Chloride binding behavior and mechanism of phosphoaluminate cement clinker and its hydration products

Abstract

As one of potential materials for extending the lifetime of reinforced concrete serving in chloride-rich environments, phosphoaluminate cement (PAC) offers excellent chloride binding capacity and superior resistance to chloride penetration. The chloride binding behavior as well as binding routes and mechanisms of the clinker and its hydration products had not been clarified so far. This study revealed that the clinker would preferentially form Friedel's salts through a dissolution-precipitation route in NaCl solutions, thereby binding chloride. This dissolution-precipitation process led to an accumulation of Al3+ in solutions, which negatively affected the subsequent hydration reactivity of the clinker. In the hydration products, the calcium hydroaluminates, mainly as C2(A,P)H8 with minor C(A,P)H10, could bind chloride rapidly via an ion exchange mechanism. Through the ion-exchange route, the chemical binding of calcium hydroaluminates to chloride could be completed in less than 1 hour. In contrast, the unhydrated clinker exhibited a slow chloride binding behavior until 72 hours. All these would provide theoretical guidance for using PAC cementitious materials in reinforced concrete to enhance resistance to chloride attack.