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Concentration of CO2 in the Atmosphere

Bio-Concrete! What Is it, Why Might We Need It and How Is it Used?

George Harvey

Natural calcium carbonate, made here by a coral reef. (NOAA image, public domain)

Calcium carbonate is really amazing stuff. Of course, we all can visualize it easily. No? Well, it is the basic material in limestone, which the Great Pyramid was built of, and of marble. It is also the thing that makes many sorts of shells, including eggshells and clamshells. Bones get their strength from it, whether in fish, or chickens, or us. It is in pearls. Now is it easy to visualize?

One thing it is found in is the cement used to make concrete. It is one of the most used commodities we have. It is made by heating materials like limestone, driving off carbon dioxide (CO₂) from its molecular structure. Both the limestone and most sources of heat to make it into cement are significant emitters of CO₂.

But consider this. In nature, calcium carbonate is created without having to heat with a fire. Egg shells, clam shells, and pearls are all made by animals. Chalk was created by various single-celled life forms that deposit calcium carbonate. They used the chemical to stiffen themselves or to glue themselves to things or each other. But they need not be CO₂ emitters to do this.

A cement plant in Maryland. (Acroterion, CC-BY-SA 3.0, www.bit.ly/2R5ti4X)

For some time now, scientists have been looking at ways to use natural biological processes to make cement without heat and emissions. Instead of making a concrete block by mixing water, Portland cement, and sand in the correct proportions and letting it set, we can allow nature to do the bulk of the work. To do this, we might put the sand in a form, flood it with a liquid with the right sorts of nutriments, and employ a few billion microbes to glue the whole together.

The process of drawing down CO₂ from the atmosphere to make calcium carbonate is called microbiologically induced calcium carbonate precipitation (MICP). This is fairly new technology, in terms of what is available on the market. Nevertheless, there are a number of clearly defined needs it can meet to reduce carbon emissions related to cement and concrete.

Products based on MICP are already in commercial use in the United States, at least for making tiles for floors and walls in buildings. These are said to be both lighter and stronger than the tiles made using firing processes, with their CO₂ emissions. Tiles of a number of sizes, textures, and colors are available from Biomason, a company in North Carolina. Biomason claims their bricks have the lowest carbon footprint of any cement product on the market, eliminating a kilogram of CO₂ emissions for every kilogram of tile installed. Their website, www.biomason.com, may be worth a visit.

There are other uses of MICP. It is possible to create self-healing concrete. In this product, any cracks that form are sealed by the action of organisms living in the concrete as dormant spores. The crack allows air and moisture to get to the spores, waking them up, and the organisms that emerge draw on calcium compounds in the concrete and carbon dioxide from the air to create calcium carbonate, which cements the crack shut, repairing the damage. The materials needed to do this are  included in the concrete when it is made.

Another product is a liquid containing nutrients to spray into cracks that have formed in concrete that is not already enabled for MICP. Similarly, products can be used to seal concrete and prevent water from leaking through it.

We looked for self-healing concrete or the materials needed to heal concrete and did not find any yet available in the United States. They are being introduced in Europe, however, by a Dutch company called Basilisk. (www.bit.ly/self-healing-basilisk)

We did find patent applications and patents that had been granted in the U.S. and internationally. We should probably expect to see these products in the U.S. market soon.

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