Effects of isothermal microwave curing on steel fibre-reinforced reactive powder concrete: Strength, microstructure and hydration products

•Isothermal microwave curing was effective for preparing RPC.•Microwaves accelerated strength development significantly.•The optimal curing process was obtained.•The contributed reactions were identified. A combined application of microwaving curing and reactive powder concrete (RPC) may provide an accelerated development of outstanding mechanical and physical properties, which has great potential to improve the quality and efficiency of production. In this work, a custom-made microwave oven was used to isothermally cure RPC with steel fibre reinforcement at 40 °C, 60 °C and 80 °C for 30 min. Compared to phased nonisothermal microwave curing, the isothermal microwave curing is equally effective, yet more energy efficient to operate and easier to control. The microwave-cured specimens at 1 day gained up 72.0% and 127.6% of the 28 days compressive and flexural strength achieved with standard curing, respectively. The acceleration rates varied with material characteristics and initial temperatures. The characterisation data obtained from STA, XRD and SEM-EDS analysis indicated that the beneficial effects of microwave curing temperature on strength development were due to the accelerated cement hydration and pozzolanic reaction of silica fume. The products resulting from this acceleration strengthened the interfacial zone between hydrated regions and aggregates, and the interfacial zone between the cement and silica fume. Besides C-S-H, AFm, AFt, CH, CC-, Al-tobermorite, a CO32−-contained fukalite-like phase was found to be formed, which may be produced by the pozzolanic reaction of silica fume at the expense of calcium carbonate under microwave curing.