Natural Carbonation of Alkali-Activated Fly Ash and Slag Pastes

The carbonation-induced degradation in alkali-activated materials (AAMs) is unknown. This is due to the current level of understanding of the carbonation mechanism in accelerated conditions, whereas the results from the long-term performance of AAMs in service are not available. In this paper, the natural laboratory carbonation of alkali-activated fly ash (FA) and blast furnace slag (BFS)-based pastes was studied. The aim of experiments was to investigate the influence of the binder composition on the carbonation mechanism. The microscope techniques, i.e. polarization, fluorescent and Environmental Scanning Electron Microscope (ESEM) enabled the identification of the carbonation front and local defects due to carbonation, such as microcracks, and pore structure changes. The pH of the simulated pore solution in carbonated and noncarbonated samples was analyzed. The pore structures after carbonation were characterized in terms of capillary and gel pores by Mercury Intrusion Porosimetry (MIP) and nitrogen sorption. The effect of the change in the porosity on the mechanical properties was examined by Nanoindentation tests. Results show that the natural carbonation did not reduce the alkalinity of the pore solution to below the pH 9. The pH was kept above 10.5 in all the mixtures. The samples with 50 wt% and less BFS content, were uniformly carbonated. The increase of the total porosity in those samples has been attributed to the simultaneous effect of the carbonation and drying shrinkage of the pastes. The change in the porosity for a consequence has a reduction in the modulus of elasticity. The samples containing more than 50 wt% of BFS were fully resistant to carbonation or carbonation was induced along the cracks as observed from the microscopic analysis.