Advancing the Interpretation of Electrical Impedance Spectroscopy for Hydration and Pore Structure Assessment in Cement-Based Materials

Electrical impedance spectroscopy (EIS) is a promising nondestructive and real-time technique for monitoring hydration and pore-structure development in cement-based materials (CBMs). However, its limited practical adoption is due to the complex interpretation of impedance parameters and their relationship to hydration and microstructural properties. This study investigated the hydration behavior and pore-structure evolution of plain and silica fume (SF) blended CBMs at water-to-binder ( w / b ) ratios of 0.35, 0.45, and 0.55 using isothermal calorimetry (IC), thermogravimetric analysis (TGA), and EIS. Hydration and pore-structure parameters including rate and total heat evolution, calcium hydroxide content, nonevaporable water content (NEWC), capillary porosity, gel-to-space ratio ( G / S ), formation factor, tortuosity, and pore connectivity were evaluated. IC and TGA results showed that hydration improved with SF incorporation and decreasing w / b ratio. EIS results indicated that bulk resistivity ( R b ) and solid-liquid interface resistance ( R 1 ) increased with decreasing w / b and SF addition, reflecting a denser, more disconnected pore structure. The dielectric constant derived from capacitance ( C 1 ) increased with higher w / b due to greater ionic mobility, but decreased with SF addition due to matrix densification. These findings were supported by TGA results. Strong correlations were observed between R 1 and both compressive strength and NEWC ( R 2 = 0.83 – 0.97 ), whereas C 1 had moderate to strong correlations with compressive strength, total pore volume, and G / S ratio ( R 2 = 0.72 – 0.99 ). Overall, this study suggests that EIS parameters effectively reflect the microstructural development and enhance the practical interpretation of EIS data for the performance assessment of CBMs.