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The unburned carbon (UBC) content of ash from coal-fired thermal power stations and plants that use massive amount of coal can sometimes be 6–7% or more; this simply means loss of carbon and decrease in efficiency. Carbon loss not only causes a decrease in efficiency but is also an economical issue. Decreasing the carbon content would provide the usage of ash from thermal power stations as cement additives. Many factors cause low combustion efficiency and a high amount of UBC. In the course of this study, fly ash and bottom ash were divided into size fractions and accumulation of UBC at coarser size fractions was observed. In this study, a ground coal sample from a mill was sieved, and the accumulation of UBC at coarser size fractions was associated to a lack of grinding of pulverized coal. Ground coal has a size distribution where 54.45% was below 75 µm, and 45.55% was still over this critical size. A significant amount (35.48%) of the UBC accumulated in the size fraction over 75 µm in fly ash, whereas almost 70% accumulated in the size fraction over 0.5 mm in bottom ash. Coal and ash samples were observed under a microscope to deduce origins of UBC. [ABSTRACT FROM AUTHOR]