Building With Bacteria. Bioconcrete. Biocement.

Concrete is one of the world’s most important materials. But making the cement that binds it generates about 8% of carbondioxide emissions. It is embedded in the very chemistry of the process. The heat is applied to limestone, to break up its principal constituent, calcium carbonate, into calcium oxide (cement’s crucial ingredient) and CO2.

In a warming world, this CO2 should be disposed of in a manner which keeps it out of the atmosphere. That is tricky. Better, then, not to generate it in the first place.

One proposal, literally as well as metaphorically green, is to recruit the services of chlorophyll laden, photosynthesising organisms called cyanobacteria. That has allowed Prometheus Materials, a firm in Colorado, to develop a cementmaking process in which the energy comes not from heat but light.

Moreover, and perhaps more importantly, photosynthesis subtracts CO2 from the atmosphere rather than adding it.

Prometheus raises its bacteria in waterfilled “bioreactors” surrounded by lightemitting diodes, to allow the bugs to photosynthesise. The water contains inorganic nutrients the bacteria need, and is perfused by streams of air bubbles which provide the CO2. It also has calcium ions dissolved in it—for the purpose of the exercise is to encourage the bacteria to generate from the ingredients provided crystals of calcium carbonate a few microns across—a process called biomineralisation.

The number of bacteria in the bioreactors would double every 4 to 6 hours if permitted to do so. Instead, quantities of them are transferred regularly to another tank. Here, they are plied with a proprietary stimulant that accelerates biomineralisation and then allowed to sit for an hour or so to mature. When the crystalrich gloop that results is mixed with an aggregate, the product is “bioconcrete”.

Bioconcrete actually comes in many varieties, depending on the aggregate employed. For the moment, Prometheus is pinning its hopes on mixing the gloop with sand, together with a socalled hydrogel (think jelly deserts for children’s parties, only more industrial), which further helps to bind the sand grains together.

To reduce the space between the grains in the mixture, and thereby strengthen the resulting material, the company first pours the mix into casts that will shape it into breeze blocks, and then uses machinery which compresses and, for about 10 seconds, “vibrates the heck out of it”, says Loren Burnett, Prometheus’s boss. The resulting blocks then take about 8 days to cure, compared with 28 days for conventionally produced breeze blocks.

Prometheus says making concrete this way emits a tenth of the CO2 generated by conventional concretemaking. Many jurisdictions, including the states of California, Oregon and Washington, are bringing forward regulations that will favour “reduced carbon” concrete.

Prometheus is going to shift the production from its laboratory to a pilot manufacturing facility nearby early next year. That said, the firm does hope to bring costs down eventually to a point where it competes with conventional cementmakers on price as well.

The stuff is certainly strong. Recent batches have withstood pressures of 380kg per square centimetre—more than some conventional concretes can tolerate.

Prometheus says its new plant will be able to turn out nearly 21000 breeze blocks a month. But, because shipping heavy products long distances is expensive, it is also working on a process that airdries both the bacteria and the crystals.

The idea, says Mr Burnett, is to produce a “just add water” biocement mixture that would be lighter than a conventional cement mix, and could thus be shipped more cheaply.

Another biocement firm, Biomason, uses a similar approach, except that its bacteria, Sporosarcina pasteurii, do not photosynthesise, so have to be fed organic nutrients, in the form of sugar and amino acids, as well as inorganic ones. Biomason’s first products are wall and floor tiles branded “Biolith”.

Applications for biocement extend beyond conventional construction, too. America’s Department of Defence, for one, has shown interest. Its aim is to be able to build things in remote areas without having to hump in cement and other materials.