Molecular Dynamics Simulation of the Precipitation of Calcium Silicate Hydrate Nanostructures under Two-Dimensional Confinement by TiO₂: Implications for Advanced Concretes

Given the environmental footprint of concrete, the demand for new concrete with higher macroscopic performances is continuously increasing to build structures with less concrete while extending the service life. With the advances in nanotechnology, comprehensive investigations have shown that nanomodification of concrete can significantly improve the macroscopic performances. However, many questions regarding the interaction between nanomaterials and cement hydrates at the nanoscale are still unclear, which greatly limits the further development of nanoengineered concrete in construction. Herein, we use reactive molecular dynamics simulation to investigate the precipitation of calcium silicate hydrate gel (C–S–H gel, the principal binding phase of conventional concrete) in the TiO2 nanochannel with different spacings. Results show that the polymerization kinetics as well as the final degree of polymerization of C–S–H can be enhanced under the TiO2 nanoconfinement. Up to 15% Q4 units form in the more polymerized C–S–H under TiO2 nanoconfinement. Because of the difference in adsorption capacity for Ca and Si ions, the chemical composition of C–S–H precipitated on the surface of TiO2 with oxygen dangling bonds will result in the nanosegregation of Ca-rich and Si-rich regions, while the highly connected Qn units (Q3 and Q4 units) are formed in the Si-rich regions. We also show that the hydroxylation of TiO2 surface drives the polymerization process of C–S–H. On the basis of the water mobility in the C–S–H gels, we demonstrate the C–S–H growth in a limited spacing and precipitated on the TiO2 surface, resulting in a more compact nanostructure

Topological Control on Silicates’ Dissolution Kinetics

Like many others, silicate solids dissolve when placed in contact with water. In a given aqueous environment, the dissolution rate depends on the composition and the structure of the solid and can span several orders of magnitude. Although the kinetics of dissolution depends on the complexities of both the dissolving solid and the solvent, a clear understanding of which structural descriptors of the solid control its dissolution rate is lacking. By pioneering dissolution experiments and atomistic simulations, we correlate the dissolution ratesranging over 4 orders of magnitudeof a selection of silicate glasses and crystals to the number of chemical topological constraints acting between the atoms of the dissolving solid. The number of such constraints serves as an indicator of the effective activation energy, which arises from steric effects, and prevents the network from reorganizing locally to accommodate intermediate units forming over the course of the dissolution

Misfit Stresses Caused by Atomic Size Mismatch: The Origin of Doping-Induced Destabilization of Dicalcium Silicate

Density functional theory (DFT) simulations are carried out to systemically investigate doping in dicalcium silicate (Ca₂SiO₄: C₂S), a major phase in calcium silicate cements. By evaluating the energetics of defect formation mechanisms for species involving Na⁺, K⁺, Mg²⁺, Sr²⁺, Al³⁺, Fe³⁺, B³⁺, and Ge⁴⁺, we find a strong site preference for all cationic substitutions. As a result, distinct defects form at low dopant concentrations (i.e., ≤ 0.52 atom %), in which, expectedly, larger dopants prefer (larger) Ca²⁺ sites, while smaller dopants favor (smaller) Si⁴⁺ sites, with charge balance being ensured by the formation of vacancies. Such site preferences arise due to local atomic distortions, which are induced when doping occurs at unfavorable substitution sites. Interestingly, we note that the formation enthalpy of each substitutional defect is proportional to the size mismatch between the dopant and the native cations. This indicates that the destabilization of the C₂S structure has its origins in an “atomic size misfit” which develops while accommodating defects in the C₂S lattice. The outcomes resulting from this work provide insights that are needed to select dopants to optimize cement performance by compositional design

Deciphering How Anion Clusters Govern Lithium Conduction in Glassy Thiophosphate Electrolytes through Machine Learning

Glasses such as lithium thiophosphates (Li2S-P2S5) show promise as solid electrolytes for batteries, but a poor understanding of how the disordered structure affects lithium transport properties limits the development of glassy electrolytes. To address this, we here simulate glassy Li2S-P2S5 electrolytes with varying fractions of polyatomic anion clusters, i.e., P2S64–, P2S74–, and PS43–, using classical molecular dynamics. Based on the determined variation in ionic conductivity, we use a classification-based machine-learning metric termed “softness”a structural fingerprint that is correlated to the atomic rearrangement probabilityto unveil the structural origin of lithium-ion mobility. The softness distribution of lithium ions is highly spatially correlated: that is, the “soft” (high mobility) lithium ions are predominantly found around PS43– units, while the “hard” (low mobility) ions are found around P2S64– units. We also show that soft lithium-ion migration requires a smaller energy barrier to be overcome relative to that observed for hard lithium-ion migration

Accessing a Forbidden Disordered State of a Zeolitic Imidazolate Framework with Higher Stiffness and Toughness through Irradiation

Metal–organic frameworks (MOFs) with nanoscale porosity have numerous potential applications, for example, within catalysis, shock absorption, ion transportation, gas sorption, and separation. Recently, the disordered state of MOFs has also attracted significant attention, since some MOF crystals can melt and be quenched into bulk glasses, as first discovered for zeolitic imidazolate frameworks (ZIFs, a subset of MOFs). Another potential route to disorder in MOFsthat so far remains unexploredis irradiation, e.g., silicate minerals are well-known to rearrange into a disordered state upon irradiation. Here, we investigate the structural disordering of both ZIF-4 crystals and glasses under irradiation using reactive molecular dynamics simulations. Our results show that the two phases converge toward a new “forbidden” disordered state that is not accessible through any thermal path. The evolution of the structure upon irradiation allows us to establish a relationship between structural disorder and mechanical properties to explain why the irradiated ZIF-4 state is both stiffer and tougher than that of both the parent ZIF-4 crystal and glass

Electronic Origin of Doping-Induced Enhancements of Reactivity: Case Study of Tricalcium Silicate

Systematic manipulation of the reactivity of silicate materials in aqueous environment remains a challenging topic. Herein, by combining first-principles and reactive molecular dynamics simulations, we present a complete picture of the influence of impurity species on hydration reactivity, using the reactive triclinic tricalcium silicate phase as an example. We show that although initial hydration is influenced by the surface’s chemistry and structure, longer-term hydration is primarily controlled by proton transport through the bulk solid. Both shorter- and longer-term hydration processes are noted as being intrinsically correlated with electronic features. These outcomes provide the first direct evidence of the linkages between electronic structure and the longer-term (i.e., on the order of several nanoseconds) hydration behavior and sensitivity of hydrophilic crystalline materials and also offer a pathway to efficient compositional design for similar materials

Predicting Fracture Propensity in Amorphous Alumina from Its Static Structure Using Machine Learning

Thin films of amorphous alumina (a-Al2O3) have recently been found to deform permanently up to 100% elongation without fracture at room temperature. If the underlying ductile deformation mechanism can be understood at the nanoscale and exploited in bulk samples, it could help to facilitate the design of damage-tolerant glassy materials, the holy grail within glass science. Here, based on atomistic simulations and classification-based machine learning, we reveal that the propensity of a-Al2O3 to exhibit nanoscale ductility is encoded in its static (nonstrained) structure. By considering the fracture response of a series of a-Al2O3 systems quenched under varying pressure, we demonstrate that the degree of nanoductility is correlated with the number of bond switching events, specifically the fraction of 5- and 6-fold coordinated Al atoms, which are able to decrease their coordination numbers under stress. In turn, we find that the tendency for bond switching can be predicted based on a nonintuitive structural descriptor calculated based on the static structure, namely, the recently developed “softness” metric as determined from machine learning. Importantly, the softness metric is here trained from the spontaneous dynamics of the system (i.e., under zero strain) but, interestingly, is able to readily predict the fracture behavior of the glass (i.e., under strain). That is, lower softness facilitates Al bond switching and the local accumulation of high-softness regions leads to rapid crack propagation. These results are helpful for designing glass formulations with improved resistance to fracture

Table_1_Glass Fracture Upon Ballistic Impact: New Insights From Peridynamics Simulations.DOCX

Most glasses are often exposed to impact loading during their service life, which may lead to the failure of the structure. While in situ experimental studies on impact-induced damage are challenging due to the short timescales involved, continuum-based computational studies are complicated by the discontinuity in the displacement field arising from the propagation of cracks. Here, using peridynamics simulations, we investigate the role of the mechanical properties and geometry in determining the overall damage on a glass plate subjected to ballistic impact. In particular, we analyze the role of bullet velocity, bullet material, and elastic modulus, fracture energy, and radius of the plate. Interestingly, we observe a power-law dependence between the total damage and the fracture energy of the glass plate. Through an auto-regressive analysis of the evolution of cracks, we demonstrate that the self-affine growth of cracks leads to this power-law dependence. Overall, the present study illustrates how peridynamic simulations can offer new insights into the fracture mechanics of glasses subjected to ballistic impacts. This improved understanding can pave way to the design and development of glasses with improved impact-resistance for applications ranging from windshields and smart-phone screens to ballistics.</p

Electrosteric Control of the Aggregation and Yielding Behavior of Concentrated Portlandite Suspensions

Portlandite (calcium hydroxide: CH: Ca(OH)2) suspensions aggregate spontaneously and form percolated fractal aggregate networks when dispersed in water. Consequently, the viscosity and yield stress of portlandite suspensions diverge at low particle loadings, adversely affecting their processability. Even though polycarboxylate ether (PCE)-based comb polyelectrolytes are routinely used to alter the particle dispersion state, water demand, and rheology of similar suspensions (e.g., ordinary portland cement suspensions) that feature a high pH and high ionic strength, their use to control portlandite suspension rheology has not been elucidated. This study combines adsorption isotherms and rheological measurements to elucidate the role of PCE composition (i.e., charge density, side chain length, and grafting density) in controlling the extent of PCE adsorption, particle flocculation, suspension yield stress, and thermal response of portlandite suspensions. We show that longer side-chain PCEs are more effective in affecting suspension viscosity and yield stress, in spite of their lower adsorption saturation limit and fractional adsorption. The superior steric hindrance induced by the longer side chain PCEs results in better efficacy in mitigating particle aggregation even at low dosages. However, when dosed at optimal dosages (i.e., a dosage that induces a dynamically equilibrated dispersion state of particle aggregates), different PCE-dosed portlandite suspensions exhibit identical fractal structuring and rheological behavior regardless of the side chain length. Furthermore, it is shown that the unusual evolution of the rheological response of portlandite suspensions with temperature can be tailored by adjusting the PCE dosage. The ability of PCEs to modulate the rheology of aggregating charged particle suspensions can be generally extended to any colloidal suspension with a strong screening of repulsive electrostatic interactions

Resolving the Conflict between Strength and Toughness in Bioactive Silica–Polymer Hybrid Materials

Simultaneously improving the strength and toughness of materials is a major challenge. Inorganic–polymer hybrids offer the potential to combine mechanical properties of a stiff inorganic glass with a flexible organic polymer. However, the toughening mechanism at the atomic scale remains largely unknown. Based on combined experimental and molecular dynamics simulation results, we find that the deformation and fracture behavior of hybrids are governed by noncovalent intermolecular interactions between polymer and silica networks rather than the breakage of covalent bonds. We then attempt three methods to improve the balance between strength and toughness of hybrids, namely the total inorganic/organic (I/O) weight ratio, the size of silica nanoparticles, and the ratio of −C–O vs −C–C bonds in the polymer chains. Specifically, for a hybrid with matched silica size and I/O ratio, we demonstrate optimized mechanical properties in terms of strength (1.75 MPa at breakage), degree of elongation at the fracture point (31%), toughness (219 kPa), hardness (1.08 MPa), as well as Young’s modulus (3.0 MPa). We also demonstrate that this hybrid material shows excellent biocompatibility and ability to support cell attachment as well as proliferation. This supports the possible application of this material as a strong yet tough bone scaffold material