Influences of additives on the properties of geopolymer matrix composites (GMCS) for high-temperature applications (1150 °C)

In this study, the influence of different additives on a geopolymer matrix composite (GMC) was explored as an alternative to a ceramic matrix composite (CMC) for thermostructural applications. Different GMCs using an N610 textile (10 × 10 cm²) and various additives (SrCO₃, BaCO₃, BaSO₄, MgO and Al₂O₃ and mullite-rich powder) were cured under 6 MPa at 120 °C for 2 h. The mechanical, microstructural and structural data of the composites were compared with those of the geopolymer matrix after thermal treatment at 1150 °C. The results showed that the addition of alkali-earth barium carbonate or sulfate to the geopolymer matrix facilitated the formation of alkali-earth aluminosilicate crystalline phases and a higher viscous flow apparition temperature (990 °C), which did not enhance its mechanical properties (50 MPa) due to poor impregnation of the textile. Refractory additives such as MgO and Al₂O₃ in the geopolymer matrix support fiber impregnation with a low-viscous-flow apparition temperature (865 °C) and the formation of crystalline phases such as forsterite, spinel and sapphirine, which act as reinforcements, allowing a flexural strength up to 80 MPa. The addition of a refractory compound, such as mullite, leads to better embedding of fibers and a flexural strength reaching 100 MPa. To conclude, mixing the geopolymer matrix with refractory compounds results in a successful thermo-structural geopolymer composite.