Synchronous vacuum hot pressing ultra-high performance geopolymers: Deformation behavior across temperatures

In order to develop ultra-high performance geopolymer products with "high strength, high density, high stability and low cost", breakthroughs in geopolymer technology are crucial for their application in high-end commercial and even military sectors. This study investigates the self-deformation mechanism at ambient temperature and the deformation behavior from low temperature to high temperature of synchronous vacuum hot pressing metakaolin fly ash-based geopolymers. In contrast to previous studies, the self-deformation of Synchro-vacuum hot pressing metakaolin fly ash based geopolymer at room temperature undergoes four distinct stages: the expansion period of the first and third stages and the shrinkage period of the second and fourth stages. The modification results in a synergistic effect between the first and second stages, effectively suppressing the volumetric deformation of geopolymers. The expansion deformation is primarily caused by an increase in internal stress within the matrix and further gelation reaction during the later stage, while shrinkage deformation arises from changes in the gel structure. Additionally, volume deformation from low to high temperatures can be divided into four stages, with 200 degrees C and 800 degrees C identified as key temperatures for such deformations. The rapid shrinkage observed at these temperatures is attributed to the swift loss of free water at 200 degrees C and transformation of the internal matrix structure at 800 degrees C. Notably, two ultra-high strength peaks of 142 MPa and 147 MPa are achieved at 200 degrees C and 500 degrees C respectively due to rapid gelation reactions prior to reaching 200 degrees C and continued reactions involving inert particles along with transformation of crystal hydration products into a more stable form. The structure of the aluminosilicate three-dimensional network remains relatively stable across all samples until feldspar formation occurs above 800 degrees C. The synchronous vacuum hot pressing metakaolin fly ashbased geopolymers synthesized in this study represents a novel category of ultra-high performance ceramic-like geopolymers, exhibiting exceptional strength and stability at both low and high temperature.