Study of the system CaO-SiO2-H2O at 30 C and of the reaction of water on the anhydrous calcium silicates

Abstract

A graph has been prepared representing the solubility of silica at 30 C in solutions containing increasing concentrations of lime up to saturation, the solutions being in "equilibrium" with solid phases composed of hydrated combinations of lime and silica. An electrometric and analytical study of both supersaturated and unsaturated solutions of silica at definite concentrations of lime, when correlated with these equilibrium solubility relationships, indicates the existence of definite compounds in the system CaO-Si02-H2 which may be the hydrated calcium salts of orthosilicic acid. Experimental values were obtained for the hydrolysis constants of these salts from which ionization constants for the four steps in the dissociation of orthosilicic acid were calculated. A study of the reaction of water upon the anhydrous calcium silicates (CaO SiO2, 3CaO-2SiO2, 7-2CaO-SiO2, 3-2CaO SiO2, 3CaOSiO2) showed that the products formed depend upon the equilibrium relationships of the system CaO-SiO2-H2O. Of these compounds all but monocalcium silicate (CaO-SiO2) form solutions which precipitate hydrated calcium silicate on standing. Application of the same methods to a study of the reaction of water upon a portland cement indicated that hydrated calcium orthosilicate is formed during the hydration of portland cement. A study of the reaction between diatomaceous silica and lime solution and between silica gel and lime solution is also included.