ABSTRACT This study reports the effect of fly ash replacement ratio on composition of C-S-H gel. The compositions of C-S-H gel were estimated by combination of Rietveld/XRD analysis, selective dissolution and ignition loss. Experiment results confirm the decreasing of CaO/SiO 2 ratio as the increasing replacement ratio of fly ash. Bonding water in hydrated gel decreased as CaO/SiO 2 ratio increased. Such changing of compositions of C-S-H gel affected its density
