Effects of mineral additives and age on microstructure evolution and durability properties of sand-reinforced cementitious mine backfills

Abstract

• Effects of sand, slag and fly ash on microstructure and strength of cemented fillings were studied. • Backfill specimens were manufactured with constant cement (5 wt%) and solid (72 wt%) contents. • Among others, 30% sand-based cementitious fillings offered the best mechanical strengths. • Sand-GBS-based backfills supports hydration reactions and prevent fill acidifying in the long-term. • Voids of fine-grained GBS and FA is well filled with sand, improving the strength of fillings. Cementitious paste/mine backfill (CMB/CPB) is a creative way to proficiently decrease the sum of sulfidic tailings. However, considering them as minor raw material and inadequacies in their gradation, OPC (costly and insufficient)-induced ecological harms, using alternative additives in CMB has become essential. This study aims to increase the short-/long-term filling performance and to reduce cement-related costs by using both sand as tailings substitute, and GBS or FA (granulated blast furnace slag or fly ash) as OPC substitution. Some mechanical (UCS, stress–strain, and modulus of elasticity) and microstructural (MIP, XRD, TG/DTG and SEM) characterization tests for CMBs were experimentally scrutinized. Samples were prepared at diverse curing ages (3–180 days), constant cement/solid content (5/72 wt%), three diverse aggregates (tailings/sand: 100/0, 90/10 and 70/30) and OPC (OPC/GBS-FA: 100/0, 90/10 and 70/30) substitution ratios. Results disclosed that 30% sand-based CMB (S30) offered the best strengths. Besides, strength loss was observed in all 90-day cured backfills, while strength gain was observed in sand/GBS-based backfills up to 180 days. This can be stated by the fact that sand/GBS supports hydration reactions and prevent fill acidifying in the long-term owing to their high CaO contents (sand: ∼54%; GBS: ∼35%). Moreover, improving sand’s gradation, filling the voids of fine-grained GBS and FA is one of the cogent reasons for strength gains. To summarize, this study runs a full report on a cost-effective and viable CMB system that will both make operations profitable/sustainable and prevent environmental damages.