Formation of scawtite from mixtures of CaO, dolomite and quartz under hydrothermal conditions

CEMENT and CONCRETE RESEARCH. Vol. 12, pp. 377-380, 1982. Printed in the USA. 0008-8846/82/030377-04503.00/0 Copyright (c) 1982 Pergamon Press, Ltd.

FORFtATION OF SCAWTITE FROMMIXTURES OF CaO, DOLOMITE AND QUARTZ UNDERHYDROTHERI~AL CONDITIONS L. ~tevula and J. Petrovi~ Slovak Academy of Sciences Institute of Inorganic Chemistry DGbravsk~ cesta, 809 34 Bratislava Czechoslovakia (Communicated by Z. Sauman) (Received Oct. 21, 1981; in final form Feb. 22, 1982) ABSTRACT Water suspension of mixtures of CaO, and quartz (CaO/SiOo = 1,2; 1,6 and 2,0) in which I0-50 wt % CaO were substituted by dolomite, were hydrothermally treated at 20O°C. X-ray phase analysis of reaction products showed that chiefly scawtite and partly calcium- and magnesium silicate hydrates were formed. The scanning electron micrographs showed variations in morphology of scawtite particles in dependence on the molar ratio of CaO/SiO2. Mit der R~ntgenphasenanalyse wurde festgestellt, dass durch den Ersatz von I0-50 Gewichtsprozent CaO mit Dolomit in einer Quarzmischung bei dem Molverh~Itnis von CaO/SiO2 = 1,2; 1,6 and 2,0 in einer Wassersuspension unter hydrothermalen Bedingungen bei 200°C ~berwiegend Scawtit und teilweise Calcium- bzw. Magnesiumsilikathydrate entstehen. Von den Aufnahmen mit dem Rasterelektronenmikroskop i s t ersichtlich, dass sich die Morphologie des Scawtits dem Molverh~Itnis CaO/SiO2 entsprechend ~ndert. Introduction By hydrothermal treatment at 200°C, a calcium silicate carbonate hydrate, scawtite Ca7(Si6018).(CO3).2HpO was prepared from mixtures of CaO and quartz (CaO/SiO2 = 1,2) in which a p~rt of CaO was substituted by magnesite (1). In a subsequent series of experiments, dolomite was used for substituting a part of CaO in the starting mixtures at molar ratio CaO/SiO2 = 1,2; 1,6 and 2,0. From systems with components in the above molar ratio only calcium s i l i cate hydrates are formed under hydrothermal conditions. The phase composition and properties of reaction products are considerably affected by the presence of gaseous CO2 or of the ions CO~- from carbonates. According to R. Javelas, et al. (2), by combination of the CSH phases with Caz+ and CO~- ions a solid solution can be formed. Formation of scawtite with satisfactory binding proper ties by hydrothermal reaction of C2S-SiO2 cements was reported (3). 377

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Experimental Calcium oxide was prepared by i g n i t i o n of CaCO3, a n a l y t i c a l grade, at IO00°C for 3 hours. Brasil quartz (99,2 wt % SiO 2) and dolomite (containing 0,4 wt % c a l c i t e , 1,4 wt % unspecified admixtures including 0.2 wt % Si02), p a r t i c l e size up to 25 um, were used. The components were then mixed with d i s t i l l e d water ( r a t i o I : I 0 ) in t e f l o n crucibles and hydrothermally treated in bombs at 200°C f o r seven days. The phase composition of the reaction products was i d e n t i f i e d with a Philips d i f f r a c t o g r a p h . The morphology and habit of p a r t i c l e s were monitored with a JEOL stereoscan. Results Phase composition of hydrothermally treated samples formed from mixtures in which varying amounts of CaO (in wt %) were s u b s t i t u t e d by dolomite (or by a homogenised mixture of CaCO3, a n a l y t i c a l grade, and s y n t h e t i c MgC03, are reviewed. Molar r a t i o CaO/Si02 = 1,2 S u b s t i t u t i o n of CaO by dolomite: I0 20 30 50

wt wt wt wt

% CaO % CaO % CaO % CaO

-

x o n o t l i t e , p a r t l y scawtite and c a l c i t e scawtite, smaller q u a n t i t y of c a l c i t e scawtite and c a l c i t e , p a r t l y t a l c c a l c i t e and t a l c , unreacted quartz

S u b s t i t u t i o n by CaCO3-MgCO3 mixture: 20 wt % CaO - x o n o t l i t e , tobermorite 30 wt % CaO = tobermorite, x o n o t l i t e Molar r a t i o .CaO/Si02. = 1,6 S u b s t i t u t i o n by dolomite: I0 wt % CaO - scawtite, small q u a n t i t y of c a l c i t e and b r u c i t e , unreacted dolomite 20 wt % CaO - scawtite, p a r t l y serpentine and b r u c i t e , increased amount of c a l c i t e , unreacted dolomite 30 wt % CaO - smaller portion of scawtite and s e r p e n t i n e , c h i e f l y c a l c i t e and also unreacted dolomite 50 wt % CaO - c a l c i t e p r e v a i l s , small amount of c r y p t o c r y s t a l l i n e serpent i n e , unreacted dolomite Molar r a t i o CaO/SiO 2 = 2,0 S u b s t i t u t i o n by dolomite: I0 wt ~; CaO - scawtite, p a r t l y C5S3H2 (4) and b r u c i t e . A portion of Ca(OH)2 remains unreacted 20 wt % CaO m o r e scawtite than in the previous sample, less C5S3H2 and b r u c i t e , unreacted . . . . Ca(OH)2 30 wt % CaO - phase composltlon s l m l l a r to that of the previous sample, except for a portion of unreacted dolomite 50 wt % CaO - scawtite and a small amount of c a l c i t e . The portion of the unreacted dolomite is greater S u b s t i t u t i o n by CaCO3-MgCO3 mixture: 20 wt % CaO - C5S3H2 and p a r t l y scawtite are formed• A part of Ca(OH) 2 remalns unreacted, the amount of the unreacted c a l c i t e exceeding s l i g h t l y than when dolomite was used forCa@ s u b s t i t u t i o n 30 wt % CaO - x o n o t l i t e and scawtite are formed. A part of CaCO3 and MgCO3 remain unreacted.

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379 SCAWTITE, HYDROTHERMALREACTION, DOLOMITE, SILICATE HYDRATES

The phases in reaction products are l i s t e d in Table I.

Substitution of CoO by dolomite (in w t % ) 10 20 3O 50 xor~tite scowt ite scawtite calcite calcite C~0/Si02=1.2 scawt ite talc calcite talc cotcite scowtite calcite ' Scowtite calcite calcite scawtite CaO/Si02= 1,6 cotcite serpentine brucite brucite scawt ite scawfite C5S3H2 scowtite S3H2 S3H2 CaO/SiO2=2.0 F,=..o..tRe calcite UClte rucite brucite Motor ratio

i,erpen~irte

~5

With the aim of checking the reaction mechanism, 15 and 30 wt % CaO were added to dolomite and the mixtures were hydrothermally treated. 15 wt % CaO added in the form of Ca(OH)2 reacted completely, dolomite was partly decomposed to form brucite and c a l c i t e . Calcium hydroxide reacted with magnesium ions from dolomite y i e l d i n g brucite and, by conversion, calcite was formed. This, together with the carbonatised calcium ions from dolomite, formed one phase. The amount of 30 wt % CaO did not cause (under the given conditions of hydrothermal process) further decomposition of dolomite, the calcium hydroxide remained uncombined. The mechanical properties of hydrothermally treated mixtures of CaO/SiO2 = 1,2 (partial s u b s t i t u t i o n of CaO by dolomite) were checked by the following procedure: a paste was prepared (water/mixture = 0,7), which was f i l l e d into ~ u l d s 20x20x20 mm and hydrothermally treated at 200°C for 3 days. The compressive strength of the t e x t cubes was then tested and X-ray diffractograms were taken. From mixtures in which varying quantities of CaO were substituted by dolomite (expressed in wt %), the following products were obtained: lO wt % CaO - scawtite and C2SH(A) approximately in equal proportions. A part of dolomite remains unreacted. Compressive strength is 0,94 MPa. 20 wt % CaO - the greatest y i e l d of scawtite obtained from these mixtures. A part of dolomite and quartz remain unchanged. Compressive strength is 2,2 MPa. 30 wt % CaO - p r a c t i c a l l y only scawtite. The portion of unreacted dolomite and quartz is greater. Compressive strength is 2,1MPa. 50 wt % CaO - small amount of scawtite, considerable amounts of calcite, unreacted dolomite and quartz. Compressive strength is 0,90 MPa. The fact that the products contained unreacted brucite indicates that the reaction between quartz and brucite is a slow one. From mixtures with CaO/SiO2 = 1,2 and 1,6 the magnesium s i l i c a t e hydrate begins to form on substitution of 50 wt % CaO by dolomite. At CaO/SiO2 = 2,0 with 30 wt :i substitution, the product contains unreacted brucite. On s u b s t i t u t i o n of 50 wt ~ of CaO the amount of available CaD is evidently too small and dolomite does not react any more. The formation of a c r y p t o c r y s t a l l i n e , serpentine-like phase does not show in d i f f r a c t i o n patterns or possibly magnesium is incorporated in scawtite. The compressive strength values of the autoclaved cubes are r e l a t i v e l y small, in the presence of scawtite, however, they increase by about twofold. This indicates that scawtite influence favourably the compressive strength of the test cubes. In Figures l and 2, morphology of the products formed from mixtures CaO/SiO2 = 1,2 (20 wt % subst.), and CaO/SiO2 = 1,6 (lO wt ~ subst.) respectively, are shown. Though they consist of identical phases, i . e . , scawtite, calcite and

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Vol. 12, No. 3 L. Stevula, J. Petrovi~

.~.~..... .

±~

FIG. 1 Morphology of products from mixtures CaO/SiO2 = 1,2 (20 wt % s u b s t i t u t i o n )

FIG. 2 Morphology of products from mixtures CaO/SiO 2 = 1,6 (I0 wt % s u b s t i t u t i o n ) brucite, they d i f f e r considerably in t h e i r habits. The sample in Fig. 3 was formed from a mixture CaO/SiO2 = 2,0 (30 wt % CaO substituted by dolomite). I t consists of column-like c r y s t a l l i t e s of scawtite with rough s t r i a t e d and forrowed surface and small i r r e g u l a r p a r t i c l e s of dolomite or brucite. Conclusion The reported experimental results can be epitomized as follows:

FIG. 3 Morphology of products from mixtures CaO/SiO2 = 2,0 (30 wt % s u b s t i t u t i o n ) 2) mixtures small. 3) affected

I) The greatest content of scawt i t e is formed in products synthetized from mixtures having C/S molar r a t i o s used throughout the present study in which 20 wt % CaO were substituted by dolomite.

The compressive strength values obtained on t e s t cubes prepared from with CaO/SiO2 = 1,2 in which CaO was replaced by dolomite, are rather The appearance of the products and the shape of scawtite p a r t i c l e s are by the molar r a t i o of the s t a r t i n g mixtures. Acknowledqements

The authors would like to thank Ing. D. Jak6bekov~ from the Research I n s t i tute of C i v i l Engineering for the stereoscan micrographs and Dr. V. Sa~kova for t r a n s l a t i o n of this paper into English. References I. 2. 3. 4.

L. Stevula, J. Petrovic, Cem. Conc. Res. I I , 549 (1981). R. Javelas, J.C. Maso, J.P. O l l i v i e r , B. T~enoz, Cem. Conc. Res. 5, 285 (1975~ G.L. Kalousek, E.B. Nelson, Cem. Conc. Res. 8, 283 (1978). A. Aitken, H.F.W. Taylor, J. Appl. Chem. lO, 7 (1960).