Chloride desorption mechanisms of cement pastes containing fly ash

Abstract

The chemical and physical bonds between cement hydrates and chlorides in concrete can immobilize ingressed chlorides and slow free chloride transport in the concrete pore solution. However, the disassociation of bound chlorides due to reduced pH is an unfavorable mechanism because it increases the concentration of free chlorides and the risk of corrosion in reinforced concrete structures. This study investigated the binding and desorption of chlorides in ordinary portland cement (OPC) and blended paste samples containing 15 % and 30 % fly ash. Paste samples were exposed to various concentrations of NaCl, CaCl2, and MgCl2 solutions to assess their chloride binding capacities. Subsequently, they were exposed to various volumes of 1 M nitric acid to study the influence of the pH of the exposure solution on chloride desorption. The results showed that incorporating 15 % and 30 % fly ash increased the chloride binding capacity of the paste samples by 1.76 and 2.16 times compared to OPC. The disassociation of bound chlorides was influenced by the fly ash replacement level, salt cation, and pH of the exposure solution. Increased fly ash replacement levels inhibited chloride desorption and led to the retention of more bound chlorides when the pH decreased.