Clay minerals are the principal reactive, non-combustible, phases present in the pulverized coal burned in power station furnace units. Despite the short heating time involved, the clay mineral impurity is wholly transformed in the ignition.

Illitic clays are partially melted, fluxed by water and potash, to produce an alumino-silicate melt phase. The melt is frothed by the synchronous production of carbon dioxide in an iron oxide catalysed oxidation of associated carbonaceous matter. This produces the hollow, gasfilled, glass-walled spheres (cenospheres) characteristic of pulverized fuel ash (PFA). Partial, primary devitrification crystallization of mullite and quartz from the high-alumina glass is general and the extent of devitrification may influence the pozzolanic efficiency of the PFA cenospheres.

Without the advantage of potash fluxing, the kaolinite lattices are dehydrated and transformed, without melting, to amorphous aluminosilicate and crystalline mullite.

Oxidation of pyrite leads to coating of cenospheres by condensed magnetite and the emission of sulphur dioxide in the flue gases.