Abstract Deformed rock aggregate from a quarry within the Clinton-Newbury fault zone in northeasterm Massachussetts has been linked to deleterious alkali-silica reactivity (ASR) in precast concrete. Deformation is evidenced by bent and broken crystals, reduction in grain size (cataclasis) and the development of foliation (schistosity). Microcrystalline quartz, produced from the dynamic recrystallization of strained quartz, is a primary petrographic feature. Accelerated mortar bar tests (ASTM P214) were made with aggregate obtained from this quarry. Subsequent to experiments, petrographic examination was carried out on mortar bar thin sections impregnated with UV-active fluorescent dye. Mortar bars containing metamorphosed basic dike rock showed minimal expansions (< 0.1% at 14 days), thereby suggesting innocuous behavior. In contrast, mortar bars prepared from other rock types had expansions implying significant ASR. Mortar bars consisting of deformed granitic rocks had 14 day expansions between 0.20% and 0.25%; petrographic examination showed that the major porosity of the aggregate was along grain boundaries of microcrystalline quartz. The most expansive samples (>0.4% expansion at 14 days) have a prominent foliation (schistosity) in addition to evidence of cataclasis. Alkali-aggregate reaction is attributed to the enhanced ingress of alkali solutions along foliated layers and with the grain boundaries of microcrystalline quartz.