Monitoring of steel corrosion and cracking in cement paste exposed to combined sulfate–chloride attack with X-ray microtomography

Abstract

The degradation process of reinforced cement paste exposed to cyclic wetting and drying with a designated salt solution was monitored through X-ray microtomography (μCT). Three exposure conditions (a 3 wt% NaCl solution, a 5 wt% Na2SO4 solution, or a mixed solution of 3 wt% NaCl and 5 wt% Na2SO4) were studied to investigate the potential synergistic effects between chloride and sulfate ions on the deterioration of reinforced cement-based materials. The time-dependent evolution of steel corrosion and cracking of cement paste cover were tracked and visualized, while the volumes of steel loss, corrosion products and cracks were quantified by image analysis. The results indicated that the presence of sulfate ions significantly delayed the initiation of chloride-induced corrosion, but did not seem to greatly affect the corrosion rate once the corrosion occurred. The existence of chloride ions, in turn, also mitigated the damage of cement paste cause by sulfate attack as evidenced by the reduced volumes of cracks. The conventional volume expansion coefficient of corrosion products can be measured by μCT, but this coefficient may not be applicable for modelling the corrosion-induced cracking process when sulfate ions were present. The use of μCT provides a new angle to directly measure the interfacial displacement in reinforced cement paste, which can be used as a valuable input for modelling non-uniform chloride-induced corrosion in the presence of sulfate ions.