A Hybrid Binder Concept for a Swift Liquid-to-Solid Transition in 3D Printing of Suspensions (2024-09)¶

Additive manufacturing changes the conventional process of concreting by enabling the construction of structures without traditional constraints such as formwork. A pivotal aspect of many additive manufacturing techniques is the sequential extrusion of material in layers. A key challenge in this process is optimizing the material's rheology; it must be sufficiently fluid for extrusion yet possess enough rigidity to maintain shape postextrusion and withstand the weight of additional layers. The rapid development of mechanical strength following extrusion is thus essential [1–3]. Our study demonstrates a novel hybrid binder system integrating organic and inorganic binders. The organic component is based on an acrylic monomer (acrylic acid or acrylamide), a crosslinker (ethylene glycol dimethacrylate), and a thermal initiator (2,2'-Azobis[2-(2-imidazolin-2-yl)propan]dihydrochloride). As such, it is heat-activated, ensuring rapid solidification and enabling a very rapid fluid-to-solid transition at the nozzle. The inorganic binder of the model system is based on carbonation hardening of Ca(OH)2. Therefore, the inorganic binder reacts slowly and is responsible for the ultimate strength development. The polymerization onset temperature of the organic binder was measured by differential scanning calorimetry (heat ramp of 2 K/min). This temperature correlates well with the onset temperature of stiffening. The rapid stiffening of the material was demonstrated by small amplitude oscillatory shear (SAOS) analyses (f = 1 Hz, ɣ = 0.1%) with plate-plate geometry while applying a 2 K/min temperature ramp (Figure 1a illustrates the temperature dependence of the storage modulus). The hybrid binder system (40/50/10 vol%) Ca(OH)2/water/organic binder, orange curve) shows a step change in storage modulus (18 MPa) compared to the reference system (40/60 vol%) Ca(OH)2/water, blue curve) at approximately 65°C (see orange arrows in Figure 1a). A significantly larger change in storage modulus can be achieved by employing a flowable paste, achieved by the addition of 1 wt% (to Ca(OH)2) polyphosphate ether (PPE) dispersant, resulting in a rapid liquid-to-solid transition with a stiffening rate of 3000 kPa/s (green curve Figure 1a). Compressive strength tests of small cubic specimens, with this mixture (hybrid binder with plasticizer), reveal a compressive strength of 0.7 MPa after 1 minute due to the reaction of the organic binder and a strength of 10.9 MPa after four days of accelerated carbonation (25°C, 80% relative humidity, 20% carbon dioxide). Using those prior results, 3D printing trials were conducted. The 3D printer used for demonstrations was a gantry-based system with a nozzle diameter of 4 mm. The binder mix was adapted to meet the printing process needs. Here, Ca(OH)2 was mixed with limestone (60/40 vol%) to achieve a higher solid volume fraction (0.65), thereby reducing porosity and drying shrinkage. The organic binder was now dosed as 10 wt% to the solid fraction. Using this setup, we can print complex geometries with overhangs (Figure 1b). This multifaceted approach allows us to assess the hybrid binder's reactivity and mechanical characteristics and demonstrate printing trials. The insights of this study should be transferable to cementitious systems, leading to potential advancements in additive manufacturing.