The concrete solution

Materials World magazine
,
2 Jun 2015

Concrete is one of the most popular materials in the construction industry, yet it is a highly energy-intensive sector. What is being done to change that? Natalie Daniels found out at Making Concrete Greener, in London.

Around three tonnes of concrete is used annually for every person in the world, which amounts to a substantial environmental cost. So what can the industry do to ensure this material is more sustainable? Concrete is energy-intensive because of the heat required – producing a tonne of cement generates almost a tonne of CO2. In a bid to     reduce emissions, both concrete specialists and the construction industry are finding new techniques and analysing all factors of the building material. 

As Sandy Patience, Architect and GreenSpec Editor pointed out, one of the problems the industry faces is in changing people’s perceptions. Even today, one of the British Science Museum’s primary suggestions to reduce CO2 emissions is to avoid using concrete. Speaking in its defence, Patience explains, ‘Concrete is getting a bad name among architects, but I don’t think people should just condemn the material. There is a prejudice against concrete that has already been made. I hope we can change that over time.’ 

Patience also noted other factors that could have an effect on the concrete sector. By 2050, there is expected to be an average temperature rise of 3°C, a 20% increase in winter rainfall and decrease in summer rainfall, as well as more frequent storms and floods. Patience argues that, ‘Using concrete in sustainable design can control heat management, prevent heat loss and act as flood resilience in extreme weather conditions – due to its fantastic durability and water resistance.’ 

The options

Concrete can be made greener in many ways. Speaking at the discussion was Dr Paula Carey, Quality Assurance and Research Director at Carbon8 Aggregates Ltd. The company uses a process known as accelerated carbonation to manufacture a lightweight aggregate from the lime-based residues created at energy from waste plants. The process converts the residual lime and other calcium silicate phases within it into carbonate, permanently capturing around 40kg per tonne of CO2 and treating the alkalinity of the waste while reducing any trace metals within the residues.

The resulting aggregate has been shown to be carbon negative – more CO2 is captured in the aggregate than is used in its manufacture. Most of the product to date has been used as a replacement for natural aggregate in medium dense and dense aggregate blocks. Carey explains, ‘The use of C8Aggregate can reduce the carbon footprint of concrete as it is manufactured from a waste, has a lower density than most natural aggregates, and has a negative carbon footprint.’ The technology can also be applied to other waste streams such as steel slags or cement by-pass dusts to manufacture aggregate for the construction industry.

It is not only innovative aggregates that could change the concrete industry. As Dr Denis Bezard from Newchem AG explained, ‘Metakaolin, I believe, is the way forward for the industry. By adding the mineral to Portland cement, it can accelerate the strength, produce extra-white precast concrete and reduce the amount of CO2 released.’ Bezard adds, ‘It is an addition to slag and fly ash and, as a material based on kaolinite, it can be produced very easily in many countries.’ Pure metakaolin contains around 44% aluminium. When the layered silicate material kaolinite is heated between a layer of silicon dioxide and aluminium, the water contained between the layers evaporates and kaolinite reacts with cement. It is then added between 600°C–850°C to form metakaolin, which can react with cement and lime. Bezard believes this method has many advantages in industry, ‘We want to have the same strength as aggregates. This technique has higher durability and strength, it reduces sintering, as well as contributing to reducing CO2.’ 

The facts: 

Standard concrete mix releases up to 79.7kg of CO2 per tonne

By 2020 concrete’s embodied CO2 is estimated to reduce to 71.8kg per tonne of concrete

Certification to responsible sourcing standard BES 6001 has risen to 91%