PhD Thesis – A supercritical water-based technology for calcium silicate hydrate nano-particles production

The main objective of the thesis project was to develop a sustainable and extrapolable method on an industrial scale for the synthesis of hydrated calcium silicates, ie, xonotlite and tobermorite. The main advantage of this methodology is linked to the ultra-fast reaction kinetics, which allows the synthesis of these mineral phases in supercritical water in just a few seconds. Also, in addition to very short synthesis times, synthetic minerals are characterized by high crystallinity and purity, unlike their natural counterparts which are often found mixed with other phases or impurities. Thanks to this property, synthetic xonotlite and tobermorite can be used to deepen the structure of hydrated calcium silicates, which is not yet fully elucidated. Subsequently, highly crystalline xonotlite and tobermorite have been used in two different fields of application. The first is linked to the use of mineral phases in the form of seeds to accelerate the hydration of the cement in order to develop a denser and more resistant cement matrix. The second application considers the use of xonotlite as a precursor to produce, following its dehydration, another calcium silicate, wollastonite. Finally, with the aim of expanding the production of synthetic minerals to industrial requirements, the environmental impact of this methodology has been studied through life cycle analysis. The impacts of the most relevant parameters, such as energy, nature and concentration of precursors, were analyzed to allow possible optimization.


PhD student (U. Bordeaux-UPV/EHU) Supercritical Fluids. Nano-tobermorite, nanoxonotlite, nanoCSH

Normal and anomalous self-healing mechanism of crystalline calcium silicate hydrates

The origin of different stability of crystalline calcium silicate hydrates was investigated. The tobermorite crystal has been used as an analog of cement hydrate that is being mostly manufactured material on earth. Normal tobermorite is thermally unstable and transforms to amorphous at low pressure. Meanwhile, anomalous tobermorite with high Al content does not significantly transform under high pressure or high temperature. Conducted X-ray absorption spectroscopy explains the weak stability of normal tobermorite which was originally hypothesized by the role of zeolitic Ca ions in the cavities of silicate chains. Atomic simulations reproduced the experimentally observed trend of pressure behavior once the ideal structures were modified to account for the Al content as well as the chain defects. The simulations also suggested that the stability of tobermorite under stress could be rationalized as a self-healing mechanism in which the structural instabilities were accommodated by a global sliding of the CaO layer.

New Kinetic Monte Carlo Model to Study the Dissolution of Quartz

Quartz dissolution is a frequent process in geochemistry and materials science. It is controlled at the atomic scale by the sequential hydrolysis reactions and breakage of siloxane bonds, the surface topography, and the Gibbs free energy difference ΔG between the solid and the solution. Atomistic simulations have provided valuable topographic information about quartz dissolution and reaction energy barriers. However, with the current interpretation of the data, serious discrepancies persist between the predicted dissolution rates Rdis and the macroscopic dissolution activation energy Ea compared to their experimental counterparts. In this work we show that both quantities can be reconciled using a kinetic Monte Carlo (KMC) atomistic model based on bond-by-bond reactions and Rdis and Ea can be jointly reproduced. In addition, the obtained etch pit shapes for different quartz planes are in agreement with the experimentally reported ones: V-shape striations in {001}, rectangular pyramidal pits in {100}, and trapezoidal semipyramidal pits in {101}. We also study the dissolution rate dependence with ΔG by introducing chemical reversibility in the KMC model, obtaining again results in good agreement with experiments. This work highlights the importance of understanding the mechanisms taking place at the nanoscale to describe macroscopic properties and provides the basic ingredients to extend this study to other minerals and/or dissolution conditions.

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THz Fingerprints of Cement-Based Materials

To find materials with an appropriate response to THz radiation is key for the incoming THz technology revolution. Unfortunately, this region of the electromagnetic spectra remains largely unexplored in most materials. The present work aims at unveiling the most significant THz fingerprints of cement-based materials. To this end transmission experiments have been carried out over Ordinary Portland Cement (OPC) and geopolymer (GEO) binder cement pastes in combination with atomistic simulations. These simulations have calculated for the first time, the dielectric response of C-S-H and N-A-S-H gels, the most important hydration products of OPC and GEO cement pastes respectively. Interestingly both the experiments and simulations reveal that both varieties of cement pastes exhibit three main characteristic peaks at frequencies around ~0.6 THz, ~1.05 THz and ~1.35 THz, whose origin is governed by the complex dynamic of their water content, and two extra signals at ~1.95 THz and ~2.75 THz which are likely related to modes involving floppy parts of the dried skeleton.

KIMERA: A Kinetic Montecarlo Code for Mineral Dissolution

KIMERA is a scientific tool for the study of mineral dissolution. It implements a reversible Kinetic Monte Carlo (KMC) method to study the time evolution of a dissolving system, obtaining the dissolution rate and information about the atomic scale dissolution mechanisms. KIMERA allows to define the dissolution process in multiple ways, using a wide diversity of event types to mimic the dissolution reactions, and define the mineral structure in great detail, including topographic defects, dislocations, and point defects. Therefore, KIMERA ensures to perform numerous studies with great versatility. In addition, it offers a good performance thanks to its parallelization and efficient algorithms within the KMC method. In this manuscript, we present the code features and show some examples of its capabilities. KIMERA is controllable via user commands, it is written in object-oriented C++, and it is distributed as open-source software.

Elucidation of Conduction Mechanism in Graphene Nanoplatelets (GNPs)/Cement Composite Using Dielectric Spectroscopy

Understanding the mechanisms that govern the conductive properties of multifunctional cement-materials is fundamental for the development of the new applications proposed to enhance the energy efficiency, safety and structural properties of smart buildings and infrastructures. Many fillers have been suggested to increase the electrical conduction in concretes; however, the processes involved are still not entirely known. In the present work, we investigated the effect of graphene nanoplatelets (1 wt% on the electrical properties of cement composites (OPC/GNPs). We found a decrease of the bulk resistivity in the composite associated to the enhancement of the charge transport properties in the sample. Moreover, the study of the dielectric properties suggests that the main contribution to conduction is given by water diffusion through the porous network resulting in ion conductivity. Finally, the results support that the increase of direct current in OPC/GNPs is due to pore refinement induced by graphene nanoplatelets.

Mechanisms and Dynamics of Mineral Dissolution: A New Kinetic Monte Carlo Model

Mineral dissolution is a fundamental process in geochemistry and materials science. It is controlled by the complex interplay of atomic level mechanisms like adatoms and terraces removal, pit opening, and spontaneous vacancy creation that can be gradually activated at different energies. Though the development of a comprehensive atomistic model is key to go deeper into the understanding of this phenomenon, existing models have failed to reproduce the abrupt dependence of the dissolution rate with the Gibbs free energy ( Δ𝐺ΔG ). Herein, a new atomistic kinetic Monte Carlo (KMC) model is presented, which, invoking the microscopic reversibility of chemical reactions, captures the experimentally observed sigmoid dependence of the dissolution rate and provides new insights on the concomitant dissolution mechanisms. As a salient result, the model predicts the possible existence of unreported close-to-equilibrium dissolution modes where spontaneous vacancies creation and pit opening can occur before adatom and terrace removal.