Bacteria That Print Buildings: Living Cement Grows Solid at Room Temperature

Cement is everywhere, and that is precisely the problem. The binding agent that holds together nearly all of the world’s concrete is produced by heating limestone and other raw materials to well over 1,000 degrees Celsius in enormous kilns, a process that releases vast quantities of carbon dioxide both from the fuel burned and from the limestone itself as it chemically decomposes. Cement manufacturing stands among the largest single sources of industrial CO2 emissions on the planet. Now a doctoral researcher at ETH Zurich’s Department of Architecture, Karen Antorveza Paez, has spent her thesis years pursuing an alternative that abandons the kiln entirely. Instead of firing rocks, she enlists living microorganisms to bond loose grains of sand into solid, self-supporting architectural components, all at room temperature.

The core of the technique is a natural phenomenon that scientists call microbiologically induced calcite precipitation, or MICP, and which engineers more plainly refer to as biocementation. When certain types of bacteria are combined with sand or other solid grains under the right chemical conditions, they trigger the formation of calcium carbonate, the same mineral that makes up limestone, chalk and mussel shells. As Antorveza Paez puts it, the team is using bacteria to produce a kind of mineral glue. The glue is not a sticky organic substance in the conventional sense. Rather, microscopic calcite crystals precipitate in the gaps between individual sand grains and interlock with one another, mechanically locking the particles into a rigid, coherent structure. The result looks and behaves, at least in its early forms, much more like a masonry solid than like anything biologists typically grow in a petri dish.

Biocementation itself is far from new. The workhorse bacterium, Sporosarcina pasteurii, has been studied for years, notably as a way of stabilising loose subsoil beneath roads and foundations, and it has also found a role in so-called self-healing concrete, where bacterial spores embedded in the mix germinate when cracks admit moisture and seal the damage with freshly precipitated mineral. What Antorveza Paez recognised was that the field had remained, in a sense, a servant to conventional materials. Bacteria were added to existing building products or injected into the ground; nobody had yet treated the biological process itself as the primary manufacturing route for architecture. Her innovation was to fuse biocementation with digital manufacturing, two research fields that had until then developed largely in isolation, and to ask what was missing. In her own words, what was lacking were manufacturing processes by which these biological processes could be controlled as required and used to make architectural elements.

Her doctoral work culminated in three scientific studies describing two distinct fabrication methods. The first is 3D biocement printing. The team formulated a bio-ink composed of a gel based on water and two natural polymers, alginate and xanthan, mixed with sand and live bacteria. Like a traditional 3D printing paste, this bio-ink can be extruded layer by layer into any digitally defined geometry. At this stage the printed object is fragile, held together only by the gel matrix, and it would not survive handling. The decisive transformation happens next, when the printed structure is immersed in a mineralisation bath containing urea and calcium chloride. There the bacteria get to work, using an enzyme called urease to break down the urea. This reaction releases carbonate ions into solution, along with ammonium as a by-product, and the carbonate ions combine with the calcium from the calcium chloride to precipitate calcium carbonate, the mineral of limestone, precisely wherever the bacteria are located inside the printed object.

The mineralisation is slow and deliberately controlled. Depending on the strength required of the finished component, the entire process takes between two and five days, and each day the object receives a fresh mineralisation bath so that the bacteria do not exhaust the chemical precursors needed to keep forming limestone. Once the bath cycles are complete, the structure is left to dry. At that point the transformation is finished: the object has become a solid mineral body, the bacteria are no longer active, and no heat source has been used at any stage. Antorveza Paez compares the sequence to baking a cake. The dough, she notes, is malleable and delicate, and only the oven can turn it into something solid. In her process the bacteria play the role of the oven, slowly setting the printed structure until it is bonded, solid and dry at ambient temperature. Among the demonstrator objects produced this way is a facade element that could serve as sun protection or as a partition wall.

The second method takes a radically different route to the same mineral, and it dispenses with the immersion bath altogether. In robotic biocementation spraying, a robotic arm applies bacteria, mineralisation substances and sand to a surface in repeated, alternating passes, building the material up layer by layer directly in place. The inspiration here is explicitly biological and geological: stromatolites, the layered sedimentary rock formations that arise in nature when microorganisms deposit minerals over immense stretches of time. The robotic process compresses that ancient rhythm into weeks. At the 2025 Biennale Architettura in Venice, the team let the spraying process run uninterrupted for six months, allowing visitors to watch a biologically mineralised structure grow in real time. As Antorveza Paez observes, the demonstration proved that the entire biocementation workflow can be automated.

The two processes are complementary rather than competing, and they were developed with different priorities in mind. Printing, Antorveza Paez explains, is geared more towards discrete architectural elements, while the spray process is designed for surfaces. The printing research concentrated on materials science: which bio-ink formulation works, and how it can be reliably extruded and mineralised. The spraying research was chiefly concerned with the machine itself, and with the considerable control problem of orchestrating three separate streams of material delivered in precise alternation. Reflecting those different emphases, the two processes also stand at different stages of maturity. The spray method in particular still requires substantial research into its mechanical properties, including how the grains and the bonding medium are arranged within the built-up object, questions that the printing studies have already begun to address for their own process.

For Antorveza Paez, the deeper significance of the work lies in the relationship it forges between architects and their materials. Architects, she points out, normally know nothing about what went into making a material, which is manufactured somewhere else entirely. She describes her fascination with the fact that nature has found ways of producing very sophisticated materials, and with the possibility of making those biological strategies applicable to the problems the built environment faces today. That orientation also carries a sobering awareness of the process’s limitations. A third study, currently underway, examines the environmental impact of biocementation and searches for solutions to its most significant drawback: the ammonium released during the urease reaction is damaging to the environment. If the disposal of this chemical by-product is not factored into the design of biocement systems from the start, Antorveza Paez warns, using biocement for construction will not be the sustainable practice it promises to be.

Equally clear-eyed is her assessment of where the material stands relative to the industrial benchmark. Biocementation, she admits, currently resembles mortar more than concrete. The comparison is instructive, because the recipe for concrete, specifying how many grains of each size are combined and in what proportions, has been refined for over a century and continues to be fine-tuned to this day. For biocementation, that systematic optimisation is only just beginning. Researchers know that the bacteria can in principle be combined with certain grain sizes, but the question of how to translate that into a long-lasting material that is as stable as possible remains open. The pivotal point, as Antorveza Paez concludes, is no longer whether biocement works, but rather how well its properties can be managed. In other words, the bacterial masons have proved they can lay the stones; the century-long work of engineering the mix now begins.

Subject of Research: Biocementation: using bacteria to bind sand into solid architectural materials

Article Title: When bacteria become masons

Article References: When bacteria become masons. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: biocementation, MICP, Sporosarcina pasteurii, 3D printing, robotic spraying, calcium carbonate, ETH Zurich, sustainable construction, digital fabrication, cement emissions, bio-ink, architecture