For each experimental batch, raw or pre-treated fly ash was mixed with distilled water or distilled water added with chemi- cals (ferrous and ferric salts). The mixture of ferrous and ferric salts were used for simultaneous stabilization of heavy metals during HT process, and the ratio of Fe (II) to Fe(III) was 2:1. The liquid-solid (L/S) ratios of the obtained suspensions were between 2 and 2.5. The suspensions were then heated to 290 ◦ C in an autoclave and main- tained for 1 hour. The HT-processed fly ash samples were cooled to below 60–80◦ with cooling water running through the cooling coils inside the autoclave, and then the cooled samples were separated into liquid phase and solid phase (ash cake) via vacuum filtrating. The ash cake was dried at 105 ◦ C for 24 hours before testing