Influence of pyrrhotite content on the mechanical and chemical behaviour of cemented paste backfill

Abstract

The chemical weathering related to sulphide minerals oxidation within mine backfill in underground stopes can cause serious problems, such as the loss of mechanical strength due to sulphate attack. This can also lead, under specific (extreme) conditions, to excessive self-heating of the backfill and to oxygen depleted atmosphere. Previous studies indicate that sulphide minerals contained in cemented paste backfill (CPB) are less reactive (i.e. less prone to oxidation) than those contained in hydraulic backfill (HB) because the favourable hydraulic properties of CPB can help maintain a high degree of water saturation (often more than 85%) in the fill material. This paper presents results from an investigation of the influence of pyrrhotite content on the physico-chemical behaviour of cemented paste and hydraulic backfills. Oxygen-consumption (OC) tests, uniaxial compressive strength (UCS) tests, and a post-testing characterisation were performed on different mixtures of cemented backfills made of tailings with 5% wt of binder and various percentages of pyrrhotite (2 to 15%). Tailings without binder were also assessed with the OC test (as control samples). All samples were cured under controlled temperature and moisture conditions (30°C and 100% respectively) to simulate curing under warm mine site conditions, for periods of 3, 7, 28, 56, 182, and 365 days. The OC tests results show that pyrrhotite-rich tailings without binder can be very reactive; the consumed oxygen flux measured can exceed 35 mol/m2/day when these are exposed to atmospheric concentration. OC tests also show that the addition of binder (Portland cement) in the backfill mixtures reduces their reactivity; a more significant reduction was observed for paste backfill samples compared with hydraulic backfill specimens. The OC tests results furthermore indicate a decrease of the control samples (tailings without binder) reactivity over time, suggesting that other factors, such as passivation of the pyrrhotite grains surface during weathering, could also influence the evolution of backfill reactivity. The UCS tests results show that paste backfill samples reached higher strength than hydraulic backfill. However, a progressive loss of strength was also observed for all backfill mixtures after a few months, probably due to sulphate attack.