Durability of biochar-cementitious composites incorporating crystalline admixture in chloride and sulphate environments

This study investigated the effects of crystalline admixture (CA) and waste wood biochar (WWB) on sulphate and chloride ions penetration resistance of coastal concrete infrastructure. Three solutions (e.g. water, 5 % Na2SO4 solution, and 5 % NaCl solution) were prepared for cement paste specimens immersion up to 156 days. Strength developments, water adsorption, and Scanning Electron Microscopy (SEM) analysis were carried out. It was found that CA and WWB addition reduced the water adsorption and promoted a slight increase of the mechanical strength in water curing condition up to 156 days. After 28 days of Na2SO4 solution immersion, due a denser microstructure, CA and WWB addition led to 15.1-16.2 % and 17.9" 20.93 % reduction in Na2SO4 solution adsorption respectively, while improving the mechanical properties when compared to the reference group. SEM-EDS presented the formation of ettringite in WWB's pore, densifying the biochar-cement composites. However, due to the continuous formation of ettringite and gypsum, excessive cracks were observed, increasing Na2SO4 solution adsorption and lowering the mechanical strength improvement for samples with CA and WWB addition. CA and WWB addition resulted in 17.16" 18.1 % and 20.85" 23.23 % reduction in 28-day NaCl solution adsorption respectively and reduced the 156-day NaCl solution adsorption by 15.2-15.8 % and 17.8" 20.1 % respectively. Meanwhile, both CA and WWB led to mechanical strength improvements when compared to the control group at all ages, samples with 1 wt% CA addition had lowest strength reduction rate. Overall, the findings suggested that the presence of CA and WWB will improve the durability properties of coastal structures immersed in seawater.