Sustainable cement

With increasing consumer awareness of global warming and CO₂ emissions, combined with possible future carbon taxes on various industries, it may be time to rethink concrete.

From the simple mortar-jointed brickwork of our homes to the industrial-scale dam walls or cast concrete pillars of bridges, cement is the glue that holds modern development together. It is ubiquitous and yet unnoticed. However, it comes at considerable environmental cost. In essence, cement is essential but environmentally unfriendly; but there are moves afoot to reduce its use and/or impact through innovative technology and design – both in the production of cement, and the use of cement in the production of concrete.

Greener cement

For every kilogram of Portland cement manufactured, almost a kilogram of CO₂ is released into the atmosphere. This is both because the process involves heating the raw materials to very high temperatures, which requires vast amounts of fuel (almost invariably fossil fuel), and also because the actual chemical process that converts limestone to cement releases CO₂ as a waste product. Double whammy!

A Danish consortium of companies and organisations has worked together to produce what they call ‘green cement’ by what appears to be a clever tweaking of the recipe. Portland Cement has for centuries been made from 90% cement clinkers and equal parts of limestone and gypsum, but the new green cement is composed of only 62% cement clinkers, 17% calcined clay and 17% limestone, with the remaining 4% gypsum. Seems like a small difference, but, the developers claim, it results in significantly reduced carbon emissions.

Engineers at Rice University in Texas have gone back to the drawing board. The ancient Romans (who invented concrete) made it using limestone and volcanic ash. Now volcanic ash is not that dissimilar to fly ash – a waste product of coal-fired power stations. So the Texans tried replicating the Roman recipe using readily available fly ash instead of mining volcanoes. It was obviously not that simple – and it took them a while to hit upon the right proportions of fly ash, lime, silica and other materials – but they have created a cement using about 80% fly ash. Hmmm – maybe we can put Medupe and Kusile to good use after all.

Neither of these is commercially available as yet, but let’s hope they will be soon.

Greener concrete

One of the three main pillars of making greener concrete is to replace some of the cement with supplementary cementitious materials (SCMs) such as fly ash or ground granulated blast furnace slag (GGBS). These not only replace the cement but also use up waste otherwise headed for landfills.

It turns out that stronger concrete is also greener, because you need less of it for the same durability. For example, you can reinforce the concrete with steel – but not the way you’re used to thinking of reinforced concrete. Steel Fibre Reinforced Concrete (SFRC) consists of thin flexible steel fibres, fly ash, slag and/or silica fume, and – of course – cement. SFRC has been used successfully in tunnels like, for example, a 45-kilometre long tunnel constructed as part of Abu Dhabi’s waste water system, and a four-kilometre long tunnel under Copenhagen Harbour to carry heating pipes. The carbon saving is both due to the enhanced strength, which allows for thinner walls, and the fact that traditional clinker has been replaced by materials that would have otherwise ended up in landfill.

Once you start looking, you will find no end of fascinating innovations:

• You don’t need to stop at replacing cement; you can replace aggregate with reclaimed concrete, or other recycled material, for example, glass. That might not reduce emissions, but saves on landfill, and it can even produce concrete that glows!
• Research at Australia’s Deakin University suggests that glass polymer concrete – in which non-recyclable glass powder replaces the sand, and polymer resin replaces the cement – is both significantly stronger and more cost effective to manufacture. And it’s waterproof.
• Self-cleaning concrete contains titanium oxide that, through a process called ‘photocatalysis’, keeps the surface cleaner. Oddly, not only does it clean itself but it also cleans the air around it, removing pollutants such as nitrous oxide.
• Probably the most interesting – and a great way to save on maintenance bills – is self-healing concrete, which contains bacteria (Bacillus pasteurii) that can heal cracks. Once the cracks appear and allow in oxygen, the bacteria wake up and produce calcite, which bonds with the concrete, healing it and returning it to close to its original strength.

Bottom line

We’re not going to find a carbon-neutral, sustainable, renewable substitute for cement any time soon but, through consistent research and improvement, we can reduce its impact – and that will, hopefully, result in reduced costs as well.