Role of Calcium Ion on Chloride Binding in Hydrated C3A Pastes Blended with CaCO3 and CaSO4•2H2O

Role of Ca2+ on chloride binding in hydrated C3A pastes blended with CaCO3 and CaSO4•2H2O was investigated by chloride binding capacity, phase assemblages and pH value based on equilibration method. The results show that pH value increases as a result of the chloride binding by hydrated C3A pastes resulting in the formation of chloride-bearing AFm phases and release of OH-. Both of CaCO3 and CaSO4•2H2O leads to the decrease of chloride binding capacity and pH value, and combined incorporation of CaCO3 and CaSO4•2H2O causes significant reduction in chloride binding capacity specially. Reduction in pH value of chloride solutions decreases chloride binding capacity of hydrated C3A pastes by less formation of chloride-bearing AFm phases, while Ca2+ leads to lower pH value but higher chloride binding capacity. The promotion in chloride binding capacity by Ca2+ is more pronounced with incorporation of CaCO3 or CaSO4•2H2O but less notable with both of them, and effect of CaCO3 is more severe. Ca2+ increases chloride binding capacity by promoting the transformation of hydrates in hydrated C3A pastes blended with CaCO3 and CaSO4•2H2O to chloride-bearing AFm, with improvement of AFt<C3AH6<AFm<carboaluminate hydrates. The phase structure of chloride-bearing AFm which are solid solutions is affected. © 2020, Editorial Department of Journal of Building Materials. All right reserved.