Self-healing of biochar-cement composites with crystalline admixture exposed to sulphate solution and simulated seawater

Although many studies considered crystalline admixtures (CA) as the self-healing agent, only limited studies investigated the potential effects of aggressive ions on the self-healing performance of marine structures. Furthermore, there is an increasing trend to use SCMs to improve concrete resilience while lowering carbon footprint. This study investigated the effects of CA and sustainable waste wood biochar (WWB) on the self-healing of cement pastes exposed to simulated seawater or 5 % sodium sulphate solution. Three-point loading was used to initiate the cracks while keeping the cracked samples unseparated. The healing rate was investigated using optical microscopy and binary image processing. The self-healing products characterisation was conducted using Scanned Electron Microscopy equipped with Energy dispersive X-ray spectroscopy (SEM-EDS), X-ray diffraction (XRD), Thermogravimetric analysis (TG), and Fourier-transform Infrared Spectroscopy (FTIR). Cracks were completely healed after 42 days for CA-based samples when exposing to wet/dry cycles in seawater (SWWD) and sulphate solution (SWD). EDS, XRD and TG results confirmed the formation of calcite as the main healing products, while a small portion of brucite was observed in healing products for SWWD. FTIR spectra further confirmed the formation of C-S-H gel and AFt in healing productions exposed to SWWD and SWD. Although WWB addition did not improve healing performance of WWB-cement composites, it led to a relatively complex cracking path, providing more nucleation sites for the self-healing process.