Water absorption characteristics and rate of strength development of mortar with slag-based alkali-activated binder and 25% fly ash replacement

Abstract

This article presents some of the results of an experimental study that evaluated water absorption characteristics and rate of strength development of geopolymer mortar where the binder consists of alkali-activated slag or slag with 25% fly ash replacement. Samples were cured in air at 22 ± 2 °C and 70% RH. From 1 to 7 days of curing, the increase in compressive strength was higher in mortar with 75%slag + 25% fly ash (S75F25) compared to samples with 100 slag binder (S100). After one day, fly ash begins to develop more N-A-S-H gel and contributes noticeably to strength development. Furthermore, the percentage change in strength development decreased continuously from 7 to 28 days of curing and became negative from 28 to 90 days. Ultimately, the 90-day strength decreased below the 28-day strength in all geopolymer samples. The decrease in strength may be due to loss of moisture causing precipitation of reactants rather than the formation of polymerization products. It was found that a strong linear relationship exists between water absorption of geopolymer mortar samples and the square root of immersion time (t0.5). Water absorption in S100 mortars decreased with an increase in the alkaline activator ratio of Na2SiO3/NaOH. This is because the free silicates from Na2SiO3 react with calcium dissolving from GBS and develop calcium-based hydrate C-(A)-S-H gel, which is typically dense and compact. When S100 mortar samples were cured for 90 days, the relationship between water absorption and t0.5 deviated from linearity, which indicates a coarser pore system and/or cracking, and is consistent with an observed decrease in compressive strength. Water absorption of S75F25 mortar exhibits a stronger correlation with t0.5 and a lower initial rate of water absorption, compared to S100, indicating a finer pore system and lower water absorption. Compressive strength at NaOH molarities of 10 mol/L was higher for S75F25 compared to S100. There is a strong correlation between the initial rate of water absorption and the compressive strength of geopolymer mortar.