Microstructure and flexural performances of glass fibers reinforced phosphate sludge based geopolymers at elevated temperatures

The focus of this study is the development of innovative and sustainable geopolymer composites with improved elevated temperatures performances. To minimize production related costs and support a circular economy model, phosphate sludge and MK are chosen as precursors for alkaline activation, and glass fibers as reinforcement material. The geopolymer composites were prepared by incorporating glass fibers in a geopolymer matrix based on a mixture of metakaolin and PS in a ratio of 50% and with amounts ranging from 0.25% to 1% of fibers. The developed composites were assessed in terms of physical characteristics, microstructural properties and flexural strength prior to and after introduction to higher temperatures. The findings demonstrated that PS based geopolymer matrix flexural strength decreased in the region of temperature between 25 and 600 °C, but increased progressively at higher temperatures, achieving a gain of 3.2% at 1000 °C. The same trend was followed after glass fiber incorporation, leading to a more significant gain of about 16.5% with a 1% fiber content up to 1000 °C. This behavior was assigned to the decomposition of carbonate content of PS at 600 °C as well as to the crystallization of the geopolymer matrix at higher temperatures.