An analytical model was developed to describe the influence of silica fume on the mechanical behavior of ultra-high-performance concrete. The model defines a single parameter,
, which estimates the thickness of paste surrounding all inert, unhydrated cement, and unreacted silica fume particles. The model considers adjustments for partially hydrated cement and partially reacted silica fume to model more accurately the total surface area in the system. The adjustments made for partial hydration and reaction enable the model to be applied to both water-starved and water-saturated mixtures. The compressive strengths of 82 UHPC mixtures containing eight different sources of silica fume were compared with model results, and results suggested that increasing paste thickness led to a decrease in the measured compressive strength.