The maximum packing fraction (φ_fm) of flexible fibers is an essential parameter for understanding the rheological behavior of flexible fiber-reinforced cement paste (FFRCP). However, direct measurement of φ_fm of flexible fibers is still lacking. In this study, a shear rheology-based method for direct measurement of φ_fm was proposed and the assumption of fiber conformation under shear was verified by micro-CT. Based on this, a yield stress model for FFRCP was constructed to explain the entanglement and friction effects in the fiber network. Finally, static yield stress tests and small amplitude oscillatory shear (SAOS) tests were carried out to explore the structural build-up of FFRCP. It was found that the proposed method enables direct determination of φ_fm through only a few viscosity-fiber content data for a given FFRCP. Furthermore, the proposed model can describe the static yield stress of FFRCP well. Finally, the relative structural build-up rate of FFRCP follows a similar trend as the relative yield stress, with a critical relative fiber volume fraction (0.299) as the boundary. Subsequently, the relative structural build-up gradually deviates from the relative yield stress due to the limiting effect of the fibers.

The fresh superfine-tailings cemented paste backfill (SCPB) exhibits strong thixotropy, but quantification of the resulting transient flow (non-steady state) and time-dependent yield stress is lacking. In this study, a simple qualitative model was developed to describe the transient flow and time-dependent yield stress of SCPB. The effect of pre-shear time on the rheological behavior of SCPB was investigated. In addition, the adaptability of conventional non-Newtonian rheological models to SCPB was also evaluated. The results showed the Herschel-Bulkley model provides the most stable dynamic yield stress estimation of SCPB compared to the Bingham and modified Bingham models. A longer pre-shear time (within 500 s) led to smaller initial static yield stress and delayed recovery kinetics of static yield stress, but hardly affected the steady state of SCPB and the time required to reach it. The proposed model provides a good quantification of the transient flow at a given shear rate and time-dependent yield stress of SCPB.