Solidification performance and mechanism of C-S-H gel for Pb(II), Zn(II), and Cd(II)

C-S-H gel is a complex cementitious system, and the properties present significant difference with various Ca/Si ratios. The immobilization mechanism of three heavy metals in C-S-H gel at various Ca/Si ratios was investigated in this work from the prospect of the physical adsorption and chemical immobilization. Results showed that higher Ca/Si ratios can result in an increase in silicon chain polymerization, and lower BET specific area, and C-S-H gel with various Ca/Si ratios showed the excellent immobilization for heavy metals. Pb(II) and Zn(II) can form hydroxyl complex polyhedra in C-S-H under strongly alkaline environments, and increase the silicon chain polymerization degree similar to the doping of C-S-H gel by Al. However, Cd(II) was immobilized in the C-S-H gel with the formation of a solid solution under the action of ion exchange with Ca(II), rather than in the form of hydroxyl complex polyhedral and cadmium hydroxide coprecipitates. The highest adsorption capacity of Pb(II) was realized in C-S-H gel with a Ca/Si molar ratio of 0.9, reaching 95.6006 mg/g. Pb(II) was mainly immobilized in C-S-H gel via physical and chemical adsorption, and the effect of chemical adsorption tended to be significant as the Ca/Si molar ratio increased. And the formation of Pb8Ca(Si2O7)3 can be detected in C-S-H gel with a higher Ca/Si molar ratio.