Effects of Biochar as a Green Additive on the Self-Healing and Properties of Sustainable Cementitious Materials

The self-healing, fresh properties, hydration, and other characteristics of the cement paste modified with biochar as a green additive were explored in this work. Evaluation of early-age hydration was conducted using setting time measurement and isothermal calorimetry, and later-age hydration was investigated using thermogravimetric analysis. The mechanical and transport properties were investigated by using compressive strength and electrical resistivity measurements, respectively. The chemical characterization of the healing products was conducted via Fourier transform infrared spectroscopy, scanning electron microscopy equipped with X-ray energy-dispersive spectroscopy, and thermogravimetric analysis. The mechanical strength recovery and crack filling were examined using the three-point bend test and optical microscopy, respectively. The paste with biochar showed an increased setting time and delayed hydration at an early age because of reduced alkali concentrations in the pore solution due to alkali uptake by biochar. However, at later ages, the hydration was improved in the paste with biochar, which is attributed to internal curing. Biochar effectively decreased autogenous shrinkage in the paste, which is attributed to internal curing. Biochar was shown to reduce the electrical resistivity and compressive strength of the cement paste due to higher overall water/cement and macrovoid formation, respectively. The chemical characterization of the healing products showed that they are primarily made up of calcium–silicate–hydrate, calcium carbonate, calcium hydroxide, and ettringite. The content of calcium carbonate was increased in the healing products of the paste with biochar relative to the control paste. The mechanical strength recovery and crack filling were improved in the paste with biochar relative to the control paste, indicating the positive impact of biochar on the self-healing of the paste.