<p>Inspired by the hierarchical nanocomposite structure of cortical bone, we developed a sustainable biomass-derived composite by incorporating hydroxyapatite (HAP) from porcine bone ash and cetyltrimethylammonium cations (CTA⁺) into a sodium carboxymethylcellulose (CMC-Na) matrix. The composite exhibited toughness comparable to that of ordinary concrete, and its water absorption was similar to that of wood–cement composites. Interactions among the carboxylate groups (–COO⁻) of CMC-Na, HAP, and CTA⁺, along with the thermomechanical properties and water resistance of the composite, were characterized using TG–DTA, DSC, FT-IR, three-point bending tests, and water immersion tests after hot uniaxial pressing. We showed that introducing CTA⁺ at a CTA⁺/–COO⁻ feed molar ratio of 1.5 under mild conditions (25 °C) effectively modified the matrix while preserving the –COO⁻/HAP interfacial interactions. This structural optimization simultaneously improved both the toughness and water resistance, increasing the fracture energy 2.2-fold compared with that of the CTA⁺-free counterpart while reducing water absorption to 28%. These results demonstrate that surfactant-mediated interfacial engineering is an effective strategy for developing bone-mimetic composites. Furthermore, combining this approach with conventional hydrophobization techniques such as acylation offers a promising route for advancing high-performance, sustainable biomass-based structural materials.</p>