This paper reports on a study of waste clay brick that was ground and used as a supplementary cementitious material (SCM) in a mortar. The effect of this ground brick, in comparison with other conventional pozzolanic materials (namely, a Class F fly ash, and a natural pozzolan), on the alkali-silica reaction (ASR) of the mortar was evaluated. The ground clay brick, fly ash, and natural pozzolan were introduced into the mortar as a cement replacement at a 20 or 35% level. Fly ash-natural pozzolan and fly ash-ground clay brick combinations were also employed as a cement replacement at a 35% level. These SCM-incorporated mortar samples were tested for ASR expansion, flexural and compressive strength, and rapid chloride permeability. The test results indicate that all the SCMs considered in this study reduced the mortar ASR expansion, rapid chloride permeability, and the ASR-associated strength loss. The finely ground waste clay brick demonstrated high potential to reduce the ASR expansion as well as the ASR-associated strength loss.