In France, Municipal Solid Waste Incinerator Fly Ash (MSWIFA) are treated as a dangerous solid waste and mostly stored in hazardous waste landfills. Due to both a high concentration of heavy metals and a high solubility of this waste material (containing salts such as NaCl and KCl), a process of solidification and stabilization is required before storage. This process usually involves mixing the solid waste with cement and water. Due to the environmental concerns related to cement production, the use of alkali-activation without any cement addition to stabilize and reduce the solubility of Fly Ashes is of interest. The addition of MSWIFA in alkali activated matrix is not completely new [1]–[3] but none of the studies conducted were focused on a mix with high enough ratio of fly ashes to be used for landfill disposal. This study therefore focuses on the stabilisation of MSWIFA by addition of Ground Granulated Blast Furnace Slag (GGBFS) under alkaline conditions to limit the leaching of salts in the environment while keeping an appropriate workability for an industrial use.
In this work, MSWIFA from one incinerator in Lyon urban area has been selected. After analysing its microstructure and chemical composition, the waste material was mixed in high ratio (> 50% in mass of waste in the total wet mix) with GGBFS, NaOH, sodium silicate and water. Samples were casted at room temperature and cured at 20°C and 80% RH for periods of time ranging from 24 h to 91 days. The concentration of NaOH in the activation solution (varying from 0 to 8 mol.L-1) is the main parameter of study to modify the conditions of alkaline activation.
Samples have been characterized using XRD, FTIR and SEM. Their physical properties have also been studied through bulk density and porosity measurements. Setting time has been followed using a Vicat needle and compressive strength measured at different times from 7 to 91 days to quantify the mechanical stabilization. Chemical stabilization has been characterized by the monitoring of sample leaching using pH measurements, ionic conductivity, soluble fraction measurements and ICP of the leachate after 7 to 91 days of curing.
This study demonstrates the feasibility of using alkali-activation to mechanically and chemically stabilize MSWIFA, validating the normative criteria for dangerous waste storage in France, with a minimal cold crushing strength of 6 MPa after 91 days and already less than 10 % soluble fraction after 7 days of cure. The dihydrogen gas production during setting of the material, due to the presence of aluminium in MSWIFA, showed a high negative impact at high pH value on both mechanical properties and salt stabilization. Mitigation strategies are proposed.