Characterisation of Bottom Ashes from Non-Woody Biomass Combustion for Application as Sustainable Supplementary Cementitious Material

Cement production is an energy- and resource-intensive industry accounting for approximately 7% of global carbon dioxide emissions. Therefore, a key decarbonisation option for the cement industry is to substitute the clinker with so-called supplementary cementitious materials (SCMs). Due to its properties and availability, the bottom ash from the biomass combustion process could be suitable as an SCM. Several agricultural residues were collected and analysed. The materials were applied for ashing experiments in a lab-scale muffle furnace, which was operated at different temperatures. The chemical, physical, and mineralogical characterisation of the ashes produced was carried out. In addition, the reactivity of the cementitious paste made from the ashes was measured through lab-scale experiments. The influence of the different ashing temperatures and the additive mixing on the properties of the ashes and cementitious paste was analysed. The results show that the spelt husk ash is the most promising biomass ash, with its high silica content and high pozzolanic reactivity. The bound water of the cementitious paste made from spelt husk ash reaches 7.3 g/100 g paste at 700 °C but decreases to 2.5 g/100 g paste at 900 °C due to the formation of a crystalline structure. Nevertheless, the addition of kaolin to the spelt husk can maintain the reactivity of the spelt husk ash produced at high ashing temperatures by stabilising the amorphous structure in the ash.