The effects of pH, time, valence, and radius of the activator cation on the reaction products and microstructure of ground granulated iron blast‐furnace slag were studied by thermogravimetry and derivative thermogravimetry, X‐ray diffractometry, scanning electron microscopy, and energy dispersive X‐ray microanalysis. Blast‐furnace slag was activated by alkali‐metal hydroxides of Li, Na, and K (12.34 pH 14.71) and alkaline‐earth hydroxides of Ca, Sr, and Ba(12.47 pH 13.53) using a water/slag ratio of 0.4 and curing for 1 day to 26 months. Reactivity of the slag was more strongly dependent on pH than on time. The reaction products were mainly varieties of C─S─H, (C,M)4AH13, and minor amounts of Ca(OH)2and C2ASH8(strätlingite). The nature of C─S─H was dependent on pH. A 1.2‐nm peak appeared in X‐ray diffractograms only when the activation pH was ⌣14.7. Water was present in the C─S─H in a way similar to water in tobermorite and synthetic C─S─H. Leached Ca from unreacted blast‐furnace slag was present around the glass particles as an amorphous layer which crystallized into Ca(OH)2with time; Mg behaved similarly. The effect of the charge or ionic radius of th