Factors Affecting Compressive Strength Development in Alkali-activated Slag Concrete

Abstract

Studies show that the production and use of ordinary Portland cement (OPC) is responsible for more than 5% of the global CO2 emission. Alkali-activated slag (AAS) has been studied for decades as potential alternative to OPC. Engineers are interested in understanding and quantifying the fresh properties, mechanical properties, and durability of concrete made used AAS as sole binder. This paper reviews some of the literature highlighting the factors affecting compressive strength development in AAS slag. These include water-binder-ratio, temperature, humidity, and amount of Na2O, molarity of the alkaline activator solution, and type of alkaline activator. For instance, certain studies reported that AAS concrete activated using sodium silicate solution (water-glass) produce higher 7-day and 28-day compressive strength compared to similar concrete that use 100% OPC, concrete activated using sodium hydroxide (NaOH), or concrete activated using potassium hydroxide (KOH). When NaOH is used as activator, mechanical properties of concrete improved with increase in molarity of the alkaline activator solution. Compressive strength of concrete cubes increases with increase in the amount of Na2O supplied by the activator solution but to an optimum beyond which further increase in Na2O results in decrease in compressive strength. However, the optimum amount of Na2O, often reported as percentage of total slag content, varied from one study to another, probably dependent on the type of activator used as well as the water-to-binder (w/b) ratio.