Alkali aggregate reaction is a chemical reaction that occurs between the reactive component of the aggregate and alkali hydroxide from the cement used in concrete or external source. In this study, the accelerated mortar bar tests were performed and their results were compared with each other in order to investigate the ratio of reactive aggregate which causes maximum expansion in mortar called a pessimum ratio in literature. For this purpose, the pessimum ratios of six different reactive aggregates (opal nodule, chert, chalcedony nodule, andesite, ignimbrite and dolomite) were determined. The effect of the particle size on the development of alkali-silica reaction was determined again by the Accelerated mortar bar tests (AMBT). AMBT were carried out with mortar bars of opal, chalcedony and chert aggregates having 4.76 mm to 0.074 mm grain size (0.074 to 4.76 mm). The experimental results showed that the highest expansion occurred in reactive aggregates, with 150- 300 square m grain size. (C) 2015 The Authors. Published by Elsevier B.V.