This study has investigated the activation of reactivity of fly ash in different blended cements such as lime-fly ash, lime-fly ash-slag and Portland fly ash cements. Experimental results have indicated that the addition of Na 2 SO 4 can significantly increase the strength of all these blended cements. The activation effect happens mainly during the first 3 to 7 days. The grinding of fly ash also increases its activity, but is not as effective as the addition of Na 2 SO 4 . On the other hand, the combination of grinding and the addition of Na 2 SO 4 shows a much higher strength than a single activation method. Finally, this paper also discusses the feasibility of the addition of Na 2 SO 4 in different cement and concrete products production processes.