Delivering carbon reduction in concrete
The world needs smart, sustainable and resilient infrastructure but challenges like climate change and limited resources mean that the production and use of heavy building materials needs to evolve.
Ready-mixed concrete is the most widely used building material in the world and is essential to society as a cornerstone of our built environment. It is flexible, versatile, durable and strong, and is used in a wide variety of applications including housing, commercial buildings, road construction and major infrastructure projects such as bridges, tunnels and airports as well as energy and water plants.
It also absorbs CO₂ throughout its life and is 100 per cent recyclable, contributing significantly to the circular economy and providing materials with lower embodied carbon.
However, it is no secret that the production of CEM I Portland cement (PC), which is a vital ingredient in concrete, is energy intensive and produces CO₂ due to the calcination of limestone and the fuels used to heat the kiln.
Heidelberg Materials UK has invested heavily to reduce process emissions through improved efficiencies and increasing the amount of alternative fuels used. But almost 70 per cent of the CO₂ emissions produced arise from the chemical reactions that take place during manufacture, so cannot be reduced by using low carbon or renewable energy sources.
That is why the company is also leading the way in developing carbon capture and storage facilities, that will prevent these emissions from entering the atmosphere in the first place. However, until this technology is widely available, developing and using supplementary cementitious materials (SCMs), which reduce the CO₂ impact of concrete by replacing some of the cement content, remains vital to minimising carbon emissions.
Best known and most effective SCM
Ground granulated blastfurnace slag (GGBS) – a by-product of iron and steel making – is one of the best known and most effective SCMs and has been used for over a century, with the first British Standard for Portland Blastfurnace Cement published in 1923 .
GGBS can be used almost anywhere concrete is needed – from sustainable housing development and soil stabilisation to wind farms and road building – and is typically used in major construction projects such as bridges and sea defences. It can replace a substantial part of the PC content in concrete – up to around 70 per cent, but can be even higher in special applications.
It is this high replacement level that means GGBS is the most effective SCM in terms of delivering carbon reduction as others, such as pulverised fuel/fly ash (PFA), calcined clays and limestone fines, can only be used to lower replacement levels without impacting on the performance of the concrete.
In addition to the environmental benefits to be gained from its use in both its production and throughout the life of the structure, GGBS does not require the quarrying of new materials and the slag used will not be disposed of as landfill.
Clarity on the environmental performance of Heidelberg Materials UK’s GGBS – evoBuild low carbon GGBS (previously known as Regen GGBS) – has been provided with a new Environmental Product Declaration (EPD). It has been published by leading EPD programme operator EPD-Norge, in accordance with EN 15804, and confirms the Global Warming Potential (GWP) of evoBuild low carbon GGBS, including an economic allocation for the granulate and the emissions from transport of imported granulate, is 155kg CO₂e/t. This compares to 840kg CO₂e/t for the average GWP for CEM I in the UK.
Added strength and durability
The use of GGBS is also proven to increase the long-term durability of concrete, further cutting a project’s environmental impact by reducing the amount of repair and maintenance needed and extending the service life of concrete structures.
The heat generated by concrete as it hydrates – particularly in large or deep pours, can affect a structure’s overall strength and durability. Concrete containing GGBS generates a lower heat of hydration compared to cement only concretes, minimising the production of heat and reducing the risk of thermal cracking. The greater the proportion of GGBS in the mix; the greater the reduction in temperature rise and the rate at which heat is developed.
GGBS also:
• enhances resistance to both the ettringite and thaumasite forms of sulfate attack;
• lowers ingress of chlorides into a structure;
• minimises the risk of alkali silica reaction;
• provides better resistance to acid attack;
• lowers propensity to delayed ettringite formation.
Secure long-term supply
In recent years the UK’s domestic iron and steel making industries have diminished and now just one steel works with blastfurnace operations remains in Scunthorpe. As a result, Heidelberg Materials needs to import granulated blast furnace slags from other countries.
Yet, despite the CO₂ emissions associated with transportation, importing into the UK is still a very efficient use of the granulate, which is a traded commodity that benefits the UK economy.
This is because the UK has several deep-sea ports which can accommodate large, 40,000 tonnes ships, to efficiently import granulate to grinding facilities which are all based in coastal regions. The UK also tends to have shorter inland transport distances than other countries, helping to keep transport-related CO₂ emissions to a minimum.
Heidelberg Materials UK has been importing granulate for decades and has wide experience of producing GGBS from a variety of sources. Through its parent company, it has access to the global granulate market for both long term contracts and spot requirements.
The anticipated long-term trend of a reduction in global blast furnace steel making cannot be ignored, however Heidelberg Materials UK has a safe and secure supply for its anticipated granulate demand for at least the next decade. To satisfy the changing demand for lower carbon concrete solutions, Heidelberg Materials is also proactively assessing and developing future SCM options, which include calcined clays and natural pozzolans.
