The piezoelectret effect in cement-based materials
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The electret effect in cement has been previously established. This thesis provides the first observation of the piezoelectret effect in cement-based materials. No aggregate was used. Both poling during the setting of the cement mix and the use of sodium silicate as an admixture helped the formation of a stable electret (as shown by a relatively stable voltage across the specimen in the absence of an applied electric field) and strengthened the piezoelectret effect. In this effect, the voltage increased upon compressive strain, with partial reversibility, such that the voltage change was up to 450 V per unit strain, corresponding to a piezoelectret coupling coefficient up to 3.1 × 10 -14 m/V. The effect was relatively strong for a sodium ion concentration of 0.5 M in the water used in the cement mix, in combination with a poling voltage of 36 V (225 V/m). The strength of the effect increased with increasing magnitude of the constant compressive stress. The direct piezoelectric effect was also observed, though it was a minor effect, with the voltage decreasing upon compressive strain, such that the voltage change was up to 6.7 V per unit strain, corresponding to a piezoelectric coupling coefficient up to -1.7 × 10 -15 m/V. A Na + ion concentration of 0.5 M gives superior performance than that of 1.0 M. For 0.5 M, the compressive modulus and piezoelectret coupling coefficient are higher. This thesis provides the first investigation of poling of cement-based materials. The poling reduced the compressive modulus and caused pore formation in the vicinity of the electrodes. However, it enhanced both electret and piezoelectret effects. The presence of silica fume as an admixture caused fracture in the specimen near the cathode during poling. No fracture occurred in the absence of silica fume.