Gallery: IS IT GREEN?: Concrete

 

Concrete is a familiar substance. Its durable nature and versatile applications have made its usage ubiquitous throughout our cities. However this primary building material is also extremely energy intensive to make and transport, and produces a significant amount of the world’s greenhouse gas emissions. Can the omnipresent grey substance ever be reconciled as a green building material? Read on for our in-depth report.

Concrete’s impact on the environment starts when limestone is blasted in quarries to make cement – the binder, or substance that sets and hardens it into a useful building material. Cement accounts for 7 to 15% of concrete’s total mass by weight and is made by superheating (in coal-fired kilns) a mixture of limestone and clay and then grinding the resulting substance into a powder. When this power mixes with water, it forms strong calcium-silicate-hydrate bonds that can bind other particulates, like sand or gravel, to make concrete. The cement-to-water ratio determines the strength of the concrete.

Once limestone has been blasted and mined it is then transported to a cement plant, where the fuels used by the plant and machinery produce CO2 emissions. Next the limestone, or calcium carbonate, releases CO2 when it is heated to make the cement. Forty percent of CO2 emissions from the cement plant come from the combustion process and Sixty Percent of CO2 emissions come from the calcination process, according to the Cement Sustainability Initiative report produced by members of the concrete industry. The report also says that since calcination is intrinsic to the process, they must focus on reducing energy use associated with the manufacture of concrete.

Concrete producers also say that as concrete ages, it carbonates and reabsorbs all the CO2 released during calcination – but this process takes hundreds of years.The general consensus is that cement manufacturing produces about 5% of global CO2 emissions generated by human activity, and 3% of global emissions of all greenhouse gases. By comparison the transport sector is responsible for about 15% of global greenhouse gas emissions, so concrete has a pretty hefty share of the pie considering it’s just one material.

The problem stems from volume: According to the World Business Council for Sustainable Development, concrete is the most widely used substance on Earth apart from water. Approximately 2.35 billion tons of concrete are produced each year. That works out to a cubic meter, or about a 3.2- by 3.2- by 3.2-foot cube of concrete for each person on Earth every year, according to researchers at MIT.

And Demand is on the Rise: Concrete production has nearly quadrupled since 1970. Global cement output may reach 5 billion metric tons by 2030 due mainly to growth in China and India, according to a December report by the global conservation organization WWF. (The same report estimates the concrete industry’s share of global emissions at 8%, and claims the industry is capable of reducing its emissions by 90% with current technology.)

So what does concrete have going for it?

It lasts. This is the stuff the Romans built their empire with. Concrete is highly resistant to heating and thawing. It’s impermeable to air and wind-driven rain. And concrete is inedible, so bugs and vermin can’t gnaw at it. This durability means that a building can preserve its concrete foundation or concrete exterior while replacing less durable parts like windows, insulation and plumbing (you get a point from LEED if you reuse a building).

A building with exterior concrete walls can also be energy-efficient, especially in climates that have daily temperature fluctuations. Even though concrete provides little insulation, it creates thermal mass that can store warmth or cold, reducing indoor temperature fluctuation. White concrete also reflects heat and can mitigate the urban heat island effect. LEED also awards points if you build concrete walls and a concrete ceiling with no coating.

A Locally-Sourced Material: Another reason concrete is so popular is because the raw materials to make it are prevalent in most of the world. For the eco-conscious builder, this means it can be locally-sourced, reducing CO2 emissions from transportation. Of course, “local” is a relative term – advocates in the cement industry claim that “the cement, aggregates, and reinforcing steel used to make concrete and the raw materials used to manufacture cement are usually obtained or extracted from sources within 300 miles of the ready mixed, precast concrete, or masonry plant.”

Concrete can also be be recycled – to a point. The concrete industry web site concretethinker.com says: “Most concrete in urban areas is recycled as fill or road base and not placed in landfills. Concrete pieces from demolished structures can also be reused to protect shorelines, for example in gabion walls or as rip rap.” Used concrete can also be reused as the aggregates in new concrete. Concrete’s recyclability is limited because its chemical properties change over time and with each processing. Other materials can be recycled as aggregates in concrete.

Some progress has been made in reducing the amount of emissions in the concrete manufacturing process. The English company Novacem says it has created a cement (patent pending) that uses magnesium silicates, which do not emit CO2 during combustion. Its production process also runs at lower temperatures and the cement absorbs far more CO2 as it hardens – enough to more than offset the amount generated during manufacturing. A California-based company, Calera, claims it has discovered a cement-manufacturing process with a negative carbon footprint. Calera’s technique, remniscent of how some corals produce reefs, is to filter carbon dioxide emissions through seawater to create a chalky carbonate byproduct. This cement substance is then mixed with aggregate and water to create concrete, sequestering carbon emissions and avoiding the need to heat the ingredients, the company says.

Is It Green?

The reality is that even if new technology truly achieves a carbon-negative manufacturing process, greener cements (like ones requiring complex facilities to shoot emissions through seawater) won’t be as affordable or accessible in the developing parts of the world that are consuming concrete most voraciously. Significant CO2 reductions won’t be possible without heavily subsidizing the industry in poor countries.

The importance of durability should not be trivialized, but today’s concrete is not green. A combination of new technology and efforts to decrease energy used in manufacturing is needed so that perhaps tomorrow’s concrete can be.

+ Concretethinker.com

+ Novacem

+ Calera

Lead photo by Jeff Kubina

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20 Comments

  1. Jack Frost September 23, 2013 at 1:25 pm

    Why not recycle your cement BEFORE it leaves the construction site with Formablok??

  2. kavingaj March 11, 2012 at 6:01 am

    The actual green concrete is the zero cement concrete. The use of slag and fly ash in a basic environment produces an altenative solution to cement.

  3. tr October 19, 2011 at 7:56 pm

    Calera corporation’s claims to have “invented a green process with net negative carbon footprint” have been widely debunked now. You can read about this, including a back and forth with those concerned, here:

    http://www.businessweek.com/investing/green_business/archives/2009/03/climate_change_2.html

  4. World's First Zero Carb... October 7, 2010 at 2:27 am

    [...] as well, as the building has a 100-year design life. A key decision was to use 6,000 tons of low-carbon concrete in construction, which significantly reduced the building’s carbon footprint, but the real [...]

  5. Stan Anderson September 20, 2010 at 4:20 pm

    I want to learn more about green concrete.

  6. Precast Concrete Bases ... August 31, 2010 at 12:38 pm

    [...] Concrete and wind turbines have a strange relationship — the material requires tremendous amounts of energy to produce and accounts for 5% of global CO2 emissions, but it is essential for the installation of large wind turbines. Several months ago a report stated that hundreds of offshore turbines could potentially collapse into the sea due to poor grout mixture being used in the bedrock and poured concrete base. Then it was reported that the base, made of cement, sand and gravel, was not holding the turbines firmly enough, causing some turbines to shift several centimeters since installation. [...]

  7. Renn Jackson March 24, 2010 at 1:30 pm

    The green philosophy has to be measured in terms of comparables. While it may seem that cement and concrete are not “green”, I’d say that by any comparable measures they are some of the greenest materials known to man in relative terms. If we simply devise an equation to consider the parameters involved and used time as a factor, concrete would dwarf all other materials in green utility and sustainability, longevity, functionality, strength, and energy use, recycables, and emmissions. Do we have any other materials that can repace it in the volume and specific types of applications in a real comparable way with better green properties? No. Is it polluting the environment so much that you can see the pollution, like coal? No. Does it need replacing every 25-50 years like many of the other building materials, even when installed properly? No.
    Is it polluting our river and streams? No.

    Are manufacturers trying to improve its processes and making progress to use more recycle materials and other advances to make it even better? Yes. Are CO2 emmissions a concern and being worked on as a means to make concrete even greener? Yes.

    Until our tiny little planet loses it vital need to shelter people, transport goods and services over rivers, streams, and harsh terrains, and provide the economic means to sustain societies, we should remember to APPRECIATE concrete and the amazing green nature of our planet’s finest material. Sand and rock are amazingly green, but they doesn’t quite fill the build for many of our more pressing requirements.

    Try visiting and living in a few countries without it…and I think you’re begin to appreciate what concrete makes possible.

  8. concrete_fan August 25, 2009 at 4:11 pm

    the terms ‘concrete’ and ‘cement’ need to be defined more clearly and not used interchangably. That’s the huge issue here. Most concrete products manufacturers pick their cement supplier based on their sustainability initiatives. Every major cement producer in the world is working as hard as possible to reduce their carbon emissions. The fact is, concrete structures use fly ash to reduce cement volume, and the fly ash would normally be landfilled if not used for this purpose. Also, most concrete mixes use less than 1% admixtures- most if not all of which are completely contained in the concrete and won’t leech out into the soil if buried. Concrete structures also use 90%-100% recycled steel for the reinforcement they require. The rest of the concrete structure is sand and stone, and again, most major concrete products manufacturers are or should be picking their aggregate and sand suppliers based on the suppliers’ committment to rehabilitating the quarry once it is no longer useful. Subdivisions, commercial parks, etc. can be built, without issue, on a rehabilitated quarry.

    Just remember, CEMENT only makes up approximately 15-17% of a CONCRETE structure, and according to the UK concrete industry sustainability report, the embodied CO2 associated with the production and transport of an average tonne of concrete is 95 kg. Very small compared to alternative materials.

  9. Hendrik May 7, 2009 at 6:21 pm

    1 cubic meter is about 35.3 cubic feet–to correct one of the commenters’ comments.

    The article is basically sound until they get to the part concerning how much concrete is made worldwide. We are making about 2.7 – 2.8 billion metric tons of hydraulic cement each year worldwide, not concrete. The concrete made using this cement (I will include mortars in this figure) would be on the order of 19 – 25 billion metric tons (let’s say, 22 Gt for a middle of the range)–this is about 3 metric tons per person on the planet–each year.

    Carbonation of concrete (i.e., the calcium silicate hydrate component) is extremely slow and usually very incomplete, even after decades to centuries. But during the hydration of the cement, free lime (becomes hydrated lime) is released and this lime amounts to roughly 20-30% of the original CaO in the cement analysis. It is this free lime that can readily carbonate if exposed to atmospheric CO2. The problem is getting the CO2 in contact with the lime–it’s a slow process (absent high pressures) in thick, intact structures. It is relatively rapid in high surface area applications (thin walls, stuccos, sidewalks, etc…) and can be fairly fast in demolition debris.

  10. cretzler March 9, 2009 at 1:10 pm

    Don’t forget about the hundreds of admixtures possible with concrete, including the ones from recycled sources such as fly ash. These can have a huge impact on the performance and environmental impact of the material. Fly ash is a byproduct of coal power plants that can be used to replace significant portions of the portland cement. It is also important to consider the vast amounts of steel reinforcement that go into concrete structures. Alternative reinforcing materials and details can help decrease the steel tonnage on a job and make the material more “green”.
    And like billolive said, we must think about what alternatives there are to concrete as well. In most cases where concrete is used, structural steel is the only viable alternative and it requires much more energy to produce.

  11. Budi Waluyo February 9, 2009 at 12:50 pm

    As if a sword, technology advancement has two edge. One edge it can enhance human life quality but another side it tend to destroy natural environment. I believe they will find their balance. I, personally appreciate all the company effort to decrease the negative effect of natural sources exploration and the streamline of building material production. While the natural sources exploration should be in tight supervision.

  12. mk February 8, 2009 at 4:54 am

    After visiting Malaysia where a lot of limestone is collected for use in concrete, it makes me cringe inside when I see concrete being used. Beautiful natural limestone hills, with trees growing all over them and around are cut down so that they can get all the limestone out of them. The end result is a flat landscape with nothing but desolate land underneath.

    I talk of this hills like the ones of the famous Batu Caves: http://www.flickr.com/photos/creazione/2592470217/

    I can see the need in our modern lives for such a material, but honestly, it puts it into perspective whenever I see it being used anymore. Beautiful landscapes being flattened so that we can build bigger buildings? The destruction of a beautiful thing.

  13. raunchpaunch February 6, 2009 at 11:14 pm

    hyssil is an alternative to precast concrete panels, aerated and thus lightweight, and it doesn’t require autoclaving and can be wet set.
    check it out here.
    http://www.hyssil.com/index.htm

  14. cpine February 6, 2009 at 3:44 pm

    Another factor to consider in concrete construction is form work. Before the “mud” can set, it must be molded, either as blocks at a plant, or on-site as form work. Slip forms can be made of steel, plywood and framing timbers, or even cardboard. Tilt-up construction, in its crudest form, simply uses the earth trenched out to the appropriate shape. It can also be used in rubble or wattle and daub construction as a binder and finish, or simply as mortar for brick and stone construction. Each of these considerations reflect local sourcing, use of other materials, transportation, waste, WATER USE, and factors frequently overlooked or “externalized” when evaluating the “greenness” of concrete alternatives.

  15. marko February 6, 2009 at 11:17 am

    Well, if the new cements require less energy to produce, they could/should also be cheaper?
    Also, it is the amount of cement in concrete (kg cement per m3 of concrete) that determines the strength of concrete, much more than the water/cement ratio. Water/cement ratios is more of a workability/strength ratio.

  16. renschede February 6, 2009 at 9:16 am

    Kudos for this kind of reporting – I would love to have much more like this.

    I believe if we are to be serious about creating “sustainability” then we must take the time to underpin our discussions and decision-making with just this kind of basic analysis – sadly lacking from most media coverage of “green” design.

    Again, thank-you.

    Ryan Enschede
    Architect, Brooklyn NY

  17. InvertedPantsMan February 6, 2009 at 3:45 am

    Nice article. 1 cubic meter is a cube of (roughly) 3.5 feet in stead of 35 however.

  18. billolive February 6, 2009 at 2:50 am

    One thing I always see missing in the “concrete is not green” argument is the simple fact that, in many cases by using concrete you don’t cut down trees! Concrete is faster, easier to use, longer lasting and replaces wood in many cases where it would take incredible amounts of lumber to achieve the same structural integrity. Translate 2.35 billion tons of concrete per year into the same number of trees being cut down and run those numbers would ya? We should just work on upping the efficiencies of its creation. A replacement might be nice, but then so would zero point energy, 150mpg cars, technological advancements instead of technologic suppression, hmmm come to think of it this list could go on and on and on………..

  19. elepski February 5, 2009 at 6:36 pm

    I feel Concrete is so important to our lives now that considering an alternative is futile. Honestly, What can you replace it with?

  20. goooglemonster February 5, 2009 at 5:02 pm

    This doesnt cover supercritical concrete that is made using co2 which draws out the water and cures the concrete. Supercritical concrete also offers the ability to absorb large amounts of co2 in the finished products.

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