Upcycling of molybdenum tailings, coal gangue and slag into sustainable repaired composites by geopolymerization technology: Workability, embodied carbon and mechanical-microstructural development

Abstract

The major disadvantage of reinforced concrete structure was multi cracks under loading, seriously affecting its normal performance, but using traditional repaired materials to strengthen reinforced concrete structures can bring some side-effects, such as higher carbon oxide and cost. Herein, this study successfully designed an eco-friendly geopolymer repaired composites developed by molybdenum tailings, ground granulated blast-furnace slag and coal gangue. Firstly, the fresh and harden properties of geopolymer repaired composites was systematically studied including water bleeding rate, fluidity, setting time, unconfined compressive strength and bonding strength. The micro performance of geopolymer repaired composites was analyzed via scanning electron microscopy, thermogravimetry-derivative thermogravimetry, X-ray diffraction and fourier transform infrared spectroscopy to give some new viewpoints for revealing the coupled geopolymerization mechanism of molybdenum tailings/coal gangue/ground granulated blast-furnace slag in geopolymer repaired composites. Meanwhile, the gray level of concrete contact surface is calculated. The results show that the properties of geopolymer repaired composites with 5% NaOH designed by 1:3 binder/sand, 0.5 w/c and 1:3 molybdenum tailings/ground granulated blast-furnace slag were optimal properties with 1.73% water bleeding rate, 192 mm fluidity, 58 min and 178 min the initial and final setting time, respectively, 21.443 MPa 28-d unconfined compressive strength and 2.642 MPa bonding strength. Excessive NaOH amount would inhibit the unconfined compressive strength and bonding strength of geopolymer repaired composites, while the increase of ground granulated blast-furnace slag would improve unconfined compressive strength and bonding strength. The primary hydration products of geopolymer repaired composites were C–S–H and C-A-S-H gels. Molybdenum tailings, coal gangue, and ground granulated blast-furnace slag showed an important synergistic effect, improving its geopolymerization reaction and strength. Besides, bonding strength of geopolymer repaired composites was highly correlated with unconfined compressive strength and fractal dimension of surface roughness. The relation coefficients are 0.92005 and 0.94006, respectively. With the higher the unconfined compressive strength of geopolymer repaired composites and the larger fractal dimension of the concrete contact surface, the bonding strength of geopolymer repaired composites is higher and the bond between geopolymer repaired composites and concrete is denser. The increase in the amount of NaOH and the increase in ground granulated blast-furnace slag have an increasing and decreasing trend on the carbon emissions of geopolymer repaired composites, respectively. The most obvious increasing and decreasing trends are that the carbon emissions of GRC increase from 66.41 kg CO₂-e/m³ to 96.28 kg CO₂-e/m³ and decrease from 79.81 kg CO₂-e/m³ to 76.37 kg CO₂-e/m³. The increase in the amount of NaOH is the main reason for the increase in the carbon emissions of geopolymer repaired composites, while the increase in ground granulated blast-furnace slag helps to reduce the carbon emissions of geopolymer repaired composites. The carbon emission of geopolymer repaired composites was reduced by 75.08%–83.4% compared with traditional mortar. The 28-day per unit unconfined compressive strength carbon emission of the geopolymer repaired composites with the optimal mix is 46.63% lower than that of the traditional mortar. Overall, this study can provide new viewpoints and novel channels for converting solid waste, including molybdenum tailings and coal gangue, into green recycled construction materials, provide highly promising insight for designing green construction products and new ideas in the development of low-carbon remediation materials for environmental sustainability, contribute an important force to environmental protection, and promote the growth of economic benefits in the process.