Influence of Silica Fume Particle Size on the Physico-Mechanical Properties and Pore Structure of Lightweight Cement with Hollow Glass Microbeads

Summary Cementing operations in deepwater formations with low fracture pressure gradients typically require the application of early-strength lightweight cement (LWC) to ensure wellbore integrity. In this paper, we compare the effects of two different sizes of silica fume (SF)—traditional SF and nanosilica fume (NSF, with finer size)—on the physicomechanical properties and microstructural features of density-controlled LWC. First, the rheological properties, settling stability, homogeneity, and compressive strength of LWC with different SF and NSF contents were experimentally evaluated. Subsequently, hydration products and microstructure characteristics were analyzed using thermogravimetric (TG) analysis and scanning electron microscopy (SEM). Multiscale pore structure analysis was performed via X-ray microcomputed tomography (μ-CT) and mercury intrusion porosimetry (MIP), achieving the quantification of pore-size distribution, fractal dimension, and porosity at different scales. In addition, the correlation between compressive strength and key pore structure parameters (e.g., Rm100 and Rn50) was comparatively investigated. The results indicate that both SF and NSF effectively mitigate the deficiencies of lightweight fillers by enhancing the stability and homogeneity of the cement pastes, promoting the formation of more and denser calcium silicate hydrate (C-S-H) gels, and reducing the overall porosity of LWC. Notably, NSF, with its finer particle size, exhibits a higher level of pozzolanic activity than SF, thereby facilitating early strength development. While the SF blends show lower overall porosity than the NSF blends, the NSF blends demonstrate a higher percentage of small pores. NSF can reduce the formation of harmful pores with sizes greater than 200 nm. A positive correlation was observed between compressive strength and the pore parameters Rm100 and Rn50. These findings provide valuable insights for optimizing SF-modified early-strength LWC, with direct implications for improving the design and reliability of deep-water cementing operations.