Abstract
Cement nanocomposites with carbon nanofibers (CNFs) are electrically conductive and sensitive to mechanical loads. These features make them useful for sensing applications. The conductive and load sensing properties are well known to be dependent on carbon nanofiber content; however, much less is known about how the conductivity of hybrid cement–CNF depend on other parameters (e.g., water to cement ratio (w/c), water saturation of pore spaces and temperatures above ambient temperature). In this paper we fill-in these knowledge gaps by: (1) determining a relationship between the cement–CNF bulk resistivity and w/c ratio; (2) determining the effect of water present in the pores on bulk resistivity; (3) describing the resistivity changes upon temperature changes up to 180 °C. Our results show that the increase in the water to cement ratio results in increased bulk resistivity. The decrease in nanocomposite resistivity upon a stepwise temperature increase up to 180 °C was found to be related to free water release from cement pores and the dry materials were relatively insensitive to temperature changes. The re-saturation of pores with water was not reversible with respect to electrical resistivity. The results also suggest that the change in the type of electrical connection can lead to two orders of magnitude different bulk resistivity results for the same material. It is expected that the findings from this paper will contribute to application of cement–CNF-based sensors at temperatures higher than ambient temperature.
Highlights
Cement nanocomposites have recently gained much attention
The very dark strikes, visible on the pictures as an extension of very bright metal connectors, are scanning artefacts associated with beam hardening
The results suggest that cement–carbon nanofibers (CNFs) material bulk resistivity is strongly dependent on both the water to cement ratio and the amount of free water present in the cement pore system
Summary
Cement nanocomposites have recently gained much attention. Nanoparticles are added to engineer the mechanical properties [1,2,3,4] of the composites and to render cement electrically conductive and responsive to mechanical load [5,6].It has been shown that cement materials with well dispersed electrically conductive fillers, such as metal fibers, graphite powder, carbon nanofibers and carbon nanotubes, show conductive properties above a percolation threshold [7,8,9,10,11,12,13,14]. The percolation threshold is a critical concentration of the dispersed material above which the dispersed particles form a continuous network [15,16] so that it conducts electric charge. Due to their conductive properties, as well as sensitivity to stress (through the piezoelectric effect), the hybrid cement materials are considered as excellent sensors in areas such as the structural health monitoring of reinforced concrete structures [8,17] and traffic. The strain sensitivity of conductive cements relies on changes in electrical resistivity upon the application of mechanical load. The closer the conductive particles are and the more interparticle connections that are created, the larger an electrical current can be established leading to a decrease in resistivity of the material [24]
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