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  • [♪ INTRO]

  • Earth has a concrete problem, and by that,

  • I mean both that the problem is very real, it is concrete.

  • But I also mean that it is caused by cement.

  • Cement is the ingredient that binds concrete together, and we use it for a lot.

  • And it has a pretty big carbon footprint.

  • According to a 2016 report put out by the European Union,

  • the cement industry is responsible for releasing

  • 4 billion metric tons of carbon dioxide into the atmosphere every year.

  • That is 8% of our global emissions.

  • And that's not just from burning fossil fuels to make it.

  • We could cut some of those emissions by using greener energy to produce cement,

  • but the chemistry of cement itself produces CO2.

  • The good news, though? That chemistry isn't set in stone.

  • In fact, we could totally turn things around and use carbon dioxide

  • to create the building materials we're gonna need anyway.

  • The main type of cement used around the world is known as Portland cement.

  • While its exact chemical proportions can vary,

  • it's pretty easy to make by throwing together some limestone and clay,

  • and then heating the daylights out of 'em.

  • Technically, this process actually makes something called clinker.

  • Cement makers add a few other ingredients, like gypsum, to create the finished product.

  • But in any case, it's the production of clinker that gives off all that carbon.

  • See, the main ingredient in limestone is calcium carbonate.

  • Heating it causes it to break down into calcium oxide and CO2.

  • Then, the clay contributes silicon dioxide,

  • and that plus the calcium oxide from the limestone creates clinker.

  • Meanwhile, that CO2 from the limestone escapes into the atmosphere.

  • Again, estimates vary, but roughly speaking,

  • a kilogram of clinker production releases around a half kilogram of CO2.

  • Over the course of a year, that works out to around 1.4 billion metric tons of carbon dioxide

  • from the chemical production of clinker alone.

  • Now, cement does absorb a small amount of CO2 as it ages.

  • In the presence of water, CO2 from the air

  • reacts with calcium oxide to make calcium carbonate again.

  • And that process continues over the lifetime of the product.

  • So, our concrete buildings are actually sucking CO2 out of the atmosphere,

  • which is nice to know.

  • But it's not anywhere near enough for the process to break even.

  • A 2016 study found that between 1930 and 2013,

  • cement materials sucked up only about 43% of the carbon

  • given off by cement production over the same period of time.

  • Except there are factors that prevent concrete from sucking up CO2,

  • like the presence of plaster or too much water.

  • And CO2 doesn't penetrate all the way through the concrete to react with it.

  • So that number might be optimistic.

  • That means cement has a substantial carbon footprint.

  • But there are ways to shrink it. Like, we could switch to greener sources of energy,

  • as well as make the process of heating the materials more efficient.

  • It's also possible to make cement using less clinker.

  • Other materials can be substituted, and as a bonus,

  • a lot of them are waste products from other industrial processes.

  • They include fly ash, which is a byproduct of burning coal, and slag from iron- and steelmaking.

  • But the supply of those is somewhat uncertain.

  • And besides, we can do even better by rethinking the chemistry of cement altogether.

  • At least, that's what a few startups want to do.

  • One of them, called Solidia, claims to have a technique

  • that will allow cement and concrete makers to use carbon as a resource.

  • That means CO2 captured elsewhere could be transformed

  • and incorporated into our building materials.

  • Solidia's cement doesn't start out that different than regular cement.

  • It's made of similar materials; the clay and limestone are still in there.

  • But by backing off the amount of limestone, they came up with a formula

  • that produces less CO2 as it's heated,

  • and can be heated to a lower temperature than Portland cement,

  • making the process more carbon-efficient.

  • But the real trick is what happens next.

  • To make concrete, you would normally mix Portland cement, water, and gravel,

  • and let the whole thing set over a period of weeks.

  • And during that time, the water isn't just drying;

  • it's reacting with components of the cement to bind everything together, in a process called curing.

  • But Solidia's process replaces water at this step with carbon dioxide.

  • They basically cast their concrete in a big chamber full of CO2,

  • which reacts with the components in the cement to harden it.

  • They've even recently developed technology to do this in the field,

  • rather than just with precast concrete in a factory.

  • Which means they need a source of CO2 from somewhere else to finish their concrete.

  • Right now they have to buy it from industrial sources,

  • but they say they could pair a concrete plant with any factory that produces CO2.

  • And these chemical reactions go way faster than traditional concrete curing,

  • finishing up in about 24 hours.

  • Solidia is hoping to sell cement makers on the idea that its concrete

  • isn't just greener than the traditional stuff, but also faster and more efficient.

  • They're trying to make something that the industry will want.

  • And theirs is only one possible strategy

  • for changing the way we make cement for the benefit of the planet.

  • But good ideas don't always catch on.

  • People advocating for new technologies sometimes lament thevalley of death

  • that exists between cool tech innovations, and that new tech actually getting adopted.

  • And the cement industry is notoriously resistant to change, which makes sense,

  • because if the specs change slightly, you don't want your building to fall over.

  • Any green cement is gonna have to convince cement makers

  • that it is commercially viable before they jump in.

  • Still, if we're gonna change the building industry, we're gonna need to act on all fronts,

  • from switching to alternative fuel sources to totally rethinking the chemistry of cement.

  • This episode of SciShow was supported by Bill Gates, who, through Breakthrough Energy,

  • is working with some of the world's top tech and business leaders to advance

  • clean, reliable, and affordable energy and to scale the technologies we need

  • to help the world reach a net-zero emissions future.

  • [♪ OUTRO]

[♪ INTRO]

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セメント。本当に難しい問題 (Cement: A Really Hard Problem)

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    林宜悉 に公開 2021 年 01 月 14 日
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