Characterising wood-derived biomass fly ash and options for its use in concrete

Wood-derived biomass fly ash (BFA) was investigated as an alternative to, or in combination with, coal fly ash (CFA) to partially replace Portland cement in concrete. Four different BFA samples were physically and chemically characterised. The colour of the BFAs was within the typical range for or slightly darker than CFA, with fineness similar to or noticeably coarser than this. The BFAs had various particle types and sizes, along with a greater surface area and particle density than the CFA. The BFAs also had high calcium and sulfate contents and low silica, alumina and iron contents, with limited amorphous phases. Alkali and chloride levels indicate that these may need to be considered with regard to concrete durability, although Le Chatelier tests suggested little implication for volume stability. While similar early mortar strengths were obtained for the BFA and CFA mixes, in some cases, by 90 days those with CFA had the highest strengths, suggesting little pozzolanicity for BFA. Their high alkalinity and soluble mineral levels suggest potential as a pozzolanic activator, which was confirmed for CFA and BFA combinations with Portland cement in mortar and concrete. With further work, this approach could provide a possible route to address availability issues for the two materials.