Effect of Sample Size on the Unconfined Compressive Strength of Cemented Mill Tailings

Abstract: To determine if the size of a cemented mill tailings sample affects its unconfined compressive strength (UCS), researchers with the National Institute for Occupational Safety and Health (NIOSH) conducted three series of UCS tests with cylindrical samples of cemented paste backfill (CPB), ranging in size from 0.5×1.0 inch (13×25 mm) to 6×12 inches (150×300 mm), after the samples had cured for 28, 90, and 180 days. The average strengths of the standard sized samples were comparable, indicating that 2×4-inch (50×100-mm) cylinder molds provide a reasonable alternative to larger sized cylinders. A comparison of UCS test results and sample aspect ratios (i.e., surface area divided by volume) demonstrated that the size of a CPB sample does not significantly affect its measured compressive strength until the dimensions of the test specimen are smaller than 1×2 inches (25×50 mm). The 0.5×1.0-inch (13×25-mm) samples exhibited a heightened response to strength gain or loss with curing age, which appeared to be related to the physical dimensions of the sample and its hydration rate. The development of more appropriate and consistent methods for collecting, preparing, and testing samples of cemented mill tailings will lead to more accurate quantification of mix design strengths and more effective quality control measures.