Cement is the glue that holds sand and gravel together, forming it into concrete. Making it, though, requires temperatures of at least 1400°C (2552°F)—and the chemical reaction that creates cement releases CO2 as well. If the cement industry were a country, it would be the third-largest producer of greenhouse gases on Earth.

In order to meet the targets set at the Paris Agreement, nations around the world must reduce cement emissions by a quarter in the next 30 years, literally reimagining the walls around us.

Here’s how cement is cleaning up its act in the present—and how it gets even better in the future.
 

Demonstration of basic reactions used in producing cement via electrolysis. | Photo courtesy of MIT/Felice Frankel

Splitting water 

Last year, researchers at MIT successfully demonstrated in the lab that it’s possible to use electrolysis instead of heat to make cement. The process splits water molecules to make an acid, which then dissolves limestone and triggers the chemical reaction. Unfortunately, this still produces CO2. But unlike the dirty gas released when cooking up cement, this CO2 is pure enough to be sequestered, or used for other purposes, such as in soft drinks or as liquid fuel.

What remains to be seen is how well the process can be scaled up to work in an actual cement plant. 

Don’t let coal ash go to waste (but it’s better to have no coal ash in the first place)

While traditional cement production heats limestone and clay in a kiln, triggering a chemical reaction that releases CO2, geopolymer cement uses industrial waste products like coal ash to accelerate the chemical reaction, which requires less heat. While it may eliminate CO2 emissions by up to 80 percent, a downside is its reliance on polluting industries. With coal plants going bankrupt, coal ash is not as available as it once was—nor should it be.

BioMASON's experimental runway. | Photo courtesy of BioMASON

Friendly bacteria

North Carolina engineering firm bioMASON raises vats of bacteria and mixes it with sand, regularly watering it until it hardens into a solid surface made of calcium carbonate structures similar to coral. The US Air Force, which is highly interested in any technology that does not involve transporting a cement mixer into a war zone, is testing biocement for military runways, but it is not yet on the market.

Recycle the old stuff

Recycling concrete keeps this nonbiodegradable material from occupying landfill space, and the technology is simple: Crush it and mix the results with fresh cement or use it to create new products like pavement and gravel. Energy is required to power a crushing machine, but when crushed, previously unexposed parts of the concrete are exposed and absorb additional CO2 from the atmosphere, through the process of carbonation. Unfortunately, recycled concrete is not universally trusted, as its strength and durability can vary.

Heliogen's experimental mirror array. | Photo courtesy of Heliogen

Mirror, mirror, mirror, mirror...

California start-up Heliogen says that by precisely angling thousands of mirrors to reflect sunlight onto a kiln, it will soon be able to generate enough heat to manufacture cement. So far Heliogen’s experimental mirror array can only consistently reach 1000°C—hot enough for the earliest steps of cement manufacturing but not enough to take it all the way.  The technology would also be limited to cement plants with enough land (and enough sunlight) for the mirrors.

Carbon sequestration

Carbon injection places CO2 into wet concrete inside the cement mixer, where it chemically reacts and transforms into a mineral, never to be released as a gas. The technology is already in use in places like Atlanta, Georgia, and Honolulu, Hawaii. The downside: Since the CO2 is added to concrete, not cement, it's not reducing the emissions from cement production itself. The sequestered gases are captured from other industrial sources, like fertilizer manufacturing.