Hydrothermal solidification of municipal solid waste incineration (MSWI) fly ash has been conducted under saturated steam pressure at 200°C for up to 48 h with quartz addition. To enhance the strength of solidified specimens further, the raw fly ash was pre-treated by water-washing and mixed with NaOH solution (2 M) as reaction solvent. Experimental results showed that curing time and temperature had significant effects on strength development. Strength development was found to be mainly due to tobermorite formation, and addition of quartz and NaOH solution promoted tobermorite formation. The raw fly ash could also be used as an additive to solidify MSWI bottom ash, and with raw fly ash addition (10%) the flexural strength of solidified specimens reached more than 21 MPa, suggesting high potential to recycle 100% MSWI ash (e.g. as 10% fly ash ? 90% bottom ash). Leaching tests were conducted to determine amounts of heavy metals dissolved from solidified specimens. The results showed that under the hydrothermal conditions of this study, leaching of heavy metals was very low. As such, the hydrothermal processing method might have high potential for recycling/reusing MSWI fly ash on a large scale.

Municipal solid waste incineration (MSWI) bottom ash could be solidified with fly ash addition at 100 °C, and the flexural strength of solidified specimens reached almost 20 MPa. The strength development was found to be due mainly to C–S–H gel formation. Leaching tests were conducted to determine the amount of heavy metals dissolved from the solidified specimens, and the results showed that, under the hydrothermal conditions of this study, the leaching of heavy metals was very low. As such, it is possible to solidify MSWI ash by 100% (80% bottom ash + 20% fly ash) in a continuous production process with a low cost.

In this study, titanium-containing slowly-cooled slag (TSS) has been solidified hydrothermally under saturated steam pressure at 200ć for 0-24 h. The experimental results showed that additions of NaOH solution and slaked lime could improve the hydraulic activity of TSS effectively, and in turn resulted in a higher strength development. A longer curing time was also found to be favorable to the strength development. The strength enhancement was believed to be due primarily to the formation of hydrogarnet. At 10 mass% slaked lime, 2 M NaOH solution introductions and 12 h curing, the flexural strength of specimens reached 15 MPa, which could be used as construction building materials. Thus, the hydrothermal solidification method may provide a high potential for recycling TSS on a large scale.