Thermal Damage of Serpentine Concrete Under Microwave Volumetric Versus Muffle Furnace Surface Heating

Serpentine concrete is a key shielding material in nuclear engineering, and its thermal damage behaviour under irradiation-induced self-heating is closely related to structural safety. This study compares microwave volumetric heating (MW), which simulates internal heat generation, with muffle furnace surface heating (MF), which represents conventional external heating, at temperatures from 150 to 800 °C. The two heating mechanisms produced opposite temperature gradients: MW generated an inward-to-outward gradient, while MF produced a top-to-bottom gradient. These contrasting gradients led to distinct damage pathways—inside-out cracking versus top-down spalling—and different damage progression rates. In the critical dehydration range of 300–450 °C, the mass loss increment under volumetric heating (7.40%) was 3.2 times that under surface heating (2.29%). At 450 °C, the strength retention of the MW group (20.4%) was substantially lower than that of the MF group (77.7%). A heating-mechanism correction factor κ ≈ 0.25 is proposed to adjust surface-heating-based strength predictions to volumetric heating conditions. A comprehensive damage index D, integrating mass loss, crack development, and mechanical degradation, showed that severe damage under volumetric heating occurs approximately 200 °C earlier than under surface heating. These findings provide a quantitative framework for assessing thermal damage in concrete subjected to internal heat generation, with implications for nuclear safety assessments.