Self-healing efficiency of sustainable biochar-cement composites incorporating crystalline admixtures

Concrete is vulnerable to cracking due to its low tensile strength and brittle nature. Cracks contribute to accelerate the penetration of aggressive ions such as chlorides into the cementitious matrix potentially leading to steel reinforcement corrosion. Self-healing concrete has been identified as a potential solution to eliminate the effect of cracks, helping to reduce the high costs related to repairing deteriorating structures. Although many studies were conducted using General Purpose cement (GPC) or blended cement-based materials, only a very limited number of studies attempted to assess the self-healing behaviour of cementitious composites incorporating biochar. This study investigates the combined effects of wood biochar, partially replacing GPC, and crystalline admixture (CA) on the self-healing performance of biochar-cement composites. Three-point bending loading was conducted to create cracks. Cracked samples were exposed to either dry condition, permanent water immersion, or wetting/drying cycles in water. Cracks self-healing was evaluated using both optical microscopy and binary image analysis. The self-healing products were analysed using SEM-EDS, XRD, TG, FTIR, and NMR. Wetting/drying cycles in water was the most effective exposure condition allowing to totally heal the cracks of CA and CA-Biochar specimens. The microstructural analysis confirmed the formation of calcite and portlandite in the healing products. FTIR and NMR spectra confirmed the formation of limited amounts of C-S-H gel. Biochar particle provided more nucleation sites for the self-healing process, healing wider crack width than in cementitious composites containing only CA.