29Si MAS-NMR spectroscopy was used to characterize the reaction products resulting from the alkali activation of a type F fly ash. Specifically, analyses focused on the degree of polymerization of the activating solution (SiO2/Na2O ratio = 0.0, 0.19, 0.69 and 1.17) and thermal curing time (from 8 h to 180 days at 85 °C). The results obtained showed that the degree of polymerization of the predominant silica species in the activation solutions plays an important role in the kinetics, structure and composition of the gel initially formed. The alkaline aluminosilicate gel formed exhibits short-range order, with the silicon appearing in a wide variety of Q4(nAl) (n = 0, 1, 2, 3 and 4) environments. In the absence of soluble silica (SiO2/Na2O ratio = 0.0, 0.19), the Al-rich gels initially generated evolved rapidly into zeolites. At higher levels of anion polymerization (SiO2/Na2O ratio ⩾0.5), zeolite crystallization was retarded.