The effects of long-term freezing-thawing on the strength properties and the chemical stability of compound solidified/stabilized lead-contaminated soil.

Solidification/stabilization (S/S) is the prevalent remediation technology for the treatment of heavy metal contaminated soils (HMCS). However, under the stress of complex surrounding environments, S/S effectiveness tends to deteriorate and freezing-thawing is one of the most influential natural forcings. The different proportions of cement, lime, and fly ash were used as the compound curing agents to treat solidified/stabilized HMCS with varying levels of lead contamination. The resulting samples were subjected to up to 180 freeze-thaw cycles (F-T) (1 day per cycle). Unconfined compressive strength (UCS) tests and semi-dynamic leaching tests were performed after F-T to explore the strength evolution of compound solidified/stabilized lead-contaminated soils (Pb-CSCS) and the chemical stability of the lead within. The results show that the F-T duration changes the strength deterioration mechanism of Pb-CSCS under F-T. There has been a shift in the main influencing factor from the promoted curing agent hydration by short-term F-T to the structural damage of the specimen induced by prolonged F-T. The variations in leachate pH, lead leachability, and diffusion ability with progressing F-T revealed a degradation effect of the changes in the physical states of water and crack propagation brought by F-T. These unfavorable changes in soil structure and chemistry reduce the acid resistance of Pb-CSCS. Notably, fly ash and cement facilitate the strength maintenance of Pb-CSCS under long-term F-T conditions. Curing formulations that included both cement and fly ash significantly increased the UCS of treated soils by up to 80.5% (3 F-T) under short-term F-T. In contrast, the curing formulation without fly ash lost 51.8% of its strength after 180 F-T conditions. For lead stabilization, cement and especially lime are favored. The results showed a 25% increase in the total proportion of lime and cement in the curing agent formulation, leading to a 41.4% reduction of lead leaching risk.