Climate change: The massive CO2 emitter you may not know about.
Concrete is the most widely used man-made material in existence. It is second only to water as the most-consumed resource on the planet.
But, while cement - the key ingredient in concrete - has shaped much of our built environment, it also has a massive carbon footprint.
Cement is the source of about 8% of the world's carbon dioxide (CO2) emissions,according to think tank Chatham House.
If the cement industry were a country, it would be the third largest emitter in the world - behind China and the US. It contributes more CO2 than aviation fuel (2.5%) and is not far behind the global agriculture business (12%).
Cement industry leaders were in Poland for the UN's climate change conference - COP24 - to discuss ways of meeting the requirements of the Paris Agreement on climate change. To do this, annual emissions from cement will need to fall by at least 16% by 2030.
So, how did our love of concrete end up endangering the planet? And what can we do about it?
In praise of concrete
As the key building material of most tower blocks, car parks, bridges and dams, concrete has, for the haters, enabled the construction of some of the world's worst architectural eyesores.
In the UK, it helped the massive wave of post-World War Two development - much of it still dividing opinion - with several of the country's major cities, such as Birmingham, Coventry, Hull and Portsmouth, largely defined by the concrete structures from that building push.
But concrete is also the reason some of the world's most impressive buildings exist.
Sydney Opera House, the Lotus Temple in Delhi, the Burj Khalifa in Dubai as well as the magnificent Pantheon in Rome - boasting the largest unsupported concrete dome in the world - all owe their form to the material.
A mix of sand and gravel, a cement binder and water, concrete is so widely embraced by architects, structural engineers, developers and builders because it is a remarkably good construction material.
"It's affordable, you can produce it almost anywhere and it has all the right structural qualities that you want to build with for a durable building or for infrastructure," explains Felix Preston, deputy research director at the Energy, Environment and Resources Department at Chatham House.
Despite known durability problems with using steel reinforcement, which can crack concrete from the inside, it is still the go-to material across the world.
"Building without concrete, although it is possible, is challenging," says Mr Preston.
Growth of cement industry
It is these unrivalled attributes of concrete that have helped boost global cement production since the 1950s, with Asia and China accounting for the bulk of growth from the 1990s onwards.
Production has increased more than thirtyfold since 1950 and almost fourfold since 1990. China used more cement between 2011 and 2013 than the US did in the entire 20th Century.
But with Chinese consumption now appearing to level off, most future growth in construction is expected to happen in the emerging markets of South East Asia and sub-Saharan Africa - driven by rapid urbanisation and economic development.
The floor area of the world's buildings is projected to double in the next 40 years, say Chatham House researchers, requiring cement production to increase by a quarter by 2030.
Concrete has a long history
While many of us assume concrete is a recent addition to our cities, architects, and builders have actually been using cement-like binders for millennia.
The earliest use is believed to have been more than 8,000 years ago, with traders in Syria and Jordan using such binders to create floors, buildings and underground cisterns.
Later, the Romans were known to be masters of cement and concrete, building the Pantheon in Rome in 113-125AD, with its 43m-diameter free-standing concrete dome the largest in the world.
But the concrete used in our modern-built environment owes much of its make-up to a process patented in the early 19th Century by bricklayer Joseph Aspdin of Leeds.
His new technique of roasting limestone and clay in an oven and then grinding it to a powder to make "artificial stone" is now known as Portland cement - still the key ingredient in almost all modern concrete.
But, despite its ubiquitous presence, concrete's environmental credentials have come under increased scrutiny in the last couple of decades.
Not only does the production of Portland cement involve quarrying - causing airborne pollution in the form of dust - it also requires the use of massive kilns, which require large amounts of energy.
The actual chemical process of making cement also emits staggeringly high levels of CO2.
The sector has made progress - improvements in the energy-efficiency of new plants and burning waste materials instead of fossil fuels has seen the average CO2 emissions per tonne of output fall by 18% over the last few decades, according to Chatham House.
The newly-established Global Cement and Concrete Association (GCCA), currently representing about 35% of the world's cement production capacity and with a focus on sustainable development, was at COP24.
Chief executive Benjamin Sporton says the fact the organisation now exists "is a demonstration of the commitment of the industry to sustainability, including taking action on climate change".
The GCCA is due to publish a set of sustainability guidelines, which its membership will have to follow.
"By bringing together global players to provide leadership and focus, as well as delivering a detailed work programme, we can help ensure a sustainable future for cement and concrete, and for the needs of future generations," Mr Sporton says.
But despite the promise, Chatham House argues that the industry is reaching the limits of what it can do with current measures.
If the sector has any hope of meeting its commitments to the 2015 Paris Agreement on climate change, it will need to look at overhauling the cement-making process itself, not only reducing the use of fossil fuels.
'Clinker' - the big polluter
It is the process of making "clinker" - the key constituent of cement - that emits the largest amount of CO2 in cement-making.
- 1. Raw materials, mainly limestone and clay, are quarried and crushed
- 2. They are ground and mixed with other materials - such as iron ore or ash
- 3. They are fed into huge, cylindrical kilns and heated to about 1,450C (2,640F)
- 4. The process of "calcination" splits the material into calcium oxide and CO2
- 5. A new substance called clinker emerges as marble-sized grey balls
- 6. The clinker is cooled, ground and mixed with gypsum and limestone
- 7. The cement is transported to ready-mix concrete companies