Cementitious mixtures for 3D printing, a contribution to the constructions of tomorrow

Armando Vargas López, Armando Vargas López, Master's student in Civil Engineering. Credit: Communications Office, Faculty of Engineering.

The construction of buildings using 3D printing technologies is an increasingly accepted practice within the industry, due to the reduction of costs and risks associated with labor and materials. However, this technology still faces great challenges, since the use of non-conventional materials for construction implies other variants to be considered to ensure the durability and resistance of the constructions. A research proposes to solve this problem, framed in a project in which two academic units of the Faculty of Engineering, Civil Engineering and Materials Engineering, participate, and to bring infrastructure to the rural area of the department of Cauca.  

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The shrinkage phenomenon and the search for a more resistant mixture for 3D printing 

As a result of work done during his undergraduate studies, in 2022 Armando Vargas López, civil engineer of the School of Civil Engineering and Geomatics of the Universidad del Valle, had the opportunity to put his knowledge at the service of the community, by being part of a project that sought to bring housing made from 3D printing to the rural area of the municipality of Cauca. This project, financed by the General System of Royalties under the name "Development of a 3D printing system of sustainable non-conventional materials for the advancement of rural infrastructure in the Department of Cauca" and led by the professor and researcher of the School of Civil Engineering and Geomatics Daniel Gómez Pizano, was the ideal opportunity for the researcher Vargas López to contribute from his area of expertise in composite materials, in connection with the Master's Degree in Civil Engineering that he is currently pursuing at the Universidad del Valle.  

The researcher is part of the Composite Materials Research Group (GMC), and his research, which is directed by Rafael Andrés Robayo Salazar, professor and researcher at the School of Materials Engineering, and co-directed by Ruby Mejía de Gutiérrez, professor and researcher at the same academic unit, aims to reduce the effects of cracking in cementitious mixtures by incorporating fine aggregates and polypropylene fibers to a Portland cement base (OPC).

  

This need to create a more resistant material is due to the fact that elements built from 3D printing are particularly exposed to a phenomenon, which in turn contributes to the origin of such cracking: shrinkage. This phenomenon is mainly due to moisture loss from the concrete during the curing process, which can generate internal stresses that result in cracks.

 

"In natural environments of traditional construction, as the mix has aggregates (gravel, sand), they take up space in the concrete mix, reducing the amount of cement paste that shrinks when drying and act as reinforcements within the cement matrix and helping to distribute internal stresses more evenly. On the other hand, with printed elements, since they are mostly made of a paste composed of cement and water, there is a minimum amount of aggregates that fulfill this barrier function," explains researcher Armando Vargas López, "and the problem with 3D printed elements is that, due to the limitations of the nozzle diameter, very coarse aggregates, such as gravel, cannot be used." The researcher adds that the inclusion of polypropylene fibers, six millimeters long, is intended to help in the transfer of this energy, dissipating the tension produced inside the mixture by the contraction of the material, which can control the shrinkage phenomenon.  

Addressing this problem is essential for the well-being and stability of buildings, because without this type of reinforcement at the level of the mixtures used for 3D printing, these can be structurally compromised.  

In addition to polypropylene fibers, the research has raised the use of two groups of additions: the first, of the supplementary type, were composed of metakaolin (MK), microsilica (MS) and calcium carbonate (CaCO₃), which were used to improve the properties of the cementitious mixture, so that it had the necessary fluidity to be treated through the printing system, in this case a piston plunger; The second used construction waste (ceramics, bricks and concrete), resulting from constructions previously made in different areas of the region, in order to make use of them. These were selected having in account the area targeted by the royalties project of which this research is part, which is more focused on the land use of this region and, therefore, it is also common to find this type of waste.   

3D printer used for the creation of structures made with the developed construction mixture. Credit: researcher's file.

Testing the resistance of this new mixture  

The research work was divided into these phases: in the first phase, a literature and research background review was carried out, which resulted in findings of 2566 articles related to 3D printing of concrete, which is an example of the growing interest in these experiments in recent years. After this, it has proceeded to characterize the raw materials to be worked with. Then, the optimum mixtures were obtained through tests in the fresh state (mini slump, flow index and buildability) and in the hardened state (compressive strength), evaluating mixtures of paste, micro-concrete (paste+sand) and fibrereinforced micro-concretes (paste+sand+fiber). The percentages of polypropylene fiber used were 0.5%, 0.75% and 1% in volume, in order to obtain prints without clogging the printer nozzle due to the agglomeration of this material in the process.   

The results were promising. "It was substantially noticed that the addition of polypropylene fiber helps to diminish the effects of this shrinkage phenomenon," comments researcher Armando Lopez Vargas. He explains that those samples that had fibers had not presented cracks until the current time of the research, six months after their manufacture.  

 Uses and impact for the region  

For researcher López Vargas, although the technology used for printing constructions from 3D printing is relatively new, his review of the state of the art so far, added to the benefits in terms of economy and loss of raw materials, allows foreseeing a future in which this type of methods will be the basis for constructions of different dimensions. According to him, the industrial sector is already starting to show signs of interest: "We have already seen, here in Colombia, that companies such as Argos and Conconcreto have large 3D concrete printers, and there is already a prototype, the Origami House, developed by Conconcreto in Medellín. On the other hand, Sika, a very important company in Colombia that manufactures admixtures, already sells materials for 3D printing. This technology has a lot of potential, although it is still in its infancy, and there is still a long way to go. 

Now that the research has resulted in the expected cementitious mixtures, it remains to make a laboratory-scale prototype of a house. It should be noted that other researchers of the GMC research group are working on other types of mixtures, particularly in soils, clays, and non-conventional materials, trying to reduce the Portland cement content in the printed material as much as possible. At present, the researcher López Vargas has printed elements that allow him to ratify the success of his research, objects with complex geometries that aim to continue contributing to the improvement of the mechanical properties of this type of constructions in the future.  

Geometric figure created from a mixture for 3D printing. Shapes like these then allow the addition of reinforcements in the final structure of the house. Credit: researcher's file.

 

If interested in being in touch with the Master's student or any further information about the investigation, please write the Faculty of Engineering Communications Office: comunicaingenieria@correounivalle.edu.co.