Concrete contains particles of different shapes. The porosity of the granular skeleton is
low since particles in a wide range of sizes (cement and aggregates) are applied in
concrete. Very small particles (e.g. micro- or nanosilica) widen the grading curve even
more, but significantly increase the total surface area. The water demand depends on
the interstices of the granular skeleton, the size, the shape and the content of each
aggregate fraction. Surface forces affect the packing density of very small particles;
frictional forces dominate the interaction between larger particles. Dispersed particles
pack tighter compared with the same particles when agglomerated. To decrease the
porosity of the granular skeleton of very fine particles, plasticisers and superplasticisers
are applied. They disperse the cement and the fillers. The fewer interstices the aggregate
skeleton contains, the less paste is required to fill them. The excess of the paste
surrounds the solids and decreases the friction. Except of the surplus of paste,
workability also depends on the characteristics of the cement paste and the surface
characteristics of the aggregates.