Influences of mine water on the properties of construction and demolition waste-based cemented paste backfill

Abstract

Construction and demolition waste (CDW) is a solid waste generated by urbanisation development around the world, and its treatment and utilization are burgeoning universally. The CDW are generally prepared into a slurry of cemented paste backfill (CPB), which is pumped to the goaf to control surface subsidence and utilize CDW. The underground mine water affects the mechanical properties of CPB. To simulate influences of mine water environment, the CPB specimens sampled onsite were subjected to soaking and drying-wetting cycles (soaking for 12 h and then drying for 12 h constituted one cycle) tests, and corroded at different durations (0, 2, 4, 6, 8, 10, 12, 14, 28, and 56d). Then, unconfined compressive strength (UCS) tests were conducted to investigate the effect of mine water corrosion on mechanical behavior of CPB, a gray model was established to predict long-term strength based on the measured compressive strength, and microstructural analysis such as X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) were performed to reveal the microstructure changes associated with mine water erosion. The results demonstrate that the erosion environments of soaking and drying-wetting cycles can both degrade the UCS of CPB, which first shows a rapidly decreasing trend (slowing later). The maximum degradation of UCS of CPB reached 6.856% when soaking duration reached 8d, and 15.311% when drying-wetting cycles reached 10d. And that the evolution of the modulus of elasticity of CPB is like that of the UCS and an exponential function was proposed to characterise the correlation between them, with a goodness of fit of 0.969. Similarly, both SEM and XRD of specimens at erosion durations of 8d and 10d indicate that, by decomposing hydrated gels in CPB, HCO3−, and SO42− ions in the mine water generate bulk calcite and columnar gypsum to expand crack structures in CPB, which reduces the strength of CPB. Furthermore, through drying-wetting cycles, the cracks blocked by corrosives are coalesced many times, facilitating the repeated migration of corrosive ions, so that the corrosion under drying-wetting cycles is stronger than that under soaking. In addition, according to the grey model, the changes in the UCS of CPB within 150 d under soaking and drying-wetting cycles were predicted and it was determined that UCS of CPB first reached the critical value (at failure) in drying-wetting cycles, and its erosion durations was 136d. This guides the environmental management of underground CPB and protection of the surface ecological environment.