Continuous fiber reinforced geopolymer matrix composites offer the potential for use in structural applications at temperatures up to 700°C, while enabling the manufacture at temperatures below 100°C. Studies have investigated a variety of high temperature structural fiber reinforcements, including carbon, SiC and Al2O3. While there has been active research into various grades of Al2O3 fibers, SiC is most commonly used for high temperature reinforcement of geopolymers in oxidizing environments. Both families of reinforcement are relatively expensive and are capable of use temperatures which exceed those of the geopolymer. Basalt fibers have the potential to be a good match for the geopolymer matrix, both in terms of upper use temperature and cost. In this study, Basalt fabric reinforced geopolymer composites were prepared with fibers having three different surface conditions, as-received (silane sized), cleaned, and carbon-coated, to investigate the effect of fiber-matrix interface on the mechanical properties. All specimens were fabricated, cured at 80°C and conditioned at 250°C for 5 hours to create the baseline specimens. More than half of the 70 specimens manufactured were exposed to an additional 5 hours at 650°C. Flexural strength, strain-to-failure and modulus were determined at ambient temperature via 4-point bend testing. The as-received and cleaned specimens showed moduli approaching theoretical predictions, indicating a strong interfacial bond, resulting in brittle failures at low loads. The carbon coating resulted in a three-fold increase in strength after the 250°C conditioning and retained a strength higher than the other specimens, even after the 650°C treatment. This strength increase did come with a reduced modulus, suggesting that the stress transfer between fiber and matrix in the carbon-coated basalt fiber reinforced geopolymer composites had also been reduced. While the carbonaceous interphase was not expected to be stable at the higher temperatures in an oxidizing environment, the results do indicate that significant Basalt fiber reinforced geopolymer strength gains are possible through the implementation of a tailored fiber/matrix interface as a crack blunting mechanism.
Effect of Fiber/Matrix Interface Modification on Basalt Fiber Reinforced Geopolymer Composites