Uniaxial compressive properties and microstructural failure mechanisms of concrete made with alkali-activated slag and rubber aggregates

Alkali-activated slag–rubber aggregate concrete (Asrac) is a novel green building material in which waste rubber particles partially or fully replace natural aggregates, and alkali-activated industrial slag is used as a substitute for traditional cement. However, studies on the axial compressive mechanical properties of Asrac are limited, restricting its structural applications. Therefore, in this study, the effects of rubber as replacements for natural aggregates and steel fibre (SF) content on the axial compressive behaviour of Asrac were investigated. The results showed that a 100% rubber replacement rate reduced the compressive strength by 58.97–80.52%, but significantly enhanced ductility, as evidenced by a 91.49% increase in peak compressive strain. Optimal performance was achieved at a SF dosage of 1–2%, with 1% SF yielding a maximum strength increase of 64.48% owing to effective crack-bridging mechanisms. A modified axial compressive constitutive model based on a previous model work was developed. By incorporating a rubber softening coefficient, the model accurately captured the complex synergistic interactions within the Asrac, with correlation coefficients exceeding 0.90. X-ray diffraction analysis confirmed that the dominant phases were calcium silicate hydrate (29–35°) and quartz (26.5°), while the absence of calcium hydroxide (Ca(OH)₂) indicated that the alkali-activation reaction governed the hydration mechanism.