Pre-treated municipal solid waste bottom ash as alkali-activated mortar precursor

Abstract

Pre-treated incinerator bottom ash was used as a precursor of alkali-activated mortars. Control mortars were produced using coal fly ash. The methodology consisted of testing different combinations of the alkaline activator, in terms of sodium oxide/binder (4%, 6%, 8%, 10%, 15%) and silica/sodium oxide ratios (0, 0.5, 1.0, 1.5, 2.0), and understanding the influence on the mortar's mechanical performance. The materials were evaluated in their fresh (density and workability) and hardened state (dry density, compressive and flexural strengths, and modulus of elasticity) after 7, 28, 91 and 182 days. The first stage corresponded to the precursor's pre-treatment in a NaOH solution to prevent the expansion of fresh specimens due to the reaction of aluminium with the alkaline activator. The pre-treatment was effective in preventing excessive hydrogen gas generation during setting, leading to dimensional stability and improved strength. Different optimum sodium oxide contents were observed after 28 days (i.e. 15% vs. 8% for fly and bottom ash, respectively). The results of a simplified life cycle assessment showed that such mixes exhibit lower overall CO2 emissions (‘Global Warming Potential’ category), when compared with cement-based counterparts. However, this trend reverses with increasing content of sodium hydroxide and sodium silicate.