Concrete is currently produced at a rate of 20 billion tonnes per year and contributes 5-10% of mankind’s CO2 production. If the strength of the calcium-silicate-hydrate (C-SH), the main binding material of concrete, could be improved, the volume of cementitious material needed for a given structure would be reduced and its environmental impact would be decreased. Here, we show that the constitutive behavior of C-S-H can be improved significantly by complexation with carrot-based cellulose nanosheets (CNSs). This environmentally friendly, reinforcing material heals the defective microstructure of C-S-H, which is responsible for structural deformation and failure at larger length scales. CNSs are built from repeating saccharide units that are covalently linked by a β-1-4 glycosidic (C-O-C) bond. The CNSs show remarkable
affinity to C-S-H due to the interfacial Ca-O coordination and H-bond interaction. The functional groups on the surface of the CNS sheet act as a root network, cross-linking the neighboring silicate calcium layers and inhibiting the water dynamics at the silicate nanochannel, thereby significantly improving the interfacial properties of the C-SH/CNS hybrid structure. The macro experimental results show that the mechanical properties of the composites increase with increasing the concentration of CNSs up to 0.4-wt%. At 28 days and CNS concentration of 0.20-wt%, the flexural strength increases by about 23.2% and the compressive strength increases by about 17.5%. The C-S-H/CNS composites show significant enhancement in strength, stiffness and ductility, and provide a foundation for the development of new high-performance construction materials with lower carbon footprint.