Facile construction of superhydrophobic porous red mud/slag-based geopolymer for thermal insulation and oil/water separation

Red mud (RM) is difficult to be comprehensively utilized and has a serious impact on the environment due to the high alkalinity, low activity, and high content of harmful substances. In this study, porous RM-based geopolymers (PRGMs) with high porosity ratio (76.83 %) and compressive strength (1.16 MPa) were prepared by controlling the ratio of slag and RM (60 %) and curing temperature (70 degrees C) and adding a mechanical foaming agent (MFA: 12 %). The flux of the PRGMs was as high as 10180.33 L m(-2) h(-1) at a thickness of 3 mm. The equilibrium cold surface temperature was reduced from 175.6 to 131.7 degrees C compared to a structure without MFA. In addition, a superhydrophobic PRGMs (SPRGMs) was successfully modified by chemical vapor deposition using water and polymethylhydrosiloxane at 120 degrees C for 3 h. It had excellent superhydrophobic properties (159.04 degrees) and low thermal conductivity (0.1922 W m(-)(1) K--(1)). The contact angle of the SPRGMs was still >150 degrees even after treatment under real simulation conditions including acid/alkali/salt solution contact/immersion, variable temperature (-20 to 450 degrees C for 2 h), ultraviolet aging (60 h), and physical abrasion (10 cycles). The SPRGMs achieved 100 % desorption of light/heavy oils with excellent and stable cycling performances. The contact angle was still larger than 151 degrees after 10 cycles. Furthermore, the SPRGMs are effective for a continuous treatment of oil/water emulsions and mixtures by gravity, which provides a new concept for the development of thermal insulation building materials, large-scale treatment of RM, and industrial treatment of oil/water pollution.