Ettringite strengthening at high pressures induced by the densification of the hydrogen bond network

Ettringite is a rare mineral with high-water content, more than half of its weight, and a relevant secondary product in Portland cement. Using density functional theory, we simulate the crystal structure and properties of ettringite under pressure. Our calculations predict a change in slope for all the lattice parameters versus pressure at about 2.5 GPa. Above such pressure, the elastic properties show a drastic increase of nearly 80% in the bulk modulus. This finding is explained in terms of a concurrent amorphization and densification of the hydrogen bond network. At low pressures, ettringite can be compressed substantially without significant repulsion in the hydrogen bond network. At high pressures, the hydrogen bonds become stiff, and their contribution to the total repulsion is then important. These changes are also supported by the evolution on the electronic density of ettringite with pressure.