Characterization of thermal resistance and mechanical strength recovery of carbon nanotubes incorporated Portland cement composites subjected to heating and rehydration: Visualization of pore structural evolutions via synchrotron 3D X-ray nanoimaging

Carbon nanotubes (CNTs) enhance the thermal resistance and post-fire mechanical recovery of cementitious composites; however, the underlying mechanism has not been fully elucidated. This study aimed to reveal the effect of the incorporation of CNTs on the physicochemical properties of cementitious composites subjected to heating and rehydration. We assessed the variations in compressive and tensile strengths, volume, bulk density, surface morphology, and hydration products of Portland cement composites based on the amount of CNTs incorporated, heating temperature (200, 400, 600, and 800 degrees C), and re-curing conditions (under 25 degrees C/65 %RH and in water). Synchrotron three-dimensional X-ray nanoimaging was performed to visualize the internal nano- and micro-sized pore structure evolutions of the composites due to heating and rehydration. The CNT-incorporated samples exhibited improved thermal resistance and noticeable mechanical strength recovery by water re-curing, even after heating temperatures higher than the CNTs decomposition temperature (>600 degrees C). The CNTs improved the stability of the volume and density changes of the paste matrix and did not affect the formation of additional hydrates during the heating process. X-ray nanoimaging results revealed that cement hydrate formation on the dispersed CNTs resulted in a fastened and interconnected structure with dense pores (<2.5 mu m(3) and rod-like shape); these pores resist thermal deterioration and are advantageous for strength recovery by rehydration.