Chemo-Mechanical Modeling of Cohesion in Structural Mortar for 3D Printing Based on the Degree of Hydration

Cementitious materials in the fresh state are commonly regarded as viscoplastic. That is, below a given yield stress, they exhibit solid-like behavior, whereas above this threshold, they behave as fluids. In this context, the shear strength of such materials has traditionally been analyzed from a rheological standpoint, considering them as fluids and using time as the primary state variable. From a structural perspective, however, relatively few studies have treated the material as a solid. With the advent of 3D printing technology, this trend has persisted. Within this framework, the present research aims to evaluate the shear strength of a structural mortar for 3D printing in its solid-like regime, by applying the Mohr–Coulomb failure criterion. Furthermore, in a novel approach, the degree of hydration of Portland cement is proposed as a state variable to replace time, enabling a more comprehensive and objective description of the material’s mechanical evolution. Thus, addressing this gap in the state of the art, a chemo-mechanical coupling is developed. To obtain the necessary data, direct shear, uniaxial compression, and isothermal calorimetry tests are performed. The results indicate that the friction angle remains constant, at approximately 33°, and that cohesion, the parameter governing strength gain, exhibits the same linear rate of increase with hydration in both mechanical tests, indicating an intrinsic relationship within the material.