Preparation of monolithic geopolymer materials from urban waste incineration slags

Abstract

Urban waste incineration slags have been applied as active fillers of geopolymer. They are classified into two groups. One is Fe-rich and the other is Fe-poor in chemical compositions. When plotted in a ternary diagram in terms of CaO–Al₂O₃–SiO₂, they aligned along with so-called the first hydraulic line and two groups were also noted. One is Ca-rich designated as BFS-group plotted near 45% CaO line close to blast furnace slag compositions. The other is Ca-poor designated as MID-group plotted near 30% CaO line apart from the blast furnace slag compositions. Na-silicate solution, Na₂O·2SiO₂·aq enriched with caustic soda was used as geopolymer liquor. When flooded, some foaming and swelling phenomena were encountered more or less presumably due to the degassing of hydrogen from contaminated Al-metal in the slags. When cured at 80°C and 100%RH, flexural strength tests for pastes revealed two groups of strength, very low and very high, ranging 3–16 MPa for 24 h curing, for instance. The strength was not depending on the iron contents but depending on the degrees of foaming and swelling, i.e., bulk densities of the hardened bodies. Sufficient work time for mixing and casting was able to take for all the slags even to the BFS-group. Matrix binder compositions of hardened pastes analyzed by SEM-EDX were discussed in terms of Al₂O₃/(Na₂O + K₂O), (CaO + MgO)/SiO₂ and SiO₂/Na₂O as well as CaO–Al₂O₃–SiO₂.