The potential of calcium aluminosilicate (CAS) glasses as supplementary cementitious materials is studied in terms of the development of compressive strength for mortars containing a mixture of portland cement, CAS glass, and limestone. In addition, the impact of internal and external alkali activation of the cementitious systems on the mortar performances is investigated. Internal alkali activation is obtained by adding alkali oxides to the CAS glass system, whereas external alkali activation is realized by hydration of the blended cements containing alkali-free CAS glasses using alkaline solutions. For the internally alkali-activated systems and the alkali-free mortars, higher strengths are achieved in comparison to the reference mortar prepared from plain ordinary portland cement. In contrast, the externally alkali-activated mortars exhibit lower compressive strengths, implying the importance of both the immediate availability of alkali ions in the cementitious system and the increased dissolution rate of the glass particles caused by the network depolymerization. The glasses are also studied by thermal analysis and the results are used to calculate the theoretical CO2 emissions. The lowest embodied CO2 emission is estimated for the blends containing alkali-activated CAS glasses.