Studies of the structure of hydroxides under pressure using neutron diffraction reveal that the high concentration of hydrogen is distributed in a disordered network. The disorder in the hydrogen-bond network and possible phase transitions are reported to occur at pressures within the range accessible to experiments for layered calcium hydroxides, which are considered to be exemplary prototype materials. In this study, the static and dynamical properties of these layered hydroxides are investigated using a quantum approach describing nuclear motion, shown herein to be required particularly when studying diffusion processes involving light hydrogen atoms. The effect of high-pressure on the disordered hydrogen-bond network shows that the protons tunnel back and forth across the barriers between three potential minima around the oxygen atoms. At higher pressures the structure has quasi two-dimensional layers of hydrogen atoms, such that at low temperatures this causes the barrier crossing of the hydrogen to be significantly rarefied. Furthermore, for moderate values of both temperature and pressure this process occurs less often than the usual mechanism of proton transport via vacancies, limiting global proton diffusion within layers at high pressure.

A critical challenge in reducing anthropogenic impacts on the environment is to decrease the carbon footprint of the cement industry. A key concern in the search for more sustainable cement designs is the understanding and control of the depolymerization process that eventually determines the integrity of their silicate chains under mechanical, chemical, or thermal stresses. Herein, we use metadynamics to show that the depolymerization of cement silicate skeletons consists of hydroxylation followed by bond-breaking. We then clarify the local effects of doping the silicate chains: a stable pentacoordinate state following hydroxylation is promoted by aluminum atoms but restrained by phosphorus additions, the presence of two dopants being related to energy landscapes less favorable to bond-breaking. The role of these dopants is explained in cement-based materials and is key to the quest for low-cost opportunities to preserve the strength of cement for high temperatures or even over time.

Silicate-chains polymerization is a crucial process in calcium silicate hydrate minerals, with large relevance for improving the durability and reducing the environmental impact of cement-based materials. To better understand the evolutionary mechanisms underlying the polymerization of silicate-chains in layered calcium silicate hydrates, we herein propose to trace the evolution of the polymerization degree by using silicon isotopes. The method requires tabulating the isotopic fractionation of several basic chemico−physical mechanisms that we obtained by performing atomistic simulations. The calculations reveal that the highly polymerized structures have longer Si−O bonds and that the Ca 2+ cations play a dual role in the stretching and bending mode properties of silicates, such as isotopic fractionation is able to discern not only between the polymerization order of calcium silicate hydrate minerals, but even between cement gels suffering calcium leaching. Silicon isotopic fractionation can, therefore, be used to quantify the different evolutions of calcium silicon hydrate phases in a sample of man-made gel cement in order to improve its sustainability along lifetime stages in the quest for green cement.