12
S. A. GREENBERG
by light scattering and the density estimated from viscosity the molecular weight is calculated to be 11.3 X lo6. The check is rather good considering that the first result should be too low and the second too high. The turbidity and the angular scattering pattern of the dilutions of the sample just discussed remained constant within experimental error throughout the period of observation which was about a year. Many similar experiments have been run. Figure 2B shows two other solutions that are as stable as low ratio solutions. Figure 2C shows stability produced by dilution of a solution that was not nearly as stable as low ratio solutions. Figures 2C and D shows results from one solution and its dilutions. The results are typical of many others that were obtained. A large quantity of solution was made; one part was sealed for future use and the other was diluted to different silicate concentrations and to different NaCl concentrations at one silicate concentration. After 74 days the sealed portion was opened, measured, and new dilutions exactly like those made the first day were made again. The stored solution gave essentially the same pH and turbidity as that which had been opened repeatedly. The new dilutions gave higher pH’s than the old ones in all cases. The turbidities were higher for the new dilutions except for those which contained added salt. The turbidity increases were in all cases much less rapid for those solutions that were diluted after appreciable aggregation had occurred. The ones lacking excess salt had essentially stable turbidities. In those that contained added salt the decrease in rate of aggregation was remarkable; compare the two solutions that contain 0.1137 M NaC1, and observe the one that contains 0.2161 J l NaCl;
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its counterpart made on the first day aggregated so rapidly that the data could not be shown on the same scale because it coincided with the ordinate. Many similar experiments were performed. All showed marked initial pH increases which probably result from rapid hydrolysis of the silicate to give some OH- ion prior t o a slower aggregation of the hydrolyzed silicate. Many showed some pH decreases at late stages probably due to minor COZ contamination during the frequent filterings and transfers. In general the results from similar solutions were the same. Higher ratio starting material gave slightly better turbidity stability for a given SiOz concentration. The addition of NaOH reversed the turbidity change in a time very small compared with that required for the increase. This was true for the very low commercial range ratios as well as the high ratio basic solutions and the acid solutions. The addition of NaOH to the basic solutions frequently briefly caused great turbidity or precipitation, but in all cases within one day the solutions had turbidities characteristic of small particle solutions. This work has been concerned primarily with aggregation and particle stability. The kinetic data have not been analyzed and particle sizes have not always been determined. It is concluded that the particles are loosely bound aggregates probably held together primarily by van der Waals forces rather than Si-0% bonds; it is believed that the density and the NaOH reversal data support this conclusion. In general in basic solution the production of a large particle as stable as the low (0-4) ratio range particles requires that one start with as high a ratio as possible (or perhaps remove as much NaCl as possible) and dilute the solution after the particles have grown appreciably.
REACTION BETWEEN SILICA AND CALCIUM HYDROXIDE SOLUTIONS. I. KINETICS I N THE TEMPERATURE RANGE 30 TO 85” BYS. A. GREEN BERG^ Contribution from the Johns-Mandle Research Center, Manville, New Jersey Received August 10, 1960
A study is described of the heterogeneous reaction between silica and calcium hydroxide solutions in the temperature range 30 to 85”. The rates were followed by determining the changes in calcium hydroxide concentrations with time by means of electric resistance measurements. The influences of the amount of silica, the calcium hydroxide concentration, temperature, type of silica, and surface areas of the silica on the rates of reaction were studied. It was concluded that of the more than six processes proceeding in the over-all reaction, the rate determining step was the solution of silica. Since the rate of solution of silica is proportional to the available surface of the silica, the over-all reaction rate is determined by this factor. The influence of aggregation of colloidal silicas on the rate is also discussed.
Introduction The mechanisms of hydrothermal reactionsa are of much interest to geochemists, cement chemists, and chemists concerned with inorganic reactions. The hydration of portland cement and the forma(1) Presented at the Colloid Division Symposium “Colloidal Silica and Siliaates,” a t the 137th Meeting of the American Chemical Society, Cleveland, April, 1960. (2) Portland Cement Assoaiation, Bkokie, Illinois. (3) G. W.Morey and E. Ingerson. Econ. Gsol., 31, 607 (1937).
tion of many minerals proceed by hydrothermal mechanisms. In the present study an example of one kind of heterogeneous reaction of this group will be described. The rates of reaction of several silicas with calcium hydroxide solutions were followed by measuring the conductances of the solutions as a function of time. The influences of the amount of silica, the calcium hydroxide concentration, temperature and type of silica on the rates were examined.
REACTION KINETICS OF SILICAAND CALCIUM HYDROXIDE
Jan., 1961
13
Theoretical for hydrated calcium silicates has been reported Morey and Ingerson* have proposed that hydroK#p = &!a+“UHsSiOc-(6) thermal reactions proceed by crystallization from to be lo-’ a t 25”. This information is useful for solution. It is apparent from the word hydrother- determining the degree of saturation of the solumal that in these reactions water must be present tions with respect to product. and the rates increase with temperature. Experimental In the present’study of the reaction of silica with Materials.-The various binds of silica, the sources, solutions of calcium hydroxide, it is necessary that the silica dissolves so that the reactions in solu- per cent. Sios content, “bound” water contents and nitrotions may proceed. Accordingly, processes 1-6 gen adsorption surface are- are listed in Table I. would be expected. TABLE I 1. Chemisorption of calcium hydroxide by NATURE OF THE SILICAS surface silanol groups. In a previous paper4 it Surface ~~. was shown that the amount of calcium hydroxide area8 chemisorbed could be correlated with the number “Bound” sq. SiOa, water, m./g. of silanol groups on the surface. Silica type Source % % Si01 2. The solution of silica in the aqueous phase. Standard luminescent Mallinckrodt 80.8 7 . 3 750 Silica reacts with water to form a saturated solu- Special bulky Mallinckrodt 84.8 6.0 380 tion of monosilicic acida6 The solubility of the Aerogel Monsanto 93.5 3.0 250 SiO4s)
+ HzO(1) = HSiOd(aq)
(1)
silica increases with pH because of the formation of HISiOd- and H2Si04-- ions.6 Silica has been reported’ to dissolve in sodium hydroxide solutions (to form a monosilicic acid solution) as a function of the surface area S of the silica
Ludox (dried)
Quartz
Du Pont
......
93.0 99.7
2.4 *.
150
