Setting time, sulfuric acid resistance, and strength development of alkali-activated mortar with slag & fly ash binders

Abstract

This study evaluated the effect of initial molar ratios SiO2/Al2O3 and (CaO + SiO2)/Al2O3, on the setting time of mortar with alkali-activated Ground Granulated Blast Furnace Slag (GBS) and fly ash (FA) binders. The study also evaluated the chemical resistance of alkali-activated mortar and the effect of NaOH alkaline activator on resistance to sulfuric acid in particular. Mortar samples were cured at 22 ± 2 °C and 70% relative humidity (RH). Three GBS/FA binder combinations were evaluated: 1:1, 3:1, and 1:0 (100% GBS). Activation was done using a combination of sodium hydroxide, with concentrations from 10 mol/L to 16 mol/L, combined with sodium silicate solution. Chemical resistance was evaluated by immersing mortar samples in sulfuric acid. Geopolymer mortar samples cured in an open system exhibited an increase in compressive strength from the age of 1 day–28 days. However, at the age of 90 days, all air-cured mortar samples, irrespective of activator concentration, decreased in strength. Similar mortar specimens immersed in sulfuric acid 24 h after casting exhibited a continuous increase in strength until the age of 90 days. Water evaporates from an open system which may cause partial precipitation of dissolved silicates rather than the formation of geopolymer products. The medium containing the sulfuric acid provided a closed system simulating a physical barrier against evaporation, and making dissolved silicates available to participate in the formation of geopolymerization products. Setting time was found to have a stronger correlation with the initial molar ratio of (CaO + SiO2)/Al2O3, than the molar ratio SiO2/Al2O3. Several mathematical models for predicting splitting tensile strength were developed and the goodness-of-fit to test data was evaluated using the integral absolute error (IAE).