Role of epoxy resin in improving the toughness of geopolymer concrete cured at different temperatures

Geopolymer concrete is proposed as an alternative to traditional cement-based concrete for supporting structures in high geo-temperature underground engineering due to its lower carbon footprint, exceptional mechanical properties and heat resistance. To address the inherent brittleness of geopolymer concrete, epoxy resin (ER) and steel fibers (SF) were incorporated to improve its toughness. The mechanical properties of hybrid modified concrete cured at simulated environments were thoroughly investigated. It was revealed that the compressive strength, flexural strength and toughness generally achieved their maximum values when cured at 60 ℃ by virtue of more C(N)-A-S-H gels produced. Regardless of whether SF were present, geopolymer concrete containing 10% ER and 15% ER exhibited the highest compressive strength of 67.93 MPa and flexural strength of 6.36 MPa, respectively. The incorporation of SF altered the brittle failure characteristic of geopolymer concrete. Furthermore, the continued incorporation of ER significantly enhanced its toughness. In comparison with residual strengths of 4.62 MPa (L/150) and toughness of 42.06 N·m (Ω150) for SF-incorporated geopolymer concrete cured at 60℃, the synergistic effect of ER significantly increased these values to 7.82 MPa and 66.2 N·m, with improvements of 69.85% and 57.39%, respectively. SEM image indicated that 10–15% content ER was uniformly distributed in geopolymer matrix, and ER existed in matrix mainly in three forms including dense bulk phase, spherical particle phase and strip-shaped film. Microscopic testing including SEM-EDS, FT-IR and XRD illustrated that the hydrogen bonds and chemical interactions existed in the interface, demonstrating nanoscale hybridization and favorable binding affinity between the ER and the geopolymer matrix. MIP test demonstrated that the incorporation of SF significantly increased the large pore and total pore volumes. However, ER could optimize the microstructure through its filling and binding effect. The synergistic toughening effect of ER/SF on geopolymer concrete was realized, and this provided a basis for its engineering applications.