Multi-objective optimal control and application of solid waste synergistically excited fluidized solidified soil

Abstract Aiming at the problems of easy settlement and insufficient durability of traditional materials under special backfilling conditions in the northwest loess area, this study prepared controllable low-strength ready-mixed flow solidified soil (CLSM) with solid waste and alkali activator (NaOH and water glass). Through multi-dimensional test and mechanism analysis, the collaborative optimization of material performance and economy was realized, and the durability mechanism of extreme environment was revealed. In this study, the original loess in Lanzhou was used as the matrix, and the L16(45) orthogonal test design was used to construct the ‘28d compressive strength-cost’ double-objective optimization model combined with the entropy weight method. The influence weights of coal gangue, carbide slag, blast furnace slag and alkali activator were quantified, and the optimal mix ratio was determined to be 15% of coal gangue, 3% of carbide slag, 15% of blast furnace slag and 60% of alkali activator. Under this ratio, the 28d compressive strength of CLSM reached 4.44 MPa, and the cost was controlled at 73.23 yuan/ton, taking into account both mechanical properties and economy. The extreme environmental durability test shows that compared with the traditional cement soil, the mass loss rate of the optimal ratio CLSM decreases by 44.83% and the strength increases by 34.89% after 25 freeze–thaw cycles. After 25 times of sulfate dry–wet cycles, the mass loss rate decreased by 55.56%, and the strength increased by 40.01%, especially the resistance to dry–wet cycles was better. The micro-mechanism study revealed that the formation of CLSM strength experienced five stages: ‘alkali-activated depolymerization-three-dimensional network construction exchange agglomeration-secondary hydration enhancement-structural densification’ and High content of solid waste and alkali activator synergistically promoted the formation of C–S–H gel and ettringite. The order of porosity inhibition effect was alkali activator > blast furnace slag > coal gangue > carbide slag, and it was significantly negatively correlated with macroscopic properties. Engineering application verification shows that CLSM has good fluidity in site pouring. Although there are adverse effects in the pouring process, it provides a new material paradigm for green backfilling in the cold and arid regions of Northwest China. In this study, “waste treatment by waste” is realized through the synergistic excitation of solid waste. The multi-objective optimization method improves the scientificity of the ratio. The systematic revelation of extreme environmental durability and micro-mechanism provides theoretical support for the engineering application of fluid solidified soil, which has both environmental protection value and engineering guiding significance.