Microstructural and pore network evolution of biochar-modified alkali-activated slag cement

The urgent need for decarbonising construction materials aligns with global efforts to achieve carbon neutrality. This study explores the potential of biochar as a carbon sink in clinker-free alkali-activated slag (AAS) systems, offering a pathway to significantly reduce carbon emissions. By incorporating biochar into AAS, the microstructural impacts were investigated using advanced characterisation techniques. The findings reveal that biochar inclusion does not alter the primary hydration products of AAS, except for the introduction of calcite. Enhanced crystallinity and structural regularity were observed in the layered double hydroxides (LDH) and C-A-S-H(I) phases, as evidenced by sharper diffraction peaks. However, the addition of 25% biochar significantly increases permeability, indicating higher pore heterogeneity, which could promote gas diffusion. At the interfacial transition zone (ITZ), a ‘wall effect’ leads to small slag particle aggregation and enhanced slag hydration, yet weak bonding between biochar and AAS paste results in ∼2 μm gaps. The chemical composition of the ITZ closely resembles the inner product (IP) of slag, highlighting the influence of particle interactions in this region. These findings offer valuable knowledge for optimising the use of biochar for carbon-negative construction materials, potentially leading to more environmentally friendly and carbon-efficient construction practices. • Carbon-negative biochar-modified AAS can be prepared by adding more than 25 % biochar. • The use of 25 % biochar in AAS gains a twofold increase in permeability. • The bonding between biochar and AAS paste is weak due to the formation of ∼2 μm gaps. • Adding biochar did not change the types of hydration products in AAS.