Study On Mechanical Properties And Pressure-sensitivity Of Cement-matrix Composites Containing Nano-sized Carbon Black

Carbon black is commonly used in rubber materials and polymers because of its high electrical conductivity, environmental stability and low cost. Presently, carbon black is added to cement matrix composites. The carbon black used is in the form of nanoparticles (with a size of 33nm). This microstructure results in a relatively large surface area per unit volume. Both the mechanical and pressure sensitive behaviors of the newly formulated composite are studied through experimental research.Results show that when the addition of carbon black in the form of nanoparticles is appropriate, both the flexural strength and compressive strength of cement-matrix composites are improved. The SEM and porosity analysis reveal that this improvement is due to the nano-size effect of carbon black. The presence of carbon black may act as the nano-filler of pores and voids, and therefore reduce the porosity of cement-matrix composites, which, in turn, increases strengths of the composite.Moreover, the newly formulated material exhibits high pressure-sensing ability. The signal provided is the fractional change in electrical resistance ((R - R0)/R0). These properties indicate that this material is an intrinsically smart material, and can be used for structural function, while at the same time act as a strain sensor itself.Results also show that the relative change in resistance ((R-R0)/R0) varies with moisture content of the composite. With very high or low moisture content, the relative change in resistance (R-R0)/R0 increases with the increase of compressive stress, and decreases during unloading, demonstrating the positive pressure sensitivity. However, when moisture content is appropriate, the relative change in resistance ((R-R0)/R0) decreases monotonically with the increase of compressive stress, and increases monotonically during unloading, showing the negative pressure sensitivity. The mechanism of "double effects" (namely, "tunneling effect" and "capacitance effect") is proposed and used for the explanation of these phenomena.