Multiscale Investigation of Modified Recycled Aggregate Concrete on Sulfate Attack Resistance.

This study investigated the sulfate resistance of modified recycled aggregate concrete (RAC) by applying carbonation and nano-silica soaking methodologies. Recycled concrete aggregates (RCA) derived from concretes of C30 and C60 strength grades were subjected to these modification techniques and subsequently utilized in the fabrication of RAC specimens. The results show notable porosity and crack density within the interfacial transition zone (ITZ) interfacing recycled aggregate and cement paste in recycled aggregate concrete (RAC). Specifically, the porosity within the ITZ of RAC is observed to be up to 30% higher than that of virgin aggregate concrete. These pathways facilitate the penetration of sulfate ions, subsequently inducing deterioration and resulting in a compression strength reduction of up to 40%. While carbonation treatment exhibits a moderate enhancement in sulfate resistance, decreasing the sulfate penetration depth by 15%, the incorporation of 2% nano-silica by weight of cement proves significantly more effective. This addition reduces the sulfate penetration depth by over 30% and lowers the sulfate concentration by 25%. Furthermore, the compressive strength of RAC modified with nano-silica increases by 15% following 28 days of sulfate exposure. Additionally, a 30% reduction in the sulfate ion mass equilibrium depth is observed in nano-silica-modified RAC, accompanied by a markedly lower sulfate concentration in the pore solution. After 56 days of sulfate attack, the compressive strength of nano-silica-modified RAC retains 85% of its initial value, whereas unmodified RAC decreases to 70%. Notably, the quality of recycled aggregate significantly impacts sulfate resistance, with high-strength RCA (exceeding 40 MPa) demonstrating superior resistance compared to low-strength RCA (below 20 MPa). Consequently, RAC produced with high-strength RCA experiences only a 20% loss in compressive strength under sulfate attack, whereas RAC containing low-strength RCA suffers a 40% loss. The novelty of this study is the effective use of nano-silica soaking and carbonation to enhance the sulfate resistance and compressive strength of recycled aggregate concrete originated from both normal and high-strength reference concrete.