With the growing general concern about the pollution by fly ash (FA), there has been global interest in its utilization. Purified FA or FA micro-beads are suitable as polymer filling materials because of their density, good dispersity and fluidity of globular particles. However, FA as a filler has not been widely used up to now on account of low whiteness values and low friction of untreated FA surface. ln order to improve the FA quality, a surface modification method by using isothermal heating is proposed in this paper. Preparation of composite fly ash (CIA) in the Ca(OH)(2)-H2O-CO2 system is described. Good coating results on FA surfaces can be achieved under suitable operating parameters. The characteristics of CFA are discussed and analyzed based on data from X-ray diffraction, scanning electron microscopy (SEM), infrared spectra, and BET multiple-point nitrogen adsorption method. Feedstocks with less than 45 mu m grain size, 2.86 m(2) g(-1) specific surface area, and 36.68 whiteness value revealed an increase in specific surface area ranging from 8.69 to 10.01m(2) g(-1) and an increase in whiteness values ranging from 63.67 to 73.13 after coating. A SEM study allowed a detailed determination of the morphology of the surface roughness. Filling tests also show that a rough surface of the CFA enhances contact opportunities and improves the interface between polymer and CFA blended with polypropylene (PP). (c) 2005 Elsevier B.V. All rights reserved.