Eco-efficient recycling of engineering muck for manufacturing low-carbon geopolymers assessed through LCA: exploring the impact of synthesis conditions on performance

With the push toward global low-carbon and sustainable development goals, an urgent need exists to find building materials that can replace cement. Engineering muck (EM) produced by foundation pit engineering in subtropical areas is rich in clay minerals, which are suitable for the preparation of low-carbon geotechnical materials. In this study, the effects of SiO2/Na2O and the liquid-solid ratio on the performance of alkali-activated EM-based geopolymers were investigated. In addition, a life cycle assessment (LCA) was performed to evaluate the environmental impacts of EM-based geopolymers. The findings showed that the geopolymer with a SiO2/Na2O ratio of 1.5 achieved the highest 7-day compressive strength of 42 MPa, which was 47.46% and 56.49% greater than that of the geopolymer with SiO2/Na2O ratios of 0.9 and 1.8, respectively because of its densest structure and fewest cracks. Moreover, increasing the liquid-solid ratio from 0.75 to 0.90 slightly increased the 28-day compressive strength from 47 to 52 MPa because of the refined pore structure. However, this modification also increased carbonate formation and mass loss at elevated temperatures. Compared with those of concrete, the CO2 and SO2 emissions of EM-based geopolymers were reduced by 4-26% and 8-19%, respectively, owing to the considerable environmental impact of cement. The overall performance of the geopolymers was assessed via multiple influence indicator methods, and the optimal synthesis conditions for the geopolymers were a SiO2/Na2O ratio of 1.5 and a liquid-solid ratio of 0.75. This study suggests that using alkaline activation technology to transform EM into geopolymers has potential as a substitute for concrete, providing a new type of green material for geotechnical engineering.