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

Abstract

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 Na 2 SO 4 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 SO 4 2- , which leading to the desorption of chemically bound chloride. The corrosion products formed during Na 2 SO 4 attack are ettringite and gypsum while the gypsum transforms to syngenite in the specimen after K 2 SO 4 attack. In the specimens with MgSO 4 attack, the formation of gypsum is promoted accompanied by the formation of brucite. The crystal size of corrosion products after MgSO 4 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 Na 2 SO 4 and K 2 SO 4 attack while it mostly transforms to M-S-H after MgSO 4 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.

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