The use of electric arc furnace slag (EAFS) as sole precursor to produce alkali-activated mortars has been experimentally investigated. EAFS, a by-product of the steel recycling industry, is a coarse material with unevenly distributed and size-extensive particles. Milling of EAFS was required to achieve a cement-like sized powder before it could be used as precursor. Different combinations of sodium hydroxide (NaOH) and sodium silicate (Na2SiO3) were used, by varying the Na2O/binder concentration (4%, 6%, 8%, 10%, 12%) and SiO2/Na2O ratio (0, 0.5, 1.0, 1.5, 2.0, 2.5) to maximize the mechanical performance. The alkaline solutions were prepared 24 h prior to mixing to unify temperatures for all mixes. The results showed that the SiO2/Na2O ratio and strength development are directly proportional. The maximum 28-day compressive strength obtained, after being subjected to an initial 24 h thermal curing at 80 °C, was 9.1 MPa in mixes with 4% Na2O/binder and 2.5 SiO2/Na2O. However, after an additional 28 days of accelerated carbonation, the maximum compressive strength (i.e., 31 MPa compared to 3.9 MPa in uncarbonated mixes, corresponding to an 800% increase) was obtained in mixes with 12% and 1.0 for Na2O/binder and SiO2/Na2O, respectively, thus showing an alteration in the optimal alkaline activator contents.

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