In this study, the following three strategies were employed to analyse the chemical structure and surface characteristics of CBF under different conditions: 1) using the Dinger-Funk particle packing model and by applying hollow glass spheres (HGS) and micro-silica (MS), a low-density cement was designed; 2) cellulose-based fibres (CBFs) were used to optimize the low-density cement, and the effects of CBF on the rheological properties, thickening time, mechanical properties, and microstructure of low-density cement were studied by scanning electron microscopy (SEM), X-ray electron spectroscopy (XPS), and Fourier-transform infrared (FT-IR) spectroscopy. The experimental results show that in the low-density cement slurry, some lignin, hemicellulose, and impurities on the CBF surface were removed to expose hydroxyl groups (-OH), CO, and CO, which increased the oxygen atom content from 35.64 at% to 41.24 at% and improved the hydrophilicity and dispersity of CBF. In the cement slurry, the MS surface was negatively charged by hydrolysis reaction; therefore, MS could be absorbed on the CBF surface by chemical action. In addition, when the pozzolanic reaction of MS was used, the interface bonding between CBF and the cement matrix improved. Furthermore, pull-out and prevention of crack propagation were also the main CBF reinforcement mechanisms. After curing for 28 days at 90 °C, the compressive strength of the low-density cement with 0.6 wt% CBF increased from 28.5 MPa to 30.85 MPa, and the tensile strength and flexural strength increased by 52.06% and 34.55%, respectively. The peak strain of the triaxial stress-strain curves increased by 72.08%. Finally, the cyclic triaxial stress-strain curves showed that CBF improved the elastic deformation of the low-density cement.
