Releases of bound chlorides from chloride-admixed plain and blended cement pastes subjected to sulfate attacks

This paper reports results of a study conducted to evaluate the releases of bound chlorides from pastes made of ordinary Portland cement alone and a mixture of ordinary Portland cement and the partial replacements of cement with 10% silica fume (SF), 30% pulverised fly ash(PFA) and 50% ground granulated blastfurnace (GGBS) subjected to Na2SO4, K2SO4 and MgSO4 attacks. The pastes were prepared by mixing water, cementitious materials and sodium chloride considering three levels of water/cementitious material ratios and four levels of sodium chloride. Powder samples collected by triturating the cement paste specimens were immersed in 5.0% Na2SO4, K2SO4 and MgSO4 solutions, respectively for allowing releases of bound chlorides. The chloride concentrations and pH values of the leachates obtained by filtering the solutions were measured, and the chemical phases of insoluble powders in the solutions were analysed by X-ray diffractometer (XRD). It has been found that the bound chlorides are partially released by sulphate attacks to form free chlorides. The released chlorides are affected by the water–binder ratio, total chloride content, mineral admixtures and sulphates with different associated cation type. After the sulphate attacks, Friedel’s salt has transformed to ettringite, which contributes to the releases of bound chlorides. Besides, by means of the index of Cl−/OH−, the influences of various mineral admixtures and sulphate types on the corrosion risk of steel reinforcement have also been discussed.