This study deals with curing of mortars in which coal ashes from both PC and FBC boilers were the sole binder components. The work focuses on the hydration products and the hydration results are correlated with the physical properties of the cured beams from 10 formulations of mortar made of sand and ash blends. Curing was extended to two to three years and the beam specimens were cured in water for swelling measurements and in moist atmosphere for shrinking measurements. Mechanical strength and dimensional stability were tested periodically and porosity measurements were also made. Separation by density of pulverized mortar samples was performed to obtain the light fractions containing the hydrates formed. The cured mortars and these separated fractions were analysed and the hydrates identified included AFt, gypsum, portlandite, a variety of hydrated calcium aluminosilicates and amorphous C-S-H. The composition of C-S -H, as characterized by atomic ratios of various elements referred to Ca indicated considerable replacement of Si by Al and S. The degree of hydration, as indicated by the percentage of light fraction in the mortar or by other properties, and the chemical characteristics of the ash blends were related to the mechanical properties of the cured mortar using two novel indices. One of these relates the silicoaluminous to sulphocalcic components and the other the available free lime to sulphate content of the mortar. (C) 2001 Elsevier Science. Ltd. All rights reserved.