Biomass ash as supplementary cementitious materials: Characterization, application, and re-conditioning New types of supplementary cementitious materials (SCM) are in demand, due to the foreseen decrease in the availability of traditional SCM (i.e. coal fly ash and ground granulated blast furnace slags). Hence, this project investigated the potential of using biomass ashes as SCM. The studied ashes came from two different sectors, pulp and paper and energy producers, and from different plants in each case. Both fly and bottom ashes were investigated in terms of chemical composition and their evolution in time, mineralogy, reactivity, and participation in the hydration of cementitious binders. The re-conditioning of the ashes was also explored to limit the presence of undesired components, such as Cl and S. The use of blends of biomass ashes of different types and origins was investigated as well. Finally, mortar bars containing ashes in different proportions were cast to check their mechanical properties. The study revealed that the composition and overall properties of ashes did not change significantly in time (i.e. for different sampling dates), but that big differences could be found between different plants (because of differences in the production processes. Chlorine and sulphur could be washed away easily by simple immersion in water, but high alkali contents remained in some cases. Generally, fly ashes tend to be more reactive than bottom ashes, but exceptions were found Some ashes were found to be hydraulic reactive. Most samples exhibited a high LOI, resulting in some cases in high water absorption and poor hydration of the cement, which resulted in poor mechanical properties. The use of blends of ashes led to a reduction of the spread in reactivity and an increase in the average reactivity. The results showed that sufficient compressive strength could be reached in mortars containing biomass ash.