A study on cemented paste backfill using red mud and coal gasification slag based on extreme vertices mixture design

The large-scale reuse of industrial by-products and the advancement of green mining are crucial for achieving sustainable construction. This study aims to develop a novel geopolymer-based cemented paste backfill (CPB) using industrial solid wastes as raw materials. Using the extreme vertex design (EVD) method, a systematic approach was developed to optimize the quaternary system of red mud, coal gasification slag, desulfurization gypsum, and cement for converting solid wastes into functional engineering materials. The effects of each component on fresh‑state properties and hardened mechanical performance, including compressive strength at different curing ages, were fully investigated. This study integrates XRD, SEM, and FTIR to reveal the early ettringite‑driven structuring phase and later C‑S‑H gel‑dominated densification phase in the hydration system. High‑precision regression models were developed from experimental data to link each response variable with raw material composition. Using multi‑objective optimization, the optimal mix proportions for mine backfill were identified by targeting high fluidity, proper setting behavior, and high compressive strength. Verification experiments demonstrate that the model predictions are in excellent agreement with the experimental measurements, with all relative errors remaining below 8.2%. This study presents a scalable way to transform red mud and coal gasification slag into functional backfill materials by applying extreme vertex design to a four-component waste system, enabling optimized performance and insight into micro-mechanisms. The approach supports the efficient use of multiple solid wastes and has practical value for civil and mining infrastructure repair, reinforcement, and maintenance.