Direct piezoelectric effect in geopolymeric mortars

In this paper the occurrence of a direct piezoelectric effect has been discovered for the first time in a metakaolin based geopolymeric mortar. Unlike any other known piezoelectric material (quartz single-crystal, PZT, PVDF), wherein the formation of local electric dipoles is due to the elastic deformation of the non-centrosymmetric crystalline structures, a new and different piezoelectric mechanism was observed in geopolymers. It is due to a complex interplay that involves not-framework cations, framework and water contained in the material pores. In particular, the model proposed by authors attributes the piezoelectric effect to the charge imbalance and local dipoles generated under compressive stress by the migration, within the network of interconnected pores of geopolymer, of the hydrated Na+ cations, coordinated in a charge balancing arrangement with the aluminium tetrahedra. The absence of a charge generation in dehydrated samples and the inexistence of a converse piezoelectric response confirmed that quartz impurities present inside the material don't contribute to the direct piezoelectric effect, thus due only to ionic mobility. Furthermore, an anisotropic behavior of the effect was observed and the measured charge coefficient in the predominant direction was found to lie in the range (4 ÷ 40) pC/N, depending on the water content in geopolymers.