Influence of NaOH Activator Concentration on Efflorescence and Compressive Strength of Sustainable Mortar with Alkali-activated Slag and Fly ash Binders

The global growth in infrastructure projects exacerbates the need for ordinary Portland cement (OPC) or other similarly effective binder. The construction industry in general and the production of OPC in particular are responsible for significant contributions to CO ₂ emissions into the atmosphere. Ground granulated blast slag (GGBS) and fly ash are industrial byproducts that can be recycled and reused as sustainable alternative binders to OPC to produce concrete. This article evaluated the effect of NaOH activator concertation on the development of 28-day compressive strength of mortar that uses combinations of GGBS and fly ash as binders and activated using Na ₂ SiO ₃ and NaOH. The Na ₂ SiO ₃ content was kept constant while NaOH concentration varied from 6 mol/L to 12 mol/L. Three groups of samples were cured in different environments including: 1) immersion in water, 2) ambient conditions, or 3) 7 days of curing under water then 21 days in ambient conditions. Mortar cured under water produced higher compressive strength when GGBS content exceeds 50% of the total binder content, compared to ambient curing. However, when GGBS content was 50% or less of the total binder, the strength of mortar cured under water was comparable to or lower than those cured in ambient conditions. An optimum NaOH concentration of 10 mol/L produced the highest 28-day compressive in mortar with 75% or 100% GGBS binder. Further increase in NaOH concentration resulted in lower compressive strength than mortar produced with 10 mol/L activator concentration. Efflorescence and strength degradation were manifested in ambient-cured mortar samples with slag binder that was activated using relatively low NaOH concentration. Increasing NaOH concentration beyond 6M decreased or eliminated efflorescence and strength degradation in ambient-cured mortar.