# Sustainable Support Material for Overhang Printing in 3D Concrete Printing Technology
## 3 Results and Discussion  
### Printability  
The printability of the material, including pumpability and buildability parameters, was characterized. Pumpability is crucial for delivering material in a 3D printing system to ensure a continuous filament product. It measures the ease of material delivery through the system. Therefore, the volumetric flow rate of the material at a constant pump input speed was used to evaluate pumpability. As shown in Figure 10, an increase in recycled glass content leads to a decrease in the material's volumetric flow rate. This reduction in pumpability is attributed to the angular shape and sharp edges of the recycled glass powder particles. The irregular shapes of the recycled glass powder hinder particle movement by interlocking, which obstructs the flow of the suspension material. In contrast, the spherical shapes of fly ash powder particles provide better flow properties for the suspension material. The spherical shape of the fly ash particles reduces friction among the angular recycled glass particles, creating what is termed the \"ball bearing effect\" [18]. Figure 10: Volumetric flow rates of mixtures under 700 rpm pump speed. A significant decrease in volumetric flow rate was observed when the recycled glass content exceeded 50%. The dominance of recycled glass powder in the suspension significantly impairs material pumpability. Mixtures with recycled glass powder content beyond 60%, such as FA35 RG65 and FA30 RG70, were found to be non-pumpable due to clogging in the pump caused by excessive recycled glass powder. This clogging results from the pump's insufficient torque to break the interlocking of the recycled glass particles. Similar to the work presented by [19], the incorporation of waste glass material results in a decrease in slump value as the amount of sand replacement increases. The lubricating effect of fly ash particles diminishes as fly ash content decreases and recycled glass content increases. The 3D printing process involves a layering operation to construct the final structure. Buildability is crucial for vertical stacking, as the material in the bottom layer must resist deformation from the weight of the layers above. Buildability was evaluated based on the material's ability to withstand the weight of accumulating layers during the printing process. Cylindrical structures with a diameter of 200 mm were printed using each mixture until the filaments exhibited high levels of deformation. A laser distance sensor was attached to the nozzle to monitor and measure the change in distance between the top surface of the filament and the nozzle opening. Deformation of the filaments increased the distance between the laser sensor and the filament surface, leading to an increase in the overall height of the structure. The printing test continued until the structure deformed beyond the laser sensor's range (approximately 35 mm). In contrast to pumpability, buildability improves with an increase in recycled glass powder content. Similar to existing literature, an increase in the glass-to-binder ratio can lead to a reduction in filament deformation up to the extrudable limits [20]. Figure 11 shows the laser distance data from the printing tests. A spike in distance is observed when the nozzle moves up a layer, which can be used to identify layer increments in the raw data. As shown in Figure 11, the FA40 RG60 mixture was able to build a larger number of layers with lower levels of deformation compared to the FA60 RG40 mixture. The FA60 RG40 mixture could not maintain its shape and deformed rapidly as layers were added, leading to structural distortion. Increasing the recycled glass powder content helps prevent deformation due to the angular shape of the glass particles, thus improving buildability. The FA55 RG45 mixture could sustain its shape upon extrusion but deformed excessively after reaching a height of 80 mm.