Mechanical properties and microstructure of nano-modified geopolymer concrete containing hybrid fibers after exposure to elevated temperature

Geopolymer concrete (GPC) has a promising future as an eco-friendly material for refractory engineering applications. It is significant to investigate the changes in mechanical properties of nano-modified geopolymer concrete containing hybrid fibers (NHF-GPC) after exposure to elevated temperature for construction fireresistant design. In this study, the macro-mechanical property tests and micro tests were carried out on geopolymer concrete containing hybrid fibers with different nano-SiO2 (NS) contents (0-2 %) after exposure to different elevated temperatures (200-800 degrees C). The coupling relationships of compressive, tensile and flexural strengths of NHF-GPC with temperature and NS content were established separately. And the relative sound velocity and damage degree were selected as acoustic parameters to construct a relationship model with the compressive strength loss rate. The results indicated that the compressive, tensile and flexural strengths of NHFGPC were enhanced at 200 degrees C, while higher temperatures significantly reduced the mechanical properties of NHF-GPC. At the same temperature, the compressive, tensile and flexural strengths of NHF-GPC all increased and then decreased with increasing NS content, and reached the peak at a NS content of 1.5 %. The matrix became denser at 200 degrees C, while at greater than 200 degrees C, the water evaporated, the number of microcracks increased abruptly, and the porosity enlarged. The mass loss of NHF-GPC after elevated temperature mainly occurred below 600 degrees C, when there was no new material phase generated inside the matrix. And the established model can be used for the identification on the strength damage degree after elevated temperature exposure of NHF-GPC, and it can offer a conceptual foundation for the assessment of elevated temperature damage and the life prediction of GPC in the practical applications.