SUMMARY
This paper reports the effect of local pozzolan on expansion of mortar bar due to local potential reactivity aggregates. The study, based on the accelerated test method, investigated capability of moderate calcium oxide fly ash to control the length change due to chemical reaction between some certain aggregates and alkalis in cement. Since the finding of ASR evidence in Thailand, this information has been essential for possible mitigation approach. Percentage variation of lignite fly ash from 20 to 50 yielded different benefits on expansion control. The highest percentage replacement showed the best result in reducing expansion from 0.17% to 0.03% and from 0.43% to 0.07% at 16 and 30 days after casting, respectively. The findings provided background information that would affect aggregate screening process adopted in the country and the potential mitigating method for local potential reactive aggregate, of greywacke lithology.