Cement-based piezoelectret

A cement-based piezoelectret is reported for the first time. Both poling during setting and sodium silicate liquid admixture strengthened the piezoelectret effect. The electret voltage increased upon compressive strain, with partial reversibility; the voltage change was up to 450 V per unit strain (i.e., piezoelectret coupling coefficient up to 4.2 × 10^-15 m/V). The effect was relatively strong for a Na⁺ concentration of 0.5 M in the water, in combination with a poling electric field of 225 V/m. The effect increased with increasing magnitude of the constant compressive stress. The direct piezoelectric effect was observed as a minor effect, with the voltage decreasing upon compressive strain; the voltage change was up to 6.7 V per unit strain (i.e., piezoelectric coupling coefficient down to -2.3 × 10 ^-16 m/V). An Na⁺ concentration of 0.5 M gave superior performance than 1.0 M. For 0.5 M, the compressive modulus and piezoelectret coupling coefficient were higher. The poling reduced the compressive modulus and caused pore formation in the vicinity of the electrodes, but it enhanced the piezoelectret effect.