With the increasing global concern over carbon emissions, geopolymers have garnered significant attention due to their energy-saving, waste utilization, and eco-friendly advantages. Metakaolin and slag, as aluminum-containing mineral materials in geopolymer production, have been widely studied and applied. Previous research has mainly focused on performance design and theoretical development, while the underlying mechanisms at the microscopic level remain unclear. In this study, we employed molecular dynamics simulations to investigate the microscale reaction behavior of geopolymers, exploring the induction process and structural evolution during the initial stages, and revealing the similarities and differences under alkali activation for different materials. Our findings indicate that the alkali activation process can be divided into two stages: mineral crystal deconstruction and oligomer polymerization. The role of NaOH differs between low-calcium and high-calcium systems, where in the low-calcium system, Na⁺ substitutes Ca²⁺ due to Ca²⁺ deficiency, participating in the formation of the network framework. Moreover, the high-calcium system exhibits a faster formation of the gel phase during alkali activation compared to the low-calcium system. This study provides valuable insights into the research and application of geopolymers.

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.

To enhance the microstructure and mechanical properties and to optimize the formulations of high-strength and high-flow ultra-high-performance concrete (UHPC), the effects of different quartz powder contents, fineness, and curing temperatures of UHPC were systematically studied. It was hypothesized that using the appropriate content and fineness of quartz powder can improve the microstructure of UHPC and, thus, improve its mechanical properties, especially at higher curing temperatures. To test this hypothesis, the flowability, compressive strength, flexural strength, and tensile strength of UHPC mixtures with different quartz powder dosages (0%, 15%, 30%) and fineness (4 µm, 8 µm), cured at 20 °C, 45 °C, and 90 °C, were investigated. The results indicate that as the dosage of quartz powder increases and the particle size decreases, the flowability of UHPC decreases. The compressive strength of UHPC first increases and then decreases with an increase in quartz powder dosage. Finer quartz powder usage considerably enhances packing density and pore structure. When the content of quartz powder is 15%, UHPC achieves optimal mechanical properties and pore structure, showing an improvement of 3.6% to 14.4% compared to UHPC with a coarse particle size. Additionally, an increase in curing temperature leads to the consistent growth of the compressive strength of UHPC. Under 90 °C steam curing, UHPC incorporating 15% fine quartz powder (4 μm) achieved a peak compressive strength of 182.1 MPa, increased by 19.8% compared to that under 20 °C, which is attributed to the enhanced pozzolanic activity of fine quartz powder. These findings provide valuable guidance for the mix design of UHPC in precast concrete components, long-span bridges, and marine structures, where high early strength and durability are critical.

The durability of reinforced concrete structures is significantly influenced by the effectiveness of small molecular inhibitors in preventing the corrosion of steel reinforcements. In a concrete environment, the passive film on steel bars serves as a critical protective component. In this study, a molecular dynamics (MD) simulation is used to study the inhibition mechanism of chloride ions by common corrosion inhibitors (2-Amino-2-thiazoline) in concrete in an excess chloride solution. The results reveal that inhibitors adsorb onto the steel surface primarily through van der Waals forces, with more than 90% of the adsorption occurring vertically. Despite this strong adsorption, inhibitors alone do not form a protective film. In the presence of chloride ions, which frequently penetrate concrete, the coverage rate of inhibitors on the steel surface decreases from 74% to 64%. Nevertheless, inhibitor molecules still provide substantial protection in chloride-rich concrete environments. Further analysis indicates that inhibitor molecules inhibit chloride ions in two ways. Corrosion inhibitor molecules actively desorb from the steel surface to capture chloride ions and prevent them from approaching. Additionally, inhibitors form a multi-electron layer on the steel surface to enhance passive film protection and hinder chloride ion diffusion through Coulombic interactions.

The use of rapid-hardening cementitious materials for the emergency repair of critical infrastructure in coastal environments is becoming increasingly widespread, and concrete surface hydrophobic protection treatment is equally necessary to improve the durability of both new and old concrete. Among them, silane-based hydrophobic materials play an important role in concrete hydrophobic protection. Graphene oxide (GO)-modified silane materials can significantly improve the hydrophobic performance of coatings, but the hydrophobic mechanism of coatings modifying the sulfoaluminate cement’s main hydration product ettringite (AFt) has not yet been explored. In this study, molecular dynamics simulations were conducted to investigate the wetting properties of water droplets on the surface of AFt, isobutyltriethoxy silane (IBTS)-modified AFt, and IBTS/GO-modified AFt. It was found that the AFt substrate had good hydrophilicity, and the droplets could wet the interface through Ca-O ionic bonds and H-bonds. The IBTS coating initially impeded droplet wetting, but the adsorption stability of IBTS on the AFt substrate surface was poor under droplet action, leading to droplet penetration and dispersion of the IBTS coating on the AFt surface. However, the IBTS/GO coating significantly restricted droplet wetting due to the stable adsorption of GO on the AFt surface and the strong stability of hydrogen bonds between IBTS and GO. In conclusion, selecting a suitable bridging material between AFt and silane is crucial for improving the hydrophobic stability of silane coatings on sulfoaluminate cement materials.