Cement recovery from mortars by microwave treatment at high temperature: an innovative way to facilitate circularity in the construction field

Abstract: The environmental burdens of cement production and the construction field play a major role in the current climate crisis. Nearly 50 % of the carbon emissions are associated with the burning of limestone and cannot be avoided unless a different raw material is used. Additionally, the construction and demolition waste is an important source of debris that typically ends up in landfills, even though it has a significant potential for upgrading into recycled cement. In this research, the thermal treatment at clinkering temperatures of hydrated Ordinary Portland cement and mortars was evaluated as a source for cement production. Hydrated blends with ground granulated blast furnace slags were evaluated as well. The thermal treatment was comparatively performed in an electric furnace at 1250 °C, 1350 °C, and 1450 °C and 2.45 GHz microwave furnace at input powers of 0.5 kW, 1 kW, 1.5 kW, and 2 kW. Microwaves are a promising technology to assure greener industrial thermal processes due to lower energy consumption, high heating rates, and low processing times. The obtained materials were characterized by X-ray Fluorescence, X-ray Diffraction, Thermogravimetric Analysis, and Differential Scanning Calorimetry. Furthermore, performance characterization was done with calorimetry and compressive strength. Finally, microstructure analysis was accomplished using Scanning Electron Microscopy and Energy Dispersive X-ray analysis. Cement type CEMI 52.5R was used as a reference. In Addition, the microwave heating was simulated in COMSOL multiphysics for a better understanding of the interaction between the cement-based materials and the microwave irradiation. The geometry of the furnace’s cavity, the input power, the heating rate, and the influence of the fraction of quartz were evaluated. The experimentation proved that microwaving is a suitable technology to recover clinker phases from hydrated samples. The materials obtained were significantly similar in mineralogic composition to the reference cement. However, the presence of quartz drastically hindered the formation of the primary clinker phases while slags additions enhanced the formation of belite rather than to alite. The mechanical characterization showed that mortars produced with 20 % of replacement of the reference Ordinary Portland Cement by recycled cement obtained by microwave treatment develop a similar compressive strength as the reference cement reaching up to 50 MPa at 28 days similar to the 52.2 MPa expected for the reference. Moreover, the simulation demonstrated the capability for short-time processing of less than 5 min for nearly 15 g of sample. In contrast, the conventional treatment for 60 g of samples took beet 7 and 10 h. The model showed that the heating is highly localized. Processing at 0.5 kW and 1 kW are recommended for a more controlled process. Finally, from the thermal point of view, additions of nearly 20 % of quartz increase the heating rate and enable more homogeneous heating. The research proved the potential of microwave treatment for a more sustainable way to produce cement from hydrated samples and could be suitable for the cement fractions from construction and demolition waste