Strength and Fresh Properties of Cemented Paste Backfill with a Low-Carbon Binder

Abstract

Cemented paste backfill (CPB) is widely used in underground mining to provide ground support and enable the safe disposal of tailings. Ordinary Portland cement (OPC), commonly employed as the binder in CPB, is associated with high energy consumption and significant CO₂ emissions, motivating the development of low-carbon alternative binders. Among these, alkali-activated slag (AAS) systems have shown strong potential; however, their application in CPB remains limited, particularly with respect to the coupled evolution of fresh properties, strength development, and hydration mechanisms. This thesis investigates the performance of CPB incorporating ground granulated blast furnace slag activated by a blended sodium carbonate-sodium hydroxide (Na₂CO₃-NaOH) system. An experimental program was conducted to evaluate the influence of activator composition and dosage on both fresh-state behavior and hardened properties. Rheological parameters, setting time, electrical conductivity (EC), volumetric water content (VWC), and matric suction were monitored to characterize early-age structuration and hydration kinetics. Unconfined compressive strength (UCS) was measured at multiple curing ages, and microstructural analyses, including X-ray diffraction were performed to link phase assemblage and pore structure to macroscopic performance. The results demonstrate that the Na₂CO₃-NaOH co-activation system governs CPB behavior through a two-stage hydration process, consisting of an initial ionization regime followed by accelerated gel formation and pore refinement. Fresh-state properties and early-age kinetics were found to be strongly coupled with long-term strength development. Compared to OPC-based CPB, the AAS systems exhibited competitive strength performance while offering significant potential for reducing the carbon footprint of backfill operations. Overall, this research provides an integrated understanding of the mechanisms controlling fresh and hardened behavior of AAS-based CPB and highlights the feasibility of sodium carbonate-sodium hydroxide activated slag as a low-carbon binder for underground mine backfill applications.