Electrothermal Activation of Carbon Nanofiber for Enhanced Early-Age Strength of Ultra-High Performance Concrete under Alternative Electric Field

Precuring stage is a pivotal period for the strength formation of ultrahigh performance concrete (UHPC), while the traditional one-day precuring duration decreases the turnover rate with low construction efficiency. This study synergistically utilized carbon nanofibers (CNFs) and ohmic heating (OH) precuring to enable rapid and low-carbon fabrication of CNF-reinforced UHPC (CNF-UHPC). Traditional room temperature (RT) curing was replaced by OH curing within 24 h, and enhanced electrical-thermal-mechanical properties were found in OH precured CNF-UHPC. Specifically, the terminal electrical resistivity was decreased from 2.51 to 1.01 k Ω · cm as CNFs increased from 0 to 0.9 vol%. Similarly, the peak temperature, heating rate, and temperature stability during OH precuring were increased to 135.6°C, 11.35 ° C / min , and 75%, respectively. Moreover, the highest one-day compressive strength of 82.4 MPa was observed in 2 h OH precured CNF0.9-UHPC, which was comparable to that of seven-day RT cured sample. The enhanced effects of CNFs and OH precuring were elucidated by microstructural tests. Hydration degree and pore structure analysis indicated that CNFs and OH precuring accelerated cement hydration, promoted the formation of hydration products, and refined the pore structure of CNF-UHPC. The conductive mechanism of CNF-UHPC during OH curing was clarified through electrochemical impedance spectroscopy analysis, 0.9 vol% CNFs plays a crucial role in stabilizing the conductive network. Further, OH precuring demonstrated superior energy efficiency and lower carbon emissions compared to the RT curing taking the strength as the indicator. This research brings innovative insights into the efficient fabrication of UHPC with modified precuring regime and advanced composites system.