The role of hydrotalcite in the chloride binding of hydrated C3A pastes containing dolomite

Abstract

To better understand the limitations of the hydrotalcite formation and the capacity of hydrotalcite to bind chloride in dolomite-containing Portland cement-based materials, the reaction of dolomite in C3A pastes and the corresponding chloride binding behavior were investigated in this study. The phase assemblages, the chloride binding capacity, and the pH value were determined. The results indicate that hydrotalcite can only form in a limited range of (active magnesium)/(available aluminum) ratios. The dissolution of dolomite, which provides the magnesium activity, rather than the dedolomitization reaction, appears to be what drives the dolomite reaction with C3A and is primarily dependent on dolomite content. The hydrotalcite formation is not affected by Ca(OH)2. Hydrotalcite seems to possess a greater capacity to bind chloride than monocarboaluminate in hydrated C3A pastes, especially at high chloride concentrations. Chemical chloride binding contributes approximately 10% to the total chloride binding capacity of hydrotalcite. The remaining dolomite reduces chloride binding by increasing carbonate ion activity, and Ca(OH)2 reacts with dolomite to decrease carbonate ion activity and thereby promote chloride binding.