Stability Of Ettringite On Heating.pdf

January 1972

Journal of The American Ceramic Society-Discussions

55

and Notes

Identification of BaO*4TiO2=Al203in the System BaO-Ti 02 -A12 03 J. P. GUHA and D. KOLAR

I

investigation of compatibility relations between BaTiO, and ALO, d lines which could not be identified as those of known phases in the system Ba0-TiOL-AI20,occurred in the X-ray diffraction patterns of several samples. Thus, a systematic attempt was made to detect an unknown phase in the system, and a new ternary compound having the probable composition Ba0.4Ti0z.A120, was identified in the Ti02-rich portion of the system. A mixture of stoichiometric amounts of reagent-grade BaCO,, TiOt, and ALOa was fired at 120OOC in a P t envelope with intermittent cooling, crushing, mixing, and pressing to ensure homogeneity. The fired sample was cooled to room temperature and subjected to X-ray analysis. Equilibrium was considered to have been attained when the X-ray diffraction patterns of successively heated samples showed no further change. X-ray powder patterns were obtained with a Geiger-counter diffractometer" using Ni-filtered CuKa radiation at a scanning rate of 1"2s/min. The 2s values were calibrated against the known peaks of pure Au. Debye-Scherrer powder diffraction patterns were obtained in a camera 114.6 mm in diameter using CuKa radiation. Simultaneous TG and DTA measurements were performed on previously prepared samples; repeated runs were conducted at temperatures up to 1530°C a t a heating rate of 10"C/min. Temperature was measured with Pt-PtlORh thermocouples, and the DTA apparatus was calibrated at the melting point of diopside, CaMgSizOo (mp 1395.1"C). The melting behavior of the compound was also investigated in a heating microscope.t Melted samples were examined metallographically. The X-ray diffraction data for the compound, BaTiaAI2Ol2, are given in Table I. These powder data are distinctly different from those for the starting materials and for binary compounds known to exist in the system Ba0-TiO2-AIz0,or mixtures thereof. This diffraction pattern was indexed on the basis of a hexagonal unit cell with a=8.860 A, c-18.458 A, and a/c=0.480. The DTA and TG results revealed only an endothermic peak at 1470°C which was not accompanied by a weight change. This pronounced endothermic effect with a sharp

Table I.

N AN

18 100 12 30 15 30 10 6 18 5 5 2 8 7 7 5 5 7 7 5 5

X-Kay Diffraction Data for Ba0*4Ti02*A1,0, 3.52 3.18 2.81 2.46 2.23 2.19 2.01 1.96 1.87 1.77 1.72 1.66 1.57 1.47 1.43 1.405 1.376 1.320 1.311 1.266 1.248

3.542 3.195 2.835 2.464 2.236 2.199 2.011 1.966 1.877 1.771 1.729 1.667 1.573 1.470 1.435 1.406 1.378 1.320 1.311 1.266 1.249

022 114 115 032 034 221 133 036 042 044 232 141 144 146 333 334 150 154 246 062 342

reversal corresponds to the melting of the compound. Observations in the heating microscope indicated that the compound melts readily at a temperature <1500°C and then slumps completely. Metallographic examination of the melted sample showed that the compound melts congruently. This result was supported by X-ray analysis of the melted sample, which produced a pattern identical to that of the unmelted compound. Work is in progress to determine the crystal structure of the compound and its compatibility relations with binary phases in the system Ba0-TiOL-AIzOa. Received November 8, 1971. The writers are with Institut "J6zef Stefan," Ljubljana, Yugoslavia. "Siemens Corp., Iselin, N. J. TLeitz, Opto-Metric Tools, Inc., New York, N. Y.

Stability of Ettringite o n Heating P. K. MEHTA

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3CaO.Al2O3.3CaSOa-32H2O,is the principal hydration product responsible for expansion in shrinkagecompensating and chemically prestressing cement concretes. Because many &O molecules are incorporated into its crystal lattice, the behavior of ettringite exposed to heat is significant in the thermal durability of concrete members which may contain significant proportions of ettringite in the hydrated cement paste. TTRINGITE,

Kuehl and Albert' reported that ettringite decomposes at 100°F; Lieber,' however, found that ettringite temperatures definitely exists in aqueous solutions heated to 194°F. It a p

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Received September 7, 1971; revised copy received October 18, lg71. Supported by Stressed Structures, Inc., Denver, Colo. The writer is with the Department of Civil Engineering, University of California, Berkeley, Calif. 94720.

56

Journal of The American Ceramic Society-Discussions

(A)

r,

u

ROOM TEMP.

il

and Notes

Vol. 55, No. 1

L il

1 11"' I1

d

AUTOCLAVED AT

Fig. 1. X-ray diffraction patterns of ettringite exposed to ( A ) drying Conditions, steam.

pears, therefore, that the thermal stability of ettringite is enhanced in an aqueous environment; this result is important in relation to the thermal durability of concrete. Published data on the properties of ettringite usually pertain to samples produced by chemical reaction in dilute solution. In actual concrete practice, however, the H,O-cement ratios used are much lower than those of dilute solutions, and the characteristics of ettringite samples produced from dilute solutions may not be similar to those of ettringite formed in pastes. Mehta' showed that the morphology of ettringite prepared from pastes with low &O-cement ratios differed from that reported by investigators4 who worked with dilute solutions. In contrast to the long, slender needles which result from dilute solutions, ettringite formed in such pastes occurred as short prisms with hexagonal cross sections having a thickness-length ratio of ~ 1 : 3 . The ettringite samples of the present investigation were produced by hydration at the H20-cement ratios normally used in commercial concrete practice. The materials and details of the procedure were described by Mehta and Klein.' Ettringite was exposed to heat under dry and moist conditions. In dry exposures, samples were heated for 1 h in a drying oven at loo', 150°, and 200°F. In moist exposures, samples in contact with distilled water were heated for 1 h at loo", 150", 200°, 300°, and 450'F (the 300' and 450" exposures were achieved in an autoclave). X-ray diffraction patterns of the samples were obtained before and after heat treatment (Fig. 1 ) using a standard diffractometera with a Cu target; the pattern of ettringite at room temperature is included for reference. As a first approximation, the heights of the principal ettringite peaks a t 9.1'20 and 15.8"20 indicate the relative proportions of ettringite present. Ettringite heated in a dry environment was stabIe a t 150° but decomposed partially at 200'F (Fig. 1( A ) ) , in contrast to reports' that it decomposed at temperatures >100"F. In moist hot environments (Fig. 1(B) ) , the ettringite peak heights were not reduced significantly after 1 h at ZOOOF, and there was no evidence of decomposition of the ettringite to other compounds. However, when it was exposed to saturated steam

,*

n

(B) moist conditions, and (C) saturated

at 300°F for 1 h, ettringite decomposed to the monosulfate hydrate. In the specimen autoclaved at 450"F, the presence of a hydrogarnet phase, 3Ca0.A1203.6R0, was apparent from the diffraction pattern; however, a considerable proportion of the ettringite remained undecomposed after this exposure. The thermal resistance of the ettringite observed in the present study is superior to that reported previously under both dry and moist conditions; this result is attributed to the improved durability of a product formed in systems with low RO-cement ratios. It is also concluded that the thermal durability of expansive cement concretes containing significant proportions of ettringite may not be affected adversely by the large amount of water of crystallization in ettringite. It is realized that dehydration of a material is both time- and temperature-dependent. No attempt was made to determine the effect on the stability of ettringite of long exposures a t dehydration temperatures; hence, the relevance of the data reported is limited to the experimental conditions described. However, the information may be useful for applications in which the temperatures to which products containing ettringite are exposed are not severe and in which the presence of many H,O molecules in the ettringite may be a cause for concern. Acknowledgment The contribution of P. S. Sethee to the experimental work is gratefully acknowledged.

QNorelco, North American Philips Co., Inc., New York, N. Y. 'H. KuehI and H. Albert, "Influence of Temperature on Structure of Calcium Sulfoaluminates," Zement, 12, 279, 285 (1923). a W. Lieber. ''Ettrinpite Formation at Elevated Temoeratures," Zem.-KaZk-Gips~lG[9] 364-65 (1963). P. K. Mehta, "Morphology of Calcium Sulfoaluminate Hydrates." J. Amer. Ceram. Soc., 52 r91 521-22 (1969). William Lerch, F. W. Ashton,-ahd R. H: B o b e , "Sulfoaluminates of Calcium," J. Res. Nat. Bur. Stand., 2 [4] 715-31 (1929). P. K. Mehta and A. Klein, "Formation of Ettringite by Hydration of a System Containing an Anhydrous Calcium Sulfoaluminate," J . Amer. Ceram. SOC.,48 [8] 435-36 (1965).