Red mud is a solid waste generated from alumina production, its massive accumulation occupying extensive land areas and leaching alkalis and heavy metals, leading to significant environmental issues such as soil alkalization and groundwater pollution. This study responds to the current demand for sponge city construction via innovatively utilizing red mud as a raw material to produce un-sintered aggregates through a novel pelletization process that leverages similarities between red mud and permeable brick materials. High-strength un-sintered permeable bricks were prepared via integrating mixing, molding, and curing techniques. These bricks improve overall compressive strength and permeability, while drastically reducing energy consumption and environmental impact during production. This approach presents a fresh perspective on red mud utilization and sponge city development, promising the environmental and economic benefits.

The preparation of red mud-based unsintered permeable bricks involved: 1) pelletization, the aged red mud chunks were crushed, then mixed uniformly with P·O 42.5 ordinary Portland cement and water, and fed into a pelletizer for pelletization. The aggregated particles were removed and naturally cured for one week; 2) mixing, different proportions were designed based on a volumetric method, with red mud-based un-sintered aggregates, cement, water, and additives calculated and weighed according to these proportions, then added to a basin and mixed uniformly; 3) pressing, the uniformly mixed slurry was placed and pressed into a mold with the dimensions of 20 cm×15 cm×10 cm by a semi-automatic brick machine; 4) curing, the pressed permeable bricks were demolded and naturally cured for 1-2 d, then placed in a standard curing box for 28 d.

The phase structure and micro-morphology of the material were analyzed by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). The heavy metal leaching performance of the permeable bricks was analyzed according to the national standards (HJ557-2010). The concentrations of ten metal elements in the leachate were determined inductively coupled plasma mass spectrometry (ICP-MS). In addition, the environmental risk of heavy metal elements in the leachate was evaluated according to the national standards (GB/T 5085.3-2007).

The bulk density (ρ_bv) of red mud-based aggregates is 1 078.50 kg/m³, with an apparent density (ρ_ap) of 1 724.14 kg/m³ and a porosity (v) of 37.45%. The analysis of the water-cement ratio (w_c) on permeable brick performance reveals that increasing w_c from 0.20 to 0.35 gradually enhances the compressive strength. This improvement stems from the increased fluidity and adhesive properties of the cement paste at a higher w_c, facilitating a better aggregate encapsulation and thereby improving the connectivity. However, increasing w_c beyond 0.35 to 0.40 results in a significant decrease in the compressive strength likely due to excessive fluidity causing settling of the cement paste.

The results of target porosity effect on permeable bricks demonstrate a negative correlation with the compressive strength and a positive correlation with the permeability coefficient. A greater porosity leads to increased interconnected pores within the bricks, enhancing water infiltration but reducing contact points crucial for stress transmission. This leads to localized increases in pressure and higher susceptibility to fracturing, thereby impacting the mechanical properties.

The results of foaming agent content effect on permeable brick performance indicate that increasing the foaming agent from 0% (in mass fraction) to 4% slightly increases the permeability coefficient, while marginally decreasing compressive strength. This effect results from hydrogen peroxide decomposition, creating additional pores within the bricks that weaken aggregate bonding and increase porosity. A further increase in foaming agent content reduces the permeability coefficient, while gradually enhancing compressive strength.

The XRD analysis confirms CaCO₃ as a common component in red mud, cement, aggregates, and permeable bricks. The characteristic peaks of red mud and aggregates remain unchanged, indicating no alteration in primary phases of red mud under the un-sintered process. In the permeable bricks, the characteristic peaks of 3CaO·SiO₂ are absent, whereas the peaks of Ca₅(Si₆O₁₆)(OH)₂ appear. This indicates that 3CaO·SiO₂ undergoes a reaction with water under high alkalinity conditions to produce Ca₅(Si₆O₁₆)(OH)₂, consequently boosting the bricks' strength. The microstructural observation of cross-sections and vertical sections reveals abundant micro-voids crucial for permeability. The cross-sections display flaky hydration gels distributed along aggregate surfaces, while the vertical sections exhibit thinner hydration products and flocculent gels.

The results of heavy metal leaching experiments reveal chromium (Cr) as the maximum concentration metal in leachates from red mud-based un-sintered permeable bricks, exceeding 40 μg/L, while other metals are below 10 μg/L. Leachate toxicity is well below the values of the national standards (GB/T 5085.3-2007), confirming compliance with environmental safety standards and minimal environmental risk for red mud-based un-sintered permeable bricks.

Red mud-based un-sintered permeable bricks were prepared through pelletization, mixing, pressing, and curing. The water-cement ratio affected brick's performance. An increase in the water-cement ratio enhanced the fluidity of the slurry and increased compressive strength, but when the ratio exceeded 0.35, the mechanical properties deteriorated. Increasing the target porosity improved permeability but reduced compressive strength. Adding hydrogen peroxide as a foaming agent effectively increased porosity at levels below 4%, but excessive addition led to the reduced performance. The XRD analysis showed that a high alkalinity of red mud promoted the formation of C-S-H gel, enhancing compressive strength. The SEM images of the cross and vertical sections of the permeable brick revealed flaky and flocculent hydration gel products attached to the surfaces of the aggregates. The results of heavy metal leaching experiments indicated that the leaching risk of heavy metals from the permeable bricks was extremely low. This work could provide an effective approach to simultaneously address the challenges of red mud resource utilization and sponge city construction.