Sintering and phase formation of ceramics based on pre-treated municipal incinerator bottom ash

Ceramics, based on 60 wt% of municipal incinerator bottom ashes (MIBA) and 40 wt% of industrial clays, were prepared by using two fractions of pre-treated ashes (above and under 2 mm) and three different industrial clays. The obtained six compositions with amounts of CaO from 12 to 14 wt%, with SiO2/Al2O3 ratio ranging from 3.5 to 1.6 and with Al2O3/CaO ratio - from 1.25 to 1.75 were investigated and characterized. These compositions were located in the crystallization field of anorthite (CaO.Al2O3.2SiO2); the ceramics with high SiO2/Al2O3 ratio are near to the eutectic with SiO2, while with low ratio – closer to the anorthite stoichiometry. The phase transformation and the subsequent melting were studied by non-isothermal DTA-TG analysis, while the densification process - by contactless optical dilatometry. Then, the optimal sintering temperatures and times were evaluated by isothermal dilatometric runs at different temperatures. At high SiO2/Al2O3 ratio the sintering temperature is lower but the sintering range is very narrow. This behavior can be explained by the formation of too high amount of liquid phase at the eutectic temperature. On the contrary, at lower ratio, the sintering temperature increases but, notwithstanding the decrease of viscosity and shorter sintering time, the densification range becomes wider. This last result is a very important technological finding with a view to industrial transferability. Additional tests performed demonstrated that a preliminary thermal treatment of ash with elevated residual organic phase is necessary, which leads to a lower firing shrinkage and to a shorter sintering time.