Concrete is all-pervading in construction and has been for many decades. According to the European Commission, 25-30% of all waste generated in the EU consists of construction demolition waste materials, many of which could be recycled – something that would prove particularly beneficial in dense urban areas, as this is where most old concrete accumulates.
While CO2 emissions and climate change are factors most commonly associated with concrete production, its environmental impact goes well beyond that, according to EC research (see www.is.gd/wopahe). “Impacts can include acid rain as a result of emissions of sulphur dioxide, nitrogen dioxide and nitric oxide; health risks of locally high concentrations of cement kiln dust; and the depletion of drinking water supplies.”
However, the tide may be turning, with the supply chain pushing forward with several initiatives to improve its green credentials, in a bid to cater to a market increasingly aware that the most common material in the building industry has a poor environmental record. The development of ‘green’ concrete, for example, with reduced impact across a range of environmental considerations, is an important step towards a sustainable construction industry. “Industrial byproducts, such as blast-furnace slag (from steel production) or fly ash (from coal combustion) can be used to replace a proportion of the cement needed for concrete,” states the EC.
OLD CONCRETE INTO NEW
One way to save carbon is to recycle waste materials. Specialty chemicals company Sika has developed a process to separate and reuse the components of old concrete, as well as increase the recycled aggregates’ quality. The old concrete is broken down into its individual parts – gravel, sand and limestone – in what Sika describes as “a simple and efficient process” which also binds about 60 kg of CO2 per ton of crushed concrete demolition waste. Under the brand name reCO2ver, this innovation will “make a significant contribution to reduce the ecological footprint of the construction industry”, Sika states.
Comparative testing of the Sika reCO2ver process has, it adds, demonstrated that new concrete-containing recycled content performs similarly to an all-new product. Thanks to additionally developed chemical additives, further process optimisations can be achieved, such as the flexibility to tailor specific concrete functionalities. “We are already developing complementary chemical additives and quality enhancers, and are pushing the industrialisation of our new technology,” said Sika chief technology officer Frank Höfflin (pictured, right). Adds CEO Paul Schuler:“In the five largest EU countries alone, roughly 300 million tons of old concrete are generated every year. With complete recycling of these materials, up to 15 million tons of CO2 emissions can be captured.”
Sika’s reCO2ver technology exploits synergy from a chemo-mechanical treatment of concrete demolition waste. It involves superficial carbonation of the cementitious matrix that is softened and removed upon attrition (grinding). With that, freshly exposed surfaces are obtained, able to further carbonate until aggregates free from cementitious material are obtained. “In that way, concrete/mortar demolition waste can be separated into ‘secondary aggregates’ for recycling at a quality level of primary material and a powdery material utilisable as secondary raw materials in a broad application spectrum,” says Sika.
CLEANER, GREENER FUTURE
One powerful advocate for advancing such change is Glen Rust, R&D engineering leader, structures & sustainability at Laing O’Rourke. In a webinar hosted by The Concrete Centre, he made it clear that any long-term solution to cement’s carbon footprint was far more complex than merely replacing more carbon-intense materials with less intense ones.
“Getting to a net zero carbon state requires focus on both infrastructure and buildings... As an industry, we are slow to adopt new innovations stuck in our ways. We like flat slabs – they are very easy to build and interface with, but the fact is they are very material-inefficient. We need to transition to highly material-efficient forms of function where we can make significant savings, before we then get on to the matter of concrete.”
Laing O’Rourke is embarking on several rail and road bridges and building solutions over the next few years that will embrace this approach, Rust states. A key factor will be the building frame solutions, D-Frame and M-Frame, that Laing O’Rourke has successfully manufactured at its Centre of Excellence for Modern Construction in Nottinghamshire (pictured, left) that could, it is claimed, reduce a typical frame programme by around 30% and achieve a 70% reduction in embodied carbon. “D-Frame has the potential to change the industry by bringing together digital modelling and manufactured precast components into an innovative solution that allows for a more efficient assembly of the frame of a building,” adds Rust. “While standardised components are used, thus improving productivity, the system has the flexibility to accommodate architectural complexity.”
FRAMING A LIFECYCLE APPROACH
According to Laing O’Rourke, D-Frame gives consideration to the entire lifecycle of a building from design, construction and through into end of life, where its demountable design feature at end of life enables circularity within the built environment. That translates into off-site manufacturing, according to Rust. He adds: “Then we can really start pushing the boundaries; performance testing components and stripping out all the superfluous materials that are not needed to perform the required function. And this has to cover all of the holistic thinking relating to building.”
From a concrete perspective, substantial savings can be made using GBBS (ground granulated blast furnace slag) based cement replacement technologies. “However, we also recognise that is limited by supply issues. So, if we are to transition to lower carbon geopolymer concrete, for example, or alkali-activated that’s based around GBBS, we are going to have to greatly increase the amount we import.”
That makes this approach a short-term one. “Longer term, we need more sustainable solutions, such as PFA [pulverised fuel ash, also known as fly ash] technology and then LC3 technology over time.”
LC3 is a new type of cement, based on a blend of limestone and calcined clay. According to One Planet: “LC3 can reduce CO2 emissions by up to 30%, is available in abundant quantities, is cost-effective and does not require capital-intensive modifications to existing cement plants.”
The route to net zero, Rust concedes, may require “small amounts of carbon capture-in-use technologies, such as the synthetic limes and aggregates available, to offset the small amounts of embodied carbon that we can’t take out of the system”.