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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.

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.

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 Ca 2+ attracts oligomers towards the calcium-rich region, thus speeding up the polymerization to some extent, but as the Ca 2+ content increases, more Ca 2+ 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, Ca 2+ 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 Q 1 dimer to Q 2 and Q 3 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.

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.

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.

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.

Correction for 'Hydrophobic silane coating films for the inhibition of water ingress into the nanometer pore of calcium silicate hydrate gels' by Jiao Yu et al., Phys. Chem. Chem. Phys., 2019, DOI: .

The super-hydrophobic nature of surfaces is greatly dependent on the interfacial molecular structure of coating materials. In this study, to understand the structure, dynamics and interfacial behavior of hydrophobic coating, molecular dynamics is utilized to study the capillary transport of water molecules through the nanometer channel of calcium silicate hydrate (C-S-H) with the interior surface impregnated with silane. The C-S-H surface is coated by connecting the bridging silicate tetrahedron with an oxygen-containing group in isobutyl-triethoxysilane (C 10 H 24 O 3 Si) with a silane molecule coverage rate ranging from 25% to 100%. We demonstrated that the silane coating with a coverage exceeding 25% can effectively inhibit the water molecule and detrimental ion invasion in the gel pore. The grafted silane groups reduce the number of non-bridging oxygen atoms in the surface silicate chains that provide plenty of sites to accept the H bonds from the surface water molecules. This results in the reduction of the dipole moment of the surface water molecules and transforms the hydrophilic C-S-H substrate to hydrophobic. The silane molecules, immobilized by the Si-O-Si bond on the C-S-H substrate, are protruded to the gel pore with the hydrophobic tail of branch-like isobutyl groups. It transforms the smooth surface to a lotus-leaf-like rough surface with distributed nanoscale papillae. The isobutyl groups, freely vibrating and rotating, further blocks the connectivity of the transport channel and weakens the interaction between penetrated ions and C-S-H substrates. Furthermore, spatial correlation analysis demonstrates that the immobilized silane molecules disturb the tetrahedron distribution of water molecules in the gel pore and break the hydration shell of the counter Ca ions that associate with less water molecules. The dramatic degradation of the time correlation function for the surface solution species in the presence of isobutyl-triethoxysilane exhibits that the coated C-S-H surface can repel the surface water molecules and calcium ions by weakening the H bond and the Ca-O ionic bond strength. These nanostructure results provide guidance for the construction of artificial super-hydrophobic surfaces and the design of cementing materials with controllable wettability.

The transport of water and ions in the gel pores of calcium silicate hydrate (C-S-H) determines the durability of cement material. In this study, molecular dynamics was employed to investigate the capillary imbibition process of CsCl solution in the C-S-H channel. The advanced frontier of CsCl solution flow inside the C-S-H capillary shows a concave meniscus shape, which reflects the hydrophilic properties of the C-S-H substrate. Reynolds number calculations show that the transport process is laminar flow and dominated by viscous forces. The invading depth of the CsCl solution deviates from the theoretical prediction of the classic Lucas-Washburn (L-W) equation, but the modified theoretical equation, by incorporating the effect of slip length, dynamic contact angle, and effective viscosity into the L-W equation, can describe the penetration curve of the solution very well. The validity of our developed theoretical equation was confirmed by additional systems with different ion concentrations. In addition, the local structure of ions was analyzed to elucidate the effect of ion concentration on the transport process. The adsorption and accumulation of ions retard the transport process of water. With an increase in the ionic concentration, the effects of immobilization and cluster accumulation became more pronounced, further reducing the transport rate of water. This study provides fundamental insight into the transport behavior of liquid in the gel pores of cement-based material.

As a promising layered double hydroxide, Friedel's salt has gained popularity. The transport and adsorption behavior of ions and water molecules at the interface is the basis for investigating the durability of concrete, in marine environments in particular. In this paper, the transport behavior of water and ions in the nanopores of Friedel's salt and the adsorption mechanism of the ions were systematically investigated by molecular dynamics. The water molecules share a larger bulk density and good orientations at the interface while the adsorption rate of chloride ions climbs to 66.62%, owing to the desorption of the surface structural anions forming Ca-Clw ion clusters. The time correlation function was employed to examine the stability of the Ca-Clw bonds formed near the Friedel's salt interface. The Ca-Clw bonds were demonstrated to be very stable, implying that the aqueous chloride ion is difficult to desorb once it is adsorbed by the interface. The surface of the ordered Friedel's salt structure could form a hydrated shell to hinder the interaction between sodium ions and oxygen atoms. In addition, Friedel's salt exhibits a poor adsorption capacity for sodium ions since it provides few adsorption sites due to the limited amount of structural chloride ions. After all, the interaction between Friedel's salt and the external environment on the nano scale was explored for a better understanding of the inherent mechanism from a molecular simulation perspective.

Sodium aluminosilicate hydrate (NASH) gel is the primary adhesive constituent in environmentally friendly geopolymer. In this study, to understand the thermal behavior of the material, molecular dynamics was utilized to investigate the molecular structure, dynamic property, and mechanical behavior of NASH gel subjected to temperature elevation from 300 K to 1500 K. The aluminosilicate skeleton in NASH gel provides plenty of oxygen sites to accept H-bond from the invading water molecules. Upon heating, around 18.2% of water molecules are decomposed and produce silicate and aluminate hydroxyls. About 87% of hydroxyls are associated with the aluminate skeleton, which weakens the Al-O bonds and disturbs the O-Al-O angle and the local structure, transforming it from an aluminate tetrahedron to a pentahedron and octahedron. With increasing temperature, both Al-O-Si and Si-O-Si bonds are stretched to be broken and the network structure of the NASH gel is gradually transformed into a branch and chain structure. Furthermore, the self-diffusivity of water molecules and sodium dramatically increases with the elevation of temperature, because the decrease in connectivity of the aluminosilicate network reduces the chemical and geometric restriction on the water and ions in NASH gel under higher temperatures. The high temperature also contributes to around 63% of the water molecules further dissociating and hydroxyl groups forming; meanwhile proton exchange between the water molecules and aluminosilicate network frequently takes place. In addition, a uniaxial tensile test was utilized to study the mechanical behavior of the NASH gel at different temperatures. During the tensile test, the aluminosilicate network was found to depolymerize into a branch or chain structure which plays a critical role in resisting the tensile loading. In this process, the breakage of the aluminosilicate skeleton is accompanied with hydrolytic reactions that further deteriorate the structure. Due to the reduction of the chemical bond stability at elevated temperature, both the tensile strength and stiffness of the NASH gel are weakened significantly. However, the ductility of the NASH gel is improved because of the higher extent of structural arrangement at the yield stage and partly due to the lower water attack. Hopefully, the present study can provide valuable molecular insights on the design of alkali-activated materials with high sustainability and durability.

The movement of water and ions in sodium alumino-silicate hydrate gel (NASH) influences the physical and chemical properties of the geopolymer material. In this paper, in order to better understand the structure and dynamics of water and ions in the interfacial region of the NASH gel, molecular dynamics was utilized to model Na2SO4 and MgSO4 solutions (both at 0.44 mol L-1) near the NASH surface. The broken silicate-aluminate surface network, with predominant percentage of randomly connected Q1 and Q2 silicate and aluminate species, provides plenty of non-bridging oxygen sites to accept the H bond from the surface water molecules, contributing toward a strongly adsorbed hydration layer with a thickness of around 5 Å. Consequently, the water molecule in the hydration layer exhibits increased density, increased dipole moment magnitude, orientation preference, and slow diffusivity. In contrast, up to 36.4% of the counter sodium ions, originally caged in the vacancies on the NASH surface, gradually dissociate from the silicate-aluminate skeleton and migrate into the bulk solution, which is consistent with the experimentally observed leaching process of alkali ions in the geopolymer material. In the MgSO4 solution, the magnesium ions-with a smaller ionic radius-penetrate into the silicate-aluminate skeleton vacancy, have 1.8 to 2.5 coordinated solid oxygen atoms, and remain on the NASH surface for a fairly longer time due to the stable Mg-O bonds. Mg species adsorbed on the inner sphere got rooted onto the hydroxyl layer, healing the damaged silicate-aluminate structures and stabilizing the network by inhibiting Na ion immigration into the solution. Mg ions in the outer layer, on average, associated with around one neighboring SO4 ion, forming ionic pairs and accumulating into large Mg-SO4 clusters, to help the immobilization of sulfate ions on the NASH surface.

In this paper, first principles and reactive force field molecular dynamics were utilized to study the mechanical properties of tobermorite 9 Å, tobermorite 11 Å and jennite. These are essential minerals in cement chemistry. The mechanical properties calculated by the first-principle method match well with previous experimental results, pointing to the introduction of vdW dispersion-type forces as able to improve the precision. The calculated elastic constants of the three minerals confirm anisotropic mechanical behavior of the layered structures. The crystals jennite and tobermorite 9 demonstrate stronger mechanical behavior in the ab plane than the interlayer direction due to the presence of stable covalent "dreierketten" silicate chains. For tobermorite 11 Å, the Q3 silicate tetrahedrons bridging the neighboring calcium silicate sheets heal the weak interlayer structure and enhance the c-direction stiffness and cohesive strength. Furthermore, analysis of uniaxial tension by reactive force MD elucidated the chemical and mechanical responses of the atomic structures in loading resistance. The stress-strain relation of the layered mineral tensioned along b direction, showing the "strain hardening" region, where stress continues to increase past the yield stage. The strain hardening and ductility enhancement for the minerals is largely due to the ability of the silicate chains to first de-polymerize into short chains or separate tetrahedrons before the broken Q species re-polymerize to form branched networks and ring structures which are able to resist loading. For all three minerals, protons transfer to oxygen and water, resulting in the formation of Si-OH and Ca-OH groups during the strain hardening stage as Si-O-Si or Si-O-Ca bonds start to break and the calcium atoms and silicate morphology is rearranged. Hydrolysis therefore accelerates structural damage and contributes to weakening of mechanical properties in the interlayer direction. Increased tensile stress level in the tobermorite 11 Å can contribute to a greater extent of water damage.

Carbonation plays an indispensable role in engineering construction, embracing mineralization, CO2 sequestration and low pH induced corrosion, but the essential mechanism of carbonation occurring in calcium silicate hydrate or portlandite can hardly be interpreted. Observation on how carbonation proceeds at the nano scale is thus critical for a better understanding of its dynamics. Here, using electron microscopy combined with first-principles calculation, a new view on carbonation in the cement system is revealed, considering morphological diversity, growth mechanism and shape evolution. Two types of crystalline forms of calcium carbonate (i.e. cubic and spindle) under room conditions were observed and determined to be calcite, both experimentally and theoretically. The mechanism of morphological evolution of calcite in a cement system was demonstrated based on the theory of aqueous chemistry. The [Ca2+] to [CO3] ratio was the principle cause for the diversity in crystal morphology instead of the types of reactants (i.e. portlandite or calcium silicate hydrates). Excess calcium species in the solution could selectively adsorb on surfaces, resulting in an inhibitive effect on the growth of specific crystal faces, (1 0 4)calcite and (2 1 1[combining macron])calcite in this case. Furthermore, a relationship between relative ionic concentration and the length to diameter ratio was established to predict the shape transformation. This work makes it possible to explore the chemical nature of carbonation from a nano scope rather than being confined to the macroscopic carbonation of concrete.

The coupled effects of mechanical loading and chemical attack can dramatically weaken the durability of a material. In this study, reactive force field molecular dynamics and the GCMC method were utilized to investigate the degradation mechanism for a deformed C-S-H gel subjected to water attack. In the elastic region of the stress-strain relation, no water molecules invaded the deformed C-S-H gel tensioned along the y-direction. On the other hand, in the failure stage, the tension loading stretched/broke the Si-O-Si bond, resulting in the distortion of the "dreierketten" silicate chain distribution and ordered zigzag sheets built by the calcium oxygen octahedron. More water molecules penetrated into the defective silicate sheets and dissociated into the Si-OH and Ca-OH surrounding the highly reactive non-bridging oxygen sites induced by the silicate chain breakage. The water invasion and hydrolytic reaction reduced the cohesive stress of the tensioned C-S-H structure. Furthermore, the cracks in the calcium silicate sheet connected with the interlayer region, enhancing the channel connectivity for the water transport. This resulted in the water dynamic transformation from the cage stage to the diffusive stage. The high mobility of confined water molecules further weakened the stability of the hydrogen bonds in the calcium silicate skeleton. Moreover, the tensile loading and water attack contributed to the silicate morphological rearrangement. The long silicate chains were first destroyed to form shorter chains and then re-polymerized to form a branch and ring structure to strengthen the weak interlayer regions.

For GO related nanocomposite design, it is of great importance to understand the behavior of water molecules ultra-confined in the interlayer region of graphene oxide (GO) sheets. In this research, to gain molecular insights into the influence of water on the properties of GO sheets, reactive force field molecular dynamics was employed to model a GO sheet with a water content of 1.3 wt%, 11.5 wt%, 18 wt% and 23.5 wt%. The epoxy and hydroxyl groups in the GO sheet exhibit high reactivity: the proton transferred from hydroxyl to dissociated epoxy contributes to carbonyl formation, which enhances the polarity of the GO sheet and strengthens the H-bond network between the functional groups. The epoxy, hydroxyl and newly formed carbonyl groups contribute to the structural hydrogen bonding with high stability. With increasing water content, the interlayer structural H-bonds between functional groups are gradually substituted by those contributed by water molecules, which weakens the interlayer stiffness and cohesive strength for GO sheets. Furthermore, the reactive force field allows coupling between the mechanical response and chemical reactions during uniaxial tensile deformation in the intra-layer direction. On the one hand, the relative epoxy bond is stretched until it is broken and transformed into a carbonyl group to resist tensile loading. On the other hand, interlayer water molecules, attacking the deformed GO sheets, are dissociated into carboxyl groups in the broken region.

Graphene oxide (GO) reinforced cement nanocomposites open up a new path for sustainable concrete design. In this paper, reactive force-field molecular dynamics was utilized to investigate the structure, reactivity and interfacial bonding of calcium silicate hydrate (C-S-H)/GO nanocomposite functionalized by hydroxyl (C-OH), epoxy (C-O-C), carboxyl (COOH) and sulfonic (SO3H) groups with a coverage of 10%. The silicate chains in the hydrophilic C-S-H substrate provided numerous non-bridging oxygen sites and counter ions (Ca ions) with high reactivity, which allowed interlayer water molecules to dissociate into Si-OH and Ca-OH. On the other hand, protons dissociated from the functional groups and transferred to non-bridging sites in C-S-H, producing carbonyl (C[double bond, length as m-dash]O) and Si-OH. The de-protonation degree of the different groups in the vicinity of the C-S-H surface was in the following order: COOH (SO3H) > C-OH > C-O-C. In the GO-COOH sheet, most COOH groups were de-protonated to COO- groups, which enhanced the polarity and hydrophilicity of the GO sheets and formed stable COOCa bonds with neighboring Ca ions. The de-protonated COO- could also accept H bonds from Si-OH in the C-S-H gel, which further strengthened the interfacial connection. On the contrary, in the GO-Oo sheet, only 8% of the epoxy group was stretched open by the Ca ions and transformed to carbonyl group, showing weak polarity and connection with the C-S-H sheet. Furthermore, uniaxial tensile test on different C-S-H/GO models revealed that C-S-H reinforced with GO-COOH and GO-OH had better interfacial cohesive strength and ductility than that observed under tensile loading. Under the reaction force field, the dissociation of water, the proton exchange between the C-S-H and GO structure, and Oc-Ca-Os bond breakage occurred to resist tensile loading. The weakest mechanical behavior observed in the G/C-S-H, GO-Oo/C-S-H and GO-SO3H/C-S-H composites was attributed to the poor bonding, dissociation of functional groups and instability of atoms in the interface region. Hopefully, the molecular-scale strengthening mechanisms could provide a scientific guide for sustainable design of cement composites.

The mechanical properties of organic/inorganic composites can be highly dependent on the interfacial interactions. In this work, with organic polymers intercalated into the interlayer of inorganic calcium silicate hydrate (C-S-H), the primary binding phase of Portland cement, great ductility improvement is obtained for the nanocomposites. Employing reactive molecular dynamics, the simulation results indicate that strong interfacial interactions between the polymers and the substrate contribute greatly to strengthening the materials, when C-S-H/poly ethylene glycol (PEG), C-S-H/poly acrylic acid (PAA), and C-S-H/poly vinyl alcohol (PVA) were subject to uniaxial tension along different lattice directions. In the x and z direction tensile processes, the Si-OCa bonds of the C-S-H gel, which were elongated and broken to form Si-OH and Ca-OH, play a critical role in loading resistance, while the incorporation of polymers bridged the neighboring silicate sheets, and activated more the hydrolytic reactions at the interfaces to avoid strain localization, thus increasing the tensile strength and postponing the fracture. On the other hand, Si-O-Si bonds of C-S-H mainly take the load when tension was applied along the y direction. During the post-yield stage, rearrangements of silicate tetrahedra occurred to prevent rapid damage. The polymer intercalation further elongates this post-yield period by forming interfacial Si-O-C bonds, which promote rearrangements and improve the connectivity of the defective silicate morphology, significantly improving the ductility. Among the polymers, PEG exhibits the strongest interaction with C-S-H, and thus C-S-H/PEG possesses the highest ductility. We expect that the molecular-scale mechanisms interpreted here will shed new light on the stress-activated chemical interactions at the organic/inorganic interfaces, and help eliminate the brittleness of cement-based materials on a genetic level.

The transport and adsorption behavior of ions and water in nanometer pores is influenced by the composition of the substrate. In this paper, to understand the effect of Al species on the properties of confined nanofluids, molecular dynamics is utilized to study the structure, dynamics and interfacial adsorption behavior of NaCl solution confined in the gel pores of C-S-H and C-A-S-H. The bridging silicate tetrahedron substituted by the aluminate species enhances the hydrophilic properties of C-S-H gel. As compared with water on the C-S-H surface, the water layered packing is densified and the magnitude of dipole moment is enlarged for water located in the vicinity of the C-A-S-H surface. This is mainly attributed to the increasing number of H bonds contributed by oxygen atoms in the aluminate silicate chains sharing more negativity. Furthermore, C-A-S-H gel immobilizes more sodium and chloride ions on the surface than C-S-H. Sodium ions can coordinate with about two to three oxygen atoms in the aluminate tetrahedron tessellated in the narrow vacancy of the silicate channel, forming inner adsorbed species. Weak interactions between Cl and the substrate are due to the few ionic pairs between Cl ions and surface-accumulated Na ions. Due to the strong Na-Os binding on the C-A-S-H surface, the diffusion coefficient of the Na ions confined in the nanometer pores is reduced by 50% and the hydration time for the Na ions associated with surrounding water is increased by 40% as compared with bulk solution.

The cement-based matrices are preferred candidates in disposing nuclear waste due to the immobilization role of the calcium-silicate-hydrate (C-S-H) gel. To better understand the immobilization mechanism of cementitious materials, molecular dynamics was utilized to investigate the intensity distribution, local structure and dynamics properties of Cs + ions in the vicinity of the calcium silicate surface. The strong inner-sphere adsorbed cesium ions were restricted by coordinated oxygen atoms in bridging and pair silicate tetrahedron and water molecules were fixed in the silicate channel by H-bonds network. On the other hand, the adsorption of chloride ion, repulsed by the negatively charged silicate surface, is mainly attributed to the formation of the cation-anion ionic pair near the interface. As compared with those of the solvated ions in the solution, the relaxation time of water in the hydration shell of adsorbed Cs + is significantly increased and the diffusion coefficient of adsorbed Cs + is dramatically reduced. Furthermore, based on the intensity profile and resident-time analysis, the adsorption capacities of monovalent cations on the C-S-H surface increase with decrease in the ionic radius, following the sequence of Na + ≫ K + > Cs + . This study provides a molecular-level understanding of the immobilization mechanism of different ions in the C-S-H gel pores.

The combination of organic and inorganic materials can result in materials with extraordinary performance. In this work, molecular dynamics was employed to investigate the structure, dynamics, interfacial behavior, and polymerization tendency of two types of monomers, aniline (AN) and ethylene glycol (EG), confined in the nanopores of calcium silicate hydrate (C-S-H) gel. The interaction mechanism between the polymer and C-S-H gel has been interpreted: the silicate chains can provide nonbridging oxygen sites to accept H-bonds from the hydroxyl in EG and -NH 2 functional groups in AN monomers. Due to stronger H-bond interactions, EG monomers are more likely to penetrate deeper into the C-S-H surface and exhibit a more pronounced orientation preference and longer resident time in the interfacial region compared with AN monomers. However, neighboring AN monomers exhibit strong spatial correlation and prefer to aggregate as large clusters, while EG monomers diffuse separately in bulk solution. Dynamically, the diffusion coefficients of monomers in the vicinity of the C-S-H surface were greatly reduced compared with those in the bulk solution due to restriction by H-bonds and attraction to surface calcium ions. Furthermore, with increasing monomer concentration, both monomers achieved greater surface adsorption density and penetration depth, and longer interfacial resident times. Higher concentrations also contributed to increased cluster size for AN monomers. The interaction mechanism in this organic-inorganic system provides useful guidelines for polymer selection, design, and fabrication of C-S-H/polymer nanocomposites.

Reactive force field molecular dynamics was utilized to simulate the reactivity, structure and dynamics of water molecules confined in calcium-silicate-hydrate (C-S-H) nano-pores of 4.5 nm width. Due to the highly reactive C-S-H surface, hydrolytic reactions occur in the solid-liquid interfacial zone, and partially surface adsorbed water molecules transforming into the Si-OH and Ca-OH groups are strongly embedded in the C-S-H structure. Due to the electronic charge difference, the silicate and calcium hydroxyl groups have binomial distributions of the dipolar moment and water orientation. While Ca-OH contributes to the Ow-downward orientation, the ONB atoms in the silicate chains prefer to accept H-bonds from the surface water molecules. Furthermore, the defective silicate chains and solvated Caw atoms near the surface contribute to the glassy nature of the surface water molecules, with large packing density, pronounced orientation preference, and distorted organization. The stable H-bonds connected with the Ca-OH and Si-OH groups also restrict the mobility of the surface water molecules. The significant reduction of the diffusion coefficient matches well with the experimental results obtained by NMR, QENS and PCFR techniques. Upon increasing the distance from the channel, the structural and dynamic behavior of the water molecules varies and gradually translates into bulk water properties at distances of 10-15 Å from the liquid-solid interface.