Abstract This study collected four ground granulated blast furnace slags from different manufacturing locations for CaO-activation to investigate the influence of the slag characteristics on strength development and the reaction products. Despite the seemingly similar characteristics of the slags, each slag developed significantly different strengths varying from 25 to 52 MPa at 28 days and different reaction products. The main reaction products were C-S-H and calcium hydroxide in all the samples; however, various forms of AFm phases appeared depending on the presence of calcite and calcium sulfates in the raw slags. Ettringite was also found in the case of raw slag with calcium sulfates. In this study, strength development was not governed by any single dominant material parameter of the raw slag, but rather by the combination of various favorable factors such as a higher content of calcium sulfates, a smaller overall particle size distribution and a higher basicity (or chemical indexes).