Research Summary Report of A01

Technical University of Braunschweig, Institute for particle technology

The fundamental goal of project A01 is to understand material process interactions in particle-bed 3D printing by Selective Cement Activation (SCA). In SCA, a particle-bed consisting of fine aggregates and cement is applied layerwise. In between the layerwise application, a liquid is applied selectively on the upper layer of the particle-bed. Thereby, the cement hydration reaction is induced locally, and the particle-bed hardens at the desired places. In the second funding period of this project, we focus on sustainability, process enhancement and material models.

Summary

At the Institute for Particle Technology, the main focus is on the material and, thus, the particles. In SCA, the particle mixture is handled in a dry state as a bulk solid until the water is applied. The application of the mixture in the printing process is heavily influenced by its bulk solid characteristics. For instance, a good flowing material like coarse sugar packs denser than a bad flowing material like powdered sugar. Therefore, one goal for SCA is to tailor the particles in order to achieve a dense particle bed, resulting higher printed concrete strength. The bulk solid characteristics are influenced by various properties. One influencing factor is the shape of the particles: Spherical particles (e.g. wooden marbles) flow better and pack denser than fibrous particles (e.g. wood chips). Another influence factor is the surface forces of the particles. Especially for small particles, the surface forces make particles stick to other particles or objects, leading to poor flowability and less dense packing. In former research, we reduced the surface forces of the particles used in SCA in two ways: 1. By applying a molecular layer of special liquids onto the particle surfaces, consequently reducing the surface forces. 2. By coating nanoparticles onto the aggregate and cement particles, slightly increasing the distance of the particles to each other. This leads to a reduction of the forces acting between two particles. Both surface modifications generate a similar increase in packing density. Hereby, we were able to improve the SCA process by increasing the particle bed density and the compressive strength of the printed specimens.

Current state of research

To better understand the mechanics of applying the aggregate/cement mixtures in the printer, we also investigated the influence of the surface modifications on the flow behavior of the mixtures. One common method is measuring the dynamic angle of repose, i.e. putting the material in a drum, rotating the drum and measuring the angle the material develops at different rotational speeds. The angle is measured by taking images through the transparent sides of the drum and evaluating these images. Commonly, a steeper angle develops at higher rotational speeds, as more material is transported up for a given time. Thus, more material needs to slide down in a given time, which is only possible with a steeper slope. For the aggregate/cement mixtures, however, we observed shallower angles at higher rotational speed. This is observable when comparing the angle at 40 1/min (Figure 2) to the angle at 5 1/min (Figure 1). We deduce this effect to the aeration of the material during dynamic movement. When air is captured between the particles, the influence of surface forces is reduced, leading to liquid-like behavior of the particle mixture in extreme cases. This also explains the much smoother surface of the material at 40 1/min.