This paper compares three methods for activation of OPC-slag mortars (OSM): (1) prolonged grinding of binders (mechanical method), (2) elevated temperature curing of mortars (thermal method), and (3) use of chemical activators such as NaOH, KOH, and Na 2 SiO 3 , 9.35H 2 O (chemical method). The proper reactivity of OSM was evaluated using a mixture of 50% OPC and 50% slag. Early and ultimate strengths were compared. All three activation methods accelerated both the slag reaction and strength development rates. However, the chemical method did not show a significant effect on the ultimate strength, while thermal activation increased the early strength by 3 days. Mechanical activation increased the early strengths of the mortar significantly, but about 6% strength loss occurred in the ultimate strength. Although, the application of mechanical and thermal activation methods needs extra equipment and energy, due to more significant of strength improvement; based on current test results, it can be said that mechanical activation is the most efficient and feasible method for the activation of OSMs.