Cement could be greener, but will it?
Between 1,500 and 1,600 million tons of CO₂ was emitted from the cement process in 2018, equal to Russia’s total CO₂ emissions. Another 1,000 million tons may be emitted from fuels.
Concrete is a widely used construction material that consists of sand and pebbles glued together with cement.
That cement is made from limestone. The lime is heated to around 1450ºC, driving the CO₂ out of the stone and transforming carbonate into oxide. This cement is called Portland Cement, after the Portland quarry in Dorset on the Jurassic Coast in England from which it was first produced in 1824. Since then, the remains of ichthyosaurs have been used to build houses and roads. It is usually heated with coal, another fossil, derived mainly from plants that grew in the Devonian era.
Concrete is a versatile material, inexpensive and predictable. It does not catch fire or mould. If reinforced, it is very strong, and provides some insulation.
But there are alternatives.
The fuel used for heating can be switched from coal to gas, waste, biomass or to electric heating.
Whichever fuel is chosen, the roasting of lime still produces CO₂.
But concrete is not the only construction material. President Macron has ordered that new public buildings financed by the French state must contain 50 per cent wood or other organic material (such as hemp or straw) by 2022.
Wood can be used for load-bearing joists and for exterior walls, even on tall buildings. An 18-storey timber building was completed in 2019, north of Oslo.
When biomass is used for vehicle fuel or heating fuel, the carbon goes back into the air. When wood is used for construction, the carbon is stored for as long as the building stands.
The construction industry could in principle require other building materials or at least lower-carbon cement. But they usually don’t, as the CO₂ from cement is not included in the environmental reports of the construction companies. Skanska, the fifth biggest construction company in the world, does not even mention cement under https://group.skanska.com/ sustainability/green/priority-areas/carbon/.
Concrete is used in the foundations of buildings, where its function is to be heavy, to keep the building in place. Part of the foundation can be stone, such as granite. Wind power foundations can substitute concrete for rock, or be anchored directly to the rock.
Foam glass can provide insulation and is at least as moisture resistant as concrete.
Concrete reinforced with steel bars uses another property of Portland cement, its high alkalinity, which protects the iron from corrosion. If the iron is allowed to oxidise it will expand and create cracks in the concrete, and then widen those cracks.
If other materials are used as reinforcement, such as glass fibre, carbon fibre, plastic fibre, stainless steel or even cellulose, there is no need for an alkaline environment.
Bridges can be built of steel which – unlike concrete – is easily recyclable. They can sometimes be made of composites, i.e. plastics, which are much lighter than concrete.
Even if concrete is preferred, its carbon footprint can vary widely.
The Pantheon in Rome was constructed 1900 years ago using low-carbon concrete made from volcanic ash. (It was naturally not reinforced, so it did not rust and crack.)
Volcanic ash can be used as an additive to Portland cement, up to 50 per cent according to MIT¹. Slag from steel production and fly-ash from coal power have long been used as “supplementary cementitious materials” blended into Portland cement.
But there is much more slag and much more ash available. There are more sources: aluminium dross, waste incineration slag, rice hull ash, silica fume, all of which have high alkalinity and can be reinforced with steel.
Why is this largely unquantified source of low-carbon cement not used?
The construction industry is not very innovative by nature. It is much less dynamic than the engineering industry, where productivity and product development have been much faster. (Just look at cars.)
It is difficult to build a house; many things can go wrong, and every change means taking risks. The risk of delays, the risk of later collapse or slow deterioration, risks to health at work, as well as subsequent health risks for the users of the building.
Logistics is complicated, so it is easier to use few, well-defined and well-known materials. Ash from industrial by-products may contain hazardous metals.
Sweden used large quantities of “blue concrete” gypsum boards for several decades. They were effectively a by-product from uranium mining, and emit radon, which caused thousands of deaths due to lung cancer, and will cause many more. This was a risk that should have been foreseen.
But a building material that is unfit in one place may be perfectly acceptable somewhere else. Living-room walls, bridges, rail sleepers, parking lots, harbours, airstrips … they all have different requirements regarding toxicity, strength, resistance to rain and salty winds etc.
With more detailed specifications for each use, the CO₂ footprint can be reduced by using more substitutes for Portland cement, which often require less cement per ton of concrete.
Why has this not happened? The answer is simple: it is cheap because the price does not include its environmental costs.
In the EU, the cement industry is part of the °C trading system. Sort of. It gets free allocations, i.e. it is paid back for all its emissions. In 2018, the cement industry received 114 million tons of free allocations
and emitted 111 Mt. Some plants actually pay for some of their emissions, but over-allocation is normal. The allocation is (in theory) benchmarked in line with the 10 per cent best performers, but this obviously does not work in practice. It is justified on the grounds of carbon leakage, i.e. the threat that if Europe and cement producers had to pay for their emissions, they would be at a disadvantage to outside competition.
The evidence for such a threat is slim². Cement is a cheap, voluminous product which is normally not transported very far. A Sandbag report summed it up “For cement, free allocation is a solution to a problem that does not exist since the sector has experienced no carbon leakage.” ³
Sandbag has noted that the industry’s carbon intensity rose between 2005 and 2014 and that the present system “offers inadequate short- and long-term incentives to reduce carbon emissions. It … makes investment in low-carbon cement unattractive.”
The cement industry – Cembureau and individual companies – has lobbied hard in Brussels and elsewhere, with great success. They lobby hard because they need to. Cement factories are usually built close to quarries. They use big mining, big kilns, big harbours and big ships. They can’t move. They can’t do anything else. So they will use all their market power and political influence to keep things as they are as long as possible. As things stand, they will keep free allocations through 2030.
As the climate debate increasingly focuses on 1.5 degrees C, the cement industry has to find some context where Portland cement can appear Paris-compatible.
How could that be done?
The International Energy Agency relies on CCS for 83 per cent of cumulative emissions reductions in the cement sector in its Energy Technology Perspectives 2017.
CCS features high on Cembureau’s low carbon web page⁴. This is in fact the only way they address the core problem, i.e. the CO₂ from lime. The rest are either things that may happen in the future (improved energy efficiency, less carbon-intensive fuels) or are up to somebody else (product efficiency and “downstream”).
Cement plants can produce a large and relatively pure stream of CO₂, so there are few places better for CCS. But nobody believes CCS will pay for itself, at least not Heidelberg Cement, which lobbies for billions of euro in government support in Norway and Sweden. A typical estimate says CCS would increase costs by over 50 per cent⁵.
A Chatham House report⁶ enumerates six alternatives to Portland cement with a potential to mitigate CO₂ by 50–100%.
- Low-clinker Portland (ash, slag etc.)
- Geopolymers (clay)
- Low-carbonate clinker with calcium silicates
- Belite clinkers
- Calcium silicate clinkers
- Magnesium-based cements
Several are now produced on an industrial scale. Costs vary with location, but are thought to be about the same as now. That would mean that much of the problem could probably be solved surer, cheaper and faster than with CCS.
There are still more options.
Another way to cut the use of cement and its emissions is to use less of it in concrete, with more fine-tuned design of buildings and concrete mixes. Some of the clinker can also be replaced with lime powder, which is mined in the same way but does not go through the kiln.
Nature, and man, have developed many ways to glue sand and pebbles together to make a strong and durable mass. Even living bacteria can be used for this purpose. The cohesion of naturally occurring materials can be quite impressive; 1900 million-year-old Scandinavian granite is still in good shape.
- Healy et al https://www.mdpi.com/1996-1073/11/5/1231