Porous biochar-assisted aqueous carbonation of steel slag as an adsorptive crystallization modifier for value-added cement applications

Carbonated steel slag shows promise as a supplementary cementitious material. However, prolonged carbonation forms a dense CaCO₃ layer, limiting carbonation efficiency and hydration activity, while uneven CaCO₃ accumulation limits its performance in cement. This study introduces biochar as a nucleating agent to enhance the carbonation and hydration activity of steel slag. Results show that biochar, particularly rice husk biochar, effectively promotes CO₂ capture (38.1 % increase) and improves mechanical properties (33.4 % improvement) by optimizing microstructure and carbonation efficiency. Reed biochar's fibrous structure optimized CaCO₃ packing best, while bamboo biochar accelerated slag hydration, raising cumulative hydration heat by 1.8 % and C-S-H gel content by 86.7 %. Results confirm that biochar's abundant active nucleation sites, functional groups, and diffuse porosity greatly promote CaCO₃ crystal growth while preventing excessive surface accumulation, enabling the filling and nucleation effects of CaCO₃ to be fully realized in cement system. These findings highlight biochar's potential as a cost-effective modifier for sustainable cementitious applications.