The effects of Na2O/SiO2 molar ratio, curing temperature and age on compressive strength, morphology and microstructure of alkali-activated fly ash-based geopolymers

Abstract Fly ashes (FA) are byproducts of electricity production from mineral coal in thermoelectric power plants. The pozzolanic properties of FA have been utilized in various applications, including structural concrete, yet the large part of FA is still discarded into the environment. To promote greater FA usage, this study aims to produce a dense matrix, with mechanical properties satisfactory for civil engineering projects, from alkali-activated fly ash-based geopolymers. Three variables were studied: the Na2O/SiO2 molar ratio (N/S 0.20, N/S 0.30 and N/S 0.40); curing temperature in the first 24 h (50, 65 and 80 °C); and age (1, 7, 28, 91 and 180 days). For this study, alkali-activated fly ash pastes and mortars were prepared. In pastes, morphology was studied using scanning electron microscopy (SEM/EDS) and microstructural properties with X-ray Diffraction (XRD) analysis. Mortars were evaluated according to their mechanical performance measured using compression strength tests. Compression strength results were analysed using ANOVA. The results show that the N/S molar ratio plays an important role in the mechanical and morphological characteristics of geopolymers. The mortars prepared with a N/S 0.40 molar ratio had the greatest compression strength. The analysis of paste morphology revealed that N/S 0.40 pastes had a denser appearance, which is in agreement with results of compression strength tests.