Effects of dielectric heating of fresh concrete on its microstructure and strength in the hardened state

•Dielectric heating of fresh concrete with radio-waves is introduced.•A uniform, rapid and accurate, temperature-controlled heating was achieved.•High early-age compressive strength was measured.•Microstructural characterisation showed changes due to heat treatment. By means of radio-wave technology it is possible to allow the uniform, rapid and accurate, temperature-controlled heating of fresh concrete. This results in the significantly accelerated release of hydration heat. The heat release from this second source can be easily compensated by the temperature-controlled reduction of RF energy input. The consequences are accelerated hydration and higher early-age compressive strength. Early-age concrete, heat-treated by radio waves, have a denser microstructure with a higher gel-pore content and more C-S-H phases than untreated concrete. When cured at the maximum temperature of 80°C, secondary ettringite can be formed in concrete, whereas this is not the case for the treatment temperatures of 40°C and 60°C. At the age of 28d, heat-treated concretes exhibit a looser microstructure than does untreated concrete. Larger portlandite crystals and secondary ettringite are its characteristic features as a consequence of the much faster reaction rates in the early-age concrete. This results in lower values of the compressive strength of the heat-treated concrete.