Microwave curing at an operating frequency of 2.45Â GHz of Portland cement paste at early-stage using a multi-mode cavity: Experimental and numerical analysis on heat transfer characteristics

In this study, microwave curing at an operating frequency of 2.45 GHz of Portland cement paste at an early-stage using a multi-mode cavity is presented. First, the dielectric evolution of the cement paste at a water-to-cement mass ratio of 0.38 during a 24-h first-hydration period was measured. Secondly, the microstructural characteristics of the hardened cement paste after heating for 45 min in microwaves at a power of 390 W, with specific attention to the temperature rise were investigated experimentally and theoretically. The obtained results show that dielectric properties decrease rapidly during the hydration reaction and formation resuming and then proceeding with a high rate. During microwave heating, the temperature increased monotonically. The micrographs of the microwave-heat paste clearly indicate that the samples consisted of hydrated phases and pores, as well as cores of Ca(OH)2 dendrite crystals, calcium silicate hydrate (C-S-H), and granular structure. The results of the temperature rise in our experiment without loss of moisture and steady heat transfer conduction, consistently agreed with the mathematical model developed for this study.