## 2 Electrical Resistivity/Impedance Measurement: Is It Really that Simple?
### Curing and Hydration  
The hydration process of cement-based materials varies with chemical composition, eventual additions and admixtures, mix proportions and curing conditions, such as T and RH. While time passes, porosity decreases because of the formation of more hydration products [157] (the decrease of porosity is associated with the difference between hydration products and hydrated cement in terms of volume [138]). In particular, in the early stages, the pore structure densities and the formed hydration products block ions conductive paths, hence electrical resistivity increases. When higher strength cement is used, porosity decreases more rapidly (while the solid phase increases) and the electrical resistivity increase is steeper [135]. Indeed, electrical resistivity is highly correlated to porosity and pore solution characteristics (conductivity reflects the volume fraction and the interconnectivity of pores, besides the conductivity of pore solution and the saturation degree [81]),hence it can be related also to hydration. This measurement can be performed in situ in real-time [158], also when concrete is in a weak and plastic state [159], avoiding the influence on the development of the microstructure [130]. Non Destructive Techniques (NDT) and in particular electrical resistivity are used to monitor concrete porosity during its early age evolution [160] and, generally, the microstructure evolution in cement-based materials during hydration processes [77]. According to [PERSON] and [PERSON] [98], during the first week of curing, electrical resistivity rapidly increase up to 60% of the final value, according to a logarithmic trend. Approximately 60 days of curing are necessary to reach an almost stationary value in mortar with carbon fibers [21]. [PERSON] et al. [77] underlined how electrical conductivity reduces with the hardening process considering 1, 10, and 100 kHz measurements. Also non-contact devices based on transformer principle [128] can be used for measuring electrical resistivity during early hydration. This makes it possible to identify five distinct phases, namely dissolution, induction, acceleration, deceleration, and diffusion-controlled periods [161]. The ionic concentration of pore solution gives a significant contribution to electrical resistivity [160]. [PERSON] et al. [130] used non-contact electrical impedance measurements [129] for the characterization of pore structure in cement pastes for three days from casting time. Through the use of pore fractal theory (combining fractal electrical network and pore structure network [162]), they obtained a relationship between the cumulative pore volume (important parameter to understand the contribution of different size pores in hydration kinetic and transportation [163]) and electrical impedance, which proved to be suitable for the assessment of pores ranging from \(\upmu\)m to mm. [PERSON] and [PERSON] [138] measured the electrical resistivity of concrete during the first 48 h of hydration; they noticed that the characteristic points on the resistivity curve over time mirror the transition processes of hydration, namely the supersaturation point (lowest point) and the final setting of concrete (first peak point of the derivative curve). [PERSON] et al. [14] evidenced that the electrical resistivity of concrete decreases with the gradual increase of ion concentration in the hydration process; moreover, there is a linear relationship between electrical resistivity and compressive strength at an early age, which means that standard compressive strength could be predicted by \(\rho\). [PERSON] et al. [87] evidenced that in the first hours of curing there is a conductivity peak, linked to the ions released from the binder to the solution; then, hydration products form, and the pore structure develops, making electrical resistivity increase. The increase of electrical resistivity over time is confirmed by [PERSON] and [PERSON] [74], who attributed this rise to drying out and hydration of concrete.