Surface Engineering of Migratory Corrosion Inhibitors: Controlling the Wettability of Calcium Silicate Hydrate in the Nanoscale

Migratory corrosion inhibitors (MCIs) are regarded as effective additives to prevent harmful ion transmission and improve concrete durability, but their behavior in the porosity of concrete is still unclarified. This paper proposes a unique perspective to evaluate the effects of surfactant-like MCIs in calcium silicate hydrate (C-S-H) nanoporosity through molecular and electronic structural information. Advanced enhanced sampling methods and perturbation theory methods were applied to evaluate the role of different MCIs. The reduced density gradient of MCI molecules was obtained by using quantum chemical calculations. This calculation is instrumental in elucidating the intensity of interactions among distinct MCI molecule head groups and the C-S-H matrix. It is found that MCIs can effectively improve the interfacial tension (IFT) between C-S-H and water, which corresponds to the inhibitory ability of transmission. Free energy indicates that the MCI has the properties of strong adsorption and weak dissolution, facilitating the improvement of IFT. The relationship between the MCI functional group and the ability of adsorption and dissolution is revealed. This study suggests that MCIs work as surface controllers of C-S-H pores and that their properties can be assessed on the nanoscale.

Understanding the wetting discrepancy in calcium alumino silicate hydrate induced by Al/Si ratio

The application of supplementary cementitious materials (SCMs) in concrete can improve its durability in the marine environment. Calcium alumino silicate hydrate (CASH) is the main hydration product of SCMs; however, to date, the mechanism of the wetting discrepancy in CASH with different Al/Si ratios has not been revealed at the molecular scale. Herein, the molecular dynamics simulation method was used to study the wettability of water nanodroplets on the surface of CASH substrates with different Al/Si ratios, aiming to reveal the influence of CASH gel with different Al contents on the wettability of water molecules. The simulation results suggested that the CASH interface with a high Al/Si ratio has better wettability for nanodroplets. The microcosmic analysis showed that the interaction between particles and the CASH substrate is affected by the Al content. The electronegativity of the CASH substrate increases due to the substitution of Al-O tetrahedrons, which makes it stronger to solidify Ca ions on its surface and easier to form hydrogen bonds with water molecules in a nanodroplet. The orientation distribution of water molecules further revealed the source of the force of the CASH substrate on nanodroplets at the atomic level. The analysis of the dynamic properties showed that the H-bonds between CASH substrate with a high Al/Si ratio and water molecules are more stable, and thus the nanodroplets have better stability on the surface of CASH.

Novel Nano-Precursor Inhibiting Material for Improving Chloride Penetration Resistance of Concrete: Evaluation and Mechanism

Durability improvement is always important for steel-concrete structures exposed to chloride salt environment. The present research investigated the influence of a novel nano-precursor inhibiting material (NPI), organic carboxylic acid ammonium salt, on the mechanical and transport properties of concrete. The NPI caused a slight reduction in the strength of concrete at later ages. NPI significantly decreased water absorption and slowed down the speed of water absorption of concrete. In addition, the NPI decreased the charge passed and the chloride migration coefficient, and the results of the natural chloride diffusion showed that the NPI decreased the chloride concentration and the chloride diffusion coefficient. The NPI effectively improved the resistance of chloride penetration into testing concrete. The improvement in the impermeability of concrete was ascribed to the incorporation with the NPI, which resulted in increasing the contact angle of cement pastes. The contact angle went up from 17.8° to 85.8° for 0% and 1.2% NPI, respectively, and cement pastes became less hydrophilic. Some small pore throats were unconnected. Besides, the NPI also optimized the pore size distribution of hardened cement paste.

Surface Functionalization of Ti3C2Tx MXene with Highly Reliable Superhydrophobic Protection for Volatile Organic Compounds Sensing

Two-dimensional (2D) transition-metal carbides (Ti₃C₂T_x MXene) have received a great deal of attention for potential use in gas sensing showing the highest sensitivity among 2D materials and good gas selectivity. However, one of the long-standing challenges of the MXenes is their poor stability against hydration and oxidation in a humid environment, limiting their long-term storage and applications. Integration of an effective protection layer with MXenes shows promise for overcoming this major drawback. Herein, we demonstrate a surface functionalization strategy for Ti₃C₂T_x with fluoroalkylsilane (FOTS) molecules through surface treatment, providing not only a superhydrophobic surface, mechanical/environmental stability but also enhanced sensing performance. The experimental results show that high sensitivity, good repeatability, long-term stability, and selectivity and faster response/recovery property were achieved by the FOTS-functionalized when Ti₃C₂T_x was integrated into chemoresistive sensors sensitive to oxygen-containing volatile organic compounds (ethanol, acetone). FOTS functionalization provided protection to sensing response when the dynamic response of the Ti₃C₂T_x-F sensor to 30 ppm of ethanol was measured over in the 5 to 80% relative humidity range. Density functional theory simulations suggested that the strong adsorption energy of ethanol on Ti₃C₂T_x-F and the local structure deformation induced by ethanol adsorption, contributing to the gas-sensing enhancement. This study offers a facile and practical solution for developing highly reliable MXene based gas-sensing devices with response that is stable in air and in the presence of water.

Water Transport Mechanisms of Poly(acrylic acid), Poly(vinyl alcohol), and Poly(ethylene glycol) in C-S-H Nanochannels: A Molecular Dynamics Study

The transport properties of water molecules in nanochannels are critical to the durability of porous materials. In this article, molecular dynamics simulations are used to study the effects of poly(acrylic acid) (PAA), poly(vinyl alcohol) (PAA), and poly(ethylene glycol) (PEG) on the durability of modified cement-based materials. By establishing ideal composite nanopores, the absorption of water molecules in the channel is simulated. The results show that PEG has the best water-blocking effect under the same simulated conditions, followed by PVA, and PAA is the most unfavorable. This difference in the water-blocking effect can be explained by two factors. On the one hand, hydrophobic alkane groups in these polymers can inhibit water molecule transport. A large number of -COOH and -OH functional groups in PAA and PVA will form a complex H-bond network with the water molecules in the nanopore, dragging the water molecules forward, thereby speeding up the water molecule transmission to a certain extent. However, PEG, which mainly contains low-polar oxygen (C-O-C), has weak hydrogen bonding with water molecules, so the water-blocking effect is more obvious. On the other hand, the van der Waals interaction and the electrostatic interaction mainly derived from O_p-Ca_w-O_s can ensure the absorption of the polymer on the C-S-H surface during the transport process. The -COOH in PAA ensures its strongest absorption. But PVA and PEG will morphologically agglomerate during the water absorption, occupying pores and hindering the transport of water molecules.