Abstract The ASTM C311 strength activity index test is a potentially flawed test because inert materials are known to be able to pass the test. The early age of testing, low fly ash replacement levels, variable water-to-cementitious materials ratio, and low strength limits all contribute to the inability of the test to successfully distinguish inert and reactive materials. Test modifications utilizing higher replacement levels (50% by mass) and higher temperatures (50 °C) are evaluated in this study for three fly ashes and two inert materials. Bulk resistivity is also measured in addition to strength. Using higher replacement levels and higher temperatures provides improved differentiation of inert and reactive materials when using strength, however, the magnitude of differences is somewhat small, when considering variability in strength measurements. On the other hand, higher replacements and especially higher temperatures result in a very substantial increase in bulk resistivity for the fly ashes but not for inert materials. Therefore, a bulk resistivity index test carried out at higher temperatures provides for clear differentiation of inert and reactive materials and should considered for standardization/specification. The calcium hydroxide contents and heat release measured in corresponding cement pastes show that increased temperature enhances fly ash reaction but does not affect inert materials. At higher replacement levels, greater effects of fly ash reaction are manifested, resulting in better differentiation from inert materials than at lower replacement levels.