Experimental study on the performance of the modified loess by polypropylene fiber mixed with bentonite and cement

Loess is the most common building material in Northwest China. However, because of its loose accumulation, it is always unable to satisfy the requirements of actual engineering. In this paper, the effect of randomly distributed polypropylene fiber length (PPL), polypropylene fiber content (PPC), bentonite (B), and cement (C) on unconfined compressive strength (UCS) and permeability coefficient were investigated, and the optimum conditions were determined. The Taguchi method was applied to the experiments to choose the optimum conditions as 6-mm PPL (1), 0.3% PPC (2), 3% B (2), and 8% C (3), which abbreviated as PPBC loess. Meanwhile, by comparing the PPBC loess with lime–loess; it was found that the PPBC loess was significantly better than the lime–loess sample in terms of both UCS and permeability. The microstructure of PPBC loess and lime–loess was investigated by scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR), and nuclear magnetic resonance (NMR). It was exhibited that the reinforcement of lime–loess was mainly realized by producing C–S–H gels through chemical reactions, and the properties of the PPBC loess were improved by the physical reinforcement of polypropylene fibers and the chemical effects of cement. Moreover, according to the results of FTIR and NMR that the improvement of the mechanical properties of PPBC loess was caused by the increase of the adsorption band intensity and the decrease of pores, the mechanical properties of lime–loess with different contents were mainly affected by the decrease of pores.