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