Biomass ash waste from agricultural residues: Characterisation, reactivity and potential to develop one-part geopolymer cement

Abstract

Biomass is burned to generate heat and electricity. The use of biomass fuel has acquired great importance in the last years owing to its carbon-neutral nature. As a result, there is a massive build-up of biomass ash that will keep growing for decades to come. This study uses biomass ash waste to fabricate cements for construction and it presents a “zero fossil energy consumption and zero waste generation" approach to cement production. We measure the reactivity of crop biomass ash (CBA) from a power plant and the properties of one-part, CBA-slag geopolymer cements made with it. CBA has pozzolanic reactivity and potential as precursor for geopolymer fabrication. Up to 30% of CBA can be blended with ground granulated blast furnace slag (GGBS) to produce sound, one-part geopolymers activated with sodium carbonate and/or sodium silicate. Beyond this ash threshold, the geopolymer matrix experiences a strength reduction due to insufficient cement content. A one-part geopolymer consisting of 30% CBA and 70% GGBS activated with 10% sodium carbonate achieved a compressive strength of 30.70 MPa at 28 days of curing at ambient temperature. However, it exhibited a prolonged setting, taking up to 5 hours to initially set and 8 hours to finally set. Adding a sodium silicate activator significantly improved mechanical performance and accelerated setting. A formulation activated with 20% sodium silicate and 5% sodium carbonate reached a compressive strength of 46.09 MPa at 28 days, with setting times reduced to 57 minutes for the initial setting and 1 hour 15 minutes for the final setting. The sodium silicate activator enhanced hydrate formation and facilitated polymerization thereby accelerating the set and enhancing mechanical performance. Owing to the low carbon emission factor of CBA, the one-part CBA-GGBS has a low carbon footprint, which can reduce carbon emission by 80% in comparison to PC-GGBS (CEM II) mortar with a similar compressive strength. The CBA-GGBS geopolymer is economically feasible, applicable, and market attractive due to its low cost compared with equivalent PC products.