Self-sensing performance of the CAC-based MWCNT/NCB composite at high temperatures

Cement-based smart intrinsic composites incorporating nanocarbon-based fillers are recognized for their self-sensing abilities at ambient temperatures. However, these cement-based composites are susceptible to spalling under high-temperature conditions. Utilizing the spalling resistance of calcium aluminate cement (CAC), this study investigates the mechanical properties and piezoresistive behavior of CAC composites that incorporate multi-walled carbon nanotubes (MWCNT) and nanocarbon black (NCB) under ambient and after exposure to peak temperatures of 200°C, 400°C, and 500°C for 2 hours. The results indicate that a higher MWCNT/NCB content leads to increased compressive strength and reduced electrical resistivity. The piezoresistive performance of the composites exhibited an initial increase across CNB1 (MWCNT/NCB: 0.25/0.20 wt%), CNB2 (MWCNT/NCB: 0.50/0.40 wt%), and CNB3 (MWCNT/NCB: 0.75/0.60 wt%). However, a decline was observed in CNB4 (MWCNT/NCB: 1.0/0.80 wt%). Notably, CNB3, with its optimal concentration, demonstrated a significant enhancement in piezoresistivity, achieving a 44 % fractional change in electrical resistivity (FCR). After exposure to 200°C, the control specimen and CNB1-CNB4 exhibited peak compressive strength due to the additional hydration of anhydrous CAC and silica fume, and CNB3 reached a peak FCR of 49 %. Subsequently, after exposure to 500°C, all mixtures displayed a loss in strength; however, CNB3 and CNB4 exhibited a lower strength loss along with a slight decline in piezoresistive performance. These results highlight the potential application of CAC-based MWCNT/NCB smart intrinsic composites under extreme environmental conditions.