HYDRATION OF TRICALCIUM SILICATE
553
The Hydration of Tricalcium Silicate
by S. A. Greenberg’ and T. N. Chang Portland Cement Association, Skokie, Illinois
(Received September 11, 1964
I n this investigation of the hydration of tricalciuni silicate, the solutions were analyzed as a function of time for the concentrations of calcium and silicic acid. The electrical conductivities and pH values were also measured. The compositions of the solutions were examined as functions of the concentration of the tricalcium silicate and stirring speed. The rates of formation of hydrated calcium silicate from solutions of monosilicic acid and calcium ions were followed by light-scattering measurements. I n the latter experiments the rate was examined as a function of the concentrations of reactants and of the pH values of the aqueous phases. The results substantiated the solution theory for hydration. The ions in crystalline tricalcium silicate, Ca+2, O-z, and Si04-4, hydrolyzed during the solution reactions. When the concentrations of calcium and HzSi04-zions in solution are high, crystallization of hydrated calcium silicates occurs with the surfaces of the tricalcium silicate acting as nuclei. After the reactant surfaces are covered with hydration product, the reaction rate decreases. The light-scattering experiments indicate that the solution reaction proceeds by the combination of calciuiii arid HzSi04-2ions.
Introduction The substances tricalciuni silicate (3CaO.SiO2) and a-dicalcium silicate (2Ca0. SiO,) are considered the most important constituents of portland cement. These constituents contribute to the formation of a hydraled calciuni silicate gel which gives concrete its strength. Because it is important for the users of cement i o control the rates of hydration and setting, it is necessary to know more about the mechanisms of hydration and gel formation for these constituents of cement. I n this paper the mechanism of hydration of tricalcium silicate is discussed. Le ChatelierZain the last century proposed that the hydraf ed calciuni silicate and calcium hydroxide products precipitated from the supersaturated solutions produced by tricalciuni silicate. I t is now believed by many investigators that the hydrated calcium silicates imniediately cover the surfaces of the unreacted tricalcium silicate particles. zb After this surface layer is deposited, the reaction rate decreases and becomes dependent upon the rates of diffusion of the reactive species through this layer. Some Russian investigators3 follow the theory of B a i k ~ vwho , ~ proposed that, after an amorphous gel of hydrated calcium silicate forms, the particles in the gel crystallize.
Graham, Spinks, and Thorvaldson5 have conducted an extensive investigation of the hydration of tricalcium silicate. Efforts are being madezbto write quantitative expressions for the kinetics of this heterogeneous reaction. The present study will provide some of the information necessary for a quantitative approach. I n the present study, samples of pure tricalcium silicate were stirred with water, and the compositions of the solutions were examined as a function of time by (1) calciuni ion determinations, (2) analyses for silicic acid contents, (3) pH determinations, and (4)measurements of the electrical conductivities of the solutions. Solutions of nionosilicic acid and calcium ions were mixed, and the rates of crystallization of the calcium silicate
(1) Merchrolab, Inc., Mountain View, Calif. (2) (a) H . Le Chatelier, “Experimental Researches on the Constitution of Hydraulic Mortars,” translated by J. L. Mack, McGraw-Hill Publishing Co., New York, N . Y., 1905; (b) for review see S. Brunsuer and S. A. Greenberg. “Proceedings of the 4th International Symposium on the Chemistry of Cements,” Washington, D. C., 1960. (3) P.A . Kehbinder, “Reports of Symposium on the Chemistry of Cements,” P. P. Budnikov, et al., Ed., State I’ublication of the Literature on Structural Materials, Moscow, 1956, pp. 125-137. ( 4 ) A I . Baikov, Compt. rend., 182, 128 (1926). (5) W. A. G. Graham, J. W. T. Spinks, and T. Thorvaldson, Can. J . Chena., 3 2 , 129 (1954).
Volume 69,Number B February 1966
S.A. GREENBERG ASD T. S.CHANG
554
product were followed by light-scattering measurements. Theoretical Before discussing the results of this investigation, it would be profitable to list the processes involved in a solution mcbchanisin for the hydration of tricalcium silicate: (1) solution of the solid; (2) reaction of calcium ions, hydroxyl ions, silicic acid, and water in solution ; (3) formation of nuclei of hydrated calcium silicate and calcium hydroxide crystals; (4) growth of the nuclei; (5) flocculation and precipitation. The crystals of tricalciuni silicate contain calcium (Ca+2),oxygen (O-Z), and silicate (Si01-4) ions.6 During the solution reactions these ions will hydrate.
+
+ Si04p4(c)+ nHzO(1) = + 4OH-(aq) + H2Si04-2(aq)
3Ca+2(c) OU2(c) 3Caf2(aq)
(1)
The H2Si04-2(aq)ions will hydrolyze to H3Si01-(aq) and H4Si04!aq) species as a function of the dissociation constants of silicic acid H4Si04, the concentrations of silicic acid, and the pH of the solutions.' Very little is known about the formation of hydrated calcium siliclates or calcium hydroxide from solutions. These reactions fall into the general class known as hydrothermal. Hydrothermal reactions are those which proceed i n the presence of liquid water and which have rates that increase with temperature. Jlorey and Ingersons proposed in 1934 that hydrothermal reactions were those i n which products crystallized from aqueous solutions. Hydrated calcium silicates have been reportedzb to precipitate froin solutions of sodium silicate and calcium salts. When the concentrations of the calcium, silicate, and hydroxyl ions exceed the solubility products of hydrated calcium silicate and calcium hydroxide, these products will tend to precipitate.9 Reactions 2 and 3 will occur.
+ 2OH-(aq) = Ca(OH)z(c) Ca+2(aq) + H2Si04-2(aq) = CaH2SiOr(c) Ca+*(aq)
(2)
(3)
Additional calcium hydroxide may enter the hydrated calcium silicate structure by the reaction
+ nCaf2(aq) + 2nOH-(aq)
CaH2Si04(c)
=
CaH2SiOdmCa(OH)z(c) (4) The solubility product for hydrated calcium silicate ") refers to the activity of each ion) is 10-1 (pK,,, = solubility product -log K,,, = 7). ~t 300 the for talcium hydroxide (a
KSp2= aca+?aoH-' The Journal of Physical Chemistry
is 8.25 X (pK,,, = 5.08).'O Therefore, solutions with pK,,, and pK,,, values less than 7 and 5.08 are supersaturated with hydrated calcium silicate or calcium hydroxide, respectively. Those solutions which exhibit pK,, values greater than the equilibrium values are unsaturated. Experimental Materials. A sample of pure tricalcium silicate with a surface area of 4400 cm.2/g. was kindly supplied by Dr. D. Rantro and Mr. C. Weise of this laboratory. This sample has been described." The saniple contained 0.75% free calcium oxide. Evidence for only a trace of dicalciuni silicate was noted in the X-ray pattern. Solutions of calcium hydroxide, calcium nitrate, and sodium silicate were prepared from Baker A.R. grade chemicals. Nallinckrodt standard luminescent grade silica gel was shaken with distilled, boiled water to prepare solutions of nionosilicic acid. Equipment. A Leeds and Sorthrup pH meter, a glass measuring electrode, and a calomel reference electrode were used for obtaining the pH values of the solutions. For measuring the electrical resistances of the solutions, an Industrial Instruments Co. bridge, Model R. C., and a dip conductivity cell with a constant of 2 were employed. The Brice-Phoenix spectrophotometer12 was used for the 135, 90, and 45' scattering measurements. The scattering was perfornied with 4360-A. light. The solutions were filtered through Millipore HA filters, size 0.45 p (Millipore Filter Corp., Bedford,' Mass.). It is possible to make water almost dust-free by this procedure. Procedures A . Reactions of Soluble Species. Solutions of moriosilicic acid or sodium silicate were mixed with solutions of calcium hydroxide or nitrate or mixtures of the latter solutions and placed in a 40 X 40 mm. semioctagonal, light-scattering cell. Light-scattering measurenients were made as a function of time. B. Hydration of Tricalcium Silicate. Solutions were placed in a three-necked flask with a stirrer in one neck; a thermometer, conductivity cell, or tube to withdraw saniples could be inserted in the other two (6) J. D. Bernal, J. R. Jeffery, and H. F. W.Taylor. M a g . Concrete Res., 1 1 , 4 9 (1952). (7) S . A. Greenberg, J . Am. Chem. Soc., 80, 6508 (1958). (8) G. W. LLlorey and E. Ingerson, Econ. Geol., 32, 607 (1937). (9) S. A . Greenberg, T. N. Chang, and E. Anderson, J . Phy8. Chem., 64, 1151 (1960). (10) S. A. Greenberg and L. E. Copeland, ibid., 64, 1057 (1960). ( 1 1) n. L. Kantro, S.Brunnuer, and C. H . Weise. Advances in Chemistry Series, No. 33, itmerican Chemical Society. Washington, D. C . . 1961, p. 199. (12) B. A. Brice, 11. Halwer. and R. Speiser. J . O p t . SOC.Am., 40, 768 (1950).
HYDRATION OF TRICALCIUM SILICATE
555
necks. The flasks were placed in a constant temperature bath a t 30 f 0.02’. The samples were stirred at moderate speeds, and at various reaction times portions of the solutions were withdrawn and filtered through a fine, sintered glass filter in the absence of carbon dioxide. Analyses were made immediately to avoid 1he possibility of precipitation from the supersaturated solutions. Efforts were made to exclude carbon dioxide by making seals in the equipment airtight and by passing nitrogen through the solutions. However, these extra precautions were found to be unnecessary. Analyses of Solutions. The calcium concentrations were determined by a versene titration with Eriochronie Black T as i1idicat0r.I~ The molybdenum blue was employed for the determination of soluble nionosilicic acid, H4Si04. The pH and electrical resistance values of the solutions were also measured. Evaluation of the Solubility Products. I t is possible to evaluate the activities of the calcium, hydroxyl, and HzSi04-2ions from a knowledge of the concentrations of calcium ions and silicic acid, the pH values of the solutions, the Debye-Huckel theory,15 the dissociation constants of silicic acids7and the constants for water. I t is corivenierit to keep in mind two equations when examining the compositions of solutions of hydrated calcium silicates. One is the electroneutrality equation 2(Ca+*)
+ (H+) = (OH-) + (H3Si04-) + 2(H2Si04-2) (7)
where the quantities in parentheses are the concentrations ill nioles/l. The hydrogen ion concentration is negligible in the basic solutions arid can be neglected. The second equation relates the total concentration of silicic acid in solution to the concentration of each species (SiOa)
=
(H4Si04)
+ (H3Si04-) + (H2Si04-2)
(8)
It is assumed that all silicic acid in solution is monomeric. The amount of nionomeric silicic acid in solution can be detected in the presence of colloidal silicic acid in solution.14 The first and second dissociation constants for silicic acid, H4Si04, may be expressed by the equations
quantities in parentheses are the concentrations in moles/l. Since H4Si04 is not ionic, the activity coefficient of this species may be assumed to be unity,’6-1* The negative logarithms of the ionization constants for silicic acid, pK1 and pKz, are 9.7 and 11.7 at 30°, respect i ~ e l y . ~ From the experimental data and solution theory the activities of the hydroxyl, calcium, and silicate ions were evaluated.
Results Solution Reactions. The dependence of reaction rate on calcium concentration was exanlined first. Solutions of sodium silicate, 0.004 M in nionosilicic acid, were mixed with equal volumes of calcium nitrate solutions. In the three experiments, the final concentrations were 0.002 M in nionosilicic acid and 0.0001, 0.0003, and 0.0005 14 in calcium ions. The pH values of the solutions were 12 f 0.1. The light scattered from the solutions was measured a t 135, 90, and 45’ as a function of reaction time. The 90’ scattering results are shown graphically in Figure 1. I t may be noted that the rate of change of scattering increases markedly with calcium concentration. The dissymmetries, 2 = i46/i136, for values greater than 1.2 were corrected for reflection of the primary beam at the exit window of the cell.I9 The dissymmetries of these solutions were in the range 6 f 1, and no correlation between the amount of reaction and reaction time was evident. The rates of reaction were measured also as functions of pH. Solutions of monosilicic acid were mixed with solutions of calcium hydroxide or mixtures of calcium hydroxide and calcium nitrate. The roncentrations of nionosilicic acid and calciuni ions were 0.001 and 0.005 M, respectively. Solutions B, C, and D exhibited pH values of 11.9, 11.7, and 11.4, respectively. To obtain a solution with a higher pH value, a solution of sodium silicate was mixed with a saturated solution of calcium hydroxide. This solution (A) exhibited a pH vaiue of 12. Figure 2 demonstrates the scattering at 90’ of these solutions as a function of reaction time. (13) H. H . Willard, N. H . Furman, and C . E. Bricker, “Elements of Quantitative Analysis,” 4th E d . , D. Van Nostrand Co., Inc.. I’rinceton, N. J., 1956, p. 138.
(10)
(14) W. E. Bunting, Ind. Eng. Chem., Anal. Ed., 16, G12 (1944). (15) P. Debye and E. Hiickel, P h y s i k . Z., 24, 185, 305 (1923). (16) S. A. Greenberg and E. 1’. Price, J . Phys. Chem., 61, 1539 (1957). (17) R. G. Bates, “Electrometric pH Determinations,” John Wiley and Sons, Inc., New York. N. Y., 1954.
where the activity a is the product of the concentration and activity coefficient, f , of each species, and the
(18) H . S. Harned and B. B. Owen, “The Physical Chemistry of Electrolytic Solutions,” 2nd Ed., Iteinhold Publishing Corp., New York, N. Y., 1950, Chapter 15. (19) R. Moore, J . Polymer Sei., 10, 551 (1953)
(9)
K 2 -
a__ H + (H2Si04-2)fH2sio, -2
(HIS~O~-)~H - ~S 104
Volume 69, JYumber 2
February 1966
556
S. A. GREENBERG AND T. N. CHANG
C, and D rise to a peak at 10 to 13 min. Solution A with a pH of 12 did not exhibit this peak. After 30 min. of reaction time, it may be noted that the rate of increase of 90' scattering is a function of pH. The dissyninietry values of the solutions A to D varied considerably with reaction time. Peaks in the dissyninietry between 9.6 and 16.0 were observed a t 10 to 13 niin. of reaction tinie. Solution C exhibited a second peak of 12.2 a t 50 min. of reaction time. After 200 min. of reaction h i e the dissymmetries of the solutions A-D decreased to 4. The peaks in the 90' scattering-reaction time curves (Figure 2) and in the dissyninietry niay be attributed partially to the changes in size of the growing particles, to the anisotropy of hydrated calciuni silicate particles, and to flocculation. The reaction times necessary to form turbid solutions were observed. At pH 12.2, 11.9, 11.7, and 11.4 the times were 3 niin., 1 , 3 , and 20 hr., respectively. Hydration of Tricalcium Silicate. The conipositions of solutions containing 0.25, 1.25,'and 5.0 g. of tricalcium silicate/l. of water were determined as a function of time. I n Tables 1-111 and Figures 3-5 the results are given. I n the tables the following quantities are listed in order: (1) reaction time, (2) and (3) concentrations of calcium and silicic acid, (4) the mole ratio CaO: SiOz in solution, (5) pH, (6) pK,,, for hydrated calcium silicate, and (7) pK,,, for calcium hydroxide.
,005 ,004
0
m
,003 .
e o.ooo3
MCO
C 0.0001 M Ca
,0002
.0001
20
60
40
80
I00
120
I
Reaclion Time,Min
Figure 1. The dependence of the rate of reaction of calcium ions and monosilicic acid in solution on the calcium concentration.
Table I: The Compositions of Solutions at 30' for 0.25 g. of 3CaO,SiOp/l. Reaction time, min.
,002 -
(Ca)t
(SiOS 1
X IO', moles/l.
moles/l.
Ca0:SiOt in soh.
2.11 2.67 2.95 3.02 3.09 3.24 3.05 3.22 3.14 3.05 3.30 3.14 3.08 3.14 3.10 2.95
6.42 8.17 9.00 9.33 9.75 9.75 9.92 8.33 10.83 8.92 10.10 10.00 10.25 10.17 10.17 6.33
3.28 3.26 3.28 3.23 3.17 3.32 3.07 3.86 2.90 3.42 3.27 3.14 3.00 3.09 3.05 4.66
X 104,
PH
PKBN
PK.P,
11.50 11.53 11.57 11.59 11.65 11.60 11.60 11.69 11.69 11.60 11.59 11.65 11.60 11.65 11.60
6.51 6.31 6.22 6.18 6.10 6.14 6.15 6.16 6.06 6.20 6.12 6.11 6.13 6.10 6.13 6.35
7.48 7.33 7.22 7.17 7.04 7.12 7.15 6.95 6.95 7.15 7.14 7.03 7.14 7.03 7.14 7.26
0 iJ.
5
001
-
r 0008 0006
.
0004
-
,0002
PO04
.
*-a c
'
'
4
6
8
0
20 R e a c t on T me
30 40 M n
I 60 BO
00
200
4W
Figure 2. The rate of formation of hydrated calcium silicate from soluble calcium ions and monosilicic acid as a function of pH.
2 5 10 15 30 42 52 62 80 100 124 150 185 223 260 21 hr
Solutions with the same silicic acid and calcium ion concentrations at pH values lower than 11.4 did not exhibit any turbidity in 24 hr. It may be seen in Figure 2 that the 90' scattering curves for solutions B, The Journal of Physical Chemietry
11.60
When 0.25 g. of tricalciuni silicate was placed in 1 1. of water and stirred at a moderate speed, the suspension
HYDRATION OF TRICALCIUM SILICATE
557
Table 11: The Compositions of Solutions at 30' for 1.25 g. of 3CaO,SiOz/l. Reaction time,
(Ca) t
min.
x ~OJ, moles/l.
(SiOz) t X 104, molea/l.
Ca0:SiOz in s o h .
pH
pKsp1
pKapY
2.5 7 18 35 60 94 120 180 285 20 hr.
3.85 4.00 3.94 4.26 4.68 5.01 4.90 5.23 5.23 5.55
11.9 11.6 11.4 19.1 5.43 5.93 2.88 1.80 1.37 0.766
3.23 3.46 3.41 2.23 8.62 8.45 17.0 29.0 38.2 72.5
11.80 11.80 11.85 11.93 12.00 12.10 12.07 11.90 12.07 12.10
5.91 5.91 5.90 5.63 6.13 6.05 6.38 6.61 6.68 6.91
6.66 6.65 6.55 6.37 6.19 5.97 6.04 6.35 6.01 5.93
Table I, column 2, that the calcium concentration of the 0.25-g./l. solution increased in 30 niin. from 2.11 x to 3.09 X lopa mole/l. The data in Table I1 show that the calcium concentration of the 1.25-g./1. solution rose to 5.23 X l o v 3 mole/l. in 180 inin. The calcium concentration of the 5-g./l. solution is shown in to Table 111 to increase gradually from 4.72 X mole/l. in 200 min. 11.0 x C. Silicic Acid Concentrations. The 0.25-g./l. solution showed a smooth, rapid increase in concentration (Figure 3; Table I, column 3) to about 9 X mole/l. in 10 to 15 niin. Figure 3 demonstrates the
A Table 111: The Compositions of Solutions a t 30" for 5 g. of 3CaO.SiOp/l. Reaction
(Ca)t
time,
X IO',
min.
moles/l.
3.5 8 15 25 30 35 40 50 60 70 80 95 110 130 160 200
4.72 5.02 5.22 6.89 7.18 7.14 7.78 7,87 8.37 8.61 8.86 9.45 9.74 10.3 10.3 11.o
(SiOd t
x IO', moles/l.
CaO: Si01 in aoln.
PH
10.5 9.47 8.10 5.83 4.53 3.90 2.93 2.03 2.77 1.33 1.17 1.oo 1.05 0,783 0.767 0,667
4.48 5.30 6.44 11.8 15.9 18.3 26.6 38.8 30.2 64.7 75.7 94.5 92.8 132 134 165
11.86 11.92 12.00 12.10 12.20 12.28 12.30 12.45 12.40 12.42 12.60 12.52 12.55 12.66 12.64 12.76
P K B ~ I ~Kapt
5.88 5.89 5.92 5.97 6.05 6.11 6.21 6.35 6.20 6.51 6.55 6.61 6.58 6.68 6.69 6.73
6.47 6.33 6.15 5.86 5.64 5.48 5.41 5.11 5.19 5.14 4.77 4.91 4.84 4.60 4.64 4.38
became clear in 20 min. Only in the case of the 0.25-g. sample was solution complete. A . p H Values of Solutions. The pH values of the solution containing 0.25 g. of tricalciuni silicate/l. remained almost constant and changed from only 11.50 to 11.60 in 21 hr. (Table I, colunin 5). Table I1 shows that with 1.25 g. of solids/l. the pH values rise smoothly from 11.80 to 12.10 in 94 inin. After 20 hr. the pH was 12.10. An increase in the pH values of the 5-g./l. niixture from 11.86 to 12.76 may be noted i n Table 111. A saturated solution of calcium hydroxide at 30" was reported by Bates, et aZ.,20to exhibit a pH value of 12.29. B . Calcium Concentrations. It may be seen in
0
25
50
100
75 R.2
.," T
n-
I25
150
175
200
Vlr
Figure 3. The silicic acid concentrations as a function of reaction time.
fluctuations in the concentration of the silicic acid in the 1.25-g./1. solution. This experiment was repeated to check the reproducibility of the fluctuations in the silicic acid concentration. Maxima may be seen in this curve at 35 and 94 niin. However, after 35 inin. the concentration gradually decreased to 0.766 X niole/l. a t 20'hr. (Table 11). The rurve in Figure 3 shows a slow decrease in concentration of the 5-g./l. solution froni 10.5 X to 0.667 X mole/l. in 200 niin. (Table 111). A small peak at 60 inin. may be noted. D . Mole Ratio CaO:Si02. The mole ratio of the 0.25-g./l. solution was approximately 3 : 1 (Table I, column 4) over 260 inin. of reaction time. After 21 hr. the ratio rose to 4.66. This indicates that the solid dissolves completely, and only soiiietinie between 260 niin. and 21 hr. does a hydrate product forin. If all the sample had dissolved, the theoretical concentrations would have been 3.3 X mole of calciuni/l. and 1.1 X mole of nionosilicic acid/l. The solutions were found to contain niaxiiiiuni concentrations of 3.3 x (20) R . G . Bates, V. E. Bower, and E. R. Smith, J . Res. S a t l . Bur Std., 56, 305 (1956).
Volume 69, Number 8
F e b r u a r y 1966
S. A. GREENBERG AND T. X. CHANG
558
10-3 mole of calciuin/l. and 1.08 X niole/l. of silicic acid. The differences between the experimental and theoretical values may be attributed to errors in the analysis or to the deviation of the tricalciuni silicate from the theoretical coniposition. The CaO:SiOz iiiole ratios of the 1.25-g./1. solution are listed as a function of time in Table 11. It niay be seen that the ratio increases froin 3.23 to 72.5 in 20 hr. Froni a knowledge of the solution coniposition and the quantity of tricalcium silicate added to the water, it is possible to calculate the mole ratio of CaO : SiOz in the solids. Initially, the solids exhibited a ratio of 3 : 1, but, as the reaction proceeded, the ratio dropped to 2 : 1 in 20 hr. Presumably, under these conditions at complete reaction the ratio would be lower than 2 : 1. An increase i n the CaO:SiOz ratio of the 5-g./1, solution to 165 in 200 min. may be seen in the data listed in Table 111. E . Silicate Solubility Products. The pK,,, values of’ the 0.2d-g./l. solution are given in Figure 4 and Table I, column 6. The product is an indication of the degree of saturation of the solution. The equilibrium pK,,, value is 7.0 A 0.1 for a silicate formed from calcium oxide, silica gel, and water.9 A line corresponding to this value is drawn in Figure 4. Values of pK,,, lower than 7.0 indicate a supersaturated solution. The pK1 values of the 0.25-g./l. solution may be seen to decrease to 6.22 in 10 niin., after which the value reniairied almost constant for 260 min. After 21 hr., the value increased to 6.35. In Figure 4 and Table I1 the pK,,, values for the 1.25-g,/1. solution niay be seen to rise from 5.91 to 6.91 in 20 hr. The pK,,, values of the 5-g./l. solution decreased froin 5.88 to 6.73 in 200 inin. (Table 111, Figure 4). F . CalciunI Hydroxide Solubility Products. In Figure 5 and Table I, column 7, the pK,,, values for calcium hydroxide of the 0.25-g./l. solution may be seen to remain essentially constant at about 7.1 after 10 niin. The 7.1 value denionstrates that this solution is unsaturated I\ ith respect to calcium hydroxide solid, which exhibits a pK,,, of 5.08.1° I t niay be observed in Table I1 that the pK,,, values of the 1.25-g./1. solution decreascb from 6.66 to 6.04 i n 2 hr. However, the pK,,, values do not decrease below the equilibriuiii value even iri 20 hr. The pK,,, data in Table I11 of the 5-g.,4. solution show a decrease from 6.47 to 4.38 in 200 inin. At about 70 niin. the pK,,, value is below the equilibrium value, which demonstrates that the solution is supersaturated with respect to calcium hydroxide. It is interwting to note that the fraction of tricalciuni The Journal of Physical Chemistry
70
-----___________
I
I
I
100 R e o c l m n T me
EOC M n
300
Figure 4. The pK,,, values for hydrated calcium silicate as a function of time.
I25q
x
A
€0
55
R e o c l ~ a n Time , M8n
Figure 5. The change with reaction time in pK,,, values for calcium hydroxide.
silicate that dissolved in 3 niin. decreased with weight of sample per liter of solution in the range 0.25 to 5 g. I n the range 0.25 to 5 g. of sample per liter, the fraction dissolved was reduced from 64 to 7%. The fraction dissolved is defined as the ratio in moles of the calcium in solution at 3 niin. to the moles of calcium in the original tricalciuiii silicate sample. The Paste Experiment. The trend shown in Figure 4 that the rate at which the solution becomes saturated with respect to hydrated calcium silicate increases with the amount of tricalcium silicate was tested again. A paste with a 0.7 : 1 water-tricalciuin silicate ratio by weight was examined. To 70 nil. of water at 13.d0, 100 g. of tricalciuin silicate was added in 15 see. with mild agitation. For 45 sec. the paste was vigorously agitated in a Waring Blendor which brought the tempera-
HYDRATION OF TRICALCIUM SILICATE
ture to 25.5'. After 1.75 niin. the aqueous phase was separated from the solids on a porous glass filter. The solution was immediately analyzed. A pH value of 12.8, a calciuni concentration of 0.032 mole/l., and a silicic acid concentration of 0.667 X mole/l. were found. The K,,, value corresponding to this solution is 1.1 X lo-' (pKap, = 6.96), which is within experimental error the equilibrium value for hydrated calcium silicate. Therefore, we may conclude that the saturation concentration was reached within 2.75 inin. The Electrical Resistance Measurements. The electrical conductivities of stirred suspensions of tricalciuni silicate in water were measured. The concentrations were varied between 0.20 and 1.25 g./l. of tricalcium silicate. I n Figure 6 the results are shown. The specific resistance of each solution may be seen to fall with time and then level off. I n each curve a plateau appears for a few minutes between 10 and 20 niin. It is interesting to note that the curves for 0.5- and 1.25-g. samples level off at the same resistance. This behavior demonstrates that, when more than 0.5 g. of sample/l. is added, the silicate does not dissolve completely. Even the 0.5-g. sample may not have dissolved completely A comparison of the specific resistances of tricalcium silicate solutions after 140 min. of hydration (Figure 7) with those of calcium hydroxide solutions having the same concentration of calciuni illustrates several things. The resistances of the calcium hydroxide solutions are
Figure 6. The change in specific resistance as a function of time.
559
I
900
1
tI
I
2 CC":e"tril
3 5n
4 i'
:a c
5
6
)I
)C-:
,,j#ei, .
Figure 7 . The specific resistances of tricalcium silicate and calcium hydroxide solutions of the same total calcium ion content.
lower than those for the silicate solutions. The calcium hydroxide solutions contain calcium and hydroxyl ions whereas the tricalcium silicate solutions consist of calciuni, hydroxyl, H2Si04-2,and H3Si04- ions. Since the ion conductance of hydroxyl ions a t 2.5' is 198.5 ohm-' ~ 1 1 1 compared .~ with the 35 ohm-' for the ion conductance of H3Si04-,' the silicate solutions should show higher resistances than the calcium hydroxide solutions. This is actually the case, but it was found, also, that the differences in resistances as shown in Figure 7 increased with the amount of tricalciuni silicate added to water. This would indicate that complete solution does not occur with coricentrations of solids greater than about 0.5 g./l. since, otherwise. the curves in Figure 7 would be approximately parallel. The E$ect of Stirring on Rate. This effect was examined in order to obtain some information on the niechanisni of solution of tricalcium silicate. To 1 1. of water, a 0.25-g. sample was added. It will be recalled that this sample will dissolve completely if the mixture is stirred. The curves in Figure 8 demonstrate the results. Curve 1 illustrates the resistance us. time relationship for no stirring, except for an initial 0.5-min. mixing. Curves 2 and 3 show the effects of increased stirring speed. I t mill be noted that the rates of solution or slopes of the curves increase with stirring speed. I t is also apparent that the resistance values at which the curves level off decrease with an increase in stirring speed. I t may be concluded that, unless the dissolving species are removed from the surface, crystallization of the product will proceed there. Volume 69, Sumber 2
February 1966
560
S. A. GREENBERG AND T. N. CHANG
25
50 75 ucui,,or r,me , M,n
100
I25
Figure 8. The effect of stirring speed on the rate of hydration.
X-Ray Adeasurement. Samples of 1.25 g. of tricalcium silicate in 1 1. of water a t 30' were stirred a t a fast rate. After 100 and 200 niin., 500-ml. portions of the solutions were removed and filtered in a carbon dioxide-free box. The solid residues were vacuum dried for 2 weeks Then X-ray patterns of the sample were made. The patterns showed strong lines for tricalcium silicate, a trace of dicalciuni silicate, and evidence for the prcserice of hydrated calcium silicate. No evidence for calciuiii carbonate was detected.
Discussion Mechanism of Reaction. The experiments show that it is possible to dissolve small amounts of tricalciuni silicate completely in water. Therefore, the hydration of the ions in tricalciuni silicate shown in eq. 1 can go to completion under the proper conditions. On the other hand, the results also illustrate that, if the calcium and silicic acid species are riot diluted by being immediately carried into the solution by agitation of the suspension, these species niay combine on the surfaces of the particles to form hydrated calciuiii silicates. Therefore, the surface of the tricalciuni silicate serves as a site for nucleation and growth. The rate a t which the solution approaches equilibrium, with respect to the hydrated calcium silicate product, increases with the weight or surface area of calcium silicate (Figure 4) until it is almost instant aneous with concentrated suspensions. The rate of nucleation is. apparently, relatively slow in the case of Th.e Journal of Physical Chemistry
the 0.25-g. sample which dissolves completely. The solution with this amount of sample begins to reach equilibrium with respect to hydrated calciuin silicate in about 20 lir. Even after 200 min., the 5-g./l. solution was supersaturated with respect to calciuni hydroxide. I t has been fairly well established that supersaturated solutions of calcium hydroxide are relatively stable. 2o Therefore, it niay be concluded from these experiments that calcium hydroxide crystals are slow to form under these conditions. After the initial rapid reaction of the surface of tricalcium silicate with water, further reaction in concentrated mixtures proceeds from a solution which is only slightly supersaturated with respect to the hydrated calcium silicate product but highly supersaturated with respect to calcium hydroxide. This, of course, demonstrates that the rate of crystallization of the hydrated silicate product from solution is much faster than the rate of hydration after the initial surface reaction. The changes in silicic acid concentration with time (Figure 3) demonstrate, also, the processes of reaction. I n the case of the 0.25-g. sample, no precipitation occurs initially. Consequently, the concentration increases steadily with time. The concentration in the 5-g./l. solution shows only a decrease with time, with the exception of a small maximum a t 60 niin. On the other hand, the concentration for the 1.25-g. sample first builds up and then drops sharply. Therefore, one may conclude that, in the presence of sufficient reactant (5-g, sample), crystallization of the product proceeds immediately. However, at intermediate coticentrations (1.25-g. sample) sonie time (30 min.) elapses before the silicic acid concentration is sufficient to cause rapid crystallization. Similar results were found for the solution reaction in the silica-calcium hydroxide solution s y s t e ~ i i . ~ The rate of formation of hydrated calcium silicate from solutions of moriosilicic acid and calcium ions was shown in this investigation to be a function of p H and calcium concentration. It niay be assumed that the reaction proceeds through the coinbinatioii of calciuni and silicate (H2Si04-2)ions (eq. 3). This is based 011the knowledge that, as the pH of the mediuni increases, the relative aniount of coiiipletely dissociated silicic acid species increases.' Therefore, the increase in rate of reaction with pH niay be attributed to the increase in concentration of H2Si0,-2 with pH. It has also been demonstrated in other studies that the hydrated calciuui silicates may be represented by the forinula CaH2SiOl arid that calcium hydroxide iiiay dissolve in this ~ u b s t a n c e . ~
HYDRATION OF TRICALCIGM SILICATE
56 1
Kinetics of Solution. Since 0.25 g. of tricalcium silicate dissolves completely in 1 1. of water, it is possible to study the kinetics of solution by means of electrical resistance iiieasurenients. As a first approximation, the tricalciuni silicate crystals were assumed to be approgimately spherical and isotropic. The rate of solution was not found to be proportional to the surface area alone. The data also did not fit the assumption that solution and precipitation were proceeding siniuli aneously
face, then the rate of solution is controlled by the rate of diffusion of the ions into solut'iori according to the Nernst-Rrunner treatmentz1
where (C3S) is the concentration of unreacted tricalciuni silicate, (C3S)d is the concentration of dissolved tricalciuni silicate, S is the surface area, which is proportional to (C3S)2'3,and kl and kz are the constants for solution and precipitation respectively. If the rate of hydration of the surface of 3CaO.SiO2 is very fast and the hydrated species form on the sur-
Acknowledgments. The authors wish to thank Dr. Stephen Brunauer for his careful reading of the nianuscript and his many helpful suggestions. Thanks are also due to Dr. Paul Seligmann for his solution of the differential equation (eq. 12).
where k is the rate constant, S is the surface area of the tricalciuiii silicate, and (C3S), (C3S),. and (C3S)dare the concentrations in moles/l. of undissolved C3S, of dissolved C3S a t the surface, and of dissolved C3S in bulk solution, respectively. The constant k is a function of the diffusion constants of the ions. However, it was not possible to fit the data to eq. 12.
(21) For discussion see c'. T.'. King, Trans. JV. Y . Acad. Sci., 10, 262 (1948).
Volume 69, Sumber 2
February 1965
