From Hydrogen Bond Disruption to Accelerated Polymerization: Atomic Mechanisms of Microwave-Accelerated C-A-S-H Gel...

Although microwave curing accelerates concrete strength development by a significant compared to steam curing, the atomistic origins of this kinetic enhancement remain elusive. Using molecular dynamics simulations, we investigate the influence of microwave irradiation on the nucleation and growth of Calcium-Alumino-Silicate-Hydrate (C-A-S-H) gels. The results show that alternating electric fieldsapplied via intermittent (IM) or sinusoidal (SM) modesinduce intense rotational and translational motion in polar water and hydroxyl groups. The alternating electric field reduces the hydrogen bond retention ratio from 0.915 in the Control group to 0.827 under SM mode and 0.725 under IM mode. Cage-jump dynamics analysis confirms the resulting "decaging" of monomers, with the non-Gaussian parameter α₂(t) rising from 0.33 (Control) to 0.64 (SM) and 0.85 (IM), while the cage residence time τ_cage decreases from 85 ps (Control) to 52 ps (SM) and 38 ps (IM). The released monomers exhibit significantly enhanced mobility, with mean square displacements increasing by 37% under SM mode and 44% under IM mode compared to the Control group.The two microwave modes drive distinct polymerization pathways. The SM mode promotes deep cross-linking into Q ₃ (7.0%) and Q ₄ (2.4%) structures through continuous energy input. The IM mode accelerates Q ₀ consumption (from 75% to 60%) and preferentially forms Q ₁ (20.2%) and Q ₂ (16.9%) chain-like oligomers through pulsed energy delivery. Correspondingly, Si-O-Si and Si-O-Al bond counts increase by 91.2% and 145.4% under SM mode, and by 128.6% and 294.5% under IM mode, respectively. These findings establish a fundamental link between electromagnetic energy input and nanoscale gel assembly, providing a molecular-level rationale for the accelerated strength gain observed in microwave-cured cementitious materials.