This study correlates the pore structure and water permeability of paste made of alkali activated fly ash (AAF) with varying silica and alkali content, curing age and curing temperature. The pore structure of AAF paste was examined by mercury intrusion porosimetry (MIP) and environmental scanning electron microscopy (ESEM), and compared with the pore structure of Portland cement paste. The results reveal that the pore structure of AAF paste with high silica content significantly differs from that of cement paste, resulting in different water permeability. For equal reaction age, the permeability of AAF paste was higher than that of cement paste with w/c = 0.4, an effect (that was) especially pronounced at later ages. However, a longer curing time at elevated temperature could significantly decreased the water permeability of AAF paste. MIP results show a consistent relationship between water permeability and total porosity, effective porosity and pore threshold diameter, respectively. Furthermore, a modified model incorporating all hereby experimentally derived parameters, is proposed to predict the water permeability of AAF. (C) 2012 Elsevier Ltd. All rights reserved.