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This paper investigates the microstructure evolution of CSA cement pastes with different water to cement ratios in various hydration stages. Microstructure information, such as porosity, cumulative and incremental pore volume of cement pastes, is revealed by the innovative non-contact electrical impedance measurement (NCEIM), low temperature differential scanning calorimetry (LT-DSC), nitrogen adsorption test and mercury intrusion porosimetry (MIP). The hydration features associated with microstructure evolution are identified by NCEIM, Micro-Raman spectra and heat evolution test. Through the overall analysis of experimental results, initial formation and evolution of microstructure of CSA cement pastes are identified.

Electrochemical impedance spectroscopy (EIS) is adopted as a nondestructive testing method for exploring the fracture feature of the carbon nanotube/cement composite materials. A novel equivalent circuit model is used to explain the change regularity of fracture toughness. Based on the charge transfer on the surface of carbon nanotubes (CNTs), the electrical resistance can be applied to evaluate the conducting efficiency in a certain orientation. The EIS results indicate that more CNTs fibers are perpendicular to the crack opening with increasing content of CNTs, which is corresponding with a higher fracture toughness of CNTs/cement composites. The experimental results also demonstrate the fracture toughness of CNTs/cement composites with different contents (0 wt%, 0.033 wt%, 0.066 wt% and 0.1 wt%) of CNTs could be accurately responded with linear relationship with the EIS results. Furthermore, an advanced equivalent circuit model is proposed to describe the impedance response of CNTs/cement composites. The EIS method therefore can be used as a reliable, convenient and non-destructive method to assess the fracture toughness of CNTs/cement composites.