It is possible to have geopolymeric gel and calcium silicate hydrate (CSH) gel forming simultaneously within a single system. Scanning electron microscopy was employed in studying the morphology and elemental composition of the two phases. The elemental composition within the different phases was consistent. However, CSH gel formed in such system had a significantly lower Ca/Si ratio than the CSH commonly formed from the hydration of ordinary Portland cement. In addition, there were some calcium precipitate along the interface between the CSH and geopolymeric gels. It is suggested that the properties ( e. g., size, elemental composition) of the geopolymeric and CSH gels forming simultaneously, and the reactivity of the calcium precipitates along the interfacial region, will hold the key in reformulating a new generation of concrete that matches the durability of ancient concrete. A chemical mechanism on how the presence of slag in the alkali activation of metakaolin would lead to the formation of both geopolymeric and CSH gels has been proposed. (C) 2003 Kluwer Academic Publishers.