Dissolution-precipitation behaviour of ettringite, monosulfate, and calcium silicate hydrate

The stability of the cement minerals ettringite, monosulfate, and calcium silicate hydrate (C-S-H) was investigated to better understand the uptake of contaminants in waste-cement mixes. Suspensions were spiked with radioisotopes of components (Ca-45 and (SO4)-S-35 for ettringite and monosulfate and Ca-45 and Si-32 for C-S-H) to observe their uptake behaviour within 0-70 days. A physical model was applied to determine dissolution-precipitation rates. An initial fast uptake was observed to occur in most systems, so the data obtained between 7 and 70 days were chosen for analysis. Dissolution-precipitation rates were in the range of 10(-11.5) to 10(-12.2) mol m(-2)s(-1) for all minerals. The whole solids would be dissolved and reprecipitated within 1-4 years. The measured dissolution-precipitation rates of pure cement minerals give the maximum rate for ion substitution processes with contaminants and are distinguishable from faster processes such as surface complexation and ion exchange processes. (C) 2003 Elsevier Ltd. All rights reserved.