Monitoring the Hydration Process in Carbon Nanotube Reinforced Cement-Based Composites Using Nonlinear Elastic Waves

There is currently an increased need for associating construction material properties and behavior with the nature of their microstructure. One of the major issues in this context is the need for understanding the curing process in freshly poured cement-based materials. This is particularly important when nanoreinforcement materials, such as carbon nanotubes, are used to enhance the mechanical behavior and multifunctionality of the final structure. The solidification point, at which the state of liquid suspension transmutes to the solid state, is of particular interest since it greatly influences the load-bearing capacity of the cement-based material and its structural behavior at the long term. The main purpose of the present work is to develop a reliable method for monitoring the hydration process during the early stages of freshly poured cementitious composites enhanced with carbon nanotubes. This methodology is based on the use of nonlinear elastic waves. To achieve this goal, a combination of contact ultrasonics with noncontact optical detection was used. The detection method for evaluating the setting process is based on the assessment of higher-harmonic amplitudes of an ultrasonic wave, with a given frequency, propagating through the cementitious material. It was observed that the material nonlinearity changes significantly during the hardening process, compared to velocity or attenuation measurements which are based on linear acoustics. These changes were more noticeable as the concentration of carbon nanotubes in the cement matrix increases, indicating that higher harmonics are more susceptible to minute microstructural changes.