Bio-con­crete and bio­gen­ic con­struc­tion mater­i­als with cyanobacteria

The con­struc­tion industry has a prob­lem. Cement, the main com­pon­ent in con­crete — argu­ably the most-used con­struc­tion mater­i­al of our time — is bad for the cli­mate. CO₂ emis­sions from cement pro­duc­tion are very high. Accord­ing to the Ger­man Envir­on­ment Agency (UBA), in 2018 cement pro­duc­tion accoun­ted for some 20 mil­lion met­ric tons of CO₂ emis­sions in Ger­many alone. That’s equi­val­ent to about ten per­cent of all indus­tri­al emissions.

Research­ers from the Fraunhofer Insti­tute for Ceram­ic Tech­no­lo­gies and Sys­tems IKTS and the Fraunhofer Insti­tute for Elec­tron Beam and Plasma Tech­no­logy FEP are now intro­du­cing an eco-friendly, bio­lo­gic­ally induced meth­od of pro­du­cing bio­gen­ic con­struc­tion mater­i­als as part of the ​“BioCar­boBeton” pro­ject. Not only does the pro­cess not emit any car­bon itself; on the con­trary, the cli­mate-dam­aging gas is used for the pro­cess and then bound inside the material.

The center­piece of the new meth­od are cyanobac­teria, formerly known as blue-green algae. These bac­teri­al cul­tures are cap­able of pho­to­syn­thes­is. As light, mois­ture, and tem­per­at­ure inter­act, they form struc­tures known as stro­matolites made from lime­stone. These rock-like bio­gen­ic struc­tures have exis­ted in nature for 3.5 bil­lion years, which attests to the resi­li­ence and dur­ab­il­ity of this bio­lo­gic­al pro­cess. Just like back then, CO₂ is cap­tured from the atmo­sphere as part of the min­er­al­iz­a­tion pro­cess and then bound in the bio­gen­ic rock.

The Fraunhofer research­ers have suc­ceeded in mim­ick­ing this nat­ur­al pro­cess with a tech­no­lo­gic­al meth­od. Under the pro­ject man­age­ment of ini­ti­at­or Dr. Mat­thi­as Ahl­helm, who also con­trib­uted the idea, Fraunhofer IKTS is devel­op­ing mater­i­als and pro­cesses, select­ing poten­tial fillers as well as bind­ing agents, and provid­ing the form and structure.

Research­ers at Fraunhofer FEP, led by Dr. Ulla König, are estab­lish­ing the meth­ods of cul­tur­ing the cyanobac­teria, the com­ple­ment­ary micro­bi­o­lo­gic­al ana­lys­is, and the upscal­ing of the bio­mass pro­duc­tion to be achieved.

Bac­teri­al solu­tion becomes solid

In the first step to pro­duce bio­mass, the light-sens­it­ive cyanobac­teria are cul­tiv­ated in a nutri­ent solu­tion. The intens­ity and col­or of the light source used affect the bac­teri­al pho­to­syn­thes­is and meta­bol­ism. To ensure that the bac­teri­al solu­tion can under­go min­er­al­iz­a­tion to pro­duce stro­matolite-like struc­tures, cal­ci­um sources such as cal­ci­um chlor­ide are added. After that, the research­ers cre­ate a mix­ture of hydro­gels and vari­ous fillers, such as dif­fer­ent kinds of sand, includ­ing sea or silica sand. Addi­tion­al CO₂ is added to increase the con­tent of dis­solved car­bon diox­ide and sup­port the process.

The bac­teri­al mix­ture is stirred to the point of homo­gen­eity and then giv­en struc­ture by being trans­ferred to molds, for example. The molds should prefer­ably be trans­lu­cent so the bac­teri­al meta­bol­ic and pho­to­syn­thes­is pro­cesses can con­tin­ue. Sub­sequent min­er­al­iz­a­tion leads to the final solid­i­fic­a­tion. The bac­teri­al mix­ture can also be shaped through spray­ing, foam­ing, extru­sion, or addit­ive man­u­fac­tur­ing, giv­ing it the form in which the final stages of min­er­al­iz­a­tion take place.

Altern­at­ively, por­ous sub­strates can also be pro­duced and sub­sequently treated with the cyanobac­teria cul­ture: ​“The devel­op­ing sol­id struc­ture is still por­ous dur­ing the pro­cess, so light enters the inside and drives the car­bon diox­ide fix­a­tion through lime­stone min­er­al­iz­a­tion. We can stop the pro­cess by remov­ing the light and mois­ture or chan­ging the tem­per­at­ure,” Ahl­helm explains. At that point, all the bac­teria simply die off. The res­ult is a sol­id product based on bio­gen­ic cal­ci­um car­bon­ate and fillers that can poten­tially be used as brick, for example. The bio-based con­struc­tion mater­i­als made from cyanobac­teria do not con­tain any tox­ic substances.

One of the goals of the BioCar­boBeton pro­ject is to determ­ine the pos­sible mater­i­al and mech­an­ic­al prop­er­ties of the bio­gen­ic mater­i­als to be pro­duced and to scale up the pro­cesses. The research­ers are already think­ing about a cir­cu­lar pro­cess design. For example, the car­bon diox­ide could be sourced from indus­tri­al waste gases. The team is cur­rently work­ing with bio­gas. Basalt and mine waste could be used as sources of cal­ci­um, but so could milk residue from dairy oper­a­tions. And aside from sand, con­struc­tion debris or renew­able resources can also be used as filler.

Applic­a­tions – from insu­la­tion to mortar

Tar­geted selec­tion of fillers and man­age­ment of pro­cess and min­er­al­iz­a­tion para­met­ers allow for man­u­fac­tur­ing of products for a wide range of dif­fer­ent applic­a­tion scen­ari­os. Poten­tial applic­a­tions include insu­la­tion mater­i­al, brick, form­work filling, and even mor­tar or stucco that cures or hardens after it is applied.

Now that the team of research­ers has estab­lished and tested the pro­cess at Fraunhofer IKTS and Fraunhofer FEP, they are work­ing to scale the volumes and determ­ine the desired sol­id prop­er­ties. The goal is to enable man­u­fac­tur­ers to pro­duce the eco­friendly bio-based con­struc­tion mater­i­als in the neces­sary volumes, quickly and cost-effectively.

Ahl­helm and König believe in the pro­cess: ​“Our meth­od shows the huge poten­tial that can be unlocked through bio­lo­giz­ing tech­no­logy. Over­all, our BioCar­boBeton pro­ject is an oppor­tun­ity for a big step toward a cir­cu­lar eco­nomy in the con­struc­tion industry and beyond.”