Macroalgae biochar as a sustainable cement additive for non-reinforced concrete

To lower concrete’s carbon footprint, biochar can partially replace cement, but commonly used wood-derived biochar has a hydrophobic, low-functionality surface (few –OH, –COOH, C O groups) that weakens bonding with cement hydrates and limits chemical interaction. To address this limitation, this study evaluates unwashed macroalgae-derived biochar as a wash-free additive for non-reinforced concrete, leveraging its oxygen-rich surface chemistry, high alkalinity (pH ≈ 10.8–11), and fine pore network while avoiding the time and cost of washing. Biochar from kelp and bladderwrack was added at 1–3% cement replacement and compared with wood biochar through workability, compressive strength, and thermal conductivity tests, supported by calorimetry, phase analysis, microscopy, and FTIR. Macroalgae biochar improved workability by 15% and compressive strength by up to 60% at 1 day and 15% at 28 days relative to wood biochar, while reducing thermal conductivity by up to 70% relative to the control. Hydration/phase analyses showed that macroalgae biochars accelerated hydration (shortened dormant period, higher silicate peak, increased C–S–H), consistent with filler/nucleation effects from their fine particle size and with high alkalinity promoting faster hydrate precipitation and clinker reactivity. Microstructurally, mixtures showed a denser matrix and compact interfacial transition zone around macroalgae biochar, matching the strength gains. FTIR identified abundant –OH and C–O groups and carbonate bands (CO₃^2-) attributable to CaCO₃ on macroalgae biochar, consistent with stronger interfacial affinity and CaCO₃-derived nucleation sites that contribute to the accelerated early hydration. Macroalgae biochar mixture had 10% lower strength-normalized CO₂ emissions than wood biochar mixture.