The Influence of Time-Dependent Rheology on the Multiple Scales of 3D Printing with Cementitious Materials

Abstract

3D printing of cementitious materials has gained tremendous momentum in recent years as a way of directly constructing full structures and infrastructure and as an enabler for bio-inspired hierarchical design, with control extending from the printed filament scale to the full structure scale. Extrusion behaviour when printing is governed by time-dependent rheology as colloidal flocculation and hydration reactions transition the material from a printable fluid to a solid. Any changes in extrusion behaviour and filament morphology due to the changing rheology will vary the macroscopic properties of the printed structures. This dynamic behaviour also means that it is not feasible to use a conventional printability metric to reflect the entire printing process. To help this production technology translate more rapidly, manufacturers need new tools and clear quality control metrics that will link their formulations to the final print performance and macroscopic properties, in advance of any large-scale trials. In order to address this need, this thesis firstly explores how to best quantify the change in rheological behaviour over time and uses this, coupled with a bespoke 3D printing system, to study extrusion and printability in detail to better understand the formulation trade-offs between time-dependent pumpability, extrudability and buildability. Numerical models were also developed to help understand the actual shear conditions within these experimental procedures. On a mesostructured scale, the cross-sectional morphology of printed filaments is experimentally and numerically studied with material ageing, and how it changes during the conventionally defined printable limit known as Open Time is demonstrated. A dramatic change in filament morphology is witnessed after a threshold time which is defined as print quality assurance time (PQAT). This is noted as a more targeted quality assurance metric that incorporates the dynamic nature of cementitious materials and defines the period during which the filament morphology remains stable, and hence macroscopic porosity. On a macroscopic scale, mechanical testing reveals that the interlayer adhesion of the full structure is reduced after the PQAT, reiterating the importance of filament-level quality control. This reduction in load-bearing capacity after the PQAT is driven by decreased load-bearing area which is the interlayer contact between filaments, with the bonding strength unchanged. More importantly, this enables a link to be identified between time-dependent rheology, extrusion behaviour and macroscopic properties. This research highlights the time-dependent effects of cementitious materials’ rheology on the multiple scales of 3D printing. It is anticipated that this study will enhance the understanding and standardisation of quality assurance in 3D printing of any age-dependent materials by bridging multiple scales of printing and giving critical guidance to avoid changes in mesostructure, a major determinator of macroscopic properties.