8 pages.-- Final full-text version of the paper available at: http://dx.doi.org/10.1111/j.1551-2916.2007.01996.x.This paper presents the microscopic characterization of two types of fly ash belite cements and their hydration products by means of scanning electron microscopy, energy-dispersive X-ray microanalysis, and X-ray diffraction analysis. The cements were obtained from ASTM class C coal fly ash by the hydrothermal–calcination route in water (FABC-2-W) and NaOH 1Msolution (FABC-2-N). The hydration was studied during a period of 180 days at 211C and 495% RH. The results showed significant incorporation of aluminum (Al) into the C–S–H gel and other minor elements, with a presumable position close to that of aluminum-tobermorite. The C–S–H composition of the FABC-2-W is more stable over the hydration time than that of the FABC-2-N cement. Portlandite is scarcely formed during hydration.Funding for the present research was provided by the Minister of Science and Technology (Spain) under Project no MAT 2002-04023-CO3.Peer reviewed