Influence of Recycled Ceramic and Concrete Fine Aggregates on the Mechanical Properties and Freeze–Thaw Resistance of Low-Carbon Cement Mortars

The reuse of construction and demolition waste in cementitious materials supports the development of sustainable low-carbon composites and circular economy strategies. This study investigated the influence of recycled ceramic fine aggregate (RCerFA) and recycled concrete fine aggregate (RConFA) on the mechanical properties, freeze–thaw durability, pozzolanic potential, and environmental performance of cement mortars prepared with different cement types. Mortars containing 20% and 40% replacement of natural sand with recycled aggregates were evaluated through strength testing and freeze–thaw resistance assessment, while SEM analysis and pozzolanic potential were assessed on separate mortars in which 25% of the cement binder was replaced with the recycled materials, alongside carbon footprint calculations based on global warming potential (GWP), using the recycled materials directly after the crushing process without additional grinding. The results showed that mortars containing recycled concrete fine aggregate generally maintained satisfactory mechanical performance and freeze–thaw resistance, particularly at the 20% replacement level. In contrast, 40% RCerFA reduced mechanical performance and freeze–thaw resistance. Neither recycled material demonstrated confirmed pozzolanic reactivity in its unground state. Although one RConFA mixture exceeded the 75% compressive strength index criterion, this result alone was insufficient to confirm a chemical pozzolanic reaction. Environmental assessment demonstrated that cement type had a greater influence on carbon footprint than recycled aggregate incorporation. The study confirms the potential applicability of recycled fine aggregates in sustainable low-carbon cement mortars and explores their possible use as low-energy supplementary cementitious components.