Unraveling complex nucleation pathways in natural and engineered cements – NUANCE

Amorphous precursors to calcitic (CaCO3) and calcium silicate hydrates (C-S-H, the main binding phase in Portland cement) were synthesized at the laboratory using wet chemistry methods. Their crystallization was followed using in situ and ex situ methods that include high-energy X-ray scatering (in situ) followed by Pair Distribtion Function analyses, laboratory-based X-ray diffraction, infrared spectroscopy, transmission electron microscopy, thermal analyses, X-ray photon correlation spectroscopy and neutron scattering. The use of all these techniques allows probing the internal structure and diffusivity of the ions and molecules forming the precursors, and to unravel the key points controlling their kinetic persistence against crystallization. A particular emphasis was put to control the humidity around the amorphous carbonates, as water is a key component regulating their crystallization.

Amorphous precursors to biomineralization contain important amounts of water. Our experiments showed that this water content regulates their internal ionic dynamics, therefore acting as a control on the crystallization kinetics. Moreover, synchrotron-based experiments have unraveled an ‘on-off’response of the ionic diffusivity when subject to cycles of different humid atmospheres.
Our studies of the disordered precursors to Portland cement have resulted in a detailed view of their internal structure. The results show also that the presence of organic additivies such as gluconate induce the formation and stabilization of the amorphous precursor. Atomic structures for the amorphous precursors have been proposed that will be used to model their interactions with organic additives.

The project has led to an academic-industrial scientific collaboration with the ESRF and Germany via the InnovaXN project (EU), which funds a PhD thesis.
An academic collaboration has been started with the Univ. Basque Country (Spain) to pursue the studies on the internal structure of the precursors to Portland cement, via the use of molecular modeling.
An European project (ITN) has been submitted to pursue the work initiated here, with the aim of understansing how other organics (polymers) control the crystallization kinetics of the amorphous precursor to C-S-H.

1. Monitoring a Mechanochemical Reaction Reveals the Formation of a New ACC Defect Variant Containing the HCO3- Anion Encapsulated by an Amorphous Matrix. Phil Opitz, Maria P Asta, Alejandro Fernandez-Martinez, Martin Panthöfer, Anke Kabelitz, Franziska Emmerling, Mihail Mondeshki, Wolfgang Tremel. Crystal Growth and Design, 20, 10, 6831–6846 (2020).

2. Nanoscale ion dynamics controls on amorphous calcium carbonate crystallization : precise control of calcite crystal sizes. Maria Asta, Alejandro Fernandez-Martinez, Juan Alonso, Laurent Charlet, Nathaniel Findling, Valerie Magnin, Beatrice Ruta, Michael Sprung, Fabian Westermeier. Journal of Physical Chemistry C, 124, 46, 25645–25656 (2020).

3. Structure of water adsorbed on nanocrystalline calcium silicate hydrate determined from neutron scattering and molecular dynamics simulations. Zhanar Zhakiyeva, Gabriel J. Cuello, Henry Fischer, Daniel Bowron, Catherine Dejoie, Valerie Magnin, Sylvain Campillo, Sarah Bureau, Agnieszka Poulain, Rogier Besselink, Stephane Gaboreau, Sylvain Grangeon, Francis Claret, Ian C. Bourg, Alexander E. S. Van Driessche, Alejandro Fernandez-Martinez. Submitted (2021).

4. Structural characteristics and kinetics of formation of an amorphous precursor to calcium silicate hydrate: interactions with gluconate. Rogier Besselink, Agnieszka Poulain, Michela La Bella, Sarah Bureau, Valerie Magnin, Leighanne C. Garrington, Carlotta Giacobbe, Alexander E.S. Van Driessche, Alejandro Fernandez-Martinez. Submitted (2022).

5. Organic controls of ionic dynamics in amorphous calcium carbonate. Maria Asta, Alejandro Fernandez-Martinez, Juan Alonso, Laurent Charlet, Nathaniel Findling, Valerie Magnin, Beatrice Ruta, Michael Sprung, Fabian Westermeier. In preparation (2022).

6. ‘Diffraction de rayons X d’haute énergie et analyses de fonction de distribution de paires : observations in situ de la formation et réactivité de minéraux et phases inorganiques nanocristallines’. A. Fernandez-Martinez, A. Lozano, S. Carrero, R. Besselink, A. Poulain. (2020). Chapitre dans ‘Rayons X et Matiere’, ISTE Editions, London. UK.

In recent years, multi-step nucleation pathways involving the formation of pre-nucleation clusters, nanocrystalline and amorphous precursors, have been reported for a wide variety of inorganic and organic phases, including natural and engineered cements. A paradigmatic example is that of the CaCO3 system, which has been shown to form in some cases via an amorphous calcium carbonate (ACC) precursor, formed itself via the aggregation of CaCO3 prenucleation clusters. In a very recent study we have shown that C-S-H, the main component of cement, is also formed in solution via amorphous precursor clusters. This amorphous precursor pathway offers a ‘cheaper’ alternative route through the free energy landscape, due to the lower interfacial energies of the intermediate phases, which considerably reduce the nucleation barrier. In addition, these pathways probably present evolutionary advantages for biominerals, due to the easiness by which the organisms can mold these precursor phases into intricate shapes. However, and in spite of the abundant literature aimed at understanding the amorphous precursor pathway and its thermodynamics and kinetics characteristics, many open questions remain unanswered, blocking the development of effective strategies for the synthesis of novel biomimetic materials and of new additives to control the crystallization process. This project aims to build some bridges between the researches performed in the field of ‘natural cements’, bringing this knowledge to the study of nucleation of ‘engineered cements’.
The key questions that still need to be resolved can be divided in two categories: (a) What is the structure of these clusters, and how is it modified in the presence of additives? Is the initial structure a signature of the final polymorph? And (b) what are the molecular mechanisms of stabilization of the amorphous precursors? What is the role of water?

Here, we plan to provide answers to these questions by using a combination of state-of-the-art scattering and spectroscopy techniques combined with detailed chemical speciation. Task 1 of this project will deal with the study of the structure of aqueous (pre-nucleation) clusters. Recent advances in X-ray scattering and detection devices have made it possible now to perform experiments in very diluted conditions, providing for the first time the possibility to describe the internal structure of CaCO3 pre-nucleation clusters using the pair distribution function method. Small-angle and pair distribution function experiments will be performed as well to characterize the formation conditions and the structure of the precursor phase to C-S-H. All these experiments will be performed under controlled chemical conditions, using a titration setup and synchrotron radiation. Task 2 includes an original approach to probe the atomistic dynamics and the stability against crystallization of amorphous precursors. Complementary techniques such as X-ray photon Correlation Spectroscopy and Inelastic Incoherent Neutron Scattering will be used to probe the atomistic dynamics of ions and water, respectively. These techniques will serve to test the long-standing simulation-based hypothesis that water acts as a stabilizer of the amorphous structure in the case of ACC. Systematic studies of the microscopic dynamics of ACC and of C-S-H amorphous precursors in the presence of different additives, and at different hydration states, will be performed.

The results of this research will have a large impact for (i) the understanding of how additives control the nucleation of naturally-occurring cements, such as ACC in biominerals, and (ii) the improved design of additives to control crystallization of engineered cements, of which C-S-H is the main component. This research will also impact other applications where a control on the crystallization kinetics is desired, such as restoration of cultural heritage, CO2 sequestration via mineral trapping or the prevention of scale formation.