Mechanism of solidification of simulated borate liquid wastes with sodium silicate activated slag cements

In this study the mineral matrix based on hydrous sodium metasilicate (NSH5) activated slag cement (AASC) was found to be suitable for solidification of borate solutions with pH 8.5-10.5 and concentration up to 200 g/L. Parameters such as setting times and compressive strength of the waste forms based on AASC and borate wastes can be influenced by the ratio of NSH5 and H3BO3 content. The dosage of 7% Na2O in the alkali activator per slag provides acceptable setting times and 28-day compressive strength of the waste forms in the range 49.7-56.1 MPa depending on pH of the borate solutions. Lowering the pH of borate solutions results in a reduced rate of setting of the fresh AASC paste, retardation in the structural formation of the hardened AASC paste, a reduced degree of hydration, and a reduction in the amount of calcium silicate hydrates and hydrotalcite. The product of AASC-based mineral matrix and simulated borate wastes interaction is ulexite (NaCaB5O6(OH)6(H2O)5).