Rice straw ash and sugarcane bagasse ash are two of the most abundantly available biomass ashes in India and can be used as supplementary cementitious materials in concrete. Unlike rice husk ash which is also a by-product of rice cultivation, the research studies on the use of rice straw ash as a pozzolan are highly limited. This study characterizes the microstructure of the as-received and processed rice straw ash in comparison with sugarcane bagasse ash. Moreover, the influence of these ashes on the rheological properties of cementitious system was investigated. The microstructural characterization reveals that both rice straw ash and sugarcane bagasse ash are constituted of dumbbell shaped silica-storage structures called phytoliths. Rice straw ash is found to be richer in phytoliths compared to sugarcane bagasse ash. However, the prismatic structures as seen in sugarcane bagasse ash is noticeably absent in rice straw ash. The unburnt carbon content is attributed to the fibrous network present in the ashes. The yield stress and viscosity of cement paste are found to increase with the addition of rice straw ash and sugarcane bagasse ash. Cements blended with sugarcane bagasse ash is found to have a significantly greater viscosity than rice straw ash blended cements at all levels of replacement. Mechanical processing of the ashes (sieving and grinding) to remove the coarse fibrous fraction is found to improve the rheological properties of cement blended with rice straw ash and sugarcane bagasse ash. The optimum dosage of superplasticizer required for rice straw ash and sugarcane bagasse ash blended cements were found to be 0.6% and 0.4%, respectively. © 2022 Elsevier Ltd