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1. Parametric Control of Melting Gel Morphology and Chemistry via Electrospray Deposition

   https://doi.org/https://doi.org/10.1115/MSEC2021-63347  

   1. Grzenda, M.; Gamboa, A. R.; Mercado, J.; Lei, L.; Guzman, J.; Klein, L. C.; Jitianu, A.; Singer, J. P (August 2021, ASME 2021 16th International Manufacturing Science and Engineering Conference)

   null (Ed.)

   Melting gels are a class of hybrid organic-inorganic silica based gels prepared via the sol-gel process that are solid below their glass transition temperatures, near room temperature, but show thermoplastic behavior when heated. While this phase change can be repeated multiple times, heating the gel past its consolidation temperatures, typically above 130 oC initiates an irreversible reaction that produces highly crosslinked glassy organic-inorganic materials via hydrolysis and poly-condensation. This ability makes melting gels uniquely compatible with processing techniques inaccessible to other sol-gels. By properly tuning their properties, it should be possible to create protective coatings for electronics and anti-corrosive coatings for metals that are highly hydrophobic and insulating. However, melting gel consolidation reactions are highly dependent on charge interactions, raising the question of how these materials will respond to a processing technique, like electrospray deposition (ESD), which is dependent on charge delivery. In this study, we focus on the role that substrate temperature and charge polarity play on film morphology, consolidation chemistry, and surface properties. Optical images, film thickness measurements, nanoindentation, and FTIR were used to characterize the sprayed melting gel with the goal of developing a robust processing space for producing highly cross linked, hydrophobic, dielectric coatings. 

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2. Atomistic origin of kinetics in hydrated aluminosilicate gels upon precipitation

   https://doi.org/10.1063/5.0165937  

   Zhao, Cheng; Yu, Jiahui; Chen, Xuyong; Wu, Qiaoyun; Zhou, Wei; Bauchy, Mathieu (October 2023, The Journal of Chemical Physics)

   Calcium–alumino–silicate–hydrate (CaO–Al2O3–SiO2–H2O, or C–A–S–H) gel, which is the binding phase of cement-based materials, greatly influences concrete mechanical properties and durability. However, the atomic-scale kinetics of the aluminosilicate network condensation remains puzzling. Here, based on reactive molecular dynamics simulations of C–A–S–H systems formation with varying Al/Ca molar ratios, we study the kinetic mechanism of the hydrated aluminosilicate gels upon precipitation. We show that the condensation activation energy decreases with the Al/Ca molar ratio, which suggests that the concentration of the Al polytopes has a great effect on controlling the kinetics of the gelation reaction. Significantly, we demonstrate that 5-fold Al atoms are mainly forming at high Al/Ca molar ratios since there are insufficient hydrogen cations or extra calcium cations to compensate the negatively charged Al polytopes at high Al/Ca molar ratios during accelerated aging. 

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3. Organic cross-linking decreases the thermal conductivity of calcium silicate hydrates

   https://doi.org/10.1016/j.cemconres.2023.107324  

   Moshiri, Amir; Morshedifard, Ali; Stefaniuk, Damian; El Awad, Santiago; Phatak, Tejasree; Krzywiński, Kamil J.; Rodrigues, Debora Frigi; Qomi, Mohammad Javad; Krakowiak, Konrad J. (December 2023, Cement and Concrete Research)

   We study the conductive heat transport through calcium silicate hydrate (C-S-H) and organically cross-linked C-S-H via experiments, micromechanical homogenization theory, and molecular simulations. We find that C-S-H's intrinsic thermal conductivity falls below its amorphous limit when cross-linked with short-chain organosilanes. The observed reduction correlates with the alkyl chain length of the bis-organosilane molecule. To understand the underlying fundamental molecular processes accountable for such a reduction, we construct realistic molecular structures of cross-linked C-S-H and validate them against the spectroscopic and pycnometery measurements. The atomistic simulations indicate that the reduction in the contribution of propagons (propagating heat carriers) and diffusons (diffusive heat carriers) to heat transport, and the amplification of locons (localized vibrational modes), are the main driving factors allowing to limit the heat conduction in C-S-H. Presented findings offer new potential directions to nanoengineering novel admixtures for cement composites and resilient lightweight cementitious mesostructures for thermally efficient building envelopes. 

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4. Drying-induced atomic structural rearrangements in sodium-based calcium-alumino-silicate-hydrate gel and the mitigating effects of ZrO ₂ nanoparticles

   https://doi.org/10.1039/c7cp07876e  

   Yang, Kengran; Özçelik, V. Ongun; Garg, Nishant; Gong, Kai; White, Claire E. (January 2018, Physical Chemistry Chemical Physics)

   null (Ed.)

   Conventional drying of colloidal materials and gels (including cement) can lead to detrimental effects due to the buildup of internal stresses as water evaporates from the nano/microscopic pores. However, for these gel materials the underlying nanoscopic alterations that are, in part, responsible for macroscopically-measured strain values (especially at low relative humidity) remain a topic of open debate in the literature. In this study, sodium-based calcium-alumino-silicate-hydrate (C-(N)-A-S-H) gel, the major binding phase of silicate-activated blast furnace slag (one type of low-CO 2 cement), is investigated from a drying perspective, since it is known to suffer extensively from drying-induced microcracking. By employing in situ synchrotron X-ray total scattering measurements and pair distribution function (PDF) analysis we show that the significant contributing factor to the strain development in this material at extremely low relative humidity (0%) is the local atomic structural rearrangement of the C-(N)-A-S-H gel, including collapse of interlayer spacing and slight disintegration of the gel. Moreover, analysis of the medium range (1.0–2.2 nm) ordering in the PDF data reveals that the PDF-derived strain values are in much closer agreement (same order of magnitude) with the macroscopically measured strain data, compared to previous results based on reciprocal space X-ray diffraction data. From a mitigation standpoint, we show that small amounts of ZrO 2 nanoparticles are able to actively reinforce the structure of silicate-activated slag during drying, preventing atomic level strains from developing. Mechanistically, these nanoparticles induce growth of a silica-rich gel during drying, which, via density functional theory calculations, we show is attributed to the high surface reactivity of tetragonal ZrO 2 . 

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5. Influence of multi-walled carbon nanotubes on the hydration products of ordinary Portland cement paste

   https://doi.org/https://doi.org/10.1016/j.cemconres.2020.106197  

   Jiaxin Chen, Ange-Therese Akono (November 2020, Cement and concrete research)

   null (Ed.)

   We elucidate the mechanisms by which multi-walled carbon nanotubes (MWCNTs) influence the microstructure, fracture behavior, and hydration of cement paste. We disperse MWCNTs using a multi-step approach that involves high-energy pre-dispersion using ultrasonic energy followed by low-energy dispersion using un-hydrated cement particles. In turn, the low-energy dispersion step involves high-shear mixing and mechanical stirring. High-resolution environmental scanning electron microscopy of cement+0.2 wt% MWCNT, cement+0.5 wt% MWNCT, and of cement+1 wt% MWCNT show that MWCNTs bridge air voids, thereby refining the pore size and strengthening the C-S-H matrix. The fracture toughness increased by 9.38% with the addition of 0.2 wt% multi-walled carbon nanotubes, and by 14.06% with the addition of 0.5 wt% multi-walled carbon nanotubes and ligament bridging was the dominant toughening mechanism. Moreover, for all reinforcement levels, MWCNTs induced a conversion of low-density C-S-H into high-density C-S-H along with a drastic drop in the capillary porosity: adding 0.1–0.5 wt% MWCNT resulted in a 200% increase in the volume fraction of high-density C-S-H. Thus, our experiments show that MWCNT enhances the mechanical properties and transport properties by: (i) promoting high-density C-S-H formation, (ii) promoting calcium hydroxide formation, (iii) filling microscopic air voids, (iv) reducing the capillary porosity, (v) increasing the fraction of small gel pores (1.2–2 nm in size), and (vi) by bridging microcracks. 

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