Compressive Behaviors of High-Strength Geopolymeric Concretes: The Role of Recycled Fine Aggregate

In this study, natural fine aggregates (NFAs) in high-strength fly ash (FA)/ground granulated blast furnace slag (GGBFS)-based geopolymer concretes were both partially and completely replaced by RFAs to prepare geopolymer recycled fine aggregate concrete (GRFC). Herein, the impacts of RFA content (0%, 25%, 50%, 75%, and 100%) on the fresh and hardened performance and microstructural characteristics of a GRFC were investigated. The results indicated that with increasing RFA substitution ratio, the setting time of the GRFC decreases. In addition, the compressive strength and elastic modulus decrease. However, owing to the enhanced adhesion of the geopolymer matrix and recycled aggregate, RFA has a relatively small impact on the compressive strength, with a maximum strength loss of 9.7% at a replacement level of 75%. When the RFA content is less than 75%, the internal structure of the concrete remains relatively compact. The incorporation of RFA in concrete has been found to adversely affect its compressive strength and elastic modulus, while simultaneously increasing its brittleness. The increase in dosage of RFA leads to a reduction in the compressive strength and elastic modulus of concrete, while partial failure occurs when the GRFC constitutes 100% of the RFA. The existing stress–strain model for conventional concrete is recalibrated for the GRFC. Observed by SEM, with increasing RFA, the damage is mainly concentrated at the interface associated with the attached cement. Although the recalibrated model predicts the stress–strain responses of the GRFC reasonably well, an acceptable range of deviation is present when predicting the residual stress due to the relatively high strength and brittle behavior of the GRFC during compression. Through this research, the applicability of RFA is expanded, making it feasible to apply large quantities of this material.