The chemistry and mechanism of mechanical strength-development at different setting-times in different weight-ratios of proclaimed commercial-cement and fly-ash mixture is studied. The main content of fly-ash is the quartz (SiO2). According to our results, as the setting time increases, the quartz present in fly-ash gets utilized during setting of the cement to form the well known C-S-H gel (3CaO.2SiO2.3H2O). Our analysis suggests that the addition of fly-ash reduces the water requirement for the hydration-process. Furthermore, the fly-ash inhibits the formation of CaCO3 phase from portlandite (Ca(OH)2) through atmospheric CO2, and more importantly, it promotes the transformation of portlandite to form the C-S-H network which provides strength and long term stability to the set cement. As the commercial cement already contains optimum amount of fly-ash, any further addition of fly ash leaves the fly-ash partially unreacted which drastically decreases the compressive strength.