Mechanical Properties and Lifecycle Assessment of a Green Alkali-Activated Mortar Based on Biomass Wood Ash

Portland cement (PC) is the most commonly used binder material for producing concrete. Nonetheless, increasing concerns have been attached to its manufacture which is highly energy-intensive and generates a large quantity of greenhouse gases. Developing alkali-activated materials as eco-binders is a sustainable replacement for PC and many investigations have been reported successfully utilizing industrial wastes as precursors. However, owing to the low reactivity, studies regarding biomass wood ashes (BWA) are still limited. To produce a green cementless alkali-activated mortar material, in this study, biomass fuel by-products – biomass wood bottom ash and biomass wood fly ash – were binarily used as precursors. Sodium hydroxide NaOH at 10 mol/L and calcium hydroxide Ca(OH)₂ at 20 % by binder mass were applied as alkali activators. Recycled sand, substituting natural sand, was adopted as fine aggregate with an aggregate/binder ratio of 2 to reduce the consumption of non-renewable natural resources. Compressive and flexural strength were tested to evaluate the mechanical performance. A cradle-to-gate lifecycle assessment was conducted to analyse the environmental impacts. The results reveal that the alkali-activated mortar has less environmental impact compared to the traditional PC mortar. NaOH solution is the primary source of environmental influence and BWA only contributes to very limited impacts. The usage of Ca(OH)₂ effectively improves the mechanical strength and compared to NaOH, it leads to decreased energy demand, requires fewer preparation steps and is less dangerous for operation.