Rare Earth Tailings Backfill: Cementation-Controlled Behaviour and ML-Based Binder Design

Cemented paste backfill (CPB) is essential for underground mine stability but relies heavily on ordinary Portland cement (OPC), contributing significantly to CO2 emissions. This study develops and evaluates a low-CO2 hybrid geopolymer backfill using rare earth element tailings (REET) and introduces a strength-driven, data-informed design framework that integrates experimental mechanics, machine learning, and environmental assessment. Material characterisation shows that REET exhibits negligible intrinsic geopolymer reactivity, rendering one-part geopolymer replacement ineffective. A two-part hybrid binder incorporating metakaolin and controlled alkali activation enables effective cementation while reducing cement demand by 37%. Comprehensive mechanical testing, including consolidated isotropic undrained and drained triaxial tests and one-dimensional consolidation, confirms that the hybrid backfill behaves as a true cemented material. Compared with OPC backfill, it exhibits cementation-controlled response with reduced excess pore pressure generation by 11 %, delayed dilation, enhanced post-peak ductility by 50%, and comparable confined stiffness at effective stresses of 800 kPa. A Random Forest model trained on a literature-derived database of 560 mixes achieves high predictive accuracy (R² = 0.97) and reveals threshold-controlled, interaction-dominated strength behaviour. Inverse design identifies a minimum binder content of ~5% to achieve backfill-grade strength (~4 MPa), while demonstrating infeasible high-strength targets under unfavorable curing or compositional constraints. Co-optimised inverse design shows that higher strengths are achievable without increased binder when chemistry, curing, and particle size are aligned. The proposed framework replaces empirical, binder-driven practice with a cementation-controlled, performance-based approach, providing a practical pathway for low-carbon CPB design without compromising mechanical performance.