro-ceeds with the formation of mainly CAH10 and AH 3 -gel phases. We provide the original illustrations of the hydrate crystals formation via amorphous phases.

2013 Elsevier B.V. All r ights reserved.

1. Introduction

Calcium aluminate cements (CACs) constitute an importantcomponent of monolithic refractory materials. CACs consistpredominantly of hydraulic calcium aluminates. Calcium monoalu-minate CaAl 2 O4 (CA; C= CaO, A= Al2 O3 ) is the principal cementingcompound in CACs with the incongruent melting point at 1602 C[1] . Two calcium aluminate hydraulic phases: CaAl 4 O7 andCa12 Al14 O33 (CA2 and C 12 A7 ) are the minor components.

The hydration of aluminates, such as CaO Al 2 O3 compounds,is a highly temperature and time dependent process. Hexagonalcalcium aluminate hydrates: CAH 10 , C2 AH8 and C 4 AH19 (H= H2 O)as the main products at low temperatures that transform becauseof a highertemperature(>40 C) into thestable hydrogarnet C 3 AH6and AH 3 phases [24] .

As it has been reported [5,6] , calcium zirconium aluminate,Ca7 ZrAl6 O18 (C7 A3 Z;Z =ZrO2 ) is the only zirconium containing thealuminate phase which simultaneously exhibits all the hydraulicproperties. The ternary compound was found to melt incongru-ently at 1550 C with a formationof solid calcium zirconate CaZrO 3and a liquid [7] . The hydraulic activity of Ca 7 ZrAl6 O18 suggestedits potential application in conjunction with CaAl 2 O4 in the newshaped and unshaped refractories. For this reason, in this paper we

Corresponding author. Tel.: +48 12 617 25 01.E-mail address: [email protected] (J. Szczerba).

investigate the thermal decomposition mechanism of CaAl 2 O4 andCa

7ZrAl

6O

18 compounds.

As a consequence of poor crystallinity of hydration productsformed after the addition of water for hydraulic setting of the cal-cium aluminate cement at the early hydration stage, the phaseidenticationby XRDis difcult.In thisregard, thedifferential ther-mal analysis (DTA) combined with the thermogravimetric analysis(TGA) and the evolved gas analysis (EGA) play an important role inqualitative and quantitative investigations.

The effect of a heat treatment on hydrated calcium alumi-nate cement paste and the calcium aluminate minerals (CaAl 2 O4 ,CaAl4 O7 , Ca12 Al14 O33 ) have been extensively studied in the past[818] by measuring the weight and thermal effects of speci-mens as they undergo heating. Alarcon-Ruiz and Harmathy [8,19]explain that during the cement paste heat treatment, a con-tinuous sequence of more or less irreversible decompositionreactions take place. Previous work [15] summarized the resultsof the thermal analysis of the binary and ternary compounds inthe CaO Al 2 O3 H2 O system. This approach offers the advantageto study both the mass losses and characteristic temperatures(DTA endothermic peaks) obtained from TGA and DTA curves,respectively. Differential thermal and thermogravimetric analysis(DTA/TGA) are widely used in evaluating the hydration productscement pastes [8] . These techniques are based on assumption thatdecomposition of a starting material occurs within a xed temper-ature range. Water vapour is given off resulting in weight changeof the test sample. Therefore, it may be simply concluded fromthis data that the qualitative and quantitative identications of

0040-6031/$ seefrontmatter 2013 Elsevier B.V. All rights reserved.

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Pleasecite this article in press as:J. Szczerba, et al., The application of DTAand TG methods to investigate the non-crystallinehydration productsof CaAl2 O4 and Ca 7 ZrAl6 O18 compounds, Thermochim. Acta (2013), http://dx.doi.org/10.1016/j.tca.2013.01.031

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Table 1Specication of the starting materials.

Specication Reagent

CaCO3(Chempur)

Al2 O3(Chempur)

ZrO2(Merck)

Pure (%) 98.81 98.50 98.08Median particle size ( m) 2.303 36.229 20.762Specic surface area (m 2 /g) 3.09 0.357 0.306

the hydrated compounds known to decompose within denedtemperature ranges can be done. Moreover, the exothermic peakson the DTA curve can be attributed to the crystallization of newphases. Theaim of this work was to recognizeand compare of mainhydration products of CaAl 2 O4 and Ca 7 ZrAl6 O18 based on ther-mal analysisandscanning electronmicroscopy. The DTATGAEGAresults of hydratedCaAl 2 O4 andCa 7 ZrAl6 O18 compoundsaccompa-nied withmicrostructureobservations led to extensive discussionson the type and crystal habit of calcium aluminatehydrates presentin the hydrating pastes.

2. Experimental

2.1. Materials and methods

The standard solid state reaction method was used to syn-thesize CaAl 2 O4 (CA sample) and Ca 7 ZrAl6 O18 (C7 A3 Z sample). Incase of the CaAl 2 O4 pure chemical CaCO 3 and Al 2 O3 were mixedtogether in the proportion of the CaO to Al 2 O3 ratio correspondingto the CaAl 2 O4 stoichiometry. Ca 7 ZrAl6 O18 was synthesized froma stochiometric mixture composed of CaCO 3 , Al2 O3 and ZrO 2 . Thespecication of the starting materials is shown in Table 1 .

The mixtures were homogenized by milling with zirconia ballsfor2 h. Subsequently, the specimenswere formedas cylinders withdiameter and height of 20mm. The reactions of the CaAl 2 O4 andCa7 ZrAl6 O18 synthesis were carried out by the two step ring pro-cedure. First, the samples were calcined at 1200 C for 10h. Afterthis stage, the samples were ground to the grain size lower than63 m and again pressed into discs under a pressure of 120MPa.Consequently, the samples were sintered at 1500 C with 10h (CAsample) or 30h (C 7 A3 Z sample) soaking time and cooled with thefurnace.

The powder X-ray diffraction was adopted in the present study.PANalytical XPert Pro MPD with a Cu K radiation was used. Thepolished fractures of sinters, produced at 1500 C, were observedunder a scanning electron microscope (SEM) and analyzed byenergy dispersive X-ray spectroscopy (EDS). The ultra high de-nition NOVA NANO SEM 200 was used for this purpose. As a nextstep, the synthesized samples were ground and then their specicsurface andgrain size distribution was measured by a laser diffrac-tionanalyzer(the Mastersizer 2000Ver. 5.60apparatus of Malvern,

UK). Hydrated CA and C 7 A3 Z pastes were prepared with a waterto solids ratio of 0.50. The size of the CA and C 7 A3 Z samples was5g, respectively. The room temperature was 20 C and humidityabove 80%. Powders and water were mixed together by hand in asealed plastic bag for 35 min. The hardened CA and C 7 A3 Z pastes,which were left at room temperature for 24h, were observedunder SEM after the experiment. The samples were ground andthe reaction was stopped by cold acetone. The use of acetone, aim-ing to withdraw free water and inhibit further reactions is knownfrom the literature [20] . Simultaneous differential thermal andthermogravimetric analyses with gas emission (DTATGAEGA)were performed with a SDT 2960 type STA (Simultaneous ThermalAnalyzer) of TA Instruments (TG, DTG, DTA, QMA) at a heat-ing rate of 10 C/min. The samples with of 54.33mg (hydrated

CA) and 42.41mg (hydratedC 7 A3 Z) masses were placed in the

Fig. 1. Measured powder XRD pattern of (a) single phase CaAl 2 O4 (unhydrated CAsample) and (b) hydrated CA sample.

corundum crucibles and heated from room temperature up to1150 C. 100.00mg of Al2 O3 was used as the reference substance.The phase composition of hydrated CA and C 7 A3 Z samples wasexamined by powder diffraction.

3. Results and discussion

3.1. Phase composition and microstructure of CA and C 7 A 3 Z materials before the hydration process

The initial materials, sintered at 1500 C, are CaAl 2 O4 and

Ca7 ZrAl6 O18 . The measured diffraction patterns of CA and C 7 A3 Zsamples are presentedin Figs.1a and 2a , respectively. In bothcasesthere is no signicant evidence of existenceof secondaryphases onthe measured XRD patterns. With the exception of the C 7 A3 Z sam-ple, which contains some amount of accessory CaZrO 3 (Fig. 2a), the

Fig. 2. Measured powder XRDpattern of (a) single phase Ca 7 ZrAl6 O18 (unhydrated

C7 A3 Z sample) and (b) hydratedC 7 A3 Z sample.
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Please cite this article in press as:J. Szczerba, et al., The application of DTAand TG methods to investigate the non-crystalline hydrationproductsof CaAl2 O4 and Ca 7 ZrAl6 O18 compounds, Thermochim. Acta (2013), http://dx.doi.org/10.1016/j.tca.2013.01.031

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Fig.3. SEMimageof theCA samplemicrostructure after10 h ofsintering at1500 C.(Spot 1) EDS analysis: 1 CaAl 2 O4 .

SEM observations revealed the presence of welldevelopedCaAl 2 O4and Ca 7 ZrAl6 O18 grains ( Figs.3and4 ). Theexplanation is conrmedby EDS analyses, shown in Figs. 3 and 4 Spots 1.

3.2. Particle size distribution of CA and C 7 A 3 Z samples

Grain size distributions of powdered CA and C 7 A3 Z samples arecharacterized by the medians ( d0.5 ) and correspond to 36.134 m(specic surface of 0.381m 2 /g) and 29.743 m (specic surfaceof 0.475m 2 /g), respectively. The samples revealed rather broad,mono-modal grain size distributions ( Fig. 5). The results of spe-

cic surface area of the CA sample were similar to the results of theC7 A3 Z sample.

3.3. Phase composition and microstructure of CA and C 7 A 3 Z materials after the hydration process

The CA and C 7 A3 Z samples exhibited high background inten-sity ( Figs. 1b and 2b ) indicating that the hydrated CaAl 2 O4 andCa7 ZrAl6 O18 contained a proportion of highly disordered mate-rials in the form of amorphous calcium aluminate hydrates( xCaO yAl2 O3 z H2 O). The effect of amorphous materials on thebroadening of the XRD patterns of the hydrated CA and C 7 A3 Zpastes is not negligible.

Three crystalline components of the hydrated CA paste were

found by XRD: the initial unhydrated material, i.e. CaAl 2 O4 , and

Fig. 4. SE M image of the C 7 A3 Z sample microstructure after 30h of sintering at1500 C.(Spot1) EDS analysis: 1 Ca 7 ZrAl6 O18 .

the newly created phases, i.e. Al(OH) 3 and C 4 AH19 (Fig. 1b). More-over, the unidentied peaks in the XRD pattern ( Fig. 1b) shouldrepresent the still remaining hydrated CaAl 2 O4 crystals or crys-tal nuclei for synthesizing calcium aluminate hydrates, rather than

Fig. 5. The particle size distributionin the CA and C 7 A3 Z powder samples.


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Fig. 6. (a and b) Microstructure of hydrated CA sample.

well-cristallizedcalcium aluminatehydrates [21] . The ternarycom-pounds of the CaO Al 2 O3 H2 O system (with the exception of C4 AH19 ), not found in the XRD pattern ( Fig. 1b), appear as an amor-phous substance.

Three crystalline components of the hydratedC 7 A3 Z paste werefound by XRD: the initial unhydrated Ca 7 ZrAl6 O18 , CaZrO3 andC4 AH19 (Fig. 2b). Other hydrates of calcium aluminates (CAH 10 ,C2 AH8 , C3 AH6 ), not found in the XRD pattern ( Fig. 2b), appear asan amorphous substance in C 7 A3 Z paste. It can be concluded that

the increased intensity of the CaZrO 3 peak in the hydrated sam-ple could be ascribed to the formation of additional amounts of CaZrO3 . In the initial, unhydrated C 7 A3 Z sample this phase occursin a negligible amount, and can be treated as an impurity. The rel-ative intensity of the CaZrO 3 peak corresponding to d = 2.83600 inthe C 7 A3 Z sample was 4.85% before hydration ( Fig. 2a). It rose upto 13.17% after hydration ( Fig. 2b).

Figs. 6a,b and7ac sh owthe SEMmicrostructureof hydratedCAand C 7 A3 Z samples, respectively. It canbe seen from theSEM imagethat partially hydrated grains of CaAl 2 O4 (Fig. 6a) and Ca 7 ZrAl6 O18(Fig. 7a) still contain unhydrated cores. The reaction proceeds asthe CaAl 2 O4 and Ca 7 ZrAl6 O18 grain cores are consumed.

SEM observations show that hydration products of aluminatephases had an amorphous structure rather than a well-crystallized

structure. Nevertheless, some crystal nuclei for synthesizing

calcium aluminate hydrates are observed under higher magni-cation ( Figs.6b and 7b ). After the formation of these crystallizationnuclei, a massive precipitation is most likelyto occur duringa time-dependent hydration process. The presence of the stable hydrateC3 AH6 in the form of a semicubic or spherical shape can be identi-ed ( Fig. 6b) which has also been reported by Das et al. [22] . Someof these crystals, such as hexagonal calcium aluminium hydrates(CAH10 , C2 AH8 , C4 AH19 ), can be also detected ( Fig. 6b). However,the SEM observations indicate that the CA sample still contains aquite large amount of the unstable amorphous phase ( Fig. 6a andb).

The SEM observations showed that the hydrated C 7 A3 Z samplecontaineda lamellaramorphous phase or other moreirregularones(Fig. 7a). The hydrated product of the Ca 7 ZrAl6 O18 compound in agrain of C 7 A3 Z sample adheres rmly to the unhydrated core of thegrains of the C 7 A3 Z sample.

Fig. 7b shows the scanning electron microscope (SEM) imageunder a higher magnication of the plate-like crystal morphol-ogy of the hydrated Ca 7 ZrAl6 O18 phase. This indicates the calciumaluminate hydrates crystallization from the amorphous phase.Hexagonal-platemorphology is the characteristic feature of CAH 10 ,which has also been reported by Parr [23] . Moreover, as it canbe seen in Fig. 7c, hydration of Ca 7 ZrAl6 O18 compound pro-ceeds with the formation of nely crystalline ( light spots ) CaZrO3(T mp =2345 C) [24,25] .

3.4. The DTATGAEGA investigation of hydrated CA and C 7 A 3 Z samples

The DTATGAEGA measurements show that the thermaldecomposition of partially hydrated CaAl 2 O4 occurs in severalsteps ( Figs. 8 and 9 ). In this process, amorphous calcium alumi-nate hydrates break down to water vapour and solid phases startto form. A DTA curve of hydrated CaAl 2 O4 shows ve endothermicpeaks with the maxima at 97.94 C, 165.93 C, 285.48 C, 451.29 Cand 548.42 C, respectively ( Fig. 8, Table 2 ). It is shown that theseendothermic maxima in the DTAcurve correspond to the mass lossdue to the release of H 2 O in the TG curve (see the EGA curve Fig. 9, CA sample) [8] . The rst step occurs up to 97.94 C and it isattributedto the dehydration process of CAH 10 andAH 3 -gel [10,26]and a 4.35% mass loss as shown by the thermogravimetric curve(Fig. 8).

The second small endothermic peak appears at 165.93 C andrepresents the loss of water (see the EGA curve Fig. 9, CA sample)due to further dehydration of CAH 10 [16] . On the other hand, thistemperature is close to the temperature value of C 2 AH8 decompo-sition, estimated by George [26] . Hence, it can be concluded thatthe presence of both CAH 10 and C 2 AH8 cannot be excluded. Thisendothermic peak position corresponds to the 3.66% mass loss onthe TGcurve ( Fig. 8).

At 285.48 C, the third step of hydrated paste decomposition

is accompanied by a sharp endothermic DTA peak ( Fig. 8) and a9.66% mass loss, as shown by the thermogravimetric curve ( Fig. 8).This could be attributed to thethermaldecompositionof theC 2 AH8residue andto thesubsequent decompositionof C 3 AH6 andAl(OH) 3during heating period [16,27] . The EGA curve of the CA sample(Fig. 9) shows the release of H 2 O (maximum at 285.48 C), indi-cating that the C 2 AH8 , C3 AH6 and Al(OH) 3 phases are degraded atthis stage (see Table 2 ).

The fourth endothermic peak appears at 451.29 C in this sam-ple with a weight loss of 6.81% which is attributed to the water lossin the solid ( Fig. 8). This could be attributed to the thermal decom-position of C 3 AH6 residue. Besides, the C 4 AH19 hydrate starts todecompose at this temperature [2] .

The last mass loss (1.02%), corresponding to the endothermic

peaksat 548.42

C, is attributed to thethermaldecomposition of the


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Table 2Comparison ofDTA peaks ofCA and C 7 A3 Z samples.

CA sample C 7 A3 Z sample [6] Phases

Temperature ( C) Mass loss (%) Temperature ( C) Mass loss (%)

Endothermic effects97.94 4.35 88.43 6.85 CAH 10 , AH3 -gel165.93 3.66 159.78 6.36 CAH 10 , C2 AH8285.48 9.66 261.67 5.79 C 2 AH8 , C3 AH6 , Al(OH)3451.29 6.81 439.20 6.64 C 3 AH6 , C4 AH19548.42 1.02 534.35 0.95 Ca(OH) 2

Exothermic effects954.22 907.06 Crystallization of

calcium aluminates

C CaO; A Al2 O3 ; H H 2 O

the CAH 10 , AH3 -gel, was assisted by the evolution of water vapour(EGA curve in Fig.9 ). What is more, theion current signals aremuchweaker than those obtained for the CA sample.

4. Summary and conclusions

From the above ndings, it can be concluded that the semi-

crystalline to nearly amorphous and thin layers of the calciumaluminate hydrates, i.e. CAH 10 , C2 AH8 , C3 AH6 and C 4 AH19 whichcover the initial CaAl 2 O4 and Ca 7 ZrAl6 O18 grains can moderate thereaction rate of the aluminate phases by reducing the diffusion of water in the direction of the aluminate phases grain cores. Nev-ertheless, the hydration proceeds as the hydration shells thickenand consume the CaAl 2 O4 and Ca 7 ZrAl6 O18 grain cores. An impor-tant microstructural aspect of the early hydration of CaAl 2 O4 andCa7 ZrAl6 O18 compounds is the formation of a shell of hydrationproducts around the aluminate grains. The occurrence of the ther-modynamically stable C 3 AH6 and AH 3 phases inthe CAand C 7 A3 Zsamples indicate partial transformation of the unstable calciumaluminate hydrates (CAH 10 , C2 AH8 , C4 AH19 ) at room temperature.Thesharp endothermicpeakson theDTAcurves areassociatedwith

decomposition of these hydrates. The TGA curves of the CA andC7 A3 Z pastes showed a ve-step water loss, including four effectsforcalcium aluminate hydrates.The decompositionof thehydrates,i.e. CAH10 , C2 AH8 and C 3 AH6 , proceeds via two stages causing thetwo thermal effects for each compound but these DTA peaks coin-cide with each other. Therefore, it can be concluded from thisdata that the qualitative identication of the hydrated compoundsknown to decomposewithindened temperature ranges wasdone.Moreover, it was found that C 2 AH8 , C3 AH6 and Al(OH) 3 phases arethemain hydration productsof CaAl 2 O4 . Under thesame laboratoryconditions, the hydration of Ca 7 ZrAl6 O18 proceeds with the forma-tion of mainly CAH 10 and AH 3 -gel phases. The sharp endothermicpeaks give evidence that hydrates possess some degree of crys-talline order butat early stages of hydrationthey arenot detectableon X-ray yet.

Acknowledgement

This work is supported by the grant no. N N507 45 76 37 of thePolish Ministry of Science and Higher Education.

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