First off, I would like to apologise – this subject really is very dry, I have tried to make the blog below interesting, but it was a struggle and so you might want to grab a strong coffee before you get started!
So here goes…..
Concrete is the most widely used building material today as a result of its strength and its durability. It is used in homes, airports, skyscrapers, tunnels and pretty much every other type of construction you can think of.
Concrete and cement are terms that are often used interchangeably, however cement is actually an ingredient that makes up concrete, along with water, sand and gravel. Cement acts as a hydraulic binding material, hardening with water and tying together all the aggregate materials.
After coal-powered electricity, cement manufacture is the next biggest emitter of greenhouse gases, accounting for approximately 5% of annual anthropogenic global CO2 production. In 2011, we used approximately 3.6 million tonnes of the stuff within the construction industry – the problem being that for every tonne of cement produced, one tonne of CO2 is also produced.
I am not going to try to use this blog as a platform to preach that we should stop building – that would be ridiculous. People need a place to live and businesses need to grow. However, I am going to look at the current manufacturing processes and identify a few technologies and areas of investigation that are being toyed with to reduce the impact of the cement / concrete industry and the impact it has on anthropogenic CO2 emissions.
How is Cement Made?
First a bit of science! To make cement, limestone (calcium carbonate – CaCO3) is heated to temperatures approaching 10000C, along with other feedstock materials such as clay (which contains silicates). At this temperature, the limestone (and other feedstock) breaks down into Calcium Oxide (known as Lime – CaO), Silicon Oxides and Carbon dioxide. The two oxides then combine to produce di & tri-calcium Silicate, which is then ground to a fine powder producing a product known as ‘clinker’.
Finally Gypsum is added to the clinker (to prevent flash setting of the cement), and this is ground to produce the cement, which can then be used as the main ingredient to make concrete.
There are two ways that making cement releases CO2:
- Burning fossil fuels to heat the kilns to achieve the reaction temperatures
- Breaking down calcium carbonate into calcium oxide and CO2
The burning of the fossil fuels accounts for about 30% of the total CO2, while breaking down the calcium carbonate accounts for 70%
Processes to Make Cement Manufacture ‘Greener’
So how can you make ‘greener’ cement, thereby helping to reduce the total volume of carbon dioxide into the atmosphere?
Well strictly, the first couple of processes I touch upon are not related to changing the manufacturing process, but instead installing carbon sequestration techniques, thereby using the CO2 rich flue gasses to produce new materials.
As covered across TheGreenAge, there are numerous techniques of using carbon dioxide to produce useful products. Artificial photosynthesis would use this gas to produce sugars that are essentially a store of energy, replicating the natural process that plants use for growth everyday. The benefit of this technique would be not only limiting CO2 emissions, but also producing a valuable by-product.
Bio-algal synthesis is another technique for using the CO2; in this process the gas is pumped through a waste water growth medium infused with microalgae. These algae use sunlight and the CO2 for growth, doubling their mass every 24 hours, which can eventually be crushed for algae oil and algae meal for cattle. The advantage of the algae meal is that there is little cellulose within it (unlike regular crop derived meal), so when digested by cattle, very little methane is produced (another greenhouse gas), helping to solve another greenhouse gas emitting problem.
In terms of changes to the process itself, there are numerous different ways that it could be carried out to produce lower carbon dioxide emissions.
Using Electrolysis to produce concrete
If electrolysis is used to react the limestone, replacing the initial kiln part of the reaction, different resulting molecules would be produced.
If carried out over 8000C, electrolysis would produce lime in addition to carbon and an oxygen molecule. If it is carried out under 8000C, the Lime is produced along with carbon monoxide and an oxygen atom. Carbon monoxide can be used to produce fuel and form plastics, and has a value of approximately $600 / tonne, allowing one to actually make money from the process (based on the lower cost to produce the lime).
Make Extra Strong Concrete, Need to Use Less!
This is a fairly simple idea, but by changing the feedstock entering the initial furnace, you can change the chemical composition of the clinker. So why not make ‘extra strong’ concrete, so you don’t need to use as much to perform the same job. Placticisers are additives that are added to the concrete that increase its strength. They are relatively complex but essentially to get strong concrete you need a low amount of water within it when it dries. These placticisers act as spacers between the cement molecules, ensuring that there is less water in contact with these molecules, hence stronger concrete.
Use Solar Power
The kilns used in the current process (to heat the clay and limestone) need to reach incredibly high temperatures. Historically, these temperatures have been reached by using the heat from burning fossil fuels (accounting for 30% of the carbon dioxide emissions from the cement manufacturing process). If the kilns were instead heated using a green electricity source, for example a concentrated solar power plant, this would obviously instantly remove much of the CO2 produced in the process.
Use Consistent mixes to make the concrete
Currently under EU standards there are 170 different types of concrete mix used in the construction industry. This poses a problem for builders, who simply do not know the strength of the different types, so in order to ‘make sure’ the construction is strong enough, excess concrete is used. If there were only say 10 types of concrete, then builders could better get to know the characteristics of each one, and they would be able to predict accurately how much of that particular type to use, thereby using less in the overall construction process.
Actually redefining the Ingredients That Make Up Concrete
There are numerous companies at the moment looking at creating a new concrete altogether. Instead of relying on limestone for the source of lime, research startups are trying to harness other techniques.
Novacem, a startup based in Imperial College, is looking to replace limestone with Magnesium based Silicates, current prototypes are beginning to demonstrate strength on par with traditional concrete.
Calera,based in California and backed by Vinod Khosla, is looking to use the flue gases of other high CO2 emitting industries to make the cement.
Louisiana Tech is looking to do away with limestone too, by creating a paste from fly ash (bits of non combustible fuel thrown out of the flue with exhaust gases), sodium hydroxide and potassium hydroxide.
As a $170bn industry, it is clear to see why these startups, and many more that have started popping up have decided to spend the time and financial resources to try to come up with a greener concrete, that could one day replace the traditional concrete.
The Future of Concrete
China currently use about half of the world’s supply of cement, and this demand is expected to rise by an additional 700,000 tonnes over the next 5 years. If current production techniques are used, an equivalent 700,000 tonnes of CO2 will enter the atmosphere.
This is just China though; hopefully in the next couple of years the worlds economies will be able to put the current financial situation behind them, and begin a new phase of growth. As a result, global demand for concrete is only going to rise, therefore we really need to come up with a way to lessen the impact of the concrete industry on the environment, potentially using one of the methods detailed above.