January 20, 2012 10:04 pm

Solid solutions

Can the development of carbon-negative cement clean up the heavily polluting construction industry?

In the early 1900s, inventor Thomas Edison had a flash of inspiration: concrete homes, cast in one piece. Concrete bathtubs and beds would be integral to the design. Occupants could even play concrete pianos. Unsurprisingly, such homes weren’t as successful as Edison’s light bulbs. Few were made – pouring concrete into a mould the size of a house proved tricky – but some still stand in New Jersey.

Concrete is made from cement mixed with water, sand and aggregate, such as gravel. The cement that had become popular in Edison’s day – Portland cement – was invented in England in 1824 and named after rock from the Isle of Portland that it resembled. Edison owned one of the first cement works in the US and needed customers. Concrete homes would require tonnes of the material. As for marketing, the Edison Portland Cement Company published a book, The Romance of Cement.

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Our affair endured. Portland cement and concrete paved the way for America’s growth, for its dams and skyscrapers, and today is one of the most commonly used building materials worldwide. Strong, long-lasting, cheap (about 15 cents per kg) and ... everywhere. According to Cembureau, the body that represents the industry in Europe, more than 3bn tonnes of Portland cement were manufactured in 2010. That’s enough for 30bn tonnes of concrete, or about 4 tonnes per person. By 2050, worldwide production is estimated to reach 4.4bn tonnes.

This causes concern for some in the construction industry. “Whole mountains are being ground down to get the raw ingredients of cement,” says Rob McLeod, a designer of Passivhaus Homes (which need no or few fossil fuels to heat or cool). “Using one tonne of [Portland] cement results in nearly one tonne of CO2 being released into the atmosphere,” he says.

The Portland cement industry acknowledges that it is responsible for 5 per cent of man-made carbon dioxide emissions worldwide, 3 percentage points more than aviation (although aviation also produces other greenhouse gases and, at altitude, they are three times as damaging). “Portland cement requires that limestone be heated to 1,450 centigrade,” says Paul Tennis of the Portland Cement Association, which represents the industry in the US. “About 60 per cent of [CO2] emissions are due to the chemical reaction that occurs when limestone – calcium carbonate – is burnt.” Heating the kiln to such a high temperature – with fossil fuels – accounts for most of the remaining 40 per cent.

The most obvious way to reduce emissions is to use less of the stuff. Passivhaus is constructing homes with little or no concrete by building them on wooden pilings with small concrete foundations (or used tyres and rocks), rather than the conventional slabs that require 40 tonnes of concrete for a typical semi-detached house.

Reducing consumption is not a strategy that the cement industry is willing to consider. Its first line of attack is to improve fuel efficiency. Lafarge, one of the world’s largest cement companies, makes 150m tonnes of cement annually. It claims to have reduced CO2 emissions by 20 per cent over 20 years: from 774kg per tonne in 1990 to 606kg per tonne in 2010.

“We’ve refurbished old plants to make them more efficient and use less fossil fuel by burning alternatives, like old tyres, and biomass, such as rice husks,” says Vincent Mages, vice-president of climate change initiatives at Lafarge.

Portland cement can also be blended with waste (Mages prefers to call it “byproducts”) from other industries – fly ash from coal-fired power stations and blast-furnace slag from the iron industry. They contain minerals that, in the presence of an activator such as a small amount of Portland cement, form a “hydraulic cement” (one that can harden underwater).

“Through blending, [CO2] emissions from cement can be reduced by 40 per cent [compared to ordinary Portland cement]; a waste product is diverted from landfill and more mining is avoided,” says Mages. But there’s only so far you can go in reducing emissions when the chemical reaction that creates cement produces CO2.

A more exciting new product from Lafarge is Aether, which has 25-30 per cent fewer CO2 emissions than ordinary Portland cement. It can be made in existing plants with the same raw materials but, crucially, needs less energy. Aether is just one of “a handful” of alternatives that emit less CO2 than Portland cement, says Dr Martin Schneider of the European Cement Research Academy (ECRA). One alternative cement that “excites” him is Celitement, claimed to emit 50 per cent less CO2 than ordinary Portland cement in its manufacture.

Developing “alternative cements” is of increasing interest (ECRA held its first conference on the topic in May 2011). After all, only a quirk of history led to the use of Portland cement. Other cements suitable for mortar have been around for millennia. They may not suit all of today’s construction: Roman concrete, used to make the Pantheon, would have needed a year or more to fully harden.

With the growth of a low-carbon building industry, an alternative class of cements, called geopolymers, may soon be widely found in buildings and pavements. Australian company Zeobond has been developing E-Crete ready-mix concrete. It is made with a geopolymer cement that, claims chief executive Peter Duxson, emits up to 80 per cent less CO2 than ordinary Portland cement in its manufacture.

Zeobond started making geopolymer paving slabs in 2008. Now a library in Melbourne has been specified solely in E-Crete for its walls and pavement. The company produces 3,000 tonnes a year of E-Crete (the average Portland cement plant produces 1m tonnes of cement annually, enough for up to 10m tonnes of concrete) but “economies of scale hinder growth”. It’s a perennial problem with materials that haven’t stood the test of time. “No one wants to be the first to build with them,” says Duxson. “If you were going to build a 100-storey building tomorrow, I’d recommend you use ordinary concrete in the support columns but for the walls and driveway, let’s use a substitute with a better emissions profile.”

One product seems to trump the lot. UK-based Novacem is developing a cement that is, it claims, carbon negative. Based on magnesium silicates rather than calcium carbonate, Novacem claims it absorbs 30kg-100kg of CO2 per tonne. A pilot plant is currently producing 4-5 tonnes of the cement annually. Lafarge has invested £1m. By 2015 Novacem hopes to be making 25,000 tonnes. Raw materials are abundant at 20 tonnes worldwide, says chief executive Stuart Evans, and Novacem cement could be manufactured in existing plants with a retrofit. “It would ‘just’ need the equivalent of a heart and lung transplant.”

Perhaps Novacem will breathe new life into the heavily polluting cement industry and transform it into one that tackles the problem of CO2 emissions. Some experts are yet to be won over. It’s still very early days in the concrete jungle.

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Build like an Egyptian

Are some of the building blocks of the Egyptian pyramids concrete rather than hewn rock? It’s a controversial theory but one that is gaining ground. Nuclear magnetic resonance studies by the University of Warwick appear to show that rock from Senefru’s Bent Pyramid is, in fact, man-made. “The ancient Egyptians knew how to make cement from local minerals and used it to bind together stones, forming a crude concrete,” says Dr John Hanna, who co-authored a 2011 paper in Materials Letters.

This ancient Egyptian cement was made from aluminosilicates and alkaline solution, says Hanna. “It was the forerunner of the modern geopolymer cements that are being developed today.”

Professor Joseph Davidovits, of the non-profit Geopolymer Institute in France, was one of the first to propose that Egyptians knew how to make cement and thus concrete. He coined the word geopolymer in the 1970s. These cements – not based on limestone – are fire and chemical-resistant.

Niche products for use in the home are being developed and some are commercially available such as a new chemical and stain-resistant grouting from BASF called PCI Geofug.

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Details

● Portland Cement Association, www.cement.org

● Lafarge, www.lafarge.com

● European Cement Research Academy, www.ecra-online.org

● Celitement, www.celitement.com

● Geopolymer Institute, www.geopolymer.org

● Zeobond, www.zeobond.com

● Novacem, www.novacem.com

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