Making this frequently used building material more environmentally friendly.
By Douglas Weinstein
CONCRETE IS A COMPOSITE MATERIAL composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time. In the past, lime based cement binders, such as lime putty, were often used but sometimes with other hydraulic cements, such as a calcium aluminate cement or with Portland cement to form Portland cement concrete (named for its visual resemblance to Portland
stone, which is quarried on the Isle of Portland, Dorset, England).
The earliest structures utilizing concrete date back to 6,500 BCE, by the Nabataea traders in regions of modern-day Syria and Jordan. They created concrete floors, underground cisterns and housing structures. A form of concrete was used to build the Great Wall of China circa 3,000 BCE. And the Romans used concrete extensively enabling revolutionary designs in terms of structural complexity – the Colosseum in Rome was built largely of concrete and the circular cella of the Pantheon is the world’s largest unreinforced concrete dome.
Mexico’s Cemex is the world’s second largest building materials company and has begun to revolutionize the concrete and cement business with the introduction of Vertua, a carbon neutral concrete solution. This cutting-edge material reduces the carbon footprint by up to 70 percent, with the remaining 30 percent footprint neutralized through offsetting efforts. Let’s deep dive into the particulars.
10-15 percent of concrete consists of cement, which is a carbon-intensive manufacturing process. Limestone has to be heated to 1,450C which requires energy from fossil fuels and accounts for 40 percent of concrete’s CO2 footprint. This process separates calcium oxide (which we want) from carbon dioxide (which we don’t want).
The calcium oxide is ground into a powder, then sand, gravel and water are added and it begins to form interlocking crystals – or, concrete.
One way that scientists have looked at to reduce the carbon emissions in concrete production is by replacing much of the conventional cement with heated clay and unburnt limestone. The Romans knew they could substitute some of the cement with volcanic ash which actually improved the concrete’s strength and durability.
This approach represents up to a 40 prcent reduction in CO2 and the real beauty is that production works with existing equipment. Several companies have begun commercial production, which is known as LC3 (limestone calcined clay cement).
What Cemex has done with their Vertua concrete is another leap forward in how to clench the sand and stone particles together, without cooking limestone into calcium oxide. The process entails a binder agent rich in aluminosilicates that are activated by chemicals in a reaction called geopolymerisation. This forms a 3D network of molecules which form a solid binder to grip sand and stone into place.
Calix is a global technology company based in Australia who has developed another forward-thinking solution to reduce CO2 emissions in cement, lime and hydrogen production. Due to the European Union targeting an 80 percent reduction in CO2 emissions by 2050, the EU introduced and began operating the largest emissions trading scheme in the world. This cap and trade system involves an open market price for CO2, which has become a benchmark for the cost of emitting CO2 in Europe.
LEILAC (Low Emissions Intensity Lime and Cement) is a project lead by Calix, and involves a consortium of industrial heavyweights Cemex, HeidlebergCement, Lhoist and Tarmac. Project LEILAC utilizes the Calix Process that changes the way limestone is heated, to enable direct capture of the produced CO2.
While the process is somewhat technical, the Calix process indirectly heats the limestone via a special steel reactor. This system enables pure CO2 capture as it is released from the limestone. In pilot programs in Europe, once captured, the CO2 is compressed, shipped in a barge to Norway and stored in an empty oil reservoir under the North Sea.
For the remaining emissions associated with heating the reactor, the technology enables the use of any type of fuel or heat source. This makes achieving a low-emissions cement kiln relatively easy, using biomass rich fuels, electricity or hydrogen. If alternative fuels, biomass, or conventional fuels are used, conventional carbon capture techniques can be applied to capture the combustion emissions, if no other alternative is found for the kiln’s main burner.
Cement makers have already reduced their carbon emissions by almost 20 percent since the 1990s, simply by making kilns more energy efficient. And while overall emissions might be reduced by 60+ percent in the coming 20 years, there will still need to be a capture and store scenario to achieve carbon neutral status.
The bigger issue facing the cement and concrete industries is getting the latest technologies to the places on the planet where concrete will be in greatest demand – China, India and Africa. Realistically, the alternatives we’ve discussed are rarely as cost-effective as mass-produced Portland cement, as well as the constraints in terms of raw material supply and the basic resistance from builders who cannot scale up to industry participation.
The long-term challenge will most likely consist of a combination of global policy mechanisms and a concerted effort on disseminating best practices. While novel cements are a cornerstone in the reduction of CO2 emissions from the deployment of concrete infrastructure, greater action is also needed on overall energy efficiency and sustainable fuels and materials in order to attract investment and interest in decarbonization.