Mechanical behavior and microscopic mechanism of fiber reinforced coarse aggregate cemented backfill

Backfills have traditionally served as artificial stope roofs during the mining of cemented undercut-and-fill, which still has many problems, such as weak tensile strength, brittleness, poor ductility, etc. To improve the stability of the backfill, fluidity tests, mechanical tests, and microscopic characteristics tests were conducted on fiber-reinforced coarse aggregate cemented backfill (FCCB) with different polypropylene fiber (PPF) contents and curing times based on the existing filling parameters of Longshou mine in China. The results showed that the slump and expansion of FCCB slurry presented a slight linear decrease with an increase in PPF content, and the slurry satisfied the requirement of self-flow transport. Additionally, the uniaxial compressive strength (UCS) and splitting tensile strength (STS) of FCCB showed growth to a different extent. When the PPF content was 0.10 %, the enhancement effect was optimal. However, the backfill deteriorated as the content was more than 0.10 %. Moreover, the effects of a single 15 mm PPF on UCS and STS were better than that of hybrid fibers with different scales. The peak strain and the compressive and splitting toughness index (Ti) of FCCB increased at different degrees with an increase in PPF content, which was significantly higher than that of the backfill without fiber. After adding PPF, the backfill appeared characteristic of a crack but not broken. Furthermore, the hydration products, including CSH gel, Ca(OH)2 (CH), and ettringite (AFt), increased and became thicker with curing time, the compactness of the interfacial transition zone (ITZ) increased, the pores decreased, and the adhesion of PPF and cement stone increased, enhancing the microscopic mechanical properties of FCCB. This study would improve the safety of mining and have high application value.