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Quantitative evaluation of interfacial transition zone of sustainable concrete with recycled and steel slag as aggregate

Utilizing steel slag and recycled concrete as fine aggregate could save the consumption of natural resource and reduce the environmental pollution. In this work, the effect of steel slag aggregate and recycled aggregate on the compressive strength and chloride migration coefficient of three types of concretes were investigated and compared with natural aggregate. The interfacial transition zone (ITZ) microstructure and the connection between aggregate and bulk cement matrix was determined by using a quantitative backscattered electron (BSE) image analysis, N2 adsorption–desorption and X‐ray diffraction analysis. Results showed that the compressive strength and chloride resistance ability of natural aggregate concrete (NAC) and steel slag aggregate concrete (SAC) was highly dependent on aggregate volume content. The performance of recycled aggregate concrete (RAC) were much poorer than that of SAC and NAC. Pore structure and phase analysis confirmed that the improved properties of SAC can be partially attributed to the densified ITZ. Regarding RAC, a porous ITZ was observed in BSE image and pore structure analysis, which caused a poor compressive strength and a large chloride migration coefficient. It was found that the improvement of chloride resistance ability by using supplementary cementitious materials in NAC was more remarkable than that in SAC.