Cement is the most energy-consuming component of concrete, leading to a high CO2 release during its fabrication. Therefore, a reduction of the cement amount used in concrete mixtures would be beneficial from an environmental point of view. One way to do this is by replacing cement with a suitable material; in this research, recycled concrete fines obtained from crushed concrete (RCF) is used to replace part of the cement in new mortar recipes. RCF subjected to various thermal treatments were used to replace part of the cement in the standard mortar samples containing either OPC or slag- or fly-ash blended cements. Both untreated and thermally-treated RCF were characterized in terms of density, PSD, composition (XRF and XRD, as well as SEM), calorimetric behavior and mechanical strength when added into mortars. Mortar samples were tested and the results show that RCFs can be a beneficial addition, especially in the case of blended cements.