The minerals found in the mixtures, after hydrothermal treatment
in H2O, NaOH (1M) and KOH (1M) but before calcination, are
given in Fig. 2. This figure also presents the mixture of fly-ash and
lime homogenized in water at 20 ◦C for 1 h (taken as the reference
here).
The only common change for all mixtures was the disappearance
of free lime. Compared to the reference mixture (20 ◦C–1 h),
the hydrothermal treatment of 5 h in 100 ◦C H2O led to a partial
dissolution of anhydrite and a precipitation of kuzelite, a calcium
aluminium hydroxide sulphate (Ca4Al2(OH)12(SO4)·6H2O).
Aluminium came from either the micas or the amorphous phase.
Part of the portlandite was carbonated to form calcite. Otherwise,
no significant changes were detectable for this treatment, including
in the quartz phase of the fly-ash.