Laboratory Study on Strength Development in Cemented Paste Backfills

Cemented paste backfills (CPBs) are widely used in backfilling underground mine voids. They are fine-grained mine tailings with significant clay content, with a small dosage of a general purpose cement (GPC) or pozzolanic binder such as blended cement (BC) containing slag or fly ash. This paper summarizes the findings from an extensive laboratory test program carried out to understand the effects of binder dosage and solid content on the strength development over 56 days of curing. Ten different mixes were studied using 80 specimens, and it was shown that the blended cement mixes give an early gain in strength and stiffness, along with higher long-term values compared with the GPC mixes. It was also shown that the E/UCS, which is the ratio of Young's modulus (E) to uniaxial compressive strength (UCS), values for the CPB throughout the curing period were in the range of 150–350, with an average value of 250. Two simple mathematical expressions were developed to express UCS as a function of time and the 14-day UCS value (UCS14). In the case of BC mixes, the relative effect of increasing the solid content or binder dosage was studied through separate UCS tests carried out at 28 and 56 days using 40 additional specimens.