The original US Clean Air Act (CAA), implemented by the Environmental Protection Agency in 1971, and the amendments to the act in 1977 and 1990 have required a considerable evolution of the quality of coal burned by utilities and in the type pollution control equipment needed to reduce SO2 and NOx emissions. Any change in coal quality or emission＇s control implies a change in the amount, type, and quality of coal combustion by-products (CCB). CCB is a collective term for fly ash, bottom ash and boiler slag, and flue-gas desulfurization (FGD) or fluidized-bed combustion produced in coal burning. Studies by the University of Kentucky in 1978, 1992, and 1997, the latter two conducted by the Center for Applied Energy Research, have focussed on the amount, quality, and end use of CCBs from Kentucky power plants, with particular emphasis on fly ash. The evolution of clean air standards has impacted the quality and volume of CCBs in a variety of ways: (1) switching from high sulfur to lower sulfur coal generates lower quantities of spinel phases and greater amounts of alumine-silicate glasses; (2) switching to Powder River Basin subbituminous coals produces Class C fly ash, compared to Class F fly ash from the combustion of the typical eastern bituminous coals; (3) the wider use of beneficiated coals reduces the amount of Ay ash and bottom ash produced; (4) use of a wider rank range into the coal blend increases the potential of unburned carbon caused by inefficient combustion of non-design coals; (5) the inclusion of non-coal fuels, such as petroleum coke and with tire-derived fuel, in the coal blend; (6) reduction of NOx emissions has generally meant an initial increase in the amount of carbon in the fly ash; (7) addition of FGD means an added CCB stream, either a calcium sulfite which in generally mixed with fly ash and landfilled, or a calcium sulfate, which is sold for wallboard manufacture. The modification of the petrology and chemistry of the fly ash impacts the potential for utilization.