Mechanism of blended steel slag mortar with CO2 curing exposed to sulfate attack

•Durability of CO2 cured blended steel slag exposed to sulfate attack was studied.•The values of corrosion resistance coefficient K were calculated every 30 days.•XRD, FT-IR, TGA, MIP and SEM were done to analyze the variation of microstructures.•Mineral phases before and after erosion were quantified by QXRD.•CO2 curing improved resistance to Na2SO4 of steel slag by reducing Ca(OH)2 content. Steel slag is a by-product of the steel manufacturing process and can be used as a filler material in cementitious composites. Past researches show CO2 curing solves the problem of poor stability of steel slag and results in good mechanical behavior but their durability characteristics are still unknown. This paper aims to evaluate the performance of CO2 cured blended steel slag exposed to 3 wt% Na2SO4 solutions for up to 120 days. As a comparison, the conventionally cured ordinary Portland cement (OPC) specimens were prepared and taken the same exposure method. The resistance ability of the specimens was evaluated by visual observation, corrosion resistance coefficient K and microstructural analysis. CO2 cured blended steel slag showed better sulfate resistance ability in terms of K values and visual observation compared with OPC for 120 days. The microstructural analysis revealed that much less expansive erosion products ettringite and gypsum were formed in CO2 cured blended steel slag specimens than OPC specimens until 120 days. The small amounts of erosion products in CO2 cured blended steel slag specimens resulted in a decrease of porosity, a denser microstructure than before and increment of strength. However, the accumulation of large amounts of erosion products in internal pores of OPC specimens led to an increase of porosity, a looser microstructure, and degradation. Thus the CO2 cured blended steel slag has good efficiency in sulfate resistance and can be adopted in the construction industry to attain its benefits.