The use of biomass for power generation is increasingly common. However, one of the most important problems when using biomass is the amount of waste produced in the combustion process such as ash, which must be transported to landfills for deposition. Biomass bottom ash may contain a large amount of organic matter which is the most restrictive property for reusing this by-product. The potential reuse is determined by chemical and physical properties. The presence of light particles and organic matter confer to biomass bottom ash certain physical and chemical characteristics which significantly reduce the possibility of reuse. Previous studies have demonstrated the possibility of using biomass bottom ash in cement-treated materials, but its properties imply a decrease of compressive and flexural strength. Therefore, this paper reported the study of the mechanical and durability properties of mortars containing biomass bottom ash applying different processing methods. Cement was replaced with different rates of substitution of biomass bottom ash and several samples were treated applying different techniques, such as grinding, elimination of lightweight particles and combustion. The results showed significant improvements in the mechanical and durability properties of mortars in which some treatment were applied. Lightweight particle extraction and organic matter removal by burning considerably improved the mechanical behaviour of the manufactured mortars. Therefore, mortars with crushed biomass bottom ash without floating particles and without organic matter led to a 10% of compressive strength decrease with respect to the control mortar. Thus, this study displays a possibility of waste valorisation by means of to the reuse of this type of by-product in mortars and concretes manufacture.