Sept., 1960
SOLUBILITY PRODUCTS OF H Y D R s T E D CALCIUM SILICATES
1151
ISVESTIGATION OF COLLOIDAL HYDRATED cLacIunISILICATES. I. SOLUBILITY PRODUCTS BY S. 4.GREENBERG, T. N. CHANGAXD ELAIXE ANDERSON Contribution from Portland Cement Bssociation Research and Development Laboratorzes, Skokze, Ill. Received Xarch 10. 1960
The solubility products of t,hese colloidal subst,ances were evaluated by determining the calcium ion and silicic acid concentrations arid pH values of the solutions and applying these data t o the activity product equations: (1) K,,, = C I C ~ + + U H ~ S ~ O ~ - and (2) K,,, = U C ~ + + ~ ~ H ~ S ~ Hydrated O~-. calcium silicates a-it,h mole ratios Ca0:SiOa between 0.8 and 1.5:1 were prepared a t 85". The data for the 1.0, 1.2 and 1.5:1 mole ratio hydrates show t h a t the pK,,t is 7.0 =t0.1 a t 25". Equation 2 agrees with data for the 0.8: 1 hydrate and the pK,p value is 8.5 zt 0.1. As expected, a 1:1 hydrate prepared hydrothermally a t 186" exhibit,s a higher pKSplvalue of i . 3 =t0.1. The Ksplvalues are assigned to the equilibrium between solid C:iH2Si01and watcr. Hydrates with CaO: Si02 mole ratios greater than one are assumed to contain calcium hydroxide in solid solution arid an equilibrium exists between the calcium hydroxide in aqueous solution and the port'ion dissolved in the solid CaH?Si04.
Introduction The solubilit,ies of hydrated calcium silicates have been examined extensively,' but only Roller and Erwinz estimat'ed a solubility product for hydrated calcium silicat'es. Because of t,he increase in available: information on hydrated calcium sili~ a t e s ~and . ~ , 5on silicic acid,6 it has become possible to characterize the structures better and t'o use more accurat,e dissociation constants for silicic acid for the evaluat,ion of the solubility products of the various calcium silicate$. Knowledge of t,he solubility products of hydrated calcium silicat,es is essential to an understanding of the reaction bet'ween solid tricalcium silicat,e and water. This reaction usually is assumed to be the most jmportaiit one proceeding in fresh concrete mixtures. Hydrated calcium silicat,es synthesized beloxT 100" are colloidal and exhibit' only a few X-ray diff ractioii lines. The variety of concentrations of calcium oxide, silica and water in these hydrates is broad. Only t'he hydrates with mole rat'ios CaO: SiOp betn-eeii 0.8:1 and 1.5:1 were examined in this study. S o effort was made to establish t8he upper or lower limits of the CaO:SiOz mole ratios. Evidence will be provided in t'his paper to support the view t,hat hydrat'ed calcium silicates with CaO: SiO, mole ratios above unity contain calcium hydroxide in solid solut'ioii in CaHzSiOl. Therefore, on the basis of the evidence obtained in the present study two equilibria appear t o be involved in the calcium silica,te hydrate-water system. One is that of the solubility of solid CaHzSiOI and the otther iiir-olves the distribution of calcium hydroxide betn-een solid CaHzSi04and aqueous solut,ion. In the present st,udy the hydrated silicat'es were synthesized from calcium oxide, silica and water niixt,ures at, 85". Products with mole rat'ios of CaO:SiOz of 0.8:1, 1:1, 12:1 aiid 1.5:1 were prepared. The solu.bilities of the products in calcium , 1) Tor r e r i * ? wsee 11. 1-1. Steinour, C h e m R 1 2 ) 1'. S . Roller and G . Erwin, J . Ani. Chem i:i) I,. Helle- and $1. I:. \\-. Taylor, "Crystallograpliic D a t a for the C'alciriin Silicai es," I1t.r Najesty's Stationery Office, London, 195fi. (4) K. G . Krasilnikux-, "Reports of Syrnposiuiii on t h e Ctiemistry of Cements," ( d i t e d by .'1 P. Riidnikor, e t a?., State Publication of Literature on Striictiiral Materials. Moscow, 1956. ( 6 ) Stephan Rrunaiier a n d S. A . Greenberg, "Proceedings of t h e Fourth International Symposiiim on the Cliemistry of Cement," Wasliington. E.. C., 19GO. ( 0 ) S. -4. Gri:enherir, J . A m . Chem. Soc., 8 0 , 6508 (1458).
hydroxide solutions of various concentratioiis were determined by analyzing the solutions for calcium and soluble silicic acid concentrations and measuring the pH values. Froin these data the solubility products were derived. For comparison a well crystallized 1:I h ~ d r a t e aiid ~ , ~ ,&wollastonite CaSiOa37 - 8 were synthesized and the solubility products of these substances were determined. Experimental A. Materials.-Mallinckrodt special bulky and standard luminescent silicas, Baker AR grade calcium hydroxide and calcium carbonate, and distilled water were used for the preparation of the hydrated calcium silicates. B. Preparation of Silicates.-Weighed calcium carbonate samples were calcined a t 1050" for tn-o hours. The ca1:ium oxide formed on ignition was slaked and then vigorously agitated for ten minutes with 400 ml. of distilled 1v:Lter in a JVaring Blendor. The amount of special bulk? silic'i t o produce the desired mole ratio of calcium oxide to silica \!:E added to 400 ml. of distilled water in a stainless steel beaker. While the CaO was mixed x-ith n-ater, the silira suspension was heated ( > 8 5 O ) in a water-bath held at 100". The slaked ealcium oxide suspension was added, a ith stirring, to the hot silica suspension and the resulting mixture was stirred again after 20 minutes. The mixture w'is allolved to react at a temperature greater than 85" for a total of three hours. The products then were filtered through a medium sintered glass filter and n-ashed first with methanol and then with acetone. Finally, the products vere dried in :%vacnuin oven under redured pressure a t 60". I n the case of an additional equimolar hldrate ( 1 : 1A) the above procedure was folloa ed, but Baker AR grade ralciuni hydroxide was used directly. -4 well crystallized equimolar hydrate x a q prcapared by aiitoclaving the wet, univashed 85' hydrate at 186" for 24 hours. The material was washed and dried 'is previously described. The &wollastonite was prepared from the pressure hydrated material b r calcining the dricd product at 1050° for two hours. Table I is a summarv of the experimental conditions used in the preparations of the hydratbs and of the temperatures at, which the solubility measurements were m:tde (Column
4).
C. Solubility Study Procedure.-Gram samples of each of the hydrates were mixed with 100 nil. of calcium hydroxide solution varying i n concentration from zero to saturation (0.02 molar) and brought to equilibrium in :I constttnt tmnperat,iire bath. The samples \\-ere shaken in polyethylene bottles for a t least seven days at solution temperatures from 25 t,o 40' (listed in Table I, Column 4 ) . Roller and Erwin? have reported that itft,er three days no appreri:hln chmipe i n c.oriwntratiori orrurs. I t is presumed that :Ift,rr thr 1iytlr:ttt.s \ w r ( ~agitated for several days the reaction W:IS F cwmpletr and no changes in the compositions of thr n-otild owiir. After equilibrium had kxrn est:Ll)lished thc solutions were drawn through a fine sintered glass filter in :L (7) S.A . Greenberg. Tms J o u ~ r a 58, ~ , 303 (14lil). (8) H. F. W. Taylor A c t a Crysr.. 10, 767 (1957).
8. A. GREENBERG, T . 3. CHANGAND ELAINE ANDERSON
I152
TABLE I EXPERIMEKTAL COSUITIOXS
1 2
0.8:1 1:l
3 5 6
1:l-i 1.2:l 1.5:l 1:I
85 85 85 85 85 186
7
1: 1
1050
4
25 25,40 30 25 25 30
30
... ... used dircetly
... Prepared by autoclaving sample No. 3 Prepared by ignition of sample No. 6
system open to the atmosphere only through a tube containing s mixture of sodium hydroxide on asbest,os(Ascarite) and anhydrous Mg( C104)2 ( Anhpdrone) to remove carbon dioxide. The pH values of the filtrat,es were measured immediately. The filtrates were examined also for calcium ion and silicic acid concentrations. Filtrates were stored in polyethylene bottles a t all times. D. Analytical.-Calcium was determined by titration with Versene.9 Analysis for silicic acid was made colorimetrically on the blue reduced form of the silico-molybdate complex by the method of Bunting.lO E. Equipment .-The pH measuremmts were performed tvith a calomel reference electrode, a glass measuring electrode, and a Leeds and Sorthrup S o . 7664 pH meter. X Beckman DU spectrophotometer was available for transmission measurements. The solutions were equilibrated in a constant temperature water bath held to 10.02O. The X-ray diffraction patterns were made with a Sorelco Diffraction unit using copper K a radiation a t 40 kilovolts and 20 milliamps. Patterns were recorded on a Brown instrument through a goniometer circuit,.
Results Nature of the Silicates.-The structure and properties of the colloidal products were examined in the present study by X-ray difTract,ion methods and water adsorption techniques. The X-ray patt,erns for samples I through 5 (Table I) exhibit~edrelatively strong 3.06 A. spacings and weak 2.81 and 1.83A. spacings characteristic of tehese poorly crystallized hydrated calcium silicates. For samples 6 and 7 the patterns of well crystallized tobermorite and /3-~0llastonite,~respect’ively,were found. The surface areas of samples 1, 2 , 4 and 5 (Table I) were measured by water adsorption. The BET areas’ were determined a t 25” by gravimetric methods.12 The surface areas of t>hesamples with mole ratios of CaO:SiOz of 0.8:1, 1:1, 1.2:l and 15:l were 330, 280, 260, and 275 sq. m. per gram igiiit,ed weight of sample. In a previous paperl3 it was reported that, the nitrogen adsorption surface area of a 1:1 mole ratio hydrate like sample 2 was 54 sq. m. g. Thus nitrogen and water adsorption results differ markedly. These differences have been attrihuted by Kalousekl4 to the penetrat,ion of t’he layers of the cryst,als by water, but not by nitrogen. I
(9) €1. €1. \Villard, N. 11. F u r m a n a n d C. E. BrickPr, “Elements of Qiiantitatii-e .Initlysiu.” 4 t h Ed., D. Van Kostrand Co., Inc., I’iinceton, Y.J.. 195C. (101 JV. E. Bunting, I n d . Eng. Chem., A n d . Ed., 16,612 (1044). (11) 6 . Bnina::tr, P. H. E i ~ n n e t a t n d E. Teller, J. Am. Chem. A’UC.. 60, 309 (1938). (12) Stephcn Rriin:iiicr, D. L. Iiantro and C. €1. IVeise, C a n . J . Chrm., 37, 71-1 (1959). ( 1 3 ) S. A . Greenberg, TIIISJOCRXAL. 61, 373 (1957). (14) G. I,. Iialousek, J . Am. Concrete Inst., 26, 233 (19%).
VOl. 64
However, Brunauer, Kantro and Copeland’s have reported evidence that water does not penetrate the layers and they ascribe the difference to the fact that the hydrated calcium silicate sheets mag be aggregated or rolled into fibers which may cause the formation of regions on the surface which are not available to nitrogen.5 It is apparent that the surface areas of samples 2 , 4 and 5 do not vary markedly from the average of 270 sq. m.jg. OH the other hand sample 1 with a CaO:SiOz mole ratio of 0.8:1, which is the lon-est in this group, shows a surface area of 330 sq. m./g. The free energy states and consequently the solubilities of the samples would be expected to be a function of the surface areas. A sample of well-crystallized hydrate like sample 6 has been reported’ to exhibit a nitrogen adsorption surface area of less than 90 sq. m./g. It is obvious that the presence of such impurities as silica, calcium hydroxide or calcium carbonate would impose additional restrictions on the composition of the solutions, but the equilibrium constants for the hydrated calcium silicates should not be affected since each equilibrium constant is independent of the constants for the other substances. Compositions of the Solutions.-In Fig. 1 and 2 the calcium ion concentrations a t equilibrium are compared with the silicic acid concentrations and pH values of the solutions. It is apparent (Fig. 1) that in general the silica concentration decreases with calcium ion concentration. The pH versus calcium ion concentration curves (Fig. 2) show an increase of pH with calcium concentration. It may be noted in Fig. 1 that the ratios of calcium (Ca), and silicic acid (SiOJT in solution are not equal to the ratios in the solids, a i d that the ratios of (Ca)F to (SiO)%Tincrease with CaO:SiOz mole ratio in the solids and concentrations of calcium in the solutions. Solutions in which the ratios of components are not equal to the ratios of components in the solids are sometimes referred to as incongruent. The removal of calcium from solution by the hydrated calcium silicates is demonstrated in Fig. 3 to increase, in general, with the ratio of SiOLto CaO in the hydrate. It is clear that the 186” and p-ivollastonite samples do not absorb as much calcium as do the other samples. The maximum amount of calcium absorbed by the hydrates increased the CnO:Si02 ratios of the solid samples by approximately 0.02, 0.01, 0.2, and 0.1 for the smiples 1, 2, 4, and 5, respectively. Kinetics Study.-An examination was made of the composition of the solution in a reaction mixture of solid silica and calcium hydroxide solution. This study was made to determine whether or not the composition of the solutions was controlled primarily by the solid hydrated calcium silicate product. A I :1 CaO: SiO? mole ratio mixture of (350 ml. of 0.0127 molar calcium hydroxide and 0.887 g. of S. 1,.silica (81.5V0 SiOJ was placed in a three-necked flask and stirred at a moderate speed. The flask tself was inserted into a constant temperature bath (15) Stephen Brunairer, D L Kantro and 1, 1: Copelmd, J. Am. Chem. Soc.. 80, 761 (1958).
SOLUBILITY PRODUCTS OF HYDRATED CALCIUM SILICATES
Sept., 1960
held at 50 =!= 0.02'. Samples were removed periodically and the calcium ion concentrations, silicic acid concentrations and pH were determined on the filtered solutions. The results are listed in Table 11.
Ixlo-~
0
2
3
4
5
G
3 10 18 25 50 75 100 125 150 200
8.35 8.27 7.Gl 7.03 7.13 6.98 6.42 6.34
2.2'5 2.21 2.57 2.81 2.83 2.9G 1.19 2.02 1.42 2.38
11.G0 11.50 11.45 11.33 11.40 11.38 11.40 11.40 11.39 11.35
2.73 2.26 2.G5 2.73 2.83 3.01 1.13 1.90 1.30 2.07
6.56 7.58 7.19 7.13 7.08 6.52 6.95 6.72 6.80 6.68
Discussion : Equilibria A. Solutions.-When hydrated calcium silicates are brought to equilibrium with aqueous solutions it is necessary to consider several relationships in the solution phases and between the solids and aqueous solutions in order to derive the solubility product~.'6.~~ First, let us consider the solutions. Two equations must be satisfied in solutions of hydrated calcium silicates. One is that of electroneutrality 2(Ca++j
+ (I-[+)=
(OH-j
+ (&SiO4-) + 2(H2Si04--)
(&sio,)+ (&Si04-)
f (I128i04--)
2
4
6
8
IO
Fig. 1.-Soluble silicic acid us. calcium concentration. I
I
1
I
c
=1 R , 1
1 1
0
(1)
Fig. 2.--The
I
I
-
~
3
I
08:I 11 I:I hydrate (l869r p wollastonite
L-__ - 1__-1__--
('5)
which nierely statts that t,he soluble silicic a d (Sic)?) is mcmorneric6 and is composed of three species, t,he mtlissociated form H4Si04and t,he t'wo ioiis HsSi04--mid H2Si04--. It is assumed in this study that the dissolved species are monomeric. By the analyticd procedure used, only the con(witrations of iiioiiomcric species were dctcrniiiiecl. KO polymeric species were noted. B. Solids-Aqueous Solutions.-Oue of the q u i librin hetween the solids and solutions will bc expressed by the eqitations
I 14
12 Colcium Ion Concentration, Moles/L.x IO3.
'
fi
where the quantities in parentheses are in moles/l. Siiice the pII values of t'he solutions are high, the hydrogen icln concentrations may be neglected. The second equat,ion is @ioz) =
1x10-5' 0
15 I hydrate 185") 12 I
B I I I IA J 0 8 I :: I I hydrate (186') E Wollastonite
ct
1
5.S6
coo 5 0 , A
TABLE I1 RESULTS OF KIXETICS STUDYAT 50"
G.CF
,
I
1153
1 - . - - L i
12 15 Calcium Ion Concentration, Moles/L x IO3.
6
9
18
pH of the solutions as a function of Ca++ COIIcentration.
coefficient in each equation. The first :utd s t ~ o i ~ l dissociation coiistniit q, K1 and K;2, for silicic ilcid, H4Si04,may be expressed by the equat ions6
ji~isio;(jII~),f i I ~ s i o i - ( ~ f I ~ a - ) alld ,f1l!si(>i (J~T~ are - ) t,hc activity coefficients; ~ H ~ Fis ~mo ~ sumed to be unity and the quantities in p:~rent~2icses (4 are in moles,:l. The other equilibrium iiis.olvcd will be discussed in The usual procedures for deriving (OH-), the a kiter sectroii. For Eq. 3 and 4 the solubility ionic strength, .fori -,f ~flia-, , (H4Si04), (€LSiO4-), products I
