The escalating concerns surrounding the growing amount of combustion ash derived from biomass residues have been further intensified by the rapid global population growth and the ever-increasing demands for energy. Among various industries, the construction sector stands out as a significant contributor to carbon emissions due to its energy-intensive nature. However, the incorporation of biomass combustion ash as supplementary cementitious materials (SCMs) and aggregates in cementitious materials presents a win-win strategy for both biomass waste management and the optimization of cement composite strength. Furthermore, the utilization of carbon-negative biochar, which is produced from biobased residues under controlled conditions, holds promise as fillers or aggregates in building materials. Despite the potential benefits, research on the application of biowaste ash and biochar as admixtures in construction materials has been scattered and fragmentary. Consequently, this chapter places significant emphasis on the selection of raw feedstocks and production conditions for biomass ash and biochar. Furthermore, this chapter provides a comprehensive overview of their impact on the fresh-state, hardened, and durability properties of cement-based composites. By recycling biomass combustion ash into SCMs and aggregates, not only can the strength of cement composites be augmented, but also valuable contributions can be made toward the mitigation of resource depletion and climate change.