July, 1957
THEREACTION BETWEEN CALCIUM HYDROXIDE AND SILICA
941
HYDROTHERMAL REACTIONS UNDER SUPERCRITICAL CONDITlONS 11. THE REACTION BETWEEN CALCIUM HYDROXIDE AND SILICA1 BY J. F. CORWIN,R. G. YALMAN, J. W. EDWARDS AND E. R. SHAW Contribution from the Department of Chemistry, Antioch College, Yellow Springs, Ohio Received February 87,1967
The hydrothermal reaction between dilute solutions of calcium hydroxide and silica flass was studied under carefully controlled experimental conditions at 400', 340 atmospheres pressure, and in a time range rom 0 to 192 hours. Through the devitrification of a controlled amount of silica glass rod, silica was added slowly to the solution during the reaction, Under these conditions xonotlite was formed first, and on continued reaction the solid phase modified through a series of crystalline and amorphous materials to P-cristobalite and finally to or-cristobalite.
Introduction The investigation of the reactions of the alkaline earth metal oxides with silica glass2 showed that calcium and strontium oxides have similar characteristics. The reaction was relatively rapid and showed signs of continuing devitrification of the glass even after equimolar quantities of silica had been removed from the source material. The reaction between Ca(OH)* and silica is so common in geological and industrial processes that further investigation seemed reasonable. Papers dealing with hydrothermal reactions of calcium oxide with silica have been published in considerable number and they cannot be reviewed here. For a general survey, however, reference is made t o summarizing work of two author^.^ Specific reference will be made throughout this paper to those authors whose work is closely parallel. The present work is concerned with the effect of dilute solutions, 0.0216 N, on Si02 which is added to the solution during the reaction by the devitrification of clear silica glass rod. Experimental
undevitrified silica glass rod was cleaned of adhering material by scraping and reweighed. The loss in weight of the rod was taken as the extent of the reaction. Silica.-Pure, transparent silica glass was obtained from the Thermal American Fused Quartz Company, and used directly in the experiments. Ca(OH)*: conformed to C.P. A. C. S. standards of purity. The liquid phase was analyzed for calcium using a Weichselbaum-Varney Flame Photometer by the method of Chow and T h ~ m p s o n . ~Silica in the dissolved form was determined using a Beckman DU spectrophotometer following the method of Kenyon and Bewick.6 The devitrified material from the centrifugation, from the autoclave sides, and from the undevitrified rod was mixed by grinding and analyzed chemically by conventional gravimetric methods for silicate analysis. The results reported are averages of several runs having equal time a t 400'. The devitrified sample which was nearest the average amount was taken for X-ray and optical analysis before the chemical analysis was made. Model Experiments.-Experiments containing measured ratios of CaSi0s:SiOz were made in which silica glass rod was pulverized and added in this form to the liquid. All other conditions were the same. The solid materials from these experiments were analyzed by optical and X-ray methods.
The experimental preparation of the autoclaves was the same as in ref. 2 but some innovations were added to allow closer control of the reactions. The silica used was in rod form and pieces were sawed to a consistent length so that the surface presented to the solution was approximately constant. The pieces weighed approximately 10 g. The time that the reaction was maintained a t 400' was varied over a wide range. A few runs were quenched in water after reaching 400' t o determine the extent of the reaction during the heating eriod. After coolng the autoclave wa8 opened and the solid material separated from the liquid by approved analytic,al methods of centrifugation and washing. The liquid was' diluted to 500 ml. in a volumetric flask and then stored in a polyethylene bottle until analysis was made. pH measurement was made on the original liquid immediately after removal from the autoclave. The solid was removed from the autoclave by scraping the sides and the bottom with a spatula and added to the solids centrifuged from the liquid phase. It was impossible to remove all of the solids without adding metal impurities to the solid. Some solid always adhered too tightly to be removed except by polishing with a motorized wire brush. The solid was dried a t 110". The remaining part of the
Table I contains the data €or the conditions under which the various experiments were conducted and chemical analysis of the solid phase present. The concentration of 0.0216 N was selected because it ensured that the retrograde solubility of Ca(OH)2 would not cause precipitation at 100". Although the solubility above 100" is not known preliminary experiment showed that reaction between Si02 and the solution started a t 130"; thus solubility to this temperature was ensured. The results in column (2) represent the loss in weight of the rod during the whole procedure of heating, holding at 400°, and cooling with the exception of the first result. To obtain the amount of reaction a t constant temperature (400') the result labeled 0-9 was subtracted from the others in column (2). The values calculat,ed in this manner for all of the react,ions except the 192 hour experiment are summarized graphically in Fig. 1 by plotting the adjusted values against time. The length of the line drawn through the points represents the range over which the amount of reaction varied. Examination of the curve shows rapid reaction up to 12 hours, then a period in which the reaction proceeds slowly, followed by a more rapid reaction up to 9G houis. Extrapolation of the curve to 192 hours shows that the value 2.4221
(1) The results and interpretations presented here are derived from work supported on contract between Antioch College and the U. 8. Army Signal Corps (Contract No. DA-36-039 SC-64605) through its Signal Corps Engineering Laboratories at Fort Monmouth, New Jersey, and the Office of Scientific Research, Air Research and Development Command, under contract No. AF 18 (GOO) 1490. Reproduction in whole or in part is permitted for any purpose of the United States Government. ( 2 ) Paper No. 1 of this series, THISJOURNAL, 61, 933 (1957). (3) H. H. Steinour, Chem. Reus., 40, 391 (19471; W. Eitel, "Physical Chemistry of the Silicates," University of Chicago Press, Chicago, tllinois, 1954.
Results and Discussion
(4) T. J. Chow and T. G. Thompson, Anal. Chem., 27, 910 (1955). (5) 0. A. Kenyon and H. A. Bewick, ibid., 26, 145 (1953).
J. F. CORWIN,R. G. YALMAN, J. W. EDWARDS AND E. R. SHAW
942
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1
11) that on heating to 400" sufficient devitrification has taken place to account for all of the that was present. That CaSiOa is formed is shown by the X-ray evidence that xonotlite (5Ca0.5SiO2.H20)is the solid phase (Table 111). 0 .BO Several authors and particularly Jander and Francke6 have found xonotlite to be the stable '0.90 solid phase between 175 and 390". A further Ej 0.70 indication of reaction is the drop in pH from 12.1 Y a to 9.1. From the pH drop another inference can 5 > 0.60 be drawn concerning the ions in solution. The 0 w silica in the presence of calcium ions exists in the orthosilicate form or some more complex ion.' 3 030 a From the ratio of CaO/SiOz after reaching 400" a 0 0.40 mixture of CaH2SiOl and Ca(H3Si0& is indicated. From the ratio of CaO/SiOZ in solution as time a t 0.30 400" is extended it is evident that more complex silicate ions are present. The pH drop is further indicative of this complex formation. Although there is considerable variation in analytical results Oq20' 0.10 ,/ a concentration minimum is reached where the 0.0s , , , , , CaO/Si02 ratio is approximately 1 :7. The method used for analysis5 eliminates the probability of colOo0 136 I2 24 48 72 96 loidal silica; however, the polymeric ion that is TIME in HOURS. present has either been depolymerized by the diluFig. l.-Devitrification rate: Si02 g. us. time in hours. tion8 or by the strong acidg solution necessary for g. adjusted in the same manner as the others falls the analytical method since the determinations show the presence of the necessary amount of silica almost exactly on the curve. in the form of the orthosilicate ion. Gravimetric TABLEI checks confirmed the colorimetric method. CONDITIONS OF REACTION AND ANALYSIS OF SOLID PHASE After sufficient devitrification has occurred to ac10 g. SiOa glass rod, pH 12.1,0.0216 N Ca(OH)2 5051, filling, count for equimolar reaction between Ca(OH)2 125 ml., 400", 340 atm. and SiOz glass, the devitrification proceeds, but (5) (2) (4) FuAmt. slows down considerably after 12 hours, then after (1) devit. (3) Analysis of devit. maexplanation ture 24 hours speeds up and continues a t the same rate No. mateterial, % ' of identiof rial, conditions ficaup to 192 hours. The slow build-up of SiOz in the runs g. HzO CaO SiOz Time tion solution and in the solid phase proceeds until a 4 0,1000 6 . 6 0 5 0 . 8 1 4 2 . 1 3 0 Quenched 0-Q new solid phase, @-cristobalite (Table 111), starts from 400' 1 hr,at400° 1 33.02 60.27 3 .I697 6.71 to form which reduces the amorphous silica in 3hr.at400° 3 26.07 70.08 3 .1893 3.85 equilibrium with the liquid, and devitrification has 6hr,at400° 6 24.71 71.53 3 .2170 3.76 to speed up in order to supply the depleted amor26.98 68.83 12hr.at400° 12 4 .2617 4.18 phous material. Although in the X-ray analysis 24hr.at40Oo 24 3.82 2 4 . 8 4 71 34 2 ,2750 4 8 h r . a t 400' 48 2.95 23.74 73.30 6 .3693 about 50% of the solid phase appears to be amor9.54 89.13 72hr.at400" 72 2 .7076 1.38 phous, optical examinations show it to be a collec4.56 94.44 96hr.at400° 96 2 1.0901 1.00 tion of very small crystals with many indices of re192hr.at400° 192 0.57 2 . 8 6 96.60 1 2.4221 fraction. The more probable explanation is that Table I1 contains analytical data for the liquid the silica in the solid phase goes through a modifiphase, and Table I11 contains molar ratios cal- cation becoming more and more complex until the culated from analytical data in column (3) of 0-cristobalite begins to form. With the presence of Table I along with X-ray data for the solid phase. nuclei of this phase which is a lower energy form than the glass, the glass then transforms to the low TABLEI1 energy form more rapidly. When the reaction was ANALYSIS OF LIQUID PHASE allowed to continue (192 hours) the yet more stable IdentifiAv. pH CaO, Si02 Ratio a-cristobalite was formed. mmoledl. CaO/Sioz final mmoles/l. cation Table IV contains data that represent an at0.70 0.80 1.10 0-Q 9.1 tempt to match conditions that exist in the pre1.50 .19 7.9 .28 1 vious reactions by starting with CaSi03 and pow0.83 .56 7.0 .45 3 dered silica glass. When CaSiOBwas used alone, .27 2.10 .12 6 7.0 xonotlite only was present, but when equimolar 12 7.0 .39 1 .80 .22 Si02 glass was present a new crystalline compound 24 7.0 .23 2.40 .09 labeled A was found with a small amount of xonot48 7.2 .27 1.80 .14 1.00
1
it
u,
/
J
tr,
72 96 192
7.2 6.7 6.2
..
..
.32 .21
2.20 2.80
..
.15
.os
Evidence of the rapid reaction of the silica with the Ca(OH)2 is indicated by observation (Table
(6) W. Jander and B. Francke, 2. anorg. allgem. Chem., 247, 161 (1941). (7) P. S . Roller and G.Erwin, J . Am. Chen. Soc., 62, 461 (1940). (8) F. Dienert and F. Wandenbulcke, Compt. rend., 176, 1478 (1923). (Q) K. Goto and T. Okura, J a p a n Analyst, 4. 175 (1955).
THEREACTIONBETWEEN CALCIUM HYDROXIDE AND SILICA
July, 1957
943
TABLE I11 SUMMARY OF SOLID PHASE DATA Moles Si02 Moles HzO
M o b CaO
1 3 6 12 24 48 72
2.47 1.58 2.18 2.11 2.08 2.09 2.59 2.29
1.91 2.69 5.46 5.70 4.94 5.61 7.61 20.08
1.291 0.587 .398 .370 .420 .373 ,347 .115
96
1.46
28.36
.051
192
1.61
50.85
.031
Ident.
0-Q
Moles CaO-___
Moles HIO
Mol-
Sios
X-Ray data
Xonotlite 50% Xonotlite 5Oy0 Xonotlite 509;b Xonotlite 50% Xonotlite 50% Xonotlite 50% Xonotlite 5Oy0 0-Cristobalite Xonotlite Amorphous material P-Cristobalite Xonotlite Amorphous material or-Cristobalite Amorphous material
Remainder amorphous Remainder amorphous Remainder amorphous Remainder amorphous Remainder amorphous Remainder amorphous Remainder amorphous 50 % 35 % 15% 50 % 25 % 25 %
The continued addition of silica results in the formation of amorphous silica in the solid phase and the build up of complex silica ions in the solution. Optical examination of the amorphous material shows that part of i t is crystalline with many inTABLE IV dices of refraction. Calcium silicate, 0.1500 g., finely divided silicon dioxide After 48 hours a new silica phase, p-cristobalite, glass, 125 ml. vol., 400°, 340 atm., 2 hr., initial pH 10.6 begins to form and with it the reaction becomes No. of Final Wt. SiOz, more rapid. With the appearance of this new, relruns pH g. X-Ray data" atively low temperature phase of silica the amount 2 8.2 None Xonotlite (S) of amorphous material in equilibrium with the 2 7.3 0.0776 Compound A (S) and solution decreases and the reaction proceeds more trace of xonotIite* rapidly. Continued heating and addition of more 2 7.0 0.1552 CompoundA(S) silica results in the transformation of the p-cristo2 7.0 0.2328 Compound A ( M ) plus balite phase to a-cristobalite. When powdered small amount of asilica is used and the reaction is much more rapid (2) cristobalite (W) Compound A (complete in two hours) a-cristobalite is the result a Refer to Table I (2) for abbreviations. when sufficient silica is present to form a new silica not identifiable from A. S. T. M. Card File. phase. Conclusions Ca(OH)2 resembles the alkali metal hydroxide The reaction of dilute solutions of CaO with reactions with silica glass to the extent that the resilica glass under supercritical conditions is rapid action is continuous. However, the reaction is and starts a t 130". When controlled by present- slower and the resulting crystalline material is in ing an almost constant surface to the solution the the cristobalite form rather than in the quartz first step in the reaction is the formation of xonot- form. The greater solubility of the alkali metal lite. Xonotlite is sufficiently soluble so that an al- silicates with their correspondingly higher pH most constant pH due to the hydrolysis of CaSiOs values accounts for this difference in reaction and is maintained. Continued heating results in fur- in product.lO ther addition of silica to the solution by the reac(10) J. F. Corwin, A. H. Herzog, G. E. Owen, R. Yalman and A. C. tion of the solution with the fused silica surface. Swinnerton, J . A p . Chem. Soc., 78, 3983 (1953).
lite. Two moles of silica glass resulted in compound A alone; while with 3 moles of a-cristobalite began to form. In these reactions all of the glass was converted to crystalline material.
