The main objective of this work was to evaluate over time the development of the compressive strength of alkali-activated fly ash mortars. A combined solution of NaOH and Ca(OH)2 was used as alkaline activator. The curing of the samples was carried out in two distinct ways. In the first one, samples were kept at 70°C during the first 12 hours after mixing and at 22°C afterwards, until the age of testing. Although compressive strengths of about 20 MPa were achieved during the first 24 hours, the strength started to decrease after 7 days. Considering this behaviour, a second method of curing was adopted. The mix proportions were the same as before, however the mortar samples were kept in an oven at 70°C until the age of testing. In this case, the compressive strength values decreased more significantly and in a shorter period of time, as compared to the results obtained for samples cured using the first method. Complementary studies were performed in order to better understand the observed reduction in compressive strength. Microstructural analyses of the fly ash, before and after the alkali-activation, have been carried out using Scanning Electron Microscopy (SEM) and Energy-Dispersive Spectroscopy (EDS). It was possible to identify three different morphologies in the alkali-activated pastes: one composed by regions with dense aspect; other showing partially solubilised particles of fly ash; and a third showing the formation of products with a needle-like shape. The information provided by the SEM/EDS experiments was not enough to enable the complete understanding of the mechanism of the deleterious reaction that took place among the alkaline activators and the fly ash. Therefore, future studies should address this question by using additional analysis tools in order to clarify this deleterious mechanism.
Fly ash; alkali-activation; waste
