Ultra-high performance concrete with carbon nanotubes and different types of superplasticizing admixtures: behavior at high temperatures

Ultra-High Performance Concretes (UHPC) are cement-based composites with enhanced properties compared to conventional concrete. Their high mechanical strength is achieved through particle packing, resulting in a very dense microstructure. In this context, these materials require improvement to be safely used under elevated temperatures, as they are susceptible to explosive spalling, which primarily occurs due to vapor pressure from internally evaporating water within the element. Given the dense pore structure, this pressure cannot be relieved, causing stresses that exceed the element's strength, leading to explosion. Research indicates that the use of carbon nanotubes (CNT) may have a preventive effect against spalling; however, their mechanisms are not yet well understood. For effective prevention, it is believed that the dispersion of CNT in the cement paste should be quite uniform, necessitating the investigation of different dispersion strategies. Considering this, the present study aims to investigate the performance of UHPC incorporating CNT using different superplasticizers, specifically lignosulfonates (LS) and polycarboxylates (PC), which can be adsorbed by the nanotubes to assist in dispersion. The results indicate that the addition of CNT to UHPC was able to modify the occurrence of spalling starting at 400 °C, with the use of LS allowing the material to be tested up to 600 °C, while the control and the material produced with PC did not resist temperatures beyond 400 °C. This performance could not be attributed to improvements in flexural strength, which were not observed to be significant. The apparent density also did not show substantial changes. Hypothetically, mechanisms can be proposed to explain the improved performance promoted by the presence of CNTs, such as modification of pore size and interconnectivity, which would reduce internal pressure, and a possible reinforcement of the microstructure by bridging effect, allowing it to withstand concentrated stresses that would cause explosion. These mechanisms were not directly revealed in the present study, so it is suggested that they be investigated in future work using complementary characterization techniques, such as pore connectivity analysis, permeability, microstructural mechanical performance, and scanning electron microscopy.