It is vital to maintain the durability of construction materials since sulfate attack is a primary factor influencing the lifespan of concrete in marine setting. Sulfate ions (SO₄^2–) degrade concrete via attacking the calcium silicate hydrate (C-S-H) that is the primary hydration product of cement and the binding phase in concrete. This process primarily occurs through calcium leaching, where Ca²⁺ reacts with sulfates to form gypsum, altering pore structure and weakening mechanical properties. C-S-H exhibits a “long-range disorder, short-range order” molecular structure, consisting of nano-scale particles. It is indicated that sulfate-induced degradation involves a competition between SO₄^2– and silicate ions (SiO₃^2–) for Ca²⁺, leading to a material destabilization. The simulations by molecular dynamics (MD) and quantum chemistry (QC) provide insights into this phenomenon, but they have limitations, i.e., MD lacks chemical reaction accuracy, while QC is restricted in a scale. Understanding the molecular interactions governing Ca²⁺ transfer from SiO₃^2– to SO₄^2– is essential for decoding sulfate-induced degradation. However, the existing experimental and computational methods struggle to capture this process comprehensively. A more effective approach is required to accurately simulate chemical reactions and address the dynamics of Ca²⁺ transport in C-S-H degradation.

This study was to construct a C-S-H model based on tobermorite’s unit cell structure, optimizing elemental balance to achieve a Ca/Si ratio of 1.5. The energy minimization was performed by the GFN-xTB method, with the model embedded within a 13 Å water sphere to simulate aqueous conditions. The simulations by Born-Oppenheimer Molecular Dynamics (BOMD) employing the GFN-xTB method investigated the C-S-H decalcification under sulfate attacks. Two 100 ps NVT ensemble simulations at 298.15 K were conducted, i.e., one in pure water and another in sulfate solution (with one SO₄^2– molecule analyzed for Ca²⁺ detachment). The atomic structures were visualized using VMD. The calculations based on Complementary Density Functional Theory (DFT) (Gaussian 16) focused on critical decalcification steps, isolating structural regions (SO₄^2–, desorbed Ca²⁺, and Si—O tetrahedra). The PBE0 hybrid functional and def2-TZVP basis set were used for single-point energy calculations, with DFT-D3 dispersion correction addressing weak interactions.

This study investigates the decalcification kinetics and structural evolution of C-S-H under sulfate attack using molecular dynamics simulations. In pure water, C-S-H experiences minor structural changes, with surface calcium ions partially dissociating but remaining adsorbed. However, in a sulfate solution, calcium ions desorb rapidly upon sulfate introduction, leading to a structural fragmentation, including Q² to Q¹ transitions and severe silicate chain distortions. The analysis by reduced density gradient (RDG) reveals sulfate’s stronger interaction with Ca²⁺, compared to silicate, facilitating calcium extraction. The analysis by electron localization function (ELF) shows a weak Ca—O_Si bonding, while Ca—O_Sul exhibits a stronger electronic localization, promoting calcium displacement. The analysis by bond critical point (BCP) indicates non-covalent interactions drive decalcification, with competitive binding between sulfate and silicate.

The analysis by electrostatic potential (ESP) indicates an increased C-S-H electronegativity post-decalcification, destabilizing the structure. The energy barrier calculations show that sulfate significantly lowers decalcification and silicate chain fracture barriers, weakening C-S-H integrity. Water further amplifies sulfate-induced degradation, enhancing polarity and self-expansion effects. These findings demonstrate that sulfate corrosion accelerates C-S-H structural failure through enhanced calcium extraction, increased electronegativity, and silicate chain fracture, ultimately compromising mechanical properties.

This study explored C-S-H decalcification and degradation under sulfate attack using quantum chemistry and DFT. Sulfate ions could extract calcium via the Coulomb interactions, confirmed by RDG and BCP analysis. The decalcification could destabilize C-S-H, increasing silicate chain electronegativity, expanding Si—O bonds, and reducing dissociation energy. Silicate chain fractures caused C-S-H gel collapse, weakening cementitious materials. Water could facilitate a degradation via lowering energy barriers and enhancing hydrogen bonding with silicate chains, accelerating structural breakdown. These findings could reveal the fundamental mechanisms of sulfate-induced C-S-H deterioration, thus providing insights into material performance under aggressive environments.