INVESTIGATIONS OF PHOSPHATE GEOPOLYMERS

Phosphate geopolymers are ceramics synthesized by reaction of a solid aluminosilicate with an aqueous acidic phosphate, typically with an overall composition of 2 SiO2 Al2O3 P2O5 n H2O. Their high thermal stability and low precursor cost make them attractive for a variety of applications, in particular as a matrix material at temperatures above which organic polymer composites degrade. However, technical barriers and knowledge gaps regarding synthesis of the geopolymer resin currently hinder widespread use. The precursor and processing conditions strongly affect the phase evolution and final properties. In this work, challenges with the neat resin involving processing, microcracking, and phase transformations were identified and methods were developed to mitigate them through analysis of hydrolytic behavior and microstructure. The impact of processing conditions on resulting thermomechanical properties, and phase evolution with temperature was investigated for selected materials. The oxide precursor characteristics, in particular five-coordinated aluminum content, were found to correlate to some degree with curing time and water stability. A post-curing heating step was found to significantly reduce cracking and porosity, improving stability at higher temperatures. While all phosphate geopolymers crystallized above 110 °C, the phase distribution and rates depended on the processing conditions and whether or not the precursor contained silica.