Characterisation of products of tricalcium silicate hydration in the presence of heavy metals

The hydration of tricalcium silicate (C₃S) in the presence of heavy metal is very important to cement-based solidification/stabilisation (s/s) of waste. In this work, tricalcium silicate pastes and aqueous suspensions doped with nitrate salts of Zn²⁺, Pb²⁺, Cu²⁺ and Cr³⁺ were examined at different ages by X-ray powder diffraction (XRD), thermal analysis (DTA/TG) and ²⁹Si solid-state magic angle spinning/nuclear magnetic resonance (MAS/NMR). It was found that heavy metal doping accelerated C₃S hydration, even though Zn²⁺ doping exhibited a severe retardation effect at an early period of time of C₃S hydration. Heavy metals retarded the precipitation of portlandite due to the reduction of pH resulted from the hydrolysis of heavy metal ions during C₃S hydration. The contents of portlandite in the control, Cr³⁺-doped, Cu²⁺-doped, Pb²⁺-doped and Zn²⁺-doped C₃S pastes aged 28 days were 16.7, 5.5, 5.5, 5.5, and <0.7%, respectively. Heavy metals co-precipitated with calcium as double hydroxides such as (Ca₂Cr(OH)₇·3H₂O, Ca₂(OH)₄4Cu(OH)₂·2H₂O and CaZn₂(OH)₆·2H₂O). These compounds were identified as crystalline phases in heavy metal doping C₃S suspensions and amorphous phases in heavy metal doping C₃S pastes. ²⁹Si NMR data confirmed that heavy metals promoted the polymerisation of C-S-H gel in 1-year-old of C₃S pastes. The average numbers of Si in C-S-H gel for the Zn²⁺-doped, Cu²⁺-doped, Cr³⁺-doped, control, and Pb²⁺-doped C₃S pastes were 5.86, 5.11, 3.66, 3.62, and 3.52. And the corresponding Ca/Si ratios were 1.36, 1.41, 1.56, 1.57 and 1.56, respectively. This study also revealed that the presence of heavy metal facilitated the formation of calcium carbonate during C₃S hydration process in the presence of carbon dioxide.