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This research examined the performance and durability of geopolymer mortars with fly ash (FA) and fly ash with recycled concrete powder (RCP) as a partial replacement for fly ash and/or the fine aggregate in the production of sustainable alternative mortars when compared with references based on ordinary portland cement (OPC). Mortar mixes were made by substituting binder and sand with RCP at varying ratios (25%, 50%, and 75%), and the mechanical and durability properties of the mortar mixtures were evaluated. The findings demonstrated that OPC-based mortars exhibited the highest performance, whereas the FA-based system activated with sodium silicate also presented significant strength with improved compactness and long-term stability. In contrast, high RCP content reduced reactivity, which highlights the importance of maintaining balanced binder proportions. There was also the use of geopolymer binders with the replacement of manufactured sand with FA fine aggregate (GFFA), 10–20% of manufactured sand, which resulted in enhanced mechanical performance and sustainability, as well as improved recycling of industrial by-products. Acid resistance tests confirmed the presence of stronger microstructural deterioration, such as gel dissolution and micro-cracking, compared with surface erosion, in influencing the reduction in strength. Furthermore, FA and RCP improved acid resistance capacity by refining microstructure and stabilizing binders. Overall, compressive strength retention proved to be a more reliable indicator of acid durability than mass loss. Therefore, properly designed FA-based mortars provide an effective, durable, and eco-friendly alternative to OPC, suitable for construction applications exposed to aggressive or challenging environments.
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