However,
gehlenite persistently exists which is undesirable due to its
low hydraulic property. Gehlenite is generally produced by the
reaction between dehydroxylated clay mineral and CaO
particularly when free silica is reduced in the system [16].
The solid-state reaction between CaO and aluminosilicate
compound such as kaolinite tends to develop belite phase rather
than gehlenite because of a transformation of kaolinite into
spinel at firing temperature range of 1000–1150 8C resulting in
a release of free silica needed for the belite formation. Lignite
fly ash used in the experiments mainly composes of
aluminosilicate glassy phase with small amounts of free silica.