Concrete is all around you – in the foundation of your home, the bridges you drive over, the sidewalks and buildings of cities. It is the by volume on Earth after water.
But the way concrete is made today also makes it a major contributor to climate change.
Portland cement, the key component of concrete, is responsible for about . That’s because it’s made by heating limestone to high temperatures, a process that burns a large amount of fossil fuels for energy and releases carbon dioxide from the limestone in the process.
The good news is that there are alternatives, and they are gaining attention.
Portland cement: A greenhouse gas problem
Cementlike substances have been used in construction for thousands of years. Architects have found evidence of their use in the and the buildings and aqueducts of the .
The Portland cement commonly used in construction today was patented in 1824 by .
Modern cement preparation starts with crushing the excavated raw materials limestone and clay and then heating them at (about 1,450 degrees Celsius) to form , a hard, rocklike residue. The clinker is then cooled and ground with gypsum into a fine powder, which is called cement.
About 40% of the carbon dioxide emissions from cement production to generate the high heat needed to run the kiln. The rest come as the heat .
In all, between of greenhouse gas is released per ton of Portland cement. Cement is a binding agent that, mixed with water, holds aggregate together to create concrete. It makes up of the concrete mix by weight.
Alternative technologies can lower emissions
As populations, cities and the need for new infrastructure expand, the , making it important to find alternatives with lower environmental costs.
Some techniques for reducing carbon dioxide emissions include substituting some of the clinker – the hard residue typically made from limestone – with such as clay, or fly ash and slag from industries. Other methods reduce the amount of cement by or recycled .
The long-term solution for reducing cement’s emissions, however, is to replace traditional cement completely with alternatives. One option is made from earthen clay and industrial wastes.
Geopolymers: A more climate-friendly solution
Geopolymers can be made by mixing claylike materials that are rich in aluminum and silicon minerals with a chemical activator through a process called . The activator transforms the silicon and aluminum into a structure that will look like cement. All of this can happen at room temperature.
The major difference between cement and geopolymer is that cement is mainly made of calcium, whereas geopolymers are made of silicon and aluminum with some possible calcium in their structure.
These geopolymers have been found to possess , including resilience in and resistance to , which are important requirements in construction. Studies have found that some geopolymers can provide and, because they don’t require heat the way clinker does, they can be produced with .
Geopolymers can also be produced from a variety of raw materials rich in aluminum and silicon, including earthen , , , , and . Geopolymer technology can be adapted depending on the clay or industrial waste locally available in a region.
An added advantage of geopolymers is that changes to the mixture can produce a range of features.
For example, at the University of Aveiro in Portugal added a small amount of cork industry waste – the leftovers from creating bottle corks – to clay-based geopolymer and found it could . The cork particles filled the spaces in the geopolymer structure, making it denser, which increased the strength.
Similarly, additives such as , and can change geopolymer properties. The additives do not participate in the geopolymerization process but act as fillers in the structure.
The structure of geopolymers can also be designed to act as adsorbents, and . Specifically, incorporating that are natural adsorbents in the geopolymer structure can make them useful for such applications as well.
Where geopolymers are used now
Geopolymers have been used in , including roads, coatings, 3D printing, coastal environmental protection, the steel and chemical industries, sewer rehabilitation and building radiation shielding and rocket launchpad and bunker infrastructure.
One of the earliest examples of a modern geopolymer concrete project was the Brisbane West Wellcamp airport in Australia.
It was built in 2014 with , which was estimated to have reduced the project’s carbon dioxide emissions by .
The geopolymer market is currently estimated to be between and , with the .
Analysts have estimated that the market could grow at a rate of to and reach about .
In several countries, are expected to support the continued growth of geopolymers in the construction industry.
Expanding the use of cement alternatives
The advantage of using industrial wastes in geopolymers is a double-edged sword, however. The composition of industrial wastes varies, so it can be difficult to standardize the processing methods. The geopolymer components need to be mixed in particular ratios to achieve desired properties.
Producing the activator for the geopolymer, typically done in chemical facilities, can raise the cost and contribute to the carbon footprint. And the long-term data about these materials’ stability is only now being developed given their newness. Also, these geopolymers can than cement, though the setting time can be sped up by using raw materials that react quickly.
Developing cheaper, naturally available activators like with sustainable supply chains could help lower the costs and environmental impact. Also, printing the recipe on the raw material packaging could help simplify the job of determining the mixing ratio so geopolymers can be more widely used with confidence.
Even though geopolymer technology has some drawbacks, these low-carbon alternatives have great potential for reducing emissions from the construction sector.
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