Abstract: In order to investigate the effect of CaO in different modes of occurrence and content on ettringite stabilization/solidification of lead-contaminated soil, artificial contaminated soils were solidified using high-alumina cement for AlO2- and gypsum and phosphogypsum for SO42-. All of the additives used include Portland cement or lime, which provide CaO. The pH value, strength of solidified samples and leaching concentration were investigated to understand the influence of different binders on physical-mechanical and leaching properties of ettringite-solidified soils with contamination. The micro-structural characteristics of solidified soils with contamination were also explored. The results show that the ettringite is able to stabilize/solidify heavy metal-contaminated soil. The cementation of calcium silicate hydrate cementing soil particles cannot be replaced by the filling effect of ettringite. Lime with higher CaO content is more beneficial not only due to the formation and stability of ettringite, but also due to the effect of stabilization/solidification of Pb2+-contaminated soil. Solidified samples with cement-contaminated curing agent have higher unconfined compressive strength but slow strength growth in later stage. Gypsum and phosphogypsum have little impact on pH value, unconfined compressive strength and the solidification result of solidified samples. The results of SEM test from microstructure features also indicate that the content of CaO affects the shape and effectiveness of ettringite. Ettringite can be transformed into calcium sulphoaluminate in case of insufficient CaO content. This research advances the on-site treatment technology of heavy metal contaminated sites, and is important in theoretical research and engineering application for remediation of heavy metal contaminated sites.

Key words: lead-contaminated soil, ettringite, stabilization/solidification, CaO content, leaching, unconfined compressive strength