Effect of superplasticizers on the consistency and unconfined compressive strength of cemented paste backfills

This paper assesses the effects of different types of high-range water reducer (HRWR) on the consistency (slump) and unconfined compressive strength (UCS) of cemented paste backfills (CPB) prepared mainly with tailings T1 at 80% solids mass content (a few tests were also conducted with tailings T2). A blended binder made of 80% ground granulated blast furnace slag and 20% GU (general use Portland cement) was prepared at three different proportions: 3.5%, 4.5%, and 6% (by dry mass of tailings). Six HRWR types were tested: four polycarboxylates (PC1 to PC4), a polymelamine sulfonate (PMS), and a polynaphtalene sulfonate (PNS). The slump values for fresh CPB and the UCS for the hardened CPB specimens were determined at different curing times. Test results showed that the addition of HRWR to CPB mixtures improved CPB consistency and UCS, regardless of HRWR type. For tailings T1, a minimal dosage of 0.121% (by dry mass of tailings + binder, for each PC type) was required to achieve a slump value of 152 (6 in., which is generally considered the lower limit in backfilling operations to achieve pumpable CPB). The PC-type HRWR more effectively increased CPB consistency compared to PNS- and PMS-type HRWR. However, UCS values at 28 days were comparable for CPB incorporating all HRWR types. The UCS increase after 28 days of curing for superplasticized CPB ranged from 21% to 29% compared to the control CPB mixture (without HRWR), depending on binder content. Results are discussed in terms of the impact of HRWR addition on the change in water-to-binder ratio, the contribution of solids content and HRWR to UCS improvement, and the potential reduction in binder consumption.