Influence of solid solutions on chloride leaching fromwasteforms

Abstract

The hydration product Friedel's salt (Ca4Al2O6Cl2.10H2O) is frequently encountered in wasteforms solidified and stabilised using Portland cement. The ability of Friedel's salt to form solid solutions plays a role in chloride binding in such matrices. This paper addresses one practical example of this influence by examining the immobilisation of chloride ions from municipal solid waste incinerator fly ash in cementitious matrices also comprising Portland cement and fly ash from coal-fired power generation. Measurements of quantities and structural parameters of Friedel's salt in pastes containing incinerator fly ashes show that chloride ion release is lowest when large quantities of chloride-deficient Friedel's salt is generated. However, chloride is more tightly bound elsewhere in the material, probably in calcium silicate hydrate (CSH) gel. The dissolution of Friedel's salt is modelled using a geochemical modelling program capable of dealing with solid solutions. These observations reinforce the need to incorporate variable phase composition in models for predicting leaching from cementitious matrices containing waste materials.