| Unidirectional carbon fiber reinforced geopolymer composite (Cf/geopolymer) is prepared by a simple ultrasonic-assisted slurry infiltration method. The effects of fiber contents, matrix composition, heat-treatment temperature and testing temperature on the mechanical properties and fracture behavior of the composite were investigated.The results show that the obtained Cf/geopolymer composites own a full infiltration and homogenous distribution of carbon fibers, and fibers are undamaged during the process. Comparing with the geopolymer matrix, the Cf/geopolymer composites exhibit drastically improved mechanical properties due to the incorporation of carbon fiber, especially to the composites reinforced by 20vol.% carbon fibers, the flexural strength, Young's modulus and work of fracture increase 11.6 times, 3.6 times and 71.5 times, separately. And all the composites fractured in non-brittle mode. With increasing Si/Al ratio of the geopolymer matrix, flexural strength and Young's modulus of the composites increased gradually due to the increased strength and stiffness of geopolymer resin. Considerable fiber pull-out was observed in composites with Si/Al≥3, indicating the significant reinforcing effects of carbon fibres in these samples.With increasing temperature, geopolymer will convert into the tetragonal- or cubic- leucite ceramic. Thermal shrinkage of geopolymer can be divided into four stages, i.e. structural resilience, dehydration, dehydroxylation and sintering, according to the dilatometer and DTA results. And it showed the maximum shrinkage during the sintering stage. For the K-based geopolymer, leucite crystallization appeared after the sintering stage and the Avarami parameter for leucite crystallization was 3.89 indicating the three-dimensional crystal growth mechanism. Comparing with the geopolymer, leucite ceramic derived from it possesed much higher mechanical properties. While the thermal expansion coefficient was much high and there was an abrupt volume expansion near 400℃due to the phase transition from tetragonal to cubic leucite phase. With increasing the cesium substitution for potassium, the amount of stabilized leucite increased and with 20at.% cesium substitution leucite was fully stabilized in cubic phase and the thermal expansion coefficient decreased sharply.After proper heat treatment, the Cf/geopolymer composite can be directly converted into the carbon fiber reinforced leucite ceramic matrix composites, and mehcnial properties were greatly enhanced. For the carbon fiber reinforced K-based geopolymer composite (Cf/KGP) heat treated at 1100℃, flexural strength, work of fracture and Young's modulus reach their highest values increasing by 102.3%, 29.1% and 84.7%, respectively, relative to their original state before heat treatment. While, for carbon fiber reinforced geopolymer with 20at.% Cs+ substitution of K+ composite (Cf/CsKGP), the mechanical properties reached the maximum values after being treated at 1200℃, and the three above values increased by 115.2%,63.3% and 123.1%, comparing with the composites before heat treatment. The property improvement can be attributed to the densified and crystallized matrix, and the enhanced fiber/matrix interface bonding based on the fine-integrity of carbon fibers. In contrast, for composite heat treated at higher temperature, the mechanical properties lowered substantially and it tended to fracture in a very brittle manner owing to the seriously degraded carbon fibers.Repeated sol-SiO2 impregnation was an efficient method to seal the cracks and pores in the heat-treated composites and thus greatly enhance their relative densities and ambient and high temperature mechanical properties. The impregnated composites possess the density of 2.15～2.2g?cm-3, and showed much high specific strength. Composites both before and after impregnation fractured in a non-brittle manner at both ambient and high temperatures. Over an elevated temperature range from 700℃to 900℃, the strength of the two composites showed anomalous gains and reached their maximum values at 900℃. The increased strength was attributed to the cracks blunding and release of residual strength based on the integrity of the carbon fiber. Compared with the composites before impregnation, the impregnated one showed superior high temperature properties, which was attributed to the improved fiber integrity due to its much denser microstructure. And at 1200℃, the impregnated composites can still retain 72.2%～87.5% of their room temperature strength. The impregnated composites also showed considerable isothermal oxidation resistance and thermal shock resistance properties. After being oxided at 900℃for 60min, the composites can still retain approximately 50% of the original value.So geopolymer technology together with high-temperature heat treatment and sol-SiO2 impregnation provides a new method to fabricate the composites of low cost and high specific strength, which might be attractive for application at elevated temperature.
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