This paper presents some recent research on the microstructural development during alkaline activation of slag pastes. Pastes of different slags activated with a range of activators have been prepared and some preliminary results from the study of the pastes activated with sodium hydroxide and waterglass solutions are presented. X-ray diffraction (XRD), differential thermal analysis (DTA), backscattered electron (BSE) imaging of polished samples in the SEM coupled with X-ray microanalysis, and secondary electron imaging of fracture surfaces were used to study the pastes. This study indicates: (i) that the products form by a dissolution and precipitation mechanism during the early stages of reaction, but at later stages the reaction may continue by a solid state mechanism; (ii) regardless of the activator used, the main hydration product is calcium silicate hydrate with low C S ratio and varying degrees of crystallinity; (iii) a crystalline phase of hydrotalcite type is formed in slag activated with either NaOH or waterglass; (iv) a crystalline phase of AFm type is also formed in slag activated with NaOH; (v) no hydrates of zeolite group or mica group were formed in slag activated with either NaOH or waterglass solution after wet curing at 20 ± 2 °C up to 15 months or at 80 °C for 14 days. © 1995.