The reason may be that, during the sintering
process, the increasing quantities of Al3+ may substitute for
the Si4+ in a network tetrahedron (23) and other unreacted
Al2O3 present as oxide acts as an accelerator using the readily
available cations (such as Cd, Cr, Cu, and Pb) to increase the
pozzolanic reactions, which contribute to the stability of the
networks andmakeheavy metals to enclose in the framework
of silicates or aluminosilicates and make them difficult to be
leached.