The availability of aggregates suitable for construction of concrete structures which can be prone to Alkali Silica Reaction (ASR) is a cause of concern to resource management community and engineering fraternity. Apart from the physical and mechanical performance of an aggregate, other properties of aggregate such as expansion potential due to presence of deleterious chemical constituents depends upon mineral composition of an aggregate, its texture and petrogenesis of source materials. The presence of siliceous aggregates when reacts with alkali present in cements leads to ASR which ultimately can cause cracking in the concrete structures in contact with moisture regularly. The paper investigates the effects of replacing a reactive aggregate from two sources (a) basalt aggregate (Khargone, Madhya Pradesh, India) and (b) sandstone (Mugadovan stone quarry-Bongaigaon, Assam, India) in cement mortar with synthetic sintered flyash lightweight aggregate on alkali silica reaction prone expansion using accelerated mortar bar test as per ASTM C-1260 standard and long term test as per IS: 2386 Part VII & IS: 383. The petrographic analysis and mineralogical examinations of reactive natural aggregates and sintered flyash lightweight aggregate have been done. To study the effect of sintered flyash lightweight aggregate on expansion due to ASR, the reactive aggregate of basalt origin with strained quartz percentage in the range of 15% and undulatory extinction angle between 16° and 18° with an accelerated mortar bar expansion of 0.24% has been replaced with sintered flyash lightweight aggregate in 20, 40 and 50%. Similar studies have been conducted on the reactive aggregate of sandstone origin with strained quartz percentage in the range of 22% and undulatory extinction angle varying from 22° to 24° with an accelerated mortar bar expansion of 0.25%. With the replacement of reactive basalt and reactive sandstone aggregate with 20, 40 and 50% sintered flyash lightweight aggregate, the decrease in expansion observed is around 34%, 58% and 60% respectively for basalt aggregate and 32%, 56% and 60% respectively for sandstone aggregate under accelerated mortar bar test. The findings of expansion results indicates that after the 40% replacement of sintered flyash lightweight aggregate in case of both the basalt and sandstone aggregate not much reduction in expansion potential is observed. The results has also been confirmed with the long term test as per IS: 2386 Part VII. Study indicates that depending upon the alkali silica reactive potential of reactive aggregate, suitable replacement percentage of sintered flyash lightweight aggregate can be one of the ways to counter the problem of alkali silica reaction in concrete structures.