The critical role of time-dependent rheology for improved quality control of 3D printed cementitious structures

3D printing of cementitious materials is gaining momentum as a method of construction across length scales, from patterned coatings to full-scale structures. The technology also enables cutting-edge research in hierarchical architectures and in carbon storage applications. Cement-based material printing faces challenges because colloidal flocculation and hydration reactions transition the material from a printable fluid to a solid over time. This drives continuous changes in material printability and can lead to unpredictable macroscopic properties. It is therefore critical to give manufacturers quality control metrics that will link their cement-based formulations to the macroscopic properties of the final printed products. Here, we report a first step to progress this with a small-scale cement paste extrusion printing study. We examine the cement paste rheological properties that link closely to flows experienced during printing, quantify the changes over time and show how these influence changes in extrusion pressure and filament cross-sectional morphology. We then use numerical simulations to help understand these experimentally observed changes. We observe a time when both the cement paste's static yield stress and the extrusion pressure suddenly increase, coinciding with a change in filament morphology. Importantly, this change in printing behaviour occurs at approximately half the Open Time, the conventionally defined metric for printability and we observe a 29 % reduction in the interlayer contact area between filaments, which can increase macroporosity and drive down load-bearing capacity. This allows us to define ‘print quality assurance time’, a metric that takes into account the dynamic nature of cementitious materials to ensure predictable mesostructures and in turn controlled macroscopic properties.