A first approach to the rheological behaviour of pastes made of fly ashes and concentrated alkaline solutions has been carried out. As in Portland cement pastes, alkaline fly ash systems can be described by means of the Bingham model. Additionally, the treatment of the rheological data has provided information on the kinetics of reaction of these systems at very early stages. The iron content in the fly ash might play a significant role in determining the reaction rate. © 2005 Thomas Telford Ltd.