Microstructural development of hydrating portland cement paste at early ages investigated with non-destructive methods and numerical simulation

Microstructure development of hydrating cement paste at early ages is not only an indicator of the reactivity of cement, but also a factor on the workability of fresh concrete. In this study, the microstructure development of hydrating cement paste at early ages is investigated with non-destructive methods including ultrasound P-wave propagation velocity measurement and non-contact electric resistivity tests, together with conventional needle penetration depth and calorimetry tests. The hydration process and microstructural development of the cement paste is modeled with the three-dimensional computer model CEMHYD3D. Evolution of microstructural parameters including the volumetric fraction of phases and their percolation status are analyzed by using the results of the numerical simulation. Microstructural mechanisms of the two non-destructive techniques (ultrasound pulse propagation and electric resistivity measurements) are discussed. The main findings of this study are that the velocity of ultrasound P-wave propagation in hydrating cement paste is a function of the propagation routes in the material and inter-particle forces. The electric resistivity is controlled by the ionic concentrations in the pore solution during the early hours and later by the connectivity of pores. A model for the development of ultrasound P-wave propagation velocity is also proposed.