Better cement for concrete results
As an indispensable part of the built environment, concrete is used twice as much as the total of all other construction materials in the world. Recently, concrete has come out from behind plasterboard and beneath ground to become a high quality technical material of choice. It can be stronger, cheaper, more durable option with low maintenance costs. Designers have embraced its flexibility to allow more uninterrupted floor space and for its excellent light reflective and thermal properties, and local production.
So what’s not to like? Well concrete is a highly resource intensive material, mainly due to the energy intensity of cement production, a key constituent of concrete. In the UK, a ton of concrete produces around 70-100 kgCO2 depending on the mix and this is a key factor in the embodied carbon impact of our buildings. The cement industry has done a lot to reduce its impact, but still accounts for around 5% of total global CO2 emissions.
1 tonne of CO2 visualised on a suburban street produced by Carbon Visuals
So how can we reduce the embodied carbon of concrete?
The answer is challenging preconceptions and choosing better ‘cement’. Traditionally Ordinary Portland Cement (OPC) or CEM I is used to bind the concrete. The more cement, the stronger the concrete, the quicker it cures.
However, a large percentage of OPC can be replaced by using two waste by-products of industry, fly ash (from coal power stations) and blast furnace slag (from steel production). At BAM’s Bradford College project, we worked with our supply partner Hanson to investigate concrete efficiency. We discovered that if we had used more cement replacement (by challenging the specification further), we could have doubled the cement replacement used, saving an additional 600 tonnes of embodied carbon.
We’ve found that up to 70% of cement in concrete can be replaced and that we can reduce embodied carbon emissions of concrete by up to 60% for some mixes. The key to achieving this though, is early engagement and collaboration with our supply chain and designers.
Bradford College is being built by BAM and due to be handed over during 2014
Despite being used for 100 years or so, it’s only recently that cement replacement demand has grown as it is cheaper than OPC and technological improvements mean it can be designed to the same, if not better, quality than traditional mixes. It’s also key to reducing the embodied carbon impact of our buildings.
Example reduction in emissions from using traditional cement vs a 50% cement replacement provided by Hanson
Flexible setting times
A concrete containing 50% slag will take only around half an hour longer to set than cement-only concrete. Advances in ad-mixtures mean that curing time, strength, workability, colour and finish can all be tailored to suit. And with early supplier engagement, these extended setting times can be worked into the programme and give greater opportunity for working the concrete, providing flexibility on site.
Less susceptible to cracking
Another natural advantage is that it’s less susceptible to cracking due to a cooler curing process gained through the cement replacement (ie the concrete gives off less heat than when using traditional cement and the higher the replacement the greater the cooling effect). The use of slag in concrete also greatly increases resistance to sulphate attack (which can cause concrete to degrade over time).
Lighter coloured concrete
Cement replacements create lighter coloured concrete, providing different finishing options. This also means they have greater reflectivity (Albedo effect), which helps keep buildings cool and reduces the urban heat island effect.
Colour comparison shows lighter colour of 50% cement replacement, provided by Hanson
Cementing a new concrete culture
With so many benefits, it’s surprising that not everyone is using cement replacements, though outdated beliefs about cement replacements still persist. The construction industry can help reduce the amount of embodied carbon by considering cement replacements as a viable alternative to traditional cement, and sharing their findings, as we are. By sharing this knowledge we can help our clients make better decisions.
Anthony Heaton is a Sustainability Advisor at BAM.
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