Alkali-activated slag & fly ash as sustainable alternatives to OPC: Sorptivity and strength development characteristics of mortar

Abstract

One approach to decreasing the environmental footprint of the construction industry is to replace ordinary Portland cement (OPC) with recycled slag and fly ash activated using alkaline materials. This article presents the outcomes of an experimental study that evaluated water absorption characteristics and strength development of mortar that uses blends of alkali-activated ASTM class F slag and fly ash binders. Mortar mixes were developed using three binder combinations: 100% slag and no fly ash (S1F0), 75% slag + 25% fly ash (S3F1), and 50% slag + 50% fly ash (S1F1). Slag and fly ash binders were activated using NaOH solution with molarity ranging from 10 mol/L to 16 mol/L mixed with sodium silicate solution. Two sets of samples were created, one set was immersed in an acidic medium after casting, which represented a closed environment, and the second was left in the lab exposed to air until the test day. For mortar cured in a closed system, the highest strength development during the first seven days after casting occurred in S1F0 mortar followed by S3F1, then S1F1. The observation indicates strength development is dominated by the fast reactivity of slag during the first 7 days. The pattern is similar for all NaOH activator concentrations evaluated in this study. Mortar samples with high slag content (S1F0 and S3F1) that were cured in air experienced a decrease in strength during the 28–90 day curing phase as the strength at the age of 90 days decreased relative to the strength at 28 days. However, S1F1 gained strength during the 28–90 day phase when NaOH molarity was the highest (16 mol/L). This is due to the dissolution and activation of fly ash at high solution alkalinity which contributed to the continued increase in strength. A strong linear correlation between the square root of time (t^0.5) and water absorption existed in all activated mortars evaluated in the study, except for mortar samples prepared using NaOH with high concentration and the highest ratio of Na₂SiO₃/NaOH. Therefore, alkali-activated slag-based mortars share similar water capillary absorption characteristics with conventional cement-based mortar. It was found that increasing the NaOH concentration increases the global warming potential (gwp) and that the mix with the least environmental impact was the S1F1 developed using the lowest NaOH concentration of 10 mol/L and 50% fly ash.