Carbon Nanotubes Reinforced Cement Composites - Applications & Performances

Prior papers have shown that AC-impedance spectroscopy (AC-IS) can be used to characterize the microstructure of fibre-reinforced cements (FRCs) with discontinuous conductive (e.g., carbon or steel) macro-fibres (mm-scale diameters) or micro-fibres (1m-scale diameters). AC-IS has been employed to investigate fibres dispersion issues in macro-and micro-composites, including orientation, coarse-scale segregation, and local aggregation. Of these issues, local aggregation (clumping) should be particularly applicable to CNT-FRCs, as adequate dispersion of carbon nanotubes in cement matrix proves very challenging to achieve. The AC-impedance spectroscopy (AC-IS) technique involves application of AC (Alternating Current) excitation of set amplitude over a range of frequencies to the specimen.
The measured response can be represented in a so called Nyquist plot (negative imaginary impedance [-Im(Z)], vs. real impedance, [Re(Z)]), with frequency increasing from right to left. The plot contains semicircles or arcs whose diameters correspond to the resistances of the different components in the composite microstructure. For AC-IS measurements, two stainless steel electrodes must be embedded at the two ends of the specimens immediately after casting.

Frequency markers are shown as darkened points with corresponding numbers. The incomplete rightmost arc corresponds to the electrochemical response of the external steel electrodes used to take measurements. The key feature of micro-fibre composite Nyquist plots is the occurrence of two bulk arcs vs. a single bulk arc (for un-reinforced matrix). Note that the low-frequency cusp (between the electrode arc and the composite bulk arc) coincides with the four-point DC resistance, RDC, of the composite (the solid symbol on the real impedance axis). Furthermore, it remains relatively unchanged (than un-reinforced OPC) upon the addition of fibres.
At DC and low AC frequencies an electrochemical double-layer (polarization resistance) on fibres surfaces electrically isolates them from the matrix. The fibres behave as if they were insulating at DC and low AC frequencies. Insulating fibres at small volume fraction will have insignificant effect on the DC resistance of the composites.
At higher AC frequencies displacement currents shorted out the double layers on the fibres, rendering them conductive. The marked decrease in resistance compared to RDC, as reflected in the high-frequency cusp (Rcusp), is a direct result of the conducting fibres. Wansom et al. applied this measurement procedure to CNT/cement pastes.

They prepared three different mixtures of un-reinforced and reinforced cement pastes (0.75%vol MWNTs, 1%vol MWNTs) at a fixed w/c ratio of 0.45. The mixture was cast into a polycarbonate mould (12.5 mm - 12.5 mm - 25 mm). Two-point AC-IS measurements were made using the embedded electrodes at 15 min, 1 h, 4 h, 6 h and 8 h after mixing. At 1 day, the specimens were demoulded and AC-IS measurements were performed using sponges soaked in aqueous solution (which approximates the conductivity of OPC pore solution at early ages) and lightly pressed against stainless steel electrodes. Four-point DC measurements, performed only at 1 day, were taken by wrapping stainless steel wires at 1/4 and 3/4 positions around the specimens to serve as voltage contacts.