The micro- and nano-structural characteristics of the reaction products of fly ash alkali activation depend, among others, on the curing conditions used: temperature, time and relative humidity. The present study focuses primarily on relative humidity. When the material is cured in air-tight containers, the silicon content of the initial aluminium-rich material gradually increases. This end product is dense and compact. When samples are in direct contact with the atmosphere, however, early age carbonation takes place, resulting in water loss and persistence of a high-aluminium content. This resulting material is granular and develops lower mechanical strength than the paste cured under high RH conditions. The main nano- and micro-structural differences observed in the materials as a result of different curing conditions have been shown through a graphical description of the reactions. (C) 2010 Elsevier Ltd. All rights reserved.