Preparation of high-strength lightweight cement-based materials with silica fume using walnut shells as partial replacement of coarse aggregate

This study systematically explored the impacts of varying walnut shell dosages (0%, 20%, 40%, 60%, 80%, and 100% replacement of coarse sand) on the physical properties, mechanical performance, autogenous shrinkage, and microstructural characteristics of cement-based materials. The results showed that the density, thermal conductivity, mechanical strength, and autogenous shrinkage of the cement composite decrease with an increase in the walnut shell content. The high-strength lightweight cement-based materials with a density of 1466 kg/m³, thermal conductivity of 0.6 W/m•K, and compressive strength of 36 MPa were obtained when walnut shells completely replaced the coarse sand. The incorporation of walnut shells lowers the early hydration heat release rate of cement, while notably increasing the later hydration degree of cement and the average chain length of C-S-H. Moreover, the inherent porous structure of walnut shells and the interface gaps formed with the cement matrix significantly elevate the porosity of the composite. Finally, nanoindentation testing revealed that the elastic modulus of the matrix adjacent to walnut shells had increased remarkably. This research provides an innovative technique for fabricating high-strength lightweight cement composites and a new pathway for the resourceful reuse of waste walnut shells.