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

Abstract

The hydration of tricalcium silicate (C 3S) 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 Zn 2+, Pb 2+, Cu 2+ and Cr 3+ 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 C 3S hydration, even though Zn 2+ doping exhibited a severe retardation effect at an early period of time of C 3S hydration. Heavy metals retarded the precipitation of portlandite due to the reduction of pH resulted from the hydrolysis of heavy metal ions during C 3S hydration. The contents of portlandite in the control, Cr 3+-doped, Cu 2+-doped, Pb 2+-doped and Zn 2+-doped C 3S 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 (Ca 2Cr(OH) 7·3H 2O, Ca 2(OH) 44Cu(OH) 2·2H 2O and CaZn 2(OH) 6·2H 2O). These compounds were identified as crystalline phases in heavy metal doping C 3S suspensions and amorphous phases in heavy metal doping C 3S pastes. 29Si NMR data confirmed that heavy metals promoted the polymerisation of C–S–H gel in 1-year-old of C 3S pastes. The average numbers of Si in C–S–H gel for the Zn 2+-doped, Cu 2+-doped, Cr 3+-doped, control, and Pb 2+-doped C 3S 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 C 3S hydration process in the presence of carbon dioxide.