This study aims to investigate the effect of limestone powder (LP) on the reaction of sodium carbonate activated slag. The results show that the incorporated LP up to 30% improves the strength development, especially at advanced curing ages. A slightly accelerated reaction is observed for samples containing low amount of LP (≤5%), while mixture with 10% LP shows the optimized results with respect to the heat release and strength development. Chemical effect of incorporating LP is observed at high replacement levels (≥15%), indicated by the formation of a new phase, natron (Na2CO3·10H2O). Besides, relatively high contents of hydrotalcite-like phases are generated when increasing the dosage of limestone powder. The chemical changes, including the volume changes of generating natron and the transformation of natron to calcite, is potentially responsible for the enhanced mechanical properties.