When water dances in cement : nanoscopic choreography

Cement is the world’s most widely used material. Despite centuries of intensive use and ever-increasing global demand, many fundamental physico-chemical questions about its nanoscale structure remain unanswered. An international research team has used neutron scattering techniques to study the dynamics of water inside concrete, one of the keys to its strength.

During the cement setting process, various nanoscopic phases, known as hydrates, are formed. Among these hydrates, calcium silicate hydrate (C-S-H) is the most important binder phase - the one that confers cohesive properties - and the key compound that controls the cement’s final properties, such as strength and durability. In contact with ambient humidity, these hydrates are covered by a thin layer of water. The properties of this ’adsorbed water’ are very different from those of water as we know it. This study has probed the properties of these water layers, revealing the ’dance’ of water molecules at the nanoscale.

Understanding the organization of water in the C-S-H phase is not only important for a better understanding of the phenomena associated with cement setting, but also because water plays a key role in the processes that are the main causes of its loss of strength. In addition, water diffusion in C-S-H nanopores is important to understand when considering other cement applications such as pollutant removal, nuclear waste storage, or for the improved development of low CO2 cements.

Figure 1. Les silicates de calcium hydratés (C-S-H) sont la principale phase liante dans le béton que nous fabriquons avec du ciment Portland, celle qui lui confère les propriétés de cohésion et mécaniques. Ce sont des structures nanocristallines en couches, avec une teneur élevée en eau (voir l’image du milieu pour une vue atomistique d’une particule de silicate de calcium hydraté). L’eau proche de la surface est fortement liée aux ions calcium, ce qui entraîne des mouvements restreints et une manière particulière de « danser » sur la surface.

In this article, published in Cement and Concrete Research, an international research collaboration studied water dynamics in C-S-H and C-A-S-H binders (containing aluminum, as in low-CO2 cements) using neutron scattering techniques at the Institut Laue Langevin (Grenoble, France) and the ISIS neutron source (Oxford, England).

"Neutron scattering techniques are ideal for studying water dynamics because of their strong interaction with hydrogen. They allow us to study a wide range of types of water motion in the C-(A)-S-H phase that cannot be measured with other techniques," explains the paper’s first author, Zhanar Zhakiyeva, from the University of Grenoble Alpes. "We were thus able to probe the vibrations of the water molecules that are present in the various C-S-H nanopores".

The team interpreted their experimental data using spectra calculated from their computer-simulated models. They observed the different ’fingerprints’ of water molecules confined in different pores, and how these molecules ’dance’ differently depending on their surroundings, more or less in contact with the cement surface, or in drier or wetter conditions.

"Overall, our experimental and simulation results indicate that different types of water are present in the thin interfacial water layers, with movements or ’dances’ that are more or less similar to so-called bulk water (water as it is in a glass of water)," explains Zhanar. "In particular, the presence of calcium ions on the surface tends to maintain water in the form of highly structured surface layers, similar to an ice-like structure."

Figure 2. Des expériences de diffusion de neutrons et des modèles d’ordinateur ont été utilisés pour étudier la « danse » des molécules d’eau à l’interface ciment-eau. Les neutrons sont particulièrement sensibles aux modes vibrationnels de l’eau (voir figure 2). Les ‘librations’ d’eau (voir figure 2, a droite) sont des modes vibrationnelles où les molécules d’eau ne peuvent pas ‘danser’ librement dû à ses fortes interactions avec les atomes de la surface du ciment, notamment les atomes de calcium. Cette étude a permis une description détaillée des vibrations de l’eau dans les constructions en béton.

This study has determined for the first time the moisture range in which bulk or ice water is present in C-(A)-S-H samples. This is a key discovery for improving the carbonation processes that take place in the presence of CO2, and which are advocated for the development of low-CO2 cements. Other processes, such as creep, are also expected to be influenced by the presence of interfacial water as opposed to bulk water.

References

Zhanar Zhakiyeva, Valérie Magnin, Agnieszka Poulain, Sylvain Campillo, María P. Asta, Rogier Besselink, Stéphane Gaboreau, Francis Claret, Sylvain Grangeon, Svemir Rudic, Stéphane Rols, Mónica Jiménez-Ruiz, Ian C. Bourg, Alexander E.S. Van Driessche, Gabriel J. Cuello, Alejandro Fernández-Martínez. Water dynamics in calcium silicate hydrates probed by inelastic neutron scattering and molecular dynamics simulations, Cement and Concrete Research, Volume 184, 2024, DOI

Local scientific contact

– Alejandro Fernández-Martínez, ISTerre/OSUG, CNRS