Early hydration of C3S in the presence of Cd2+, Pb2+ and Cr3+ and the immobilization of heavy metals in pastes

The effects of Cd2+, Pb2+ and Cr3+ with various dosage on early hydration of C3S and their immobilization in pastes were investigated by the micro calorimeter, XRD, 29Si MAS NMR, FE-SEM, EDS mapping and XPS. Results show that Cd2+/Pb2+-doping shortens the induction period of C3S hydration. The effect of Cr3+ on early hydration of C3S varies with its dosage, lower dosage Cr3+-doping accelerates the early hydration of C3S, whereas, higher dosage Cr3+-doping leads to the formation of new well crystalline Cr-rich phase and exhibits severe retardation effects. 29Si NMR data confirm that, high dosage Cr3+ greatly hinders the formation of CSH and polymerization of the silicate tetrahedra in the structure of CSH. This study also revealed that Cr was more easily immobilized in CSH than that in CH, and more Cr was immobilized in outer CSH.