AbstractSupplementary cementitious materials (SCMs) are beneficial for improving concrete’s long-term performance, durability, and environmental impact. Impure clay sources are abundant and when calcined, may serve as suitable SCMs. This study presents and evaluates a method for enrichment of low-purity kaolinite clays to improve their reactivity and performance in concrete. A byproduct clay containing 27% kaolinite, 10% muscovite, and 52% quartz was obtained by drying aggregate wash water slurry from a commercial sand and gravel pit. Several methods were evaluated for separating the clay and nonclay minerals. Among others, dispersant-assisted sedimentation, using 0.06% sodium hexametaphosphate, was shown to be effective in enriching the clay content which, upon calcination, produced a purified calcined clay (PCC) containing 64% metakaolinite, 26% muscovite, and 2% quartz. Both the PCC and the nonpurified calcined clay (CC) were evaluated for their physical and chemical properties, compliance with ASTM C618, pozzolanic reactivity, and their performance in mortar and concrete mixtures. The results show both SCMs to be valuable, whereas PCC is clearly a higher performance pozzolan.