Effect of immersing geopolymer slag-fly ash mortar in sulfuric acid on strength development and stability of mass

Abstract

This article presents the findings of a study aimed at assessing the effect of immersing mortar with alkali-activated ground granulated blast furnace slag and fly ash binders in highly acid solution, on compressive strength development and stability of mass. Effects of slag & fly ash contents on strength development and mass change were evaluated by testing mortars with three groups of binders including 100%slag, 75%slag + 25% fly ash, and 50% slag + 50% fly ash. The precursors were activated using a combination of sodium silicate (SS) and sodium hydroxide (SH) solutions. It was found that immersing samples in 10% sulfuric acid solution, one day after casting, didn’t not cause decrease in strength or mass from 7 days to 90 days, and the pattern was consistent for NaOH concentrations from 10 M to 16 M. This was attributed to presence of water in the sulfuric acid solution which provided a closed curing environment. Similar samples cured by direct exposure to air developed lower compressive strength at the age of 28-days than samples immersed in sulfuric acid solution. At lower solution molarity (NaOH concentration = 10 M), mortars with 75% slag and 25% fly ash binder developed the highest compressive strength of 87 MPa after 28 days of immersion in sulfuric acid solution, compared to 49.85 MPa for samples cured in air, further confirming the positive effect of water as curing medium. The study also found a strong correlation between splitting tensile strength and compressive strength of air-cured mortar when the binder is 100% slag. The relationship weakens as fly ash content is increased from 0% to 50%.

Keywords: Fly ashSlag, Sodium silicate, Sodium hydroxide, Compressive strength, Tensile strength, Geopolymer mortar