Exploring the Mechanism of Microstructural Changes in Ultra-High-Performance Concrete Under Microwave Influence: Experiments and Molecular Dynamics Simulation.

To elucidate the mechanisms of microstructural changes in ultra-high-performance concrete (UHPC) under microwave exposure, this study characterizes the microstructure at multiple scales using a combination of microscopic experiments and molecular dynamics simulations. The hydration products, pore structure, morphology, and interface transition zone (ITZ) of UHPC specimens were analyzed using mercury intrusion porosimetry (MIP), X-ray diffraction (XRD), and scanning electron microscopy (SEM). Molecular dynamics simulations were employed to investigate the uniaxial tensile behavior, free volume, and radial distribution of calcium silicate hydrate (C-S-H) gel, the primary hydration product. The results indicate that microwave curing significantly reduces the pore volume of specimens, with a daily average reduction of 0.15% in the early stages. This accelerated reduction in porosity effectively diminishes the number of high-risk pores. The hydration products formed under microwave curing exhibit higher density and enhanced internal pore optimization. Simulation findings suggest that the non-thermal effects of microwaves play a more significant role in the structural evolution. The molecular orientation of C-S-H changes after oscillation, leading to more ordered molecular arrangements. Mechanical oscillation also expels free volume from the crystal cells, promoting a more compact overall structure and increasing the tensile strength by up to 1 GPa.