Agro-waste ashes can be used as supplementary cementitious materials in concrete. Although many studies have been reported on the use of rice husk ash as a pozzolan, only limited studies are attempted on the use of rice-straw ash in concrete. The composition of rice straw and the influence of thermochemical processing on the amorphous nature and microstructure of rice-straw ash are reported in this study. Moreover, compressive strength, water penetration test and rapid chloride permeability tests were conducted for rice-straw ash blended concrete. Results from the study indicate that rice-straw ash obtained from pyrolysis temperatures up to 700°C were amorphous and contained reactive silica. From considerations of strength, workability and durability, 10% rice-straw ash is observed to be the optimum level of replacement. Rice-straw ash blended concrete with 20% replacement level exhibited better performance against water penetration and chloride ingress than control concrete. © 2021 American Society of Civil Engineers.