Strength of the hardened paste

In discussing the hydration process it was stated that setting and hardening of the cement paste is brought about by the formation of a CSH gel. The gel fills the space between the cement grains, bridges between them, and thereby causes stiffening of the paste and its subsequent hardening. The continued formation of the gel gradually fills the capillary pores, the porosity of the paste decreases, and its strength is increased. However, the mechanical strength of the paste and its stability in water require further discussion with respect to the nature of the bond between gel particles. Generally speaking, the strength of the paste may be attributed to cohesion forces (van der Waals forces) acting between the gel particles (secondary bonds), or to the intergrowing of the crystallites and the formation of chemical bonds (primary bonds) at their points of contact. In this context it is generally meant that a chemical bond is a solid to solid contact similar to that existing at the grain boundary of a polycrystalline material, where some of the atoms approach the spacing of the atoms in the crystals, and a close fit exists between the lattices of the neighbouring crystals. Such bonds could be formed during a crystallisation process accompanying a chemical reaction when the mobility of the atoms allows for a regular arrangement. The strength of a material characterised by such bonds (e.g. gypsum) is determined by the number of bonds per unit volume, the bond strength, and the strength of the crystals themselves.