The design theory of ultra-high performance concrete is based on the Maximum Density Theory. According to this theory, the constituent materials of different particle sizes are combined in optimal proportions to achieve the densest packing. Specifically, the gaps between millimeter-sized particles (aggregates) are filled by micron-sized particles (cement, fly ash, and slag powder), and the gaps between micron-sized particles are filled by sub-micron-sized particles (silica fume). Silica fume can react with hydration products (calcium hydroxide) to form even finer particles, which provide secondary filling, resulting in a more compact and dense structure.
