In this research, four industrial wastes were used for up to 80% as supplementary cementitious materials (SCMs) in cement mortar systems: ground granulated blast furnace slag, electric arc furnace slag, basic oxygen furnace slag, and waste limestone powder. Quaternary cementitious blends were prepared and studied for up to 120 days. Workability, compressive strength, durability, microstructures, and sustainability studies were performed and compared with Portland cement references. Results showed that more than 30 MPa in compressive strength can be achieved by > 50% replacement with SCMs; only 9% below the reference. Neither H₂SO₄ nor MgSO₄ attacks resulted in critical damages; nevertheless, curing under NaCl solution showed detrimental behavior. C-S–H with a low Ca/Si ratio was identified in the mortars as the main hydration product, possibly intermixed with stratlingite, C-A-S–H and/or hydrotalcite. Environmental impact for the blended cements was determined as the CO₂eq. factor from a simple life cycle assessment. The embodied greenhouse gasses varied in 260.2–541.4 kg CO₂eq./ton of binder depending on the formulation. This was 40–70% less than Portland cement (922.6 kg CO₂eq./ton). The production of the raw materials dominated the polluting emissions, while freight, grinding, and sieving had little environmental impact.