Ability of Hardened Paste of Cementitious Calcium–Aluminophosphate Mineral to Bind with Chloride Ions

Abstract

Phosphoaluminate cement (PAC) is a green cementing material with wide application prospects. Its main mineral compositions are calcium aluminate (CA), tricalcium phosphate (C3P), and calcium aluminophosphate (CAP). In this study, calcium aluminophosphate mineral was prepared in the laboratory, and its binding with chloride ions was investigated as a separate binder. The microstructure and the hydration and hardening process of the CAP paste were observed by X-ray computed tomography (XCT) and scanning electron microscopy (SEM), and the phase analysis of the hydration products before and after being soaked in sodium chloride solution was conducted using X-ray diffraction. The mechanism for the binding of the hardened CAP paste with chloride ions was also studied. The results show that the Freundlich isotherm well models the adsorption of chloride ions onto the hydration products. The binding of chloride ions with the CAP is chiefly attributed to the chemical bonding of the hydrated calcium aluminate with the ions, which is different from that with portland cement, largely caused by the physical adsorption of them onto the C─ S─ H gel.