Cement is one of the construction industry’s biggest carbon polluters. It’s responsible for approximately 5 to 7 percent of global carbon dioxide emissions caused by humans. But green practices could reduce cement’s environmental impact, and new alternative products may eventually shrink that impact even more.

“Greener” cement

Currently, cement substitutes aren’t nearly as versatile as the tried-and-true product on which, as they say, “civilization was founded.” But these three approaches aim to make cement more Earth-friendly.

1. Autoclaved aerated concrete blocks 

These blocks aren’t cured and finished in kilns like most concrete masonry units (aka cinderblocks). They’re produced in an autoclave, a heated chamber that prompts a chemical reaction from a special mix of ingredients. This doesn’t reduce the amount of energy used in making the blocks, but it does cut down, in several other ways, on carbon emissions tied to the end product. In addition to requiring no baking, the blocks — a sort of foam — are lighter to transport and better at insulating.

• Drawbacks: Aerated blocks are brittle, which makes them more likely to break during construction or when drilled into. They’re also susceptible to water damage, so they must be protected from the rain during installation and then sealed. And the carbon savings are relatively modest

2. CarbonCure blocks

CarbonCure collects carbon dioxide from industrial emitters like cement factories and injects it, via a hose, into the mold for each block to permanently embed it. In addition to reducing the net carbon in the end product, the technique strengthens the material, which allows the block manufacturer to use less cement. CarbonCure claims its method is no more expensive than a conventional process. It also says its business doubled over the last year. The patented process is now being used in nearly 40 concrete plants.

• Drawbacks: While a significant step toward reducing the overall carbon footprint, CarbonCure is limited to pre-formed products.

3. Concrete made with supplemental materials

Concrete companies are adding materials such as fly ash, ground rice husk and blast-furnace slag to reduce the amount cement in various mixes so they release less carbon per pound of concrete.

• Drawbacks: In some applications, very few of the additives can be used because they can change the properties of the cement. There’s also a risk that some additives, especially coal ash, will release toxic chemicals in certain applications. And like the two concrete block products listed above, the additives don’t fundamentally reduce the amount of carbon released in the initial process of breaking down limestone.

Concrete alternatives

Change isn’t coming just in the form of new cement products but also in alternatives to cement. The perfect eco-friendly alternative has yet to be discovered or invented, but here are two emerging materials to watch.

1. Rammed earth is as old as the hills — literally. But it’s getting popular again. The technique involves pounding a carefully selected mixture of different types of earth into a mold onsite to create a retaining wall or the exterior wall of a building. Builders often use local materials excavated onsite, and the result can be unique and beautiful.

• Drawbacks: Rammed-earth construction is much more labor-intensive than pouring concrete. Rammed-earth also must be finished with a material (usually concrete) that prevents water from getting into it. Plus, it’s tricky to use below grade and doesn’t lend itself to foundations or pavement.

2. Ferrock is an alternative to cement made almost entirely from steel dust and broken glass, both of which are waste products. University of Arizona grad student David Stone discovered it in a lab in the early 2000s, got a series of grants to test and develop it and has been working to commercialize it ever since. Like cement, Ferrock takes carbon out of the atmosphere in the process of drying — except at a much greater rate. Stone also says it’s much stronger than concrete because it dries into a substance with the blast-resistant properties of iron.

• Drawbacks: After more than a decade, Stone hasn’t proven that Ferrock can scale up mass production. Another big question is whether the price of the world’s limited supply of steel dust will rise if Ferrock does become popular. If that happens, it might become too expensive to compete with concrete.