The evolution of composition and thermal conductivity of the ITZ in the early age concrete: model and experiment

The Interfacial Transition Zone (ITZ) thermal properties are critical for early-age thermal crack initiation in concrete. This study developed a model combining equivalent water-cement ratio (w/c) theory and the Mori-Tanaka method to predict the evolution of the ITZ's composition and thermal conductivity during the hydration process. Meanwhile, an innovative testing method by using special layered specimens was proposed based on the Serial-Parallel to verify the model. The results show that as the w/c ranges from 0.3 to 0.5, the decline rate of the thermal conductivity ranges from 17% to 21%. Furthermore, this reduction exhibits strong time-dependency, characterized by a rapid decrease during the initial 7 days of hydration followed by a gradual deceleration up to 28 days. The ITZ thermal conductivity evolution is affected by the competition between pore and solid phases, and the decrease in the solid phase thermal conductivity due to the hydration-induced chemical composition transformation plays a dominant role. Finally, an explicit simplified reduced-order model was derived based on extensive data generated by a physics-based surrogate model.