Understanding erosion resistance mechanisms of sodium aluminate silicate hydrate in erosion environments: a molecular dynamics study.

Sodium-aluminate-silicate-hydrate (NASH) gel, as the primary reaction product stimulated by alkali in silica-aluminum-rich minerals, influences the mechanical and durability properties of geopolymers. In erosion environments, NASH demonstrates superior compressive strength and erosion resistance compared to hydration products of ordinary Portland cement. However, the underlying erosion resistance mechanism of NASH under such conditions remains unclear. Therefore, this study employs molecular dynamics research methodology to investigate the alteration in performance and deterioration mechanism of NASH in erosive environments. The findings reveal that in Na2SO4 solution, the infiltration of H2O molecules and Na+ ions into the three-dimensional mesh structure of NASH results in slight expansion and reduced tensile strength. Although H2O intrusion induces hydrolysis of the three-dimensional skeleton, the adsorption sites within NASH possess the capability to capture externally introduced Na+ ions. During tensile loading, Na+ ions can interact with reactive oxygen species produced through stretching or H2O molecule-induced decomposition of the internal framework, facilitating the repair of fractured structures. Consequently, this process partially alleviates tensile rupture, modifies the fracture damage mode, enhances overall toughness, and improves resistance against sulfate attack.

The mechanism of fluidity improvement of cement slurry by graphene oxide: a study on nanofriction.

Graphene oxide (GO) as a nano-reinforcing material has received extensive attention in cement composite materials. This paper employed molecular dynamics to simulate the friction process of calcium silicate hydrate (CSH) particles in the presence of double-sided and single-sided GCOOH (graphene oxide with a -COOH functional group, covering 10% of the surface). The investigation uncovered the lubricating effects of bifacial and unifacial GCOOH on the CSH interface. The findings indicate that the interfacial friction among CSH particles follows the sequence of double-sided GCOOH > pure CSH > single-sided GCOOH. In the double-sided GCOOH system, a greater external force is needed on the opposing side to alter the interaction with water molecules, calcium ions, and silica-oxygen tetrahedra, thereby enhancing friction. In contrast, the majority of the carboxyl groups on the single-sided GCOOH surface are strongly adsorbed onto the CSH surface, facilitating the entry of additional water molecules into the interlayer. Conversely, the unmodified side of the GCOOH has lower interactions with water molecules, hence improving its lubricating properties.

Unraveling the molecular freezing behavior of water on a calcium silicate hydrate matrix.

The hydration process of cement-based materials primarily results in the formation of calcium silicate hydrate (CSH), which is crucial in deciding how long concrete will last. This study utilizes molecular dynamics simulation technology to explore the freezing behavior of pure water solutions within various calcium silicate hydrate (CSH) matrices. The investigated matrices encompass four different Ca/Si ratios. According to the simulation, as ice crystals develop close to the surface of CSH, the CSH matrix strengthens its hydrogen and ionic interactions with water molecules, which effectively prevents water molecules from crystallizing and nucleating. Consequently, these molecules compose an unfrozen water film structure that bridges between ice crystals and the CSH matrix. The research also reveals an intriguing relationship between silica chain behavior on the Ca/Si ratio and the CSH surface. Surface flaws arise as a result of the silica chains of CSH breaking into shorter segments as the Ca/Si ratio increases. These surface defects manifest as grooves on the matrix's surface, effectively capturing and retaining specific water molecules. The CSH matrix's hydrogen bonds with water molecules are weakened as a result of this process, facilitating their participation in the crystallization process, and leading to a thinner unfrozen water film thickness with an increased Ca/Si ratio. This study contributes to a greater knowledge of the performance and dependability of cement-based products by offering molecular-level insights into the freezing actions of liquids in gel pores.

Atomic Insights into the Relationship between Molecular Structure and Dispersion Performance of Phenyl Polymer on Graphene Oxide.

The adsorption of organic polymers onto the surface of graphene oxide is known to improve its dispersibility in cement-based materials. However, the mechanism of this improvement at the atomic level is not yet fully understood. In this study, we employ a combination of DFT static calculation and umbrella sampling to explore the reactivity of polymers and investigate the effects of varying amounts of phenyl groups on their adsorption capacity on the surface of graphene oxide. Quantitative analysis is utilized to study the structural reconstruction and charge transfer caused by polymers from multiple perspectives. The interfacial reaction between the polymer and graphene oxide surface is further clarified, indicating that the adsorption process is promoted by hydrogen bond interactions and π-π stacking effects. This study sheds light on the adsorption mechanism of polymer-graphene oxide systems and has important implications for the design of more effective graphene oxide dispersants at the atomic level.

The inhibitory effect of excess calcium ions on the polymerization process of calcium aluminate silicate hydrate (CASH) gel.

Calcium ion, as an essential component in CASH, affects the aggregation and formation process of CASH, thereby influencing its microstructure and mechanical properties. However, the mechanism by which calcium ions affect the polymerization process of CASH is not yet fully understood. In this study, the effects of calcium ions on the polymerization process, coagulation state, and microstructure of CASH are investigated via molecular dynamics simulation. The results indicate that the presence of a trace amount of Ca2+ attracts oligomers towards the calcium-rich region, thus speeding up the polymerization to some extent, but as the Ca2+ content increases, more Ca2+ binds to the oxygen atoms in silica-oxygen tetrahedra and aluminum-oxygen tetrahedra, forming tight ion pairs and occupying the hydroxyl binding sites required for the polycondensation reaction. This inhibits the continuous aggregation of CASH gel and slows down the rate of polymerization. Additionally, Ca2+ attracts oxygen atoms from free water molecules and free OH-, forming Ca(OH)2 dispersed in the spatial structure, which hinders the formation of larger clusters. As a result, the higher the Ca ion content in the system, the lower the overall polymerization degree of the CASH gel, resulting in a decrease in the conversion of the Q1 dimer to Q2 and Q3 chain structures, a shorter average chain length, poorer overall connectivity, and a transition from large clusters in a better-aggregated state to dispersed small clusters. This study sheds light on the polymerization reaction mechanism of CASH gels.

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.

Molecular insights into the interfacial adhesion mechanism between carbon nanotubes and epoxy resin.

In recent years, carbon nanotubes (CNTs) have garnered widespread attention and have been deemed the preferred option for the creation of epoxy composites, owing to their outstanding mechanical properties. Despite this, the interaction between pure CNTs and epoxy resin is primarily dependent on van der Waals forces and therefore, the interfacial forces are weak, making it challenging for effective load transfer. To enhance the mechanical properties of the composites, surface functionalization is often deemed a more favorable method for improving interfacial bond strength. This study employs molecular dynamics simulations to examine the interfacial bonding characteristics between functionalized CNTs and epoxy resin. The results demonstrate that functional group modification can significantly improve the interfacial adhesion between CNTs and epoxy resin, and the incorporation of functional groups can enhance the crosslinking degree of the epoxy resin at the interface. The hydrogen bond network established between the CNTs and epoxy resin after functional group modification, and the high stability of the bond cooperation, are factors contributing to the excellent interfacial performance.

Acid Radical Tolerance of Silane Coatings on Calcium Silicate Hydrate Surfaces in Aggressive Environments: The Role of Nitrate/Sulfate Ratio.

Silane is known as an effective coating for enhancing the resistance of concrete to harmful acids and radicals that are usually produced by the metabolism of microorganisms. However, the mechanism of silane protection is still unclear due to its nanoscale attributes. Here, the protective behavior of silane on the calcium silicate hydrate (C-S-H) surface is examined under the attack environment of nitrate/sulfate ions using molecular dynamics simulations. The findings revealed that silane coating improved the resistance of C-S-H to nitrate/sulfate ions. This resistance is considered the origin of silane protection against harmful ion attacks. Further research on the details of molecular structures suggests that the interaction between the oxygen in the silane molecule and the calcium in C-S-H, which can prevent the coordination of sulfate and nitrate to calcium on the C-S-H surface, is the cause of the silane molecules' strong adsorption. These results are also proved in terms of free energy, which found that the adsorption free energy on the C-S-H surface followed the order silane > sulfate > nitrate. This research confirms the excellent protection performance of silane on the nanoscale. The revealed mechanism can be further used to help the development of high-performance composite coatings.

Atomistic Insights into the Deposition of Corrosion Products on the Surfaces of Steels and Passivation Films.

The deposition of corrosion products on the surface of the steel is a key step for understanding the generation of corrosion products. To clarify the molecular mechanism for corrosion product deposition, the reactive molecular dynamics were utilized to study the deposition process of ferric hydroxide (Fe(OH)3) on iron and passivation film substrates. It is shown that the deposition phenomenon mainly occurs on the iron surface, while the surface of the passivation film cannot adsorb Fe(OH)3. Further analysis indicates that the interaction between hydroxyl groups in γ-FeOOH and Fe(OH)3 is very weak, which is unfavorable to the deposition of Fe(OH)3. Moreover, the degree of ordered water in the two systems is affected slightly by deposition but the oxygen in water corrodes Fe(OH)3, breaking its Fe-O bonds, which is more obvious in the Fe system due to its instability. This work has revealed the nanoscale deposition process of corrosion products on the passivation film in a solution environment by reproducing the bonding and breaking of atoms at the molecular level, which is a case in point to the conclusion of the protection of steel bars by passivation film.

Adhesion Performance of Ettringite at the Interface with Silane and GO/Silane: Insights into Molecular Dynamics Simulations.

The application of silane in sulfoaluminate cement repair materials can improve its waterproof, permeability, freeze-thaw, and other properties, but it would reduce the mechanical properties of sulfoaluminate cement-based materials, making it unable to better meet the engineering requirements and durability indices. The modification of silane with graphene oxide (GO) can effectively address this issue. However, the failure mechanism of the interface between silane and sulfoaluminate cement-based materials and the modification mechanism of GO remain unclear. In this paper, the interface-bonding mechanical models of isobutyltriethoxysilane (IBTS)/ettringite and GO-IBTS/ettringite are established by molecular dynamics method to study the source of interface-bonding properties of IBTS, GO-IBTS, and ettringite, as well as the failure mechanism of interface bonding, to reveal the mechanism of GO-modifying IBTS to improve the interface-bonding properties of IBTS and ettringite. This study finds that the bonding properties of the IBTS, GO-IBTS, and ettringite interface are derived from the amphiphilic nature of IBTS, which can only produce unilateral bonding with ettringite, thus becoming a weak link in interface dissociation. The double-sided nature of GO functional groups enables GO-IBTS to interact well with bilateral ettringite, thus enhancing the interface-bonding properties.

Molecular Insight into the Pozzolanic Reaction of Metakaolin and Calcium Hydroxide.

The reaction mechanism of the pozzolanic reaction of metakaolin (MK) from the atomic point of view has not yet been explored. To explain the process and mechanism of the pozzolanic reaction from the atomic point of view, molecular insight into the pozzolanic reaction of MK and calcium hydroxide (CH) was analyzed through the reaction molecular dynamics (MD) simulation. The results show that the pozzolanic reaction of MK and CH can be essentially regarded as the CH decomposition and penetration into MK. Also, the structure evolution after the pozzolanic reaction shows that the water molecules cannot penetrate the MK structure till the participation of Ca2+ and OH- ions of CH. The Ca2+ and OH- ions have strong interaction with MK and drill into the MK structure, followed by the destruction of a part of the MK structure and water penetration. The final structure of CH removed by MK can be regarded as the precursor of the CASH gel structure.

Molecular Insights into the Reaction Process of Alkali-Activated Metakaolin by Sodium Hydroxide.

When metakaolin (MK) is alkalized with an alkaline activator, it depolymerizes under the action of the alkali. However, the process of MK alkalinization is still unrevealed. Here, we supplied a molecular insight into the process of MK alkalinization through reaction molecular dynamics (MD) simulation. The structure, dynamics, and process of MK alkalinization are systematically investigated. The results showed that the layered structure of MK was destroyed and the silicates in MK were dissolved by sodium hydroxide solution during the alkalinization reaction of MK. The aluminates in MK are not dissolved, indicating that aluminates are more stable than silicates. Moreover, the equilibrium structures of MK with H2O and MK with NaOH solution show that only when both sodium hydroxide and water are involved in the alkalinization reaction, the silicates in MK undergo depolymerization. Also, the observed final state of MK alkalinization can be recognized as the precursor of alkali-activated materials (AAMs).

Investigation of Composite Protective Coatings Coregulated by Core-Shell Structures and Graphene Oxide Interfaces.

The construction of multiple microstructures is a significant measure in improving the protective performance of composite polymer coatings. In this paper, a novel polystyrene acrylate-highly hydrophobic polysiloxane composite emulsion was fabricated by innovatively integrating the core-shell emulsion method and Pickering emulsion method through the interfacial stabilization and molecular polymerization regulation of graphene oxide, achieving a significant improvement in the compatibility of the thermoplastic core with a thermoset shell. The bonding degree between the polystyrene acrylate (PSA) component and the siloxane component is significantly improved in the synthesized composite emulsions, achieving the dual protection of the cementitious substrate with surface shielding and internal crystalline hydrophobicity. The capillary water absorption of the concrete treated with Pickering emulsions is reduced by over 98.3% with high hydrophobicity and low permeability. Meanwhile, the absolute ζ-potential and impedance of composite membranes reach over 45 mV and 109 ohms, respectively, giving the cementitious substrate excellent resistance to ionic attack and acid/alkaline corrosion. In addition, the composite membranes have excellent resistance to tensile cracking and physical erosion, maintaining a favorable adhesion level and plastic deformation under acid/alkaline attack and thermal aging, respectively.

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.

The Optimal Design on the Molecular Structure of a Fluid Transport Inhibitor Applied to Reinforced Concrete Structures.

Inhibiting the penetration of water molecules and aggressive ions is of considerable significance in improving the durability of reinforced concrete structures. In this work, molecular dynamics(MD) is employed to design a high-efficiency organic fluid transport inhibitor. MD results indicate that there is mutual complementation between the hydrophilic and hydrophobic functional groups in the chemical structure of this polymer. One end with the carboxyl groups can stably adsorb on the surface of the cementitious matrix due to the strong attraction from calcium ions. Simultaneously, the rest of the hydrophobic part of the polymer can stand up to maximize the repelling effect on the penetration of fluids. Furthermore, for high cost-effectiveness performance, the minimum number and the optimum position of the carboxyl groups of one polymer inhibitor have been determined. As the molecular structure contains two hydrophilic groups, only if located at the same end, the polymer chain can display the most preferable adsorption morphology.

Concentration-induced wettability alteration of nanoscale NaCl solution droplets on the CSH surface.

Due to the hydrophilic nature of concrete, moisture and corrosive ions adsorbed on its surface pose a severe challenge to the durability of concrete structures; therefore, investigating the wettability of a concrete surface is an indispensable prerequisite for designing durable and sustainable concrete. This paper utilizes molecular dynamics to simulate the concentration-induced wettability alteration of nanoscale NaCl droplets on a CSH surface, and verifies the feasibility and rationality of predicting the contact angle of a droplet on the CSH surface based on the surface tension. Results suggest that the wetting ability of droplets on the CSH surface is weakened with the increase of the NaCl mass fraction. Microscopic analysis reveals that water molecules clustered around Na+ and Cl- ions to form an ion hydration shell (Na+-Ow and Cl--Hw pairs); the binding energy barrier of these ion pairs is much larger than the dissociation energy barrier, which enhances the associative ability of the NaCl droplets. The particles on the CSH surface attract Na+ and Cl- ions by forming Oh(Os)-Na+ and Ca(Ho)-Cl- connections with droplets, which further weakens the spreading ability of water molecules on the CSH surface.

Structure, dynamics and transport behavior of migrating corrosion inhibitors on the surface of calcium silicate hydrate: a molecular dynamics study.

The incorporation of a corrosion inhibitor into a cement-based material can enhance the durability of the reinforced concrete. In this study, molecular dynamics simulation is utilized to study the interfacial structure and dynamic behavior of a solution with three migrating corrosion inhibitors (MCI) functionalized by hydroxyl (-OH), carboxyl (-COO-), and phenyl (-PH) groups in calcium silicate hydrate (CSH) gel pores. The transport rate of inhibitors is greatly dependent on the polarity of the functional group: -PH > -OH > -COO-. The slow migration rate of the inhibitor with -OH and -COO- is attributed to the chemical bond formed between CSH and MCI. The silicate chains near the CSH surface can provide plenty of non-bridging oxygen sites to accept the H-bond from the hydroxyl group in the inhibitor molecule. The surface calcium atom can capture the -COO- by forming an ionic COO-Ca bond. Furthermore, the hydration structure of the inhibitor molecule also influences its transport properties. The inhibitor functionalized by the carboxyl group, associating with the neighboring water molecules, forms ion-water clusters, and the inhibitor molecule and its hydration shell with a long resident time retard the migration rate. Hopefully, this study is able to provide molecules for the development of a migration-type corrosion inhibitor to elongate the service life of cement-based materials.

Microscopic insight into nanodiamond polymer composites: reinforcement, structural, and interaction properties.

Nanodiamond (ND)-reinforced polymer composites attract a great deal of attention; however, insufficient understanding of their reinforced behavior is greatly limiting their further design and application. In this paper, a microscopic investigation of the stretched processes of aminated ND and resin composites is undertaken to elucidate the reinforcement mechanism of ND in the polymer matrix by employing molecular dynamics and semi-empirical quantum chemistry simulations. The stretching process and reinforcement behavior are observed on the nanoscale, and the nano-pinning effect of ND on the resin matrix is clarified. Further analyses indicate that due to the increase of system volume, the interactions between the ND and resin molecules increase with stretching; this facilitates the combination of ND and resin molecules and thus recovers the breakage of the resin matrix. This recovery process is regarded as the origin of the ND reinforcement of the polymer. The strong interaction between the ND and resin molecules is quantitatively assessed by the interpenetration of the van der Waals surface and the reduced density gradient analysis of electrons using quantum chemistry calculations. The high stability of their noncovalent combination is also revealed. This investigation provides an excellent approach for revealing the origin of ND reinforcement behavior and thus facilitates further optimization of the performance of the ND-polymer composites.

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 Op-Caw-Os 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.

Nanoscale insight on the durability of magnesium phosphate cement: a molecular dynamics study.

The sustainable green building material magnesium phosphate cement (MPC) is widely used in the fields of solidifying heavy metals and nuclear waste and repair and reinforcement. Magnesium potassium phosphate hexahydrate (MKP) is the main hydration product of MPC. The transport of water and ions in MKP nanochannels determines the mechanical properties and durability of MPC materials. Herein, the interface models of MKP crystals with sodium chloride solution in the [001], [010] and [100] direction were established by molecular dynamics. The interaction of the MKP interface with water and ions was studied and the durability of MPC in sodium chloride solution was explained at the molecular level. The results show that a large number of water molecules are adsorbed on the MKP crystal surface through hydrogen bonds and Coulomb interactions; the surface water molecules have the bigger dipole moment and the dipole vector of most of the water molecules points to the solid matrix, when the crystal surfaces of the three models all show hydrophilicity. In addition, plenty of sodium ions are adsorbed at the MKP interface, and some potassium ions are desorbed from the matrix. In the MKP[001] model, the amount of potassium ions separated from the matrix and diffused into the solution is the highest and the interface crystal is the most disordered. Due to the attack of water and ions, the K-Os bond loses its chemical stability and the order of the MKP crystal is destroyed, which explains the decline of MPC performance after the erosion of sodium chloride solution at the molecular level. Besides, in the three models, the Na-Cl ion bond is more unstable than the K-Cl ion bond due to the smaller radius of the sodium atom. The stability of ionic bonds in the models is as follows: MKP[010] > MKP[100] > MKP[001].