Production of high-early-compressive-strength Portland cement paste using low-pressure microwave-accelerated heating and curing: processing characteristics and factors affected

The use of low-pressure microwave (MW)–accelerated heating and curing in the production of high-early-strength Portland cement paste (CP) in relation to the processing characteristics and factors affected is investigated. The effects of the pressure in the MW cavity, the feed direction of the MW, and the number of CP specimens per MW curing batch on the temperature increase and moisture content (i.e., the water-cement ratio (w/c)) of the CP and its compressive strength after MW-curing CP are considered. A double-feed MW-vacuum system is used to generate MW by applying 800 W (1 magnetron) or 1,600 W (2 magnetrons) and to convey MW to the CP specimens. The CP pastes are designed and mixed at specific initial w/c ratios of 0.25, 0.35, and 0.45. The CP specimens are cured using a low-pressure MW cavity at 30 and 50 kPa and fed in symmetrically or asymmetrically perpendicular directions of the CP specimens in batches of 12 (3.95% of the volume of microwave cavity) and 24 specimens (7.90% of the volume of the microwave cavity) per MW-curing batch with the following dimensions: 5 cm long × 5 cm wide × 10 cm thick. The experimental results show that with the maximum MW curing temperature of 70 °C from the initial stage (approximately the first 30 min of applying low-pressure MW curing) until 100 min, the temperature of the CP increases continuously at a high rate; then, the rate at which the temperature increases decreases slightly, which is consistent with the remaining w/c of the CP. The pressure in the MW has a slightly different temperature increase and remaining CP. A perpendicular symmetric magnetron placement in respect to the horizontal position of the specimens can lead to a steady increase in the strength development of all the CP specimens. Further, over the course of 28 days, compared with the water-cured CP, the MW-cured CP develops more compressive strength.