| Summary: | The thinning of tailings gradation during ore processing leads to a sizeable fall in the strength of cementitious paste backfill (CPB), increases operational risks, and encourages researchers to use alternative economic products. This study aims to increase the strength performance by improving CPB’s gradation while cutting costs and reducing the sum of the binder employed per unit volume. An evolution of the slump/strength/structural properties of sand-substituted CPBs was explored experimentally. Samples were made with a fixed cement content (7 wt.%), diverse tailings/sand fractions (e.g., 100/0, 90/10, 80/20, 70/30, and 50/50), and diverse solid contents (e.g., 72 and 76 wt.%). After curing for 3–56 days, several experiments, such as slump, uniaxial compressive strength (UCS), mercury intrusion porosimetry (MIP), and scanning electron microscopy (SEM), were undertaken for the filling samples. The results demonstrate that adding sand to the backfill greatly increases CPB’s strength (up to 99%), but the replacement rate of sand was limited to 30% due to its segregation effect. Microstructural tests reveal that CPB’s void volume decreases as the added amount of sand increases. To sum up, it was concluded that calcareous sand made a major contribution to the filling strength, incorporating the effects of enhancing the fill gradation’s readjustment and reducing the sum of cement being used in the unit volume for CPB manufacturing.
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