Calcium silicates and their surface layers in aqueous electrolyte solutions

In a survey of Ca silicates and hydrated silicates it is shown that the following compds., some of which are important in portland cement, occur in the CaO-SiO2 phase diagram: modifications of SiO2 with O2-ions in bridging positions; CaSiO3 with endless chains of SiO32-tetrahedrons repeating after 3 units and held together by Ca2+ ions, with a high-temp. form with the tetrahedrons united to Si3O96- units or as a glass; 2 modifications of Ca3Si2O7; 4 modifications of Ca2SiO4 with SiO44- tetrahedrons; Ca3SiO5 with O2- ions, SiO44- tetrahedrons, and Ca2+ ions. Ca3SiO5 and Ca2SiO4 give compds. that include water, with structures derived from b-CaSiO3 in which drifting O2- ions are replaced by OH groups, or when the CaO:SiO2 ratio is >1 with structures derived from Ca2SiO4. When Ca silicates and hydrated silicates are in contact with alk. solns. contg. low Ca2+ concn., they have a neg. surface charge, but higher Ca2+ concns. can reverse the sign. The z-potential is not high (potentials >20 mv. are exceptions). Suspensions coagulate rapidly. The Ca2+ concn. required for zero charge depends on the structure of the solid. Electrokinetic results for hydrated and anhyd. silicates are different, so that the anhyd. form can only be superficially hydrated. Ca3SiO5 and b-Ca2SiO4 are hydrated to a greater extent. [on SciFinder (R)]