Effect of dispersion behavior of silica fume on the rheological properties and early hydration characteristics of ultra-high strength mortar

Silica fume plays a crucial role in ultra-high performance cementitious materials, but its dispersion, influenced by varying densification levels, leads to different degrees of agglomeration. However, the precise impact of this on rheological properties and early hydration characteristics of ultra-high performance cementitious materials remains unclear. This study assesses the impact of highly densified silica fume (HDSF), moderately densified silica fume (MDSF), and raw silica fume (RSF) on the rheological prperties of ultra-high strength mortar (UHSM). Additionally, the early hydration characteristics of UHSM influenced by these silica fumes are investigated through both heat of hydration and penetration resistance methods. The results showed that HDSF contained numerous large agglomerates, contrasting with MDSF and RSF, where the agglomerate sizes were comparable to those of cement particles. As the content of HDSF increased, the yield stress, viscosity, and thixotropy of UHSM gradually rose, accompanied by a notable decrease in hydration rate. Conversely, increasing MDSF and RSF content initially decreased and then increased these rheological properties, while the hydration rate remained relatively unchanged. The distribution behavior of the silica fumes influences the rheological properties of UHSM through water film thickness (particle spacing). Large agglomerates in HDSF reduce the activity of the entire cementitious system, significantly decreasing the hydration rate. In contrast, MDSF and RSF primarily influence the particle spacing within the cementitious system, subsequently impacting the hydration rate. This study provides theoretical insights and practical guidance for construction professionals in selecting appropriate silica fume for designing and regulating ultra-high performance cementitious materials performance.