Comparative Study of Alkali-Activated Natural Pozzolan and Fly Ash Mortars

Abstract

Owing to the considerable CO2 emission resulting from portland cement (PC) production, a great deal of research has been devoted to finding alternative binders, among which alkali-activated binders have shown promising potential. Fly ash and natural Pozzolan are two choices of the starting ingredients for alkali-activated mateials, with the latter having rarely been studied. The current study compares the flow, strength, and transport properties of natural Pozzolan– and fly ash–based alkali-activated mortars. To this end, various mortar mixtures were prepared using fly ash or natural Pozzolan as aluminosilicate source (precursor) and 2.5, 5, 7.5, 1, and 12.5 M sodium hydroxide solution as alkaline activator. Several experiments were conducted on produced mortars to evaluate their fresh and hardened properties including including flow, compressive and flexural strengths, elastic modulus, absorption and total void volume, and rapid chloride migration. These tests were also conducted on PC mortars to compare the performance of alkali-activated mortars to that of PC mortars. The results of this study revealed that natural Pozzolan can be activated and perform to the same level of fly ash. However, the activation rate was slower and the minimum level of alkaline concentration for a proper activation was higher for natural Pozzolan than fly ash. While alkali-activated mortars displayed noticeably lower compressive strength than PC mortars, the studied mortars activated with sodium hydroxide molarities of 7.5–12.5 M produced strength adequate for structural applications. The transport properties of alkali-activated mortars having high NaOH molarity were similar to those of PC mortars.