Cement is the most widely used material in the world. When mixed with sand, gravel and water it produces concrete. Despite several centuries of intensive use and ever-increasing global demand, many fundamental questions about its structure remain unanswered. An important example is the organisation and dynamics of water in cement-based materials, which is still a matter of debate, in spite of its major influence on properties such as durability and its relevance for other cement applications (e.g. pollutant removal) or in the development of low CO2 cements.
The process through which water combines with cement particles (called cement hydration) is indeed complex. It involves several chemical and physical reactions and is influenced by multiple factors such as temperature and water-to-cement ratio. As hydration reactions proceed, their products gradually bond together the individual particles to form a solid mass.
During the process, different ‘hydrates’ (the products of hydration) are formed. Calcium silicate hydrate (usually denoted C-S-H) is the most important binder phase – the glue that keeps everything together. On the other hand, water remains in the final product, either as an integral part of the different components or trapped in the porous structure of concrete. The properties of this ‘adsorbed water’ can be very different from those of water as we know it.
In a study published in Cement and Concrete Research Journal, an international research collaboration studied the dynamics of water in hydrate binders using neutron scattering techniques at the Institut Laue Langevin (Grenoble, France) and the ISIS neutron source (Oxford, England).
‘Neutrons 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 movements in hydrate binders that cannot be measured using other techniques,’ explains the paper's first author, Zhanar Zhakiyeva, who recently concluded her PhD at the ILL, University of Grenoble Alpes (UGA)* ‘We were thus able to probe the vibrations of the water molecules present in the various C-S-H nanopores’.
‘The neutron scattering spectra measured using the IN1-Lagrange instrument at the ILL provide insights into the intramolecular vibrations of water molecules and the strength of the hydrogen bond network they form, which varies significantly between confined and bulk-like water. This information can be obtained because neutron spectroscopy allows all vibrational modes to be measured,’ adds Mónica Jiménez-Ruiz, ILL scientist responsible for the spectrometer IN1-Lagrange.
The team interpreted their experimental data in the light of spectra calculated from their computer-simulated models.
“Molecular dynamic simulations allow us to interpret the data, pinpointing the different spectral ‘fingerprints’ of the water molecules confined in pores, and understanding how these molecules move depending on their surroundings: their distance to the surface of cement, in dry or wet conditions” explains ILL scientist Stéphane Rols.
This study has determined for the first time the moisture range at which bulk water or ice water is present in the hydrate samples.
‘Overall, our experimental and simulation results indicate that different types of water are present in the thin layers of interfacial water, with movements that are distinct from 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 the water in the form of highly structured surface layers, similar to an ice-like structure.’
* The PhD work of Zhakiyeva was supported by ILL, ISTerre (CNRS/IRD/UGA/Univ. Savoie Mont BlancNRS, Univ. Gustave Eiffel) and Bureau de Recherches Géologiques et Minières,
ILL Instrument: IN1/Lagrange
ILL contact person: Mónica Jiménez-Ruiz
Reference: 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, 184, 2024, 107616, https://doi.org/10.1016/j.cemconres.2024.107616