Exploring the link between ultrasonic and strength behavior of cementitious mine backfill by considering pore structure

Abstract

Understanding the porosity/pore linkage and strength properties of cementitious mine backfill (CMB) is essential for the design of underground mining filling. Adoption of new production techniques in parallel with the development of technology in the mining sector has also increased the orientation to easier and less costly applications for evaluating CMB performance. This study aimed to evaluate the strength development of CMB and the change in its pore structure by using ultrasonic pulse velocity (UPV) technique. The influence of two types of pyritic tailings (low pH and high pH) obtained because of diverse production techniques on the quality of filling specimens was also investigated. The link between the microstructure formation (e.g., MIP), mechanical (e.g., UCS) and ultrasonic (e.g., UPV) characteristics of backfilling was explored by using a fixed ratio of fly ash-slag as a cement substitute (cement/slag-fly ash: 70/30). Filling samples were prepared at constant solid content (74 wt%), different curing times (3–90 days), and binder ratios (3–5-7 wt%). The results revealed that the total porosity of CMBs decreased with increasing age and binder ratio. One could also observe that the total porosity of the fill samples containing high pH tailings was lower (up to 56 %) compared to those containing low pH tailings. The acidic nature of low pH tailings negatively affected the aged strength of CMBs, destructing its microstructure. However, it was observed that the negative effects of low pH added backfills decreased in slag substitution and high binder dosages. Excellent correlation curves (power fitting) were found between UPV and UCS according to the variables used (e.g., different types of processing tailings and mineral additives). The measured and calculated UCS values were largely (R2 = 94.8 %) consistent. Finally, this study provides a complete report to support operationally low cost, easy and feasible fill design for underground metallic mines.