How blended cement, used everywhere from ancient Rome to modern London, could help counter climate change

What do the Pantheon, the Burj Khalifa and the Shard have in common?

The answer is not architecture, height or Instagram appeal. It’s what’s holding them together.

They are made from concrete, which thousands of years after its invention, is still all around us. It forms our homes and schools, roads and bridges, hospitals, and skyscrapers. But producing cement, the ingredient that binds concrete together, comes with a substantial climate cost. Cement and concrete production account for around 8 per cent of global carbon dioxide emissions.

Clinker is the key ingredient in conventional Portland cement – and a major source of its carbon footprint. Producing clinker requires heating limestone and other raw materials to very high temperatures. Carbon dioxide is released both from the energy needed to generate that heat and, crucially, through the chemical transformation of limestone itself.

But clinker does not have to do all the work.

Blended cements reduce the amount of clinker needed by combining it with other materials with cementitious properties. Depending on where and how cement is produced, these can include fly ash, slag, natural pozzolans, calcined clay and limestone. Blended cements available today can produce up to 50 per cent fewer emissions per tonne than ordinary Portland cement, with comparable strength, durability and cost.

Using blended cements is taking on increased urgency. The Global Status Report for Buildings and Construction, produced by the United Nations Environment Programme (UNEP), finds that buildings and construction account for 34 per cent of global energy-related carbon dioxide emissions. Around half of the buildings expected to exist in 2050 have yet to be built – making the materials we choose to build them increasingly important.

Today, the climate benefits of blended cement are attracting increasing attention. But the underlying idea of combining materials to make concrete perform better is anything but new. Here are five remarkable buildings that bear that out.

1. The Pantheon: an ancient idea (Rome, Italy)

[p]Almost 2,000 years after it was completed, the Pantheon still has the world’s largest unreinforced concrete dome.

Building the world’s tallest tower is one thing. Getting the concrete up there is another. Rising 828 metres above Dubai, the Burj Khalifa required concrete that was exceptionally strong, durable and workable.
Engineers achieved this by combining Portland cement with supplementary cementitious materials, creating a high-performance blended cement system that could meet the project’s demanding requirements. Concrete had to be pumped vertically more than 600 metres during construction, setting a world record at the time.
The Burj Khalifa illustrates an important point about blended concrete: these materials are not used only for environmental benefits. They are also valued for the way they can improve concrete performance in some of the world’s most challenging construction projects.

London’s Shard is instantly recognizable for its glass façade and pointed silhouette.
The building’s extensive foundations used a cement blend containing 70 per cent ground granulated blast-furnace slag, a material derived from slag generated during ironmaking.
Using the slag helped limit the early heat generated as the foundation concrete cured, reducing the risk of thermal stresses in such large concrete elements. It also reduced the amount of Portland cement required.
The result sits beneath one of Europe’s most recognizable contemporary buildings: an example of a high proportion of supplementary material being used not despite demanding engineering requirements, but partly because of them.

At 541 metres, One World Trade Center presented engineers with extraordinary structural demands. Its reinforced concrete core forms the spine of the tower, carrying gravity loads while helping it withstand wind and seismic forces. It needed exceptionally strong concrete. But stronger didn’t simply mean more cement.

Near Lausanne, a tower brings the story firmly into the present. Nearly two millennia after the Pantheon, the ingredients have changed considerably. The idea that one material doesn’t have to do all the work has not.