Insight into multi-ionic adsorption behavior of recycled cement paste exposed to chloride solutions

Abstract

The multi-ionic adsorption behavior of recycled cement paste (RCP) exposed to NaCl and CaCl2 solutions is investigated in this study by comparing with that of the ordinary cement paste (OCP). The adsorption of Na+, K+, Ca2+, and Cl- in RCP and OCP are quantified by ICP-OES and potentiometric titration. RCP has larger adsorption capacity for Na+, K+, and Cl- compared to OCP, which is attributed to a lager specific surface area of RCP. For the multi-ion adsorption of OCP and RCP in both NaCl and CaCl2 solutions, the adsorption amount of minor cation ions are found to be influenced by the concentration of primary cation ion, reflecting a competitive adsorption among cation ions. Besides, the maximum Na+ adsorption amount is proportional to the product of the specific surface area and the percentage of the silanol groups in CSH measured by 29Si NMR spectroscopy. Additionally, molecular dynamics simulation is conducted to investigate the Na+ and Cl- adsorption behavior of CSH in OCP and RCP at a molecular level. The simulation results confirm the higher adsorption capacity of OCP than RCP.