ETH Zurich grows sand into solid building material with microbes

Researchers use a limestone-forming bacterium to bind sand by 3D printing or robotic spraying, but strength and toxic waste still limit it.

Researchers at ETH Zurich, led by Karen Antorveza Paez, have developed two ways to turn plain sand into a solid architectural material using bacteria. The microbe is Sporosarcina pasteurii, which deposits calcium carbonate between sand grains and locks them together. Calcium carbonate is the same compound found in limestone, chalk and mussel shells. The work is at research stage, and the team produced a solid-sand facade element as part of the study.

In the first method, a gel-like bio-ink of water, sand, living bacteria and some chemicals is 3D printed. The object then sits for three days in a bath of urea and calcium chloride, where the bacteria trigger crystal formation, followed by two days of drying at room temperature. The whole process takes up to five days, and the bacteria are no longer active in the finished piece. In the second method, a robotic arm sprays a sand-and-bacteria mix onto a surface and builds it up layer by layer. That system can be fully automated and ran continuously for six months. Printing suits complex elements, while spraying suits surfaces.

The appeal is the contrast with Portland cement, whose production needs very high temperatures and large energy input, making it a major source of CO2. A material made at room temperature from abundant sand could, in principle, let communities build with local inputs and less embodied carbon.

The limits are real. The material could currently replace mortar but is not as strong as concrete, so it is not suited to floors, columns or foundations. Printing leaves behind toxic ammonium that must be neutralized before scaling up. The team is still studying how grain size affects the result, and the article gives no strength figures or cost comparison with cement. Watch for those numbers and a fix for the ammonium.

Source: New Atlas