ASTM (American Standard Testing and Materials) C 618 prohibits use of biomass fly ash in concrete. This document systematically investigates the strength, microscopic study and durability (mitigation of Alkali Silica Reaction (ASR) expansion) of biomass fly ash concrete (cement partially replaced by fly ash) and kinetics of the mixture of biomass fly ash and calcium hydroxide. The biomass fly ash in the investigation comes from cofired (herbaceous with coal) fly ash, which includes different kinds of coal and biomass. All the results show that biomass fly ash with cofiring concentration within the interest to commercial coal-biomass cofiring operations at power plants have equal or much better performances than those of coal fly ash; therefore, the exclusion of biomass fly ash in concrete by ASTM C 618 seems inappropriate and more quality research on the applicability of biomass fly ash in concrete should be conducted. (C) 2007 Elsevier B.V. All rights reserved.