Influence of recycled fine aggregate on sulfate attack resistance of alkali‐activated <scp>slag/fly ash</scp> concrete: macro, micro properties and thermodynamic modeling

Abstract The influence of different replacement ratios of recycled fine aggregate (RFA) on the sulfate resistance of alkali‐activated slag/fly ash recycled aggregate concrete (AARAC) was investigated. The incorporation of RFA accelerated the reaction kinetics. At 28 days of curing, the sample with 70% RFA achieved the maximum early‐age strength of 60.2 MPa, representing a 3% increase compared to the reference group. Mercury intrusion porosimetry results indicated that the addition of RFA refined the pore structure, with the porosity reaching a minimum value of 11.38% at a 50% replacement level, which was 32.98% lower than that of the control sample without RFA. However, replacement levels exceeding 50% led to a reduction in matrix density. After sulfate attack, an appropriate amount of RFA promoted the polymerization of the gel network within the AARAC, stabilized its microstructure, and mitigated the deterioration caused by micro‐expansion of ettringite. Thermodynamic modeling further elucidated the evolution of sulfate products following RFA incorporation.