The stability of bound chlorides in cement paste with sulfate attack

This paper presents an experimental investigation on the stability of bound chlorides in chloride-contaminated cement pastes with and without FA/GGBS when subjected to Na2SO4 and MgSO4 attack. It is shown that bound chlorides were released in the chloride-contaminated pastes when exposed to Na2SO4 or MgSO4 solution. This is mainly attributed to the decomposition of Friedel's salt (FS), where Cl− bound in FS is replaced by SO42−. However there were fewer released chlorides found in the pastes exposed to MgSO4 solution than in those exposed to Na2SO4 solution. This is partly due to the low pH in the pore solution and partly due to the blocking effect of brucite on ionic transport caused by MgSO4. The inclusion of FA/GGBS in concrete can increase the decomposition of FS and thus the release of bound chlorides. However, it also resists the penetration of Na2SO4 and thus reduces the attack of Na2SO4.