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. Xray 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 2Ok2 "C up to 15 months or at 80 "C for 14 days.