Physical and chemical chloride binding characteristics of the hydration products for phosphoaluminate cement

Phosphoaluminate cement (PAC) has the potential to prepare the anti-corrosive coatings for rebar due to with high compactness and resistance to chloride. Whereas, the reports of the binding type, its ratio and binding capacity of PAC for chloride ions are not found up to now. Thus, the chloride binding performance of PAC hydration products, as well as the binding isotherms and kinetic models, were investigated in this work. The experimental results showed that the binding capacity of the fully hydrated sample reached 21.03 mg/g in 0.56 mol/L NaCl solution. Up to 80 % of this total was non-water-soluble chloride, dominated by the chemical binding of calcium hydroaluminates (i.e., C(A,P)H10 and C2(A,P)H8). The water-soluble chloride at 20 ℃ accounted for only 20 % and was mainly adsorbed physically by Al(OH)3 and Fe(OH)3 gels. The whole chloride binding process of hydration products was well described using Freundlich isotherm and Elovich kinetic models. Its non-water-soluble bound chloride was more suitable for D-R isotherm and Elovich kinetic models, while the water-soluble bound chloride was more applicable to Freundlich isotherm and PSO kinetic models. These data will offer theoretical support for the rational use of PAC in constructions exposed to high chloride environments, such as the ocean.