Effect of microfibers or SRA on the shrinkage and mechanical properties of alkali activated slag/fly ash-based mortars incorporating recycled fine aggregate

Alkali activated slag and fly ash (AASF) systems present satisfactory mechanical properties compared to portland cement systems; however, the high shrinkage and cracking potential of these binders require preventive strategies. Furthermore, to improve the sustainability of such systems, using recycled fine aggregate (RFA) in concrete or mortar can help reduce the problem of depletion of aggregate reserves and disposal of laboratory concrete wastes. This study aims to evaluate the shrinkage characteristics of AASF mixtures incorporating RFA reinforced with polypropylene (PP) or steel fiber (SF) or containing shrinkage reducing admixture (SRA). In this regard, the compressive and uniaxial tensile strength and drying, autogenous, and restrained shrinkage properties of AASF mortars containing RFA were studied. Furthermore, in order to investigate the effect of RFA, natural sand was replaced with RFA by 20%, and as a result of this replacement, a slight decline in mechanical properties and free shrinkage was observed. The inclusion of microfibers enhanced the tensile strength of mortars, and the utilization of 0.6% microfibers reduced the drying and autogenous shrinkage of mixtures up to 20% and 15%, respectively, while the incorporation of 0.3% of microfibers was insufficient to reduce the free shrinkage. The addition of 1.5 and 3% SRA significantly decreased the drying and autogenous shrinkage up to 28% and 53% compared to the reference mixture, respectively. The restrained shrinkage test results indicated that incorporating microfibers or SRA extended the crack initiation time up to 6.4 and 14.6 times compared to the reference. The stress rate and crack width values of mortars decreased by 74% and 91% with the inclusion of 0.6% microfibers or 3.0% SRA, respectively. The overall test results showed that AASF mixtures incorporating 0.6% SF, 0.6% PP fiber, or 3% SRA exhibited satisfactory results in terms of shrinkage properties without compromising the mechanical performance.