Nickel retention by calcium silicate hydrate phases: Evaluation of the role of the Ca/Si ratio on adsorption and precipitation processes

The objective of the present study is to investigate Ni retention processes in Calcium Silicate Hydrate (CSH) phases, as the main cement hydration products, and to describe them in a generalized form for the first time. In particular, the effects of the CSH Ca/Si ratio (from 0.8 to 1.6) on the equilibrium water chemistry, Ni speciation, adsorption and solubility have been addressed. Nickel adsorption tends to decrease as the Ca/Si ratio increases, especially when Ca/Si is < 1.2, and then Ca/Si becomes less relevant. The "adsorption " region is limited to low final aqueous Ni concentrations (< 5.10(-9) M), and when the Ni concentration exceeds this limit, an increase in the (apparent) distribution coefficient, K-d, is observed, which is related to precipitation processes. The onset of precipitation depends on Ni solubility but also on the Ca/Si ratio: at the lowest ratio, the lowest Ni concentration occurs when the apparent K-d starts to increase. The existence of Ni-Si phases as surface precipitates could explain the observed experimental behaviour, which is not totally explicable assuming Ni(OH)(2)(s) as the unique phase limiting Ni solubility. Nickel retention on the CSH phases was analysed and interpreted with the help of surface complexation modelling using an electrostatic double layer approach and accounting for the major species present in solution for each CSH. The uncertainties in the available Ni thermodynamic database under alkaline conditions and in the presence of Si are underlined.