Highly conductive and sensitive piezoresistive cement mortar with graphene coated aggregates and carbon fiber

Traditionally, conductive fillers are mixed directly with cement matrix before binding with aggregates to develop piezoresistive cement-based sensors. This results in the most vulnerable region, interfacial transition zone (ITZ), from which microcracks are initiated, merely located at the periphery of the conductive network and thus limits the sensitivity of the smart sensor. This study proposes a strategy to construct a three-dimensional (3D) conductive network in the mortar with ITZ directly embedded in it, thus greatly increasing both the conductivity and piezoresistivity without significantly sacrificing mechanical property. Highly conductive graphene-coated fine aggregates (termed conductive G@FAg particles) are prepared by adsorption of graphene oxide (GO) onto the fine aggregates (FAg) surface, followed by simple annealing and microwave treatment. The combined usage of conductive G@FAg particles and 0.1 wt% 6 mm-CF results in an outstanding electrical conductivity (resistivity of 580 Ω cm) and an excellent fractional change in resistivity (FCR of 30%) under cyclic compressive loading, with a negligible compressive strength loss of 3.1%. Remarkably, the addition of conductive G@FAg particles together with 0.5 wt% 10 mm-CF leads to an excellent conductivity (resistivity of 165 Ω cm) and self-sensing ability (FCR of ∼90%), which outperforms the previously reported mortar directly incorporated with the same concentration of CF and graphene. The much-improved conductivity and FCR value with such a low weight percentage of conductive carbon materials are attributed to the unique 3D network of conductive channels.