Stabilization of soils containing sulfates by using alternative hydraulic binders

Excavation operations during construction produce several tons of soil, which frequently contain high concentrations of sulfates (>0.5 wt%). In accordance with the requirements of the French decree for waste classification for disposal, a soil containing sulfates is classified as “inert and non-hazardous waste” if the leachable sulfate concentration is lower than a mass fraction of 0.1%. To prevent sulfate leaching from excavated soils, solutions for immobilizing sulfates are needed and the associated stabilization mechanisms must be understood. On the other hand, the reuse of soils containing sulfates for civil engineering purposes can lead to significant risks after their treatment with ordinary cementitious binders because of chemical reactions involving sulfates. These reactions can promote the formation of massive ettringite crystals resulting in expansion, cracking and eventually catastrophic damage of materials or structures. For these reasons, the stabilization of soils containing sulfates by adding alternative hydraulic binders is studied in this paper. Several binders were used to treat a sulfate-spiked soil. It was observed that treatment with cementitious binders having high C3A content led to volume expansions greater than 5%, while treatments with binders containing a high fraction of ground granulated blast furnace slag (GGBS) showed volume expansions of less than 5% and about 89% of sulfates were immobilized in the solid matrices. These preliminary results suggest that GGBS binders are effective for the treatment of soils containing sulfates. Moreover, numerical calculations using PHREEQC were compared with experimental results to improve the understanding of sulfate immobilization mechanisms.