Effect of sodium hydroxide concentration on strength and microstructure of alkali-activated natural pozzolan and limestone powder mortar

Abstract

The present study evaluates the effect of sodium hydroxide concentrations on strength, bond properties, reaction products and the microstructures of binary blended alkali-activated volcanic natural pozzolan and limestone powder (AANL) mortar. Six AANL mortar were prepared with alkaline activators of 4–14 M NaOH(aq) and Na2SiO3(aq) such that the activators ratio was 1:1. Curing was done at 75 °C for 24 h duration. The maximum 90-day compressive strength of 31.3 MPa was achieved with 10 M NaOH(aq) while 76% of the strength could be achieved within three days when 10 M NaOH was used. The 28-day tensile strength of 2.32 MPa was achieved with 14 M NaOH(aq). XRD characterizations revealed that activation with a strong concentration of sodium hydroxide (10 M NaOH(aq)) led to the formation of anorthite (CaAl2Si2O8) and gehlenite (Ca0.Al2O3SiO2). SEM + EDX indicated that strong alkali (10 M NaOH) enhanced microstructural density compared to that of mild alkali (4 M NaOH(aq)).