Preliminary Study of New Sustainable, Alkali-Activated Cements Using the Residual Fraction of the Glass Cullet Recycling as Precursor.

Featured Application: The use of a residue that is currently disposed of in landfills was considered in this study, to obtain binder materials more sustainable and respectful to the environment. The recycling glass industry is identified as a model to advance towards a circular economy. The recycling glass rate in Europe is around 74%; meanwhile, in Spain it is approximately 70%, as reported elsewhere. However, in Spain, there is a problem with a residue obtained during the glass recycling process. The residue is named CSP (ceramic, stone, and porcelain). The results of this contribution show that is possible to use this residue as the precursor for developing alkali-activated cements (AAC). Due to the small amount of aluminum in CSP, it is desirable to use it as a non-hydraulic binder for specific purposes, such as prefabrication, decoration, insulation walls, or flooring material. Otherwise, for future works, it will be necessary to include aluminum for hydraulic binder development. During the glass selection process by optical sorting equipment, a rejection material called CSP (ceramic, stone, and porcelain) is generated, which is lower than 2 wt % of the glass cullet collected in Catalonia (Spain). Although this process should only separate non-glass impurities from the glass cullet, around 84 wt % of glass is found in the CSP. The CSP characterization reveals that CSP is mainly compound by SiO₂, Al₂O₃, alkali metals, and CaO, which are key components for the alkali-activated cement (AAC) development. Consequently, this study is focused on the potential of CSP as a precursor to synthesize AAC. The concentration of the alkali activator (NaOH: 1 M, 4 M, and 8 M) and the liquid-to-solid (L/S) ratio were tested in the formulation of the AAC. The AAC specimens at 28 days cured were evaluated using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FIR), scanning electron microscopy (SEM), apparent density (ρ_app), and compressive strength (σ_s). The results obtained showed that the L/S of 0.5 and 4.0 M for NaOH concentration are the best conditions, due to the mechanical properties (ρ_app = 1.75 g·cm⁻³; σ_s = 52.8 MPa), cohesion (SEM), and formed phases (XRD and FT-IR). Therefore, CSP can be a precursor for developing new, sustainable binders. [ABSTRACT FROM AUTHOR]