Effect of acid concentration and curing conditions on microstructure, mechanical and electrical properties of phosphate geopolymers

Abstract

This study focuses on how curing conditions and H₃PO₄ concentration effect on microstructural changes, physical-properties, compressive strength, and electrical properties of phosphate geopolymers. Firstly, phosphate geopolymers were prepared by metakaolin and H₃PO₄ (8, 10 and 12 M molar-concentrations). Two series of geopolymers were produced: the first series was cured directly in air, while the second series was activated at 75 °C before being cured in air. The phase composition of was investigated by X-ray diffraction technique, and confirmed by infrared spectroscopy. Apparent porosity, bulk density and water absorption were determined according to Archimedes-principle. Scanning electron microscope was utilized to follow the microstructural changes. The compressive strength and electrical resistivity were also investigated. The results revealed that after geopolymerization some amorphous, semi-crystalline and crystalline phases are formed. For example; amorphous alumino-silicate-phosphate-hydrate phases, aluminum hydrogen phosphate 2AlH₃(PO₄)₂.3H₂O and berlinite (AlPO₄). Moreover, the variation of curing-conditions and acid-concentration had a significant effect on the geopolymerization process which consequently reflected on all properties. The apparent porosity was decreased while the bulk density and compressive strength were increased with increasing the acid concentration up to 10 M, then reduced at 12 M. The electrical resistivity was increased with increasing the acid concentration. Moreover, the geopolymers activated at 75 °C, showed better properties than that cured directly in air. The sample S4 that prepared by 10 M acid and activated at 75 °C exhibited the best properties among the other geopolymers. Their compressive strength, bulk density and apparent porosity values were 68.7 MPa, 2.15 g/cm³, and 20 % respectively.