Hydrochar-cement composites: Valorisation of food waste digestate for sustainable construction

Low-carbon and resource-efficient cementitious composites are crucial for reducing environmental footprints, while integrating waste-derived materials enables effective waste valorisation in sustainable development. Here, we developed novel cementitious composites incorporating hydrochar derived from food waste digestate to reduce carbon emissions in the construction industry. The results indicated that hydrochar unavoidably retarded early hydration reactions of cement and GGBS pastes and inhibited silicate polymerisation. However, in GGBS-rich systems (GHM-20), hydrochar enhanced pure silicate chain connectivity (MCL(Si) = 3.04 compared to 2.89 in neat paste) despite an initial delay of hydration. Micromechanical results from nanoindentation showed that hydrochar incorporation shifted the C-(A)-S-H phases from high-density to low-density, reducing elastic modulus yet enhancing overall matrix densification. Life cycle assessment indicated a significant environmental benefit of hydrochar-cement mortar, reducing global warming potential (GWP) from ∼300 kg CO₂-eq/m³ in conventional mortar to 171 kg CO₂-eq/m³. Further inclusion of 50 wt% GGBS achieved even lower GWP values of 149 kg CO₂-eq/m³. At 20 wt% hydrochar, the composites achieved the most pronounced environmental benefits, with microstructural evidence of modified C-(A)-S-H gel connectivity and matrix densification. Further performance validation would be recommended. These findings highlight hydrochar-cement composites as potentially promising, eco-friendly materials that can balance mechanical performance, hydration kinetics, and environmental sustainability in the construction sector.