1984 Volume 57 Issue 6 Pages 1591-1595
Kinetic study of the hydrolysis of silicon phosphate, Si3(PO4)4 (hexagonal), was carried out in borax buffer solutions (pH 5–12, temperature 20–50 °C) to investigate how it acts as a hardener for alkali silicate adhesive. The silicon phosphate was found to be hydrolyzed with a resulting release of orthophosphate ion, soluble silicate ions, and colloidal silica under mild conditions; the decomposition rate increased with decreasing the particle size of the phosphate and with increasing the pH and temperature of the buffer solution. From the analysis of the decomposition rate on the basis of the shrinking core model, it was found that the decomposition rate could be adequately expressed by the rate equation based on the mixed control of diffusion through a residual layer and surface chemical reaction. This indicates that a part of the colloidal silica which resulted from the hydrolysis remains on the surface of the silicon phosphate particle and then partially limits the overall rate of the hydrolysis. The dependence of both rate-determining steps for the decomposition rate was quantitatively indicated according to the reaction conditions.
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