The effect of high salt concentration on the integrity of silica-fume blended cementitious matrices for waste immobilization applications

Silica Fume is a commonly used pozzolanic additive for cementitious matrices used for immobilization of Low Level Waste (LLW). Cementitious systems containing silica-fume are used to reduce the leachability of various hazardous species. However, during the last years several publications have shown that commercially available densified silica-fume (DSF) does not fully disperse within cementitious pastes and concrete mixes, but rather tends to form agglomerated particles which range in size from tens to hundreds of microns. Cementitious matrices containing such agglomerates are prone to the alkali-silica reaction (ASR). As radioactive waste streams often contain high alkali salt concentrations, the occurrence of ASR, deleterious osmotic pressure or other degradation mechanisms in cementitious waste matrices must be considered. The aim of this research was to study the effect of high salt content in DSF bearing pastes on the integrity of the immobilized waste form and its efficiency to immobilize low level radioactive waste. The dependence of matrix integrity on both salt and silica fume concentration is presented.