This paper aims to study the adhesion mechanism of polyvinyl alcohol (PVA) fiber within alkali-activated slag/fly ash (AASF) matrix using molecular dynamics (MD) simulation in combination with systematic experimental characterization. The adhesion of PVA to C-(N-)A-S-H gel with different Ca/(Si+Al) and Al/Si ratios was modeled using MD simulation, with the related adsorption enthalpy calculated and the adhesion mechanism explored. The experimentally attained chemical bonding energy of PVA fiber in AASF coincides well with the simulation results. In both cases, the adhesion enhances primarily with increasing Ca/(Si+Al) ratio of C-(N-)A-S-H gel. Additionally, MD simulation indicates preferential element distributions of Ca around PVA molecule, which was confirmed experimentally by the detection of the Ca-rich C-(N-)A-S-H gel in the interfacial transition zone (ITZ). This study provides further insights into the adhesion mechanism of PVA fiber to C-(N-)A-S-H gel formed in AASF, which is particularly valuable for the future development of PVA-based high-performance alkali-activated composites.

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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.

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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 ((Formula presented.)). 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.

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Shortly after mixing cement grains with water, a cementitious fluid paste is formed that immediately transforms into a solid form by a phenomenon known as setting. Setting actually corresponds to the percolation of emergent network structures consisting of dissolving cement grains glued together by nanoscale hydration products, mainly calcium-silicate-hydrates. As happens in many percolation phenomena problems, the theoretical identification of the percolation threshold (i.e. the cement setting) is still challenging, since the length scale where percolation becomes apparent (typically the length of the cement grains, microns) is many times larger than the nanoscale hydrates forming the growing spanning network. Up to now, the long-lasting gap of knowledge on the establishment of a seamless handshake between both scales has been an unsurmountable obstacle for the development of a predictive theory of setting. Herein we present a true multi-scale model which concurrently provides information at the scale of cement grains (microns) and at the scale of the nano-hydrates that emerge during cement hydration. A key feature of the model is the recognition of cement setting as an off-lattice bond percolation process between cement grains. Inasmuch as this is so, the macroscopic probability of forming bonds between cement grains can be statistically analysed in smaller local observation windows containing fewer cement grains, where the nucleation and growth of the nano-hydrates can be explicitly described using a kinetic Monte Carlo Nucleation and Growth model. The most striking result of the model is the finding that only a few links (~12%) between cement grains are needed to reach setting. This directly unveils the importance of explicitly including nano-texture on the description of setting and explains why so low amount of nano-hydrates is needed for forming a spanning network. From the simulations, it becomes evident that this low amount is least affected by processing variables like the water-to-cement ratio and the presence of large quantities of nonreactive fillers. These counter-intuitive predictions were verified by ex-professo experiments that we have carried out to check the validity of our model.

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Dissolution of minerals is a complex process that not only depends on the nature of the material, but also on its topography and the equilibrium with the solution1. In the case of cements, for instance, understanding the dissolution of alite and belite is of great importance from a technological point of view. In this sense, atomistic Kinetic Monte Carlo (KMC) simulations have been already employed in different fields of geochemistry to investigate the dissolution of materials at the nano-micro-scales. Unfortunately, current models usually assume far equilibrium conditions2,3 and fail to capture the dissolution mechanisms that take place close to chemical equilibrium between the dissolved ions in and the material. In order to account for the subtle reversible mechanisms that take place close to equilibrium, the commonly used KMC model, in which an atom in the material has a dissolving probability, is further complemented with another equivalent equation which also considers the probability of an atom from the solvent to enter the material. The method was tested in a general Kossel crystal, and to the best of our knowledge it is capable for the first time to reproduce the different mechanisms described in the literature as a function of the free energy and their dissolution flux. This is a general study applicable to any dissolving material, but with clear impact in cement materials where large changes in the free energy take place in short periods of time. @en