Strength development in underground high-sulphate paste backfill operation

A laboratory investigation was undertaken to evaluate the effectiveness of different binding agents in a paste backfilling operation suitable to be us ed in a sulphide-rich environment. Different combinations of Portland cemen t and fly ash type C as well as ferrous slag were used as the binding agent s. The unconfined compressive strength of the samples was measured for exte nded curing periods. The results indicate that the addition of 3% slag in t he binding agent mix could produce strengths up to 0.5 MPa in the samples u nder investigation. Far higher binding contents, the addition of Portland c ement will lead to higher availability of calcium hydroxide and calcium alu minate, which are susceptible to deterioration from the phenomenon known as sulphate attack. Using slag, on the other hand, will reduce the permeabili ty, availability of calcium hydroxide, and dilute the calcium aluminate ion s reducing the susceptibility of the samples to sulphate attack. However, g ypsum will form which could have the same detrimental effect on the strengt h of the treated matrix. Therefore, special attention must be given to the content and properties of agents used to activate the pozzolanic properties of the slag.