Massive amounts of quarry fines are available worldwide. These materials are generally inert, thus limiting their applications. At the same time, shortfalls in conventional supplementary cementitious materials (SCMs) have become an increasing challenge. We can extend SCM usage and reduce filler landfilling by blending these materials together. This paper investigated the proportions of fly ash and pumice that could be replaced with inert basaltic fines without degrading properties significantly. Blended SCM reactivity, and cement paste compressive strength, bulk resistivity, and calcium hydroxide content over 91 days, and alkali-silica reaction mitigation in mortars were investigated. No/minimal synergistic effects between basaltic fines and fly ash or pumice were found. Thus, by measuring initial reactivity or bulk resistivity of end members, an estimate of reactivity, resistivity, and the effects of the blended SCMs on concrete can be established. Replacement of fly ash or pumice by inert materials is likely limited to 40%, depending on the specific applications.