Effect of graphite on the self-sensing properties of cement and alkali-activated fly ash/slag based composite cementitious materials

This study investigated the self-sensing performances of cement and alkali-activated fly ash/slag (AAFS) systems with graphite powders as a conductive filler. Graphite powders must be evenly dispersed through the cement-based and AAFS matrices to obtain an adequate electrical response. The effects of graphite fineness on the fluidity, mechanical, rheological behavior, and cement hydration of cement paste were investigated. The finer graphite was found to have less effect on the mechanical property but seriously damaged the workability. Therefore, this research explored using four surfactants including sodium dodecyl sulfate (SDS), sodium dodecyl benzene sulfonate (SDBS), sodium salt of polynaphthalene sulphonic acid (NNO), and Triton X-100 (TX100) as dispersing agents for fine graphite in cement. The combination of NNO and TX100 was found to be the ideal admixture for improving the fluidity of graphite-containing cementitious materials. Furthermore, cement- and AAFS-based composites were produced with the graphite dispersions, and several tests such as electrical impedance spectroscopy (EIS), energy dispersive X-Ray (EDX) map analysis, mercury intrusion Porosimeter (MIP), X-ray diffraction (XRD), and thermogravimetric analysis (TGA) were used to investigate their intrinsic characteristic. The results concluded that AAFS had a better conductivity than cement due to the higher porosity in the range of very small pores (<6 nm) and the higher ion concentration contribution. Finally, the self-sensing behavior of the mortars was investigated under cyclic loading, and the self-sensing mechanism was proposed. The sensing properties of graphite composite mortars under compression load can be attributed to the combination of ionic, contact, and tunneling conduction.