The construction industry's quest for sustainability has heightened interest in alternative construction materials, particularly the adoption of fine recycled concrete aggregates (FRCA) as a viable substitute for natural sand in concrete and mortar. The carbonation of FRCA, attributed to the adhered cement paste, has demonstrated the ability to enhance the properties of fine RCA, thereby broadening its applications. This study delves into an underexplored research area concerning carbonated FRCA, specifically examining their influence on the long-term sulfate and acid resistance of mortar. This inquiry stems from concerns about potential thaumasite sulfate attack, which could arise due to the presence of carbonates resulting from the carbonation process of fine RCA, coupled with moisture exposure. In this investigation, carbonated FRCA serves as a complete replacement for natural sand in mortar mixes, subjected to magnesium sulfate solution and hydrochloric acid. Furthermore, the study evaluates the sulfate resistance of carbonated FRCA mortar with a reduced water/cement ratio and explores the inclusion of ground granulated blastfurnace slag (GGBS) as a partial replacement for Portland cement. Additionally, XRD analysis of samples extracted from deteriorated specimen surfaces provides insights into their mineralogical compositions. This research offers a good understanding of long-term performance implications. By contributing to discussions on sustainable construction practices, it underscores the necessity for further exploration in this promising field.