Effect of replacing Portland cement with limestone, metakaolin and blast furnace slag on the unconfined compressive strength of cemented paste backfill

Cemented paste backfill (CPB) is widely used in the mining industry to stabilise underground voids while allowing the re-use of mine tailings. This approach reduces the environmental risks linked to surface storage of tailings. Nevertheless, stabilisation capacity relies on the unconfined compressive strength (UCS) of CPBs. The UCS, a critical property, is influenced by factors such as the physical and chemical characteristics of the tailings, the solids mass concentration, and the type and proportion of binder used. However, the important carbon footprint and increasing cost of conventional general use (GU) Portland cement, as well as the depletion of ground granulated blast furnace slag (GGBFS) have prompted the need for more cost-effective and environmentally friendly materials. This paper investigates the incorporation of alternative materials, such as limestone (LS) and metakaolin (MK), into binder formulations. The study utilises a mixture design approach to identify the optimal proportions of these alternative materials to enhance the UCS of the CPB at 28 and 56 days. The methodology includes five reference mixes and 10 mixes of two binary binders (GU-MK and GU-LS), 13 mixes of ternary binder (GU-LS-MK) and 33 mixes of quaternary binder (GU-LS-MK-GGBFS) Unconfined compression tests were performed on these different mixtures after curing times of 7, 28 and 56 days under underground mining conditions. The 61 UCS results at 7, 28 and 56 days of curing demonstrate that formulations incorporating LS, MK (calcined clay) and GGBFS exhibit promising UCS performance. Although further improvements are required to compete with the GGBFS/GU blend, these formulations are anticipated to hold significant potential for reducing both costs and the carbon footprint associated with CPB in mining operations relying solely on GU.