Effect of industrial multi-walled carbon nanotubes on the mechanical properties and microstructure of ultra-high performance concrete

To enhance the safety and functionality requirements of engineering structures, carbon nanotubes are used to improve the performance of concrete. However, their high cost limits their large-scale application. In this study, industrial multi-walled carbon nanotubes (IMWCNT) were employed to ultra-high performance concrete (UHPC) to achieve a balance between nanomodification and economy. The effects of different IMWCNT contents on the flowability, mechanical properties, and water resistance of UHPC were investigated. Moreover, the hydration products, microstructure, and fiber-matrix interface characteristics of UHPC specimens were analyzed using thermogravimetric analysis and scanning electron microscopy. The incorporation of appropriate amounts of IMWCNTs could effectively improve the mechanical properties and crack resistance of UHPC and partly prevent the infiltration of water into the matrix. Adding 0.1 wt% IMWCNTs resulted in optimal mechanical properties, and the flexural/compressive strengths of fiberless UHPC mortar and fibrous UHPC (2 vol% steel fibers) were increased by 6.7/5.2 % and 8.5/11.3 %, respectively. Microstructural analysis of the samples showed that uniformly dispersed IMWCNTs can enhance cement hydration and bridge the cracks at the microscale and nanoscale. In addition, incorporating an appropriate amount of IMWCNTs in UHPC reduced the porosity of its fiber-matrix interface and optimized steel fiber distribution in the matrix. Cost-benefit analyses results showed that although the addition of IMWCNTs increases the manufacturing cost of fibrous UHPC, their addition in moderate amounts (0.1 wt%) does not adversely affect the economic index due to the improvement in mechanical properties.