Enhancing compressive strength of cement with biomass and coal ash mixtures: The role of gypsum

This study focuses on the compressive strength of cement hardeners, wherein the proportion of ash used in the binder is increased to approximately 80% to emphasize the compressive strength when biomass and coal ashes are used in the mixture, which has not been extensively studied. The effects of gypsum on compressive strength and hydrate formation were investigated in aggregate-free pastes with varying contents of biomass ash and coal ash. In the case of using only coal ash, this study focused on gypsum, which increased the compressive strength of the hardened body. A negative correlation existed between the compressive strength and pore volume when biomass fly ash was used alone or with coal ash flour, whereas the compressive strength reached a 60% higher value without gypsum. The presence or absence of gypsum affects the formation of hydrates, such as ettringite and hydrocalumite. The use of biomass ash alone or mixing it with coal ash without gypsum resulted in a higher crystallinity of C-S-H owing to the increased degree of polymerization of the Si chains compared to that when using coal ash alone, suggesting the possible formation of more C-S-H. This could be a factor contributing to the higher compressive strength. These findings offer valuable insights for further research into the effective utilization of biomass ash that is disposed as industrial waste. • Use of biomass fly ash as a substitute in cement and fly ash • Gypsum addition to the cement and BFA mixture influences hydrate formation • Compressive strength higher without gypsum when BFA used alone or with FA • Negative correlation between compressive strength and pore volume