The change of phase assemblage and desorption of bound chloride for seawater cement paste under sulfate attack

In this study, the effect of sulfate attack on the change of phase assemblage and desorption of bound chloride for seawater mixed cement paste with different w/b ratios and supplementary cementitious materials (SCMs) is investigated. It is shown that the decrease of w/b ratio from 0.4 to 0.2 and use of SCMs with low aluminum content such as silica fume (SF) mitigates the Na2SO4 attack by reducing the content of corrosion products. After sulfate attack, Friedel's salt has been all transformed to ettringite by the substitution of Cl− with SO42-, which leading to the desorption of chemically bound chloride. The corrosion products formed during Na2SO4 attack are ettringite and gypsum while the gypsum transforms to syngenite in the specimen after K2SO4 attack. In the specimens with MgSO4 attack, the formation of gypsum is promoted accompanied by the formation of brucite. The crystal size of corrosion products after MgSO4 attack is the largest due to the large growth space through the formation of large cracks. Note the fact that the degree of silicon polymerization for C–S–H gel is increased by sulfate attack. The C–S–H gel partly transforms to sodium or potassium silicate-like gel after Na2SO4 and K2SO4 attack while it mostly transforms to M-S-H after MgSO4 attack. This leads to the desorption of physically absorbed chloride. The use of MK, GGBS, and FA improves the stability of physically absorbed chloride while SF contributes to the further release of physically absorbed chloride after sulfate attack.