3D printing of bio-based concrete composites for construction

This thesis presents a proof concept study on using hydrogel binders as cement replacement in 3D printing mortars to enable advanced flow properties and therefore, the realization of highly optimized structures for minimum material consumption.

The construction sector is one of biggest polluters and emitters of CO₂ worldwide. Therefore, focus of recent research lays on minimization of waste and emissions, and reuse. 3D printing of concrete shows the potential of increasing the productivity of the sector and, to the same time, increasing its environmental performance by enabling the realization of highly optimized structures. In addition, no formwork is required for building, which decreases the amount of waste for concrete structures significantly.  

Since the printing of concrete materials requires advanced rheological properties that let the material stiffen rapidly after extrusion, the printing of cementitious materials is usually connected with the use of a high share of cement, presenting a drawback in its environmental performance. Its slow setting characteristics limit the placing of material to a, broadly seen, vertical build up, which restrain the possibilities of minimizing material use by realization of optimized structures.

This research project therefore investigated the potential of replacing the cementitious shares with biologically based materials. The research focused on natural polymers, which use low extraction temperatures. Some of the natural polymers showed thermoreversible properties both in pure form, as well as in a concrete composite with mineral aggregates. This allowed for use within a temperature-controlled print head and rapid stiffening after extrusion.

14 natural polymers were evaluated for the use as binder in printing mortar. Both polysaccharides and proteins proved elevated potential for the application. The materials were assessed for their mechanical strength, and rheological- and printing properties. 

In a composite, gelatin showed the highest mean compression strength (37MPa and 59.5MPa, respectively for mammal and cold water fish gelatin), as well as the largest yield strength development in the fresh state under cooling from 50-20ºC (0.1kPa-106kPa). 

The research could prove that a concrete composite from natural polymers and mineral aggregates can be used as structural material and as filament in a temperature controlled extrusion process. Its mechanical strength could be measured in the same order of magnitude as for cementitious printing mortars. Its temperature controlled rheological stiffening as fresh material made it possible to print advanced free form constructions up to an unsupported inclination of 80º. This enables the construction of freely shaped geometries and highly optimized structures.