A predictive model for interlayer water evolution and experimental validation of 3D printed cementitious materials

The physical and mechanical properties of interlayer zones in freshly 3D printed cementitious (3DPC) specimens are highly dependent on the time evolution of water content inside. This study develops a predictive model for water evolution, considering water bleeding, evaporation, adsorption and desorption processes affected by hydration and capillary pressure change, before and after overlays are printed. In particular, to overcome the insensitivity of humidity to time for the first a few minutes and hours after printing, a new water content equation for the desorption process of the substrate is proposed, based on a new humidity - capillary pressure relation and the microscale pore size distribution. In total, 24 3DPC specimens with different water-to-cement ratios and pass times were measured, regarding bleeding and evaporation rates, capillary pressure evolution and hydration degrees etc, to validate the model. The water evolution curves before overlay printing predicted by the new model were found in good agreement with the measured data, and parametric studies demonstrated significant effects of the water-binder ratio and the pass time on the interlayer water content after overlay printing. The new model thus provides a quantitative tool to predict the interlayer water content that may significantly affect the interlayer microstructures and bond strength in 3D-printed cementitious specimens.