973 2.2 Sulfide-rich tailings and admixture use Sulfide-rich tailings, such as those from base-metal deposits, pose specific challenges: high sulfur contents and reactive minerals can lead to sulphate generation, delayed ettringite formation, swelling and strength loss if mixes are not properly designed. Fine particle size distributions increase specific surface area and typically push both water and cement demand upwards. For these materials, simple increases in cement dosage can control early strength but at a disproportionate cost in CO₂ and operating expenditure. High-range water-reducing and rheology-modifying admixtures originally developed for concrete, have increasingly been evaluated for CPB. The formulations have been adjusted to adapt to the effects of the pernicious phases and Particle Size Distribution (PSD) variability of different tailings. Laboratory and field case studies show that they can maintain CPB pumpability (yield stress/slump) at a higher solids content, allowing lower water contents and improved strength efficiency. Translating these benefits from controlled tests into long-distance backfill systems with complex piping and variable tailings remains an area where mine-specific trials and partnerships are essential. 3. MATERIALS AND METHODS 3.1. Binder The CPB operation uses sulphate-resistant Portland binder, which meets ASTM C1157 standard. 3.2 Cerro Lindo tailings and existing backfill practice The actual backfill operation uses two paste plants to feed cemented paste to sublevel stopes via steel pipelines up to about 2 km long. Actual CPB production is approximately 250 ton/h, which is achieved with a continuous mixing and pumping process (Figure 1). Figure 1 – Cerro Lindo paste fill plant flowsheet Filtered tailings are delivered as a dense cake with solids contents around 90% by weight. X-ray fluorescence and mineralogical analysis indicate a material dominated by high sulfur contents linked to pyrite; quartz is the principal gangue mineral, with significant pyrite and minor barite, plagioclase, micas and chlorite (see below table 1 and Figure 2). Tailings particle size distribution (PSD) was measured using the laser diffraction method with a Mastersizer 3000 device, showing a fine material with more than 20% of
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