Bio-inspired polydopamine modification of recycled carbon fibers for improving the performance of recycled carbon fiber reinforced mortars

The hydrophobicity nature of the waste carbon fiber felt (WCF) surface results in the formation of a weak interface with the paste during the curing process of WCF concrete, which seriously affects the performance of WCF concrete and the reuse of WCF. To address this problem, this paper utilized polydopamine (PDA) for the functionalization of WCF to enhance bonding between WCF and paste. The wettability, Ca2+ complexation, and functional groups of PDA coating on WCF were analyzed by contact angle, ICP-OES, and XPS, respectively. Additionally, the effects of incorporating PDA-functionalized WCF on the flowability, compressive strength, hydration kinetics, pore structure, water absorption capillary, and SEM of mortars were tested. Experimental results showed that the PDA coating improved the wettability of WCF and enhanced the flowability of WCF mortar. What's more, the PDA coating could chelate Ca2+ and promote the formation and deposition of hydration products on the WCF surface, resulting in a denser interfacial transition zone (ITZ). Compared with ordinary WCF mortar, the compressive strength of the PDA-modified WCF mortar was increased by 18% at 28d. Furthermore, the capillary water absorption of the modified group mortar was also decreased by up to 13%, significantly enhancing the durability of mortars. This paper presents a green and simple bio-inspired method to strengthen the ITZ between WCF and paste, which is fundamentally different from the traditional modification methods, and also offers a theoretical basis and technical foundation for the sustainable utilization of WCF.