Effects of post-fire curing on the mechanical properties of cement composites containing carbon black nanoparticles and multi-walled carbon nanotubes

Concrete elements exposed to fire can partially recover strength and stiffness if recured in water or in a moist environment. This paper reports a pioneering investigation of the influence of post-fire curing on residual mechanical properties and microstructure of cement mortars containing different concentrations of carbon-black nanoparticles (CBN) or multi-walled carbon nanotubes (MWCNT). A total of 84 specimens were subjected to various maximum exposure temperatures (200, 400, or 600 degrees C) and post-fire curing (water-recuring for 1 day followed by air-recuring for 27 days). X-ray diffraction (XRD), scanning electron microscopy (SEM), and energydispersive spectrometry (EDS) were used to investigate the microstructure of different types of specimens. Experimental tests were carried out to determine the compressive strength, static modulus of elasticity, and dynamic modulus of elasticity of rehydrated specimens. Nanofillers enhanced the strength recovery of composites exposed to temperatures up to 400 degrees C, since they improved rehydration reactions. In this case, the rehydration process provided values of relative residual strength and elastic modulus factors higher than 85%. Composites containing 0.4% MWCNT and 3% CBN presented the best strength and stiffness recovery. Therefore, small contents of carbon nanomaterials provided additional nucleation sites for dehydration products to sediment and being rehydrated. Moreover, water adsorbed by the nanomaterials during the water-recuring process was used later to provide greater rehydration. Composites containing 1.2% MWCNT and 9% CBN presented lower mechanical properties recovery, as high contents of nanomaterials made it harder for water to reach decomposed C-S-H gel, hindering the development of rehydration reactions. After 600 degrees C, lower relative residual factors were obtained, but always higher than 40%. It happened because nanofillers were not able to provide those rehydration improvements, since their thermal decomposition started at about 500 degrees C.

Automated summary

The study found that post-fire curing with water or in a moist environment can partially restore the strength and stiffness of concrete elements. Nanofillers, such as carbon-black nanoparticles or multi-walled carbon nanotubes, were found to enhance the strength recovery of composites exposed to temperatures up to 400 degrees C by improving rehydration reactions. Composites containing 0.4% MWCNT and 3% CBN showed the best strength and stiffness recovery.