Jul 26, 2014· Class F is fly ash produced from burning anthracite or bituminous coal, and Class C is produced from the burning of subbituminous coal and lignite. Class F is low in lime, under 15 percent, and contains a greater combination of silica, alumina and iron (greater than 70 percent) than Class C fly ash. Class C fly ash comes from coals which may produce an ash with higher lime content — generally .