Fly ash is a waste material from coal-burning power plants that consume pulverized solid fuels. Two fly ashes from Asturias (Spain) were activated mechanically by wet milling and chemically by leaching with sulfuric acid. The activated fly ashes were characterized in terms of physico-chemical characterization, granulometry, density, blaine, BET, XRD and SEM. A comparative study was carried out of several mortars, in some cases using different additions of silica fume or activated fly ash. The influence that these additives have on the mechanical resistance of the mortars was studied. As well as the possible use of these activated fly ashes as a replacement for silica fume in producing high-strength mortar or concrete. It was found that mortars containing activated fly ash presented higher compressive strengths. A mercury intrusion porosimetry study was carried out on cement mortars made with mineral additives such as silica fume and activated fly ashes. In general, the porosities values of these mortars showed that mineral admixtures improved mechanical resistance due to the decrease in pore size. (C) 2006 Elsevier Ltd. All rights reserved.