In this study, a hybrid mixture of graphene nanosheets and carbon nanotubes was incorporated into cement-based mortar at a mass fraction not exceeding 0.15% of the cement content. The objective was to verify that the hybridization of multi-dimensional nanomaterials is a feasible approach for improving dispersion. The prepared mortar specimens were subjected to sustained elevated temperatures up to 600 °C. After cooling to room temperature, residual mechanical properties were tested with three replicates per group. The air-void network, microstructure, and morphology were characterized using X-ray diffraction, scanning electron microscopy, and mercury intrusion porosimetry. The results indicate that incorporating this hybrid carbon nano material enhances residual strength and retards strength loss caused by elevated temperatures. After 28 days of standard curing, the G5C10 group (containing 0.05 kg/m3 graphene nanosheets and 0.10 kg/m³ carbon nanotubes) exhibited compressive and flexural strengths 37% and 28% higher, respectively, than the reference group without nanomaterials. Following exposure to elevated temperatures up to 400 °C, the hybrid mixture demonstrated superior performance. Specifically, the G5C10 group achieved a compressive strength of 38.09 MPa, which was 33% higher than that of the reference group. Microscopic characterization revealed that the hybrid structure in the G5C10 group helped suppress the agglomeration of individual nanomaterials, thereby achieving better dispersion within the cement matrix. This enabled effective filling and bridging effects, resulting in a denser microstructure. The pore structure was refined, which played a key role in preventing residual strength loss after high-temperature exposure.
Zhou et al. (Thu,) studied this question.