Programming Earthen Materials' Shape Retention with Synergistic Biopolymers for 3D Printing

Additive manufacturing of bio-stabilized earthen materials offers a low-carbon, waste-free path to sustainable construction. Although various biopolymer additives have been used to improve printability, their ability to deliver cohesion, high mechanical strength, and shape retention in a unified formulation remains limited. We report a synergistic combination of biopolymers-xanthan gum (XG) and locust bean gum (LBG)-that meets all these requirements. XG and LBG form a reversible supramolecular gel network that couples strong mineral surface binding with enhanced internal structuration. Compared to single-polymer systems, XG-LBG mixtures exhibit four- to tenfold increases in yield stress, an order-of-magnitude higher storage modulus, and markedly improved thixotropic breakdown and recovery-key properties for extrusion-based additive manufacturing. To uncover the underlying interaction mechanisms, we develop an experimental workflow that integrates polymer rheology, sequential physicochemical characterization, and suspension rheology. This approach enables us to decouple the roles of polymer-polymer, polymer-clay, and clay-clay interactions. Based on these findings, we identify two critical characteristics of polymer-based rheology modifiers that improve buildability in 3D-printed earthen materials: a strong binding affinity to mineral surfaces and sufficient intrinsic gelation to maintain structural integrity during and after extrusion.