Chloride Binding Capacity of Calcium Silicate Hydrates and Friedel’s Salt Influenced by Fly Ash, Slag, and Silica Fume

Abstract

Chloride binding capacity is an important factor affecting the durability of concrete, which is directly related with the hydrates of paste. Therefore, this research investigated the chloride binding capacity of calcium silicate hydrates (C-S-H) and Friedel’s salt (Fs) to demonstrate the impact of three common mineral admixtures on the binding behavior of pastes. The results show that the binding capacity of pastes can be enhanced when the dosage of fly ash (FA), slag (SL), and silica fume (SF) does not exceed 30%, 45%, and 5%, respectively. The reinforcement effect of SL is the most significant; with 30% dosage, for example, the chloride binding capacity can be increased by 32.0%. Moreover, the single incorporation of FA/SL/SF increases the overall content of C-S-H and decreases the chlorides adsorbed in unit mass C-S-H in general, leading to the result that the binding capacity of paste does not develop positively with the content of C-S-H or Fs. It is thus necessary to take the content and chloride binding performance of phases into consideration when explaining the chloride binding capacity of pastes. Furthermore, Fs fixed chlorides per unit mass paste decline with the increase of FA/SF, while SL dosage has limited impact; further, C-S-H adsorbed chloride per unit mass paste first grows and then reduces for the three series pastes. Moreover, when the NaCl solution concentration is 3.0 mol/L, the chloride content combined by C-S-H is significantly higher than that by Fs in all pastes, with the proportion of the former above 75% in total bound chloride.