Utilization of microwave-accelerated heating and dewatering in low-pressure conditions to accelerated-cure Type-I cement paste for early-age compressive strength development

Abstract

With selective and volumetric heating, microwave (MW) heating and dewatering can improve efficiency and considerably reduce the time needed to complete the dewatering process and thereby save energy. On this basis, MW heating has great potential in terms of the accelerated-curing of cement-based materials. In this paper, the investigation focuses on the use of MW-accelerated heating and dewatering for the early-age compressive strength development of Type-I cement paste (CP) under the low-pressure conditions of 30 kPa (29.6% of ambient atmospheric pressure (101.325 kPa)). CP specimens prepared at water–cement (w/c) ratios of 0.25, 0.35, and 0.45 associated with compensating for water in the admixture used are considered. The investigation focuses on the temperature increase and moisture content of CP specimens in relation to compressive strength development during MW heating and dewatering. For this purpose, a prototype is used: specifically, a combined unsymmetrical 2-magnetron-double-fed MW and a vacuum system that constantly generates 780 ± 10 W with one magnetron installed symmetrically perpendicular to the horizontal position of the specimens (magnetron 1) and the other asymmetrically perpendicular to the horizontal position of the specimens (magnetron 2). The results show that at an early stage of MW heating and dewatering, the temperature increases continuously at a high rate. The results also show that after this early stage, strength development continues but at a slightly lower rate consistent with a change in the moisture content of the CP. Further, this change in moisture content results in the temperature of the CP increases continuously at a high rate while simultaneously causing a slight decrease in the moisture content of the CP. The results show that greater strength development accrues when CP specimens are cured in quantities of 12 per batch in the cavity (3.95% of the volume of the MW cavity) than when CP specimens are cured in quantities of 24 per batch (7.90% of the volume of MW cavity). When both magnetron 1 and magnetron 2 are used, MW heating is less efficient than when only magnetron 2 is used (780 ± 10 W). Moreover at 28 days, compared with the conventional water-cured CP (i.e., with lime-saturated water)), the MW-cured CP develops more compressive strength throughout the first 28 days, especially at 28 days: 100.2, 112.4, and 119.2% with w/c = 0.25, 0.35, and 0.45, respectively, consistent with the X-ray diffraction patterns and matched phases at day 1.