Eco-cement, the cheapest carbon sequestration on the planet

Cement production is one of the dirtiest industrial processes on the planet. It produces nearly 9% of global carbon emissions. This increases every year with the extraordinary demands for building materials in China and India. But it is set to become much greener: cements and concretes of the future will sequester vast amounts of carbon dioxide (CO₂) and utilise huge volumes of industrial wastes in the form of additives.

Cement (and the concretes made with it) are about to become carbon negative – absorbing more carbon that they produce. It will happen by mimicking nature – in this case, the process through which marine organisms build shells.

It is not widely appreciated that the most substantial process of carbon sequestration on the planet is accomplished by myriad marine organisms making their exoskeletons, or shells. Shells are produced biologically from calcium and magnesium ions in sea water and carbon dioxide from the air, as it is absorbed by sea water. When the organisms die, their shells disintegrate and form carbonate sediments, such as limestone, which are permanent, safe carbon sinks.

We can emulate this natural process by producing cement, concrete and other composite construction materials that are “shell-like”. They contain synthetic calcium and magnesium carbonate, and are produced at temperatures far below those employed in conventional kilns.

The calcium and magnesium have to be sourced industrially (and there are various promising routes, quite apart from mining the stuff). The carbon comes from the air – from our releases via combustion of fossil fuels and ordinary cement production. This is biomimicry in action – or what the Australian geoscientist John Harrison calls geomimicry - emulating geological processes such as weathering.

Almost all cement used today is Portland Cement (PC), a convenient and cheap material that reacts with water to bind aggregates like gravel and sand. PC was patented in 1824, and has become by far the dominant technology, ousting traditional rival construction materials.

Now China and India are gearing up for a huge industrialisation and infrastructure building effort of world-historical scale. China and India between them in the next year will use 40 times the amount of cement used in the United States.

This is surely the time to rethink the dominant technology, with its excessive dependence on fossil fuels, high temperatures and resulting carbon emissions.

There are several green alternatives to Portland cement, based in different ways on biomimicry. The most straightforward alternative is to bypass the use of limestone altogether. The other important point is to utilize magnesium in cements to give them carbon absorption characteristics.

Harrison advocates a process of carbonation of magnesium ions found in seawater, brine or other waste streams, to produce a product that uses the carbonates as aggregate and a cement to bind them that absorbs carbon dioxide as it sets. This technology eliminates the burning of limestone (calcining), a potent source of CO₂ emissions, and has the potential to sequester huge amounts of CO₂.

These “eco-cements” can be produced at much lower temperatures. This means manufacturers can use renewable sources of energy (such as solar furnaces), again eliminating carbon emissions associated with the traditional use of fossil fuels. Eco-cements absorb CO₂ as they set, making them carbon-negative if the CO₂ released during manufacture is used to make synthetic carbonate aggregate.

Magnesium cements can have greater compressive and tensile strength, greater capacity to “breathe” and to bond to cellulosic (woody) materials. They can be used to build lighter structures with better insulating properties.

Another promising approach is catalytic flash calcination technology, being developed by the Australian company Calix. This separates the calcination step in cement production from the process of producing clinker. It allows the carbon dioxide otherwise emitted to be trapped, and perhaps utilised by some subsequent industrial process.

Calcination is the oldest industrial process that we have, dating back to the development of the first fertiliser (lime produced by heating limestone was found to improve soil fertility). Developing a green version of calcination in cement production has a beautiful connection back to the very origins of our industrial civilization.

The problem is that no cement company has to date exhibited any interest in moving off the dominant Portland technology, with its cost-minimising business model. And the alternatives have not yet been able to prove themselves in commercial-scale operations. This is what has to change if India and China are to move along a different industrial trajectory.

The standard approach to carbon capture and storage involves pumping carbon dioxide through lengthy pipelines to be disposed of down mineshafts and geological reservoirs – a risky, uncertain and extremely expensive option.

By contrast, sequestering carbon in a cement that absorbs it as it cures (eliminating up to 3 billion tonnes of carbon dioxide emissions while producing the green cement) is a straightforward process. It pays for itself via the cement produced. It is a classic instance of industrial ecology at work.

There is clearly enormous scope for carbon sequestration via industrial ecology solutions rather than the simplistic “bury it and it will disappear” approach to carbon capture and storage. The production of carbon-negative cement and building materials is one of the most attractive and forceful of these industrial ecological initiatives, because it addresses the problem directly and can be scaled to the required dimensions.

Co-locating facilities that produce oil, power, cement and other industrial materials, and linking them so waste is converted into feedstock and new carbon-negative products result, is one of the most exciting aspects of industrial evolution. Australian technology can play an important role in this essential transition.

The author wishes to acknowledge extensive and rewarding discussions held with John Harrison, Principal of TecEco, and Dr Mark Sceats of Calix.