Researchers funded by the European Union's Horizon 2020 program have created a new type of lightweight concrete made from recycled glass that is significantly lighter than traditional mixtures and can be 3D printed into structurally sound buildings.

The team was able to decrease the weight of the concrete by up to 40% by replacing some or all of the natural aggregate with recycled waste glass. Notably, the new, lightweight material still meets the current insulation requirements in the EU but, thanks to the 3D printing process, is more versatile than conventional materials. The study, published May 15 in the Journal of Building Engineering, is one of just a few published articles on lightweight printable building material.

"3D printing is kind of a revolutionary technology, which can really change the way we perceive construction and give us a lot of geometrical freedom to build whatever you want, theoretically. With your imagination, you are free to design any structure," corresponding author and project lead Pawel Sikora, a researcher at Technische Universität Berlin, told The Academic Times.

Two major environmental issues exist in concrete production, Sikora explained. The first is the material's contribution to anthropogenic climate change. According to Bloomberg, the cement industry is responsible for 8% of the world's carbon dioxide emissions. In 2019, The Guardian called concrete "the most destructive material on Earth."

"The second thing is, we have a big problem with natural sand," Sikora said. Many nations are worried about using up their dwindling reserves of construction sand. "We have to look for solutions in how to find alternatives for the sand before it becomes a global problem," Sikora continued. He and his colleagues are confident that recycled materials can markedly improve the sustainability of concrete.

Concrete made from recycled and waste materials could alleviate the burden on new, natural materials that are becoming scarcer. The nature of 3D printing means it creates less waste. No framework is needed for pouring the concrete into the shape of a building, which reduces the number of materials required for an oddly shaped foundation or even just a typical cement wall. The installation of a building also requires less physical labor, as a machine is extruding the material into a predetermined form.

The co-authors found that replacing 50% of the natural aggregate with waste glass led to a composite material with greater flexural and compressive strength than traditional concrete. Sikora and his team also replaced roughly 30% of the cement with limestone powder.

The waste glass and limestone powder decreased the density of the final material and, at the same time, reduced its thermal conductivity, which improved it by making it more insulating. Though less dense materials are usually less strong, the new lightweight concrete is unique, as its lower density still allows for structural integrity. This balance between the material's insulating properties and strength is important in a building that will house people – safety is always an important concern, Sikora notes.

The team noticed that when 100% of the basalt aggregate was replaced with waste glass, the material did in fact lose some composite strength. Because of the diminishing returns of structural integrity with waste glass, they recommended 50% waste glass as a target for future 3D-printable lightweight concrete.

Sikora believes materials science has the potential to modernize civil engineering. In his view, the fundamentals have not budged for centuries.

"If you look at construction in medieval times [compared to today], the process didn't change too much," he said. "You still need hard labor and the people who will build a house brick by brick."

In Europe, implementing the latest innovations in materials science in actual construction projects is an even greater challenge, as structures are built to last for hundreds of years, Sikora said. He added that designing a material that will hold up to the EU's standards can be a daunting task. 

"One of the challenges we have in the EU is strict requirements about the thermal transmittance of the building," explained Sikora. "Therefore, we need to look for new material solutions for 3D printing in order to produce high-insulating materials."

Countries are slowly starting to produce 3D-printed houses in Europe and beyond. In April 2021, a Dutch couple made history as Europe's first residents of a 3D-printed house. Sikora noted that Germany is also planning a 3D-printed apartment building. 

"I know there's also a big interest in the U.S. Army, who are trying to print shielding barriers, bunkers or places where a person on the field cannot be exposed," he added.

Aside from recognizing its usefulness for designing larger homes and buildings, Sikora sees 3D printing technology as potentially valuable to short-term relief operations. "If you have a catastrophe like a tsunami, you have to build shelters for people very fast," he said. "I think [3D printing] may be the solution for big emergency situations."

The study, "3D printable lightweight cementitious composites with incorporated waste glass aggregates and expanded microspheres – rheological, thermal and mechanical properties," published May 15 in the Journal of Building Engineering, was authored by Karla Cuevas, Falk Martin, Dietmar Stephan and Pawel Sikora, Technische Universität Berlin; Mehdi Chougan and Seyed Hamidreza Ghaffar, Brunel University.