Impact of Alkaline Activators on the Compressive Properties of Steel Slag Mortar

Abstract

This study investigates the effectiveness of sodium hydroxide (NaOH) and sodium silicate (Na₂SiO₃) as alkaline
activators in steel slag-blended mortar, with the dual aims of valorizing industrial waste and reducing the
environmental burden associated with Portland cement. A two-phase experimental programme was implemented.
In Phase 1, steel slag was incorporated at replacement levels of 0%, 5%, 10%, 15%, 20%, and 25% to identify
the optimal dosage. Phase 2 employed this optimal 10% blend and examined the influence of varying NaOH and
Na2SiO3 concentrations (2%, 4%, 6%, 8%, and 10%). Compressive strength was measured at 7, 14, 28, and 56
days. The physico-chemical characteristics of the slag were characterized using X-ray diffraction (XRD) and
scanning electron microscopy (SEM). The findings reveal that although the steel slag possesses moderate or low
pozzolanic reactivity, incorporating 10% slag improved the compressive strength compared to the control mortar,
primarily due to its notable silica (SiO₂) content and filler effects. However, alkaline activation using NaOH and
Na2SiO3 did not produce meaningful strength enhancements over the slag-only mixes within the tested
concentration ranges. The findings suggest that ground steel slag is a sustainable partial cement substitute, but
its limited pozzolanic reactivity constrains the effectiveness of conventional alkali activation.