The effects and mechanisms of low-energy consumption microwave curing on the microstructure and strength development of cement-based materials

Abstract

Microwave curing is a promising method for concrete curing. This study investigates the effects of microwave curing on the hydration and microstructural development of Portland cement. Compressive strength tests were conducted to compare the mechanical properties of Portland cement specimens under various microwave and thermal curing regimes. Additionally, techniques such as XRD, ¹H LF NMR, ²⁷Al NMR, ²⁹Si NMR, and ICP-OES were employed to characterize the composition of hydration products and the microstructure of the Portland cement under different curing conditions. The results indicate that the Portland cement specimens achieved optimal compressive strength under an appropriate microwave curing regime (MC30). Remarkably, even when the temperatures reached by microwave curing were comparable to or lower than those of thermal curing, the strength of the specimens was significantly superior to those of thermal cured and normal cured specimens at most ages. The analysis suggests that microwave curing not only exerts a thermal effect on the hydration of Portland cement but also has non-thermal effects. These non-thermal effects promote the leaching and migration of Al and Si from the clinker minerals, leading to a higher formation of AFm phases and a more polymerized C-(A)-S-H gel in the hydration products at equivalent ages. Furthermore, the appropriate microwave curing regime results in a more uniform temperature distribution within the specimens, which mitigates the damage and increased porosity commonly associated with large temperature gradients during thermal curing. Therefore, through the combined effects of these factors, microwave curing can enhance the microstructure and improve the strength development of cement-based materials.