Hydration mechanisms and strength development of hybrid sodium carbonate–sodium hydroxide activated slag-cemented tailings backfill

Abstract

Cemented paste backfill (CPB) is a sustainable construction material produced by mixing mine tailings, water, and a small proportion of binder to fill underground mine voids. Widely used in modern underground mining operations, CPB provides critical ground support, enhances mining safety by reducing void spaces, improves ore recovery, and minimizes the environmental risks associated with surface tailings storage. In this context, slag-based alkali-activated binders have emerged as promising low-carbon alternatives to Portland cement for CPB applications; however, the hydration mechanisms governing hybrid carbonate–hydroxide activation under the high water-to-binder conditions typical of CPB remain insufficiently understood. This study investigates the hydration mechanisms, microstructure evolution, physical property development, and strength performance of slag-based CPB activated with blended sodium carbonate (SC) and sodium hydroxide (SH). A systematic evaluation of SC/SH ratios and total activator dosages was conducted under realistic CPB conditions. To elucidate early-age hydration kinetics and matrix densification processes, a multi-parameter monitoring framework integrating electrical conductivity (EC), volumetric water content (VWC), and matric suction (ψ) was implemented. Hydration products and phase assemblages were characterized using X-ray diffraction (XRD), while physical properties and uniaxial compressive strength (UCS) were evaluated over extended curing periods. Results show that hybrid SC–SH activation fundamentally alters hydration pathways relative to Portland cement systems, promoting the formation of a composite phase assemblage dominated by amorphous C-(A)-S-H gels, calcite, and hydrotalcite-like phases. The hydration process follows a distinct two-stage mechanism consisting of an initial ionization–retardation stage with elevated ionic concentration and limited water consumption, followed by a deionization–acceleration stage characterized by rapid gel precipitation, progressive water binding, and pore structure refinement. Balanced SC/SH ratios significantly shorten the retardation period and enhance matrix densification, as indicated by decreasing EC and VWC and increasing matric suction. Among the formulations investigated, the SC/SH 50/50–10% mixture was the best-performing formulation, exhibiting the most efficient hydration kinetics, lowest porosity, and highest long-term UCS, achieving strength levels comparable to Portland cement-based CPB. These findings demonstrate that hybrid sodium carbonate–sodium hydroxide activation provides an effective strategy to mitigate carbonate-induced retardation while enabling controllable hydration behavior and sustainable strength development in slag-based CPB. The EC–VWC–ψ monitoring framework provides new mechanistic insight into hydration processes under realistic backfill conditions and supports the rational design of next-generation low-carbon slag-based cementitious binders for underground construction applications.