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Nowadays, protecting various structures including commercial, medical, industrial, and residential infrastructures against fire is a very complex issue. High heat causes microstructural changes and decreases compressive strength of the concrete containing conventional portland cement, but geopolymers as the third generation of cement due to amorphous structure and aluminosilicate 3D networks lead to more stable behavior under high heat conditions considering the conventional concrete. Calcium silicate hydrate (C-S-H) and calcium aluminosilicate hydrate (C-A-S-H) nanostructures are products of the hydration and geopolymerization processes that play an important role in increasing the strength of conventional and geopolymeric concrete, But heat, either in transient or steady state, changes the mechanical properties and microstructure of the concrete. Hence for a deeper understanding of the behavior of C-S-H and C-A-S-H nanostructures affected by high temperatures, geopolymer concrete has been compared with conventional concrete. In this regard, about 300 samples were cured in the humidity bath for 1, 3, 7, 14, and 28 days. All samples were then put in of 25, 50, 100, 200, 300, 500, 700, and 900°C temperatures for 2 hours. Length and weight change percentages, compressive strength, and ultrasonic and cracking behavior tests were performed on all samples. Images from the scanning electron microscope (SEM) and the energy-dispersive X-ray (EDX) analysis were also used to evaluate the microstructural behavior of samples in various temperatures. According to the results, the strength of both types of concrete decreases with increasing temperature. By increasing the temperature to more than 700 °C, the geopolymer concrete structure has transformed to a porous and semi-stable ceramic structure. This change in the ceramic structure has made a difference in the high heat compressive strength of geopolymer concrete vs. conventional concrete. The compressive strength of 28-day aged geopolymer concrete and conventional concrete samples at 900 °C was 7.35 and 4.31 MPa, respectively.

Keywords: High Temperatures, Conventional Concrete, Geopolymer Concrete, Compressive Strengths, C-A-S-H.

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