Epitaxial templating and amorphous carbonate trapping: Distinct reinforcement mechanisms in nanoparticle-cement composites

• Hydrophilic carbon nanoparticles induce epitaxial templating to increase silicate polymerization by 189% and gel formation by 50.7%. • Anhydrous carbon variants amplify atmospheric CO2 sequestration by 69% through the kinetic trapping of metastable amorphous carbonates. • Nanoscale interfacial stiffening enables high-performance concrete using 100% recycled aggregates by improving the transition zone modulus by 35%. The construction industry faces dual challenges, including infrastructure demands and CO₂ emissions. This study presents calcium-functionalised pyrolytic carbon nanoparticles that enhance recycled aggregate concrete while enabling atmospheric CO₂ sequestration. Advanced characterisation (HR-TEM, NMR, XPS, nanoindentation) revealed two distinct mechanisms. Hydrophilic functionalisation enhances conventional hydration through organised water activation, and anhydrous functionalisation establishes alternative bonding pathways. Optimal formulations achieved 17-29% improvements in mechanical properties with 100% recycled aggregates. Solid-state ²⁹Si NMR demonstrated a 189% increase in silicate chain length, while XPS confirmed 69% enhancement in CO₂ capture through amorphous carbonate formation. Nanoindentation mapping revealed 35% strengthening of the interfacial transition zone, the traditional weakness in recycled concrete. This approach transforms concrete from a carbon source to a sequestration sink while enabling superior performance with 100% recycled aggregates, establishing scalable pathways for decarbonising construction.