Hydrothermal solidification of underground construction wastes into building materials: Waste slurry recycling, industrial application and evaluation

Abstract

A novel hydrothermal technology was developed to recycle underground construction wastes into tobermorite-based building materials and immobilize polyvalent Cr. The hardening mechanism involved in tobermorite synthesis and immobilization of Cr were studied by various techniques, including XRD, SEM, STA, XPS and ICP-OES. A simulated waste slurry was converted hydrothermally to well-crystalline tobermorite for both promoting strength and structurally accommodating Cr. A higher curing temperature and longer time facilitated the tobermorite formation, with a high proportion of tobermorite (∼93.67 wt%) being obtained at 200 °C for 24 h at a Ca/Si molar ratio of 0.6. Along with tobermorite formation, the in-situ immobilization of Cr in the tobermorite structure was also examined. Cr(Ⅲ) was found to replace the Ca2+ in the Ca–O octahedron to form Cr–O bonds or connect with H2O in the interlayer as Cr–OH bonds; whereas Cr(Ⅵ) groups linked with Ca2+ in the interlayer as stable CaCrO4, which adversely affected tobermorite formation. Both Cr(Ⅲ) and Cr(Ⅵ) had very low leaching rates from the solidified specimens, although the lower leaching rate of Cr(Ⅲ) (∼4.05% of 5000 ppm) compared to Cr(Ⅵ) (∼7.23% of 5000 ppm) confirmed that tobermorite had a stronger immobilization capacity for Cr (III). An actual waste slurry was used as the dominant raw material (∼70 wt%) for the hydrothermal synthesis of a tough tobermorite-based building material with a flexural strength (∼25.3 MPa), which was even higher than that of common concrete materials. The hydrothermal technology developed here has been successfully applied to the industrial production of standard bricks containing 50–70 wt% waste slurry that were fully compliant with Chinese standards. Life cycle assessments revealed that the hydrothermal slurry-recycled bricks effectively reduced environmental impacts by ∼13%–152% compared to similar building products, thereby demonstrating economic benefits and sustainability.