Emissions from cement-making

One supporter sent this information in, in response to our note that 40% Co2 emissions come from buildings.

Making “Ordinary Portland Cement”, which is the grey stuff known to almost everyone as plain “cement”, is a matter of turning limestone (calcium carbonate) into calcium oxide. This means producing carbon dioxide (in its simplified form: CaCO3 -> CaO + CO2).

The chemical process, known as calcination, releases around 500 kilograms of the gas for every metric tonne of cement it makes. The raw materials must also be ground and then heated to 1450 degrees. Altogether, according to a study published in the Annual Review of Energy and Environment, the manufacture of 1000 kilograms of cement emits, on average, 814 kilograms of carbon dioxide. This does not take into account the energy costs of quarrying and transport. It is probably fair to say that a tonne of cement produces about a tonne of carbon dioxide.

According to David Ireland, of the Empty Homes Agency, a British house requires, on average, 25 tonnes of concrete for the foundations and floors, and 4 tonnes of mortar and rendering. One tonne is wasted. That’s 30 tonnes in total. Now, the concrete used in house building consists of one part of cement to five parts of sand and gravel, so if the 30 tonnes is correct, every new home requires about 5 tonnes of cement, for which 5 tonnes of carbon dioxide is produced during its manufacture.

Depending on whose figures you believe, cement produces between 5 and 10 percent of the world’s manmade carbon dioxide.

The carbon dioxide could be captured during the calcination process, as could that from the combustion process of the fuel that fires the kilns, but it would push up the price.

One thing we could do is to make cement go further. It was thought that something called AirCrete (or ‘autoclaved aerated concrete’) may provide the answer. When cement is mixed with quicklime, sand, water and aluminium powder, it rises in the mould like a loaf of bread. AirCrete blocks are strong and they keep the heat in. Because between 60 and 85 percent of their volume is air, they contain much less cement than solid blocks. Unfortunately, the carbon savings appear to be thrown into reverse by the aluminium powder. This is the component that acts as the yeast in the concrete dough, generating the bubbles which make it rise. Though much smaller quantities are used, the energy costs of smelting aluminium are around forty times as great as the energy costs of manufacturing cement.

Another approach is to use High Strength Concrete, which contains additives such as silica fume and finely ground fly ash. Because it is extremely strong, it allows builders to halve the weight of the materials they use. The additives are quite expensive, but because the volume of materials and the transport costs are smaller, the overall cost is generally lower. The problem is that the industries that produce silica fume are moving out of rich nations, which means that the carbon costs of transport will rise, while fly ash is, on the whole, the product of burning coal in power stations, which we have to reduce if we are serious about global warming. You could also reduce the carbon content of cement by mixing it with slag from blast furnaces, but once again, steel making is going off-shore or switching to new technologies whose slags don’t have the right properties.

Turning unwanted cement back into limestone, by forcing it to reabsorb the carbon dioxide it loses during the calcination process, might be an answer, but a cost-effective process is not there yet. This happens anyway, but very slowly and according to New Scientist “a large slab of concrete could take 30000 years to carbonate fully”.

Magnesium carbonate needs to be heated to only 650 degrees to make cement, which reduces the energy costs of manufacturing it. It is also claimed that it is stronger that calcium cement, but magnesium carbonate is much rarer than limestone, so it is more expensive and much more carbon would be produced by having to ship it further.

There is hope, however, with materials similar to the pozzolan cements used by the Romans to build the domed roofs of many of their buildings. It is claimed that these materials set quickly and have great strength, last longer than ordinary cement, shrink less and are more fire resistant. These materials are called ‘Geopolymeric cements’. They can be manufactured from several kinds of clay and industrial waste, and quite a few common sedimentary rocks. They are cheap and their fabrication produces between 80 and 90 per cent less carbon dioxide than Portland cement. This is because they are formed at lower temperatures, about 750 degrees, and the chemical process doesn’t depend on releasing carbon dioxide. According to the CSIRO, they can be used for every major purpose for which ordinary cement is used. So why aren’t they used more? Artificial geopolymers were invented in the late 1970s. George Monbiot says that the construction industry is notoriously conservative and the cement companies have a powerful financial incentive to maintain their existing plants, rather than to start up somewhere else with a different process.

(KF – This is probably where governments should step in.)

So there may be hope, that by 2030, all concrete in the rich countries will come from plants that are capturing their carbon or producing geopolymers instead of Portland cement.

Monbiot refers to various sources for his figures, which I can supply if you are interested.

I hope you found this interesting and informative.
Keith Frudd