Dispersion behavior of silica fume in cementitious suspensions

The dispersion characteristics of silica fume are pivotal factors influencing the rheological properties, mechanical properties, and durability of cement-based materials. This study investigated the dispersion behavior of different types of silica fume including highly densified silica fume (HDSF), moderately densified silica fume (MDSF), and raw silica fume (RSF) in water and cement paste filtrate mediums. Optical microscopy (OM) was utilized to monitor the silica fume agglomerations in suspensions, and scanning electron microscopy (SEM) was employed to observe the agglomerates in hardened ultra-high performance concrete (UHPC) containing 25 wt% silica fume. The results revealed that RSF, MDSF, and HDSF have similar particle size distributions in water and cement paste filtrate, with D50 values of 10.9 μm, 15.0 μm, and 112.2 μm, respectively. The agglomeration degree of silica fume is virtually unaffected by superplasticizer. Additionally, the effects of dispersion methods (i.e., ultrasonic, aerodynamic, and aggregate premixing) on silica fume dispersions were also investigated. It was shown that the aerodynamic dispersion method significantly reduced silica fume agglomeration, and adequate ultrasonic treatment almost eliminated physical agglomeration. By contrast, premixing silica fume with aggregates and steel fibers partially mitigated agglomeration. This study provides a theoretical foundation and technical guidance for the effective utilization of different types of silica fume in concrete applications.