Alkali activation of bottom ash from municipal solid waste incineration: Optimization of NaOH- and Na 2SiO3-based activators

Considerable amounts of municipal solid waste incinerator bottom ashes are produced yearly without an adequate outlet. Given the established reactivity of the ash when subjected to an alkali medium, there is considerable scope to valorize it within the Construction industry in an alkali-activated material, thereby reducing the significant environmental pressure of the cement industry. This study assesses the optimization of alkali-activated mortars, using milled municipal solid waste incinerator bottom ash as a precursor, from mechanical and durability behaviour perspectives, many properties of which have never been analysed. The alkaline solution is comprised of either NaOH or Na2SiO3 (Na2O/precursor ratio varying between 9.1% and 24.2% and SiO2/Na2O ratios of 0 or 1.0); fly ash from a coal-fired power plant was also used as control precursor. X-ray fluorescence and powder X-ray diffraction analyses of cement, fly ash, bottom ash and of the resulting pastes were carried out. All specimens were evaluated in terms of their mechanical and durability-related performance (i.e. flexural and compressive strength, carbonation, water absorption by capillary action, and shrinkage). Considerable generation of H2 gas was observed during production, resulting from the reaction between the bottom ashes’ metallic aluminium fraction and the NaOH solution, leading to significant expansion and an overall increase in porosity and thus a strong decline in performance. Alkali-activated bottom ash specimens presented fast carbonation, resulting in a considerable strength increase. Despite the low performance of most of these mixes, the strength enhancement of specimens subjected to some conditions infers that there is significant scope for further mix design optimization (e.g. aluminium reduction, mixing time, Na2O/precursor and SiO2/Na2O ratios) that can lead to the manufacture of fit-for-industry construction materials.