Chloride binding of cement pastes with fly ash exposed to CaCl2 solutions at 5 and 23 °C

Abstract

In cementitious materials exposed to solutions containing chloride, chloride binding typically results from the chemical reactions between chloride ions and aluminate phases to form Friedel's salt, and the interaction between chloride ions and calcium silicate hydrates (CSH). Calcium oxychloride can also form when Ca(OH)2 in cementitious materials reacts with CaCl2 solutions. This paper examines the chloride binding of hydrated cement pastes containing fly ash exposed to CaCl2 solutions of varying concentrations at 5 and 23 °C. Thermogravimetric analysis was used to quantify the chloride binding associated with Friedel's salt and calcium oxychloride. The amount of bound chloride by Friedel's salt is relatively independent of the exposure temperature, and as the chloride concentration [Cl−] increases, it increases until a plateau is reached at [Cl−] greater than 2 M. The addition of fly ash results in an increase in the chloride binding due to Friedel's salt. A lower exposure temperature leads to a greater amount of bound chloride associated with calcium oxychloride. In this study, no chloride binding associated with calcium oxychloride was observed in the cement pastes with 40% and 60% fly ash. The temperature-dependent chloride binding associated with CSH is a significant portion of the total chloride binding (19.8 %–70.8%) when cement pastes are exposed to CaCl2 solutions. As the replacement level of fly ash increases, the chloride binding by CSH increases first and then decreases. The amount of bound chloride by CSH increases linearly as the pH of the exposure solution decreases.