Building materials from CO2

Building materials are the one utilization route where the carbon stays put. Concrete, aggregate and related products have service lives measured in decades, and the carbon fixed in them as carbonate is chemically stable rather than merely stored. That is why construction materials stand out in assessments of utilisation as capable of both using and removing CO2, where chemical and fuel routes can only reduce emissions [1].

The mineralization chemistry itself is covered under mineral storage. This page is about the product and its place in the cement and concrete industry.

Why cement is the target

Calcination of carbonate rock to make cement clinker produced 5 percent of global CO2 emissions from all industrial processes and fossil-fuel combustion in 2013 [3]. Roughly half of that is process emissions, released from the limestone itself rather than from the fuel, and process emissions cannot be removed by changing the energy supply. Demand is projected to keep rising with urbanisation [2].

Two things can be done with CO2 here. It can be injected into fresh concrete or used to cure precast elements, where it reacts with calcium phases and is fixed as carbonate. Or it can be used to carbonate aggregate, demolition waste or alkaline industrial residues that then go into concrete as a component.

The finding that complicates the claims

Concrete already absorbs CO2 on its own. Carbonation over the life cycle of cement materials, through service life, demolition and secondary use of concrete waste, is a large and growing net sink, rising from 0.10 gigatonnes of carbon per year in 1998 to 0.25 in 2013. Cumulatively, 4.5 gigatonnes of carbon were sequestered in carbonating cement materials between 1930 and 2013, offsetting 43 percent of the process emissions from cement production over the same period, and this sink is not currently counted in emissions inventories [3].

This matters directly for how CO2-curing claims should be read. A process that accelerates carbonation is partly bringing forward uptake that would have happened anyway over the material's life. The additional storage is the difference between the two, not the total carbonated, and that difference is the number worth asking about.

Feedstocks that are already mined

The most efficient version of this route uses alkaline material that has already been extracted and ground for another purpose. Blast furnace and steel slag, cement kiln dust, concrete demolition waste, coal and biomass ash, red mud and ultramafic mine tailings amount to around 7 billion tonnes produced globally each year, with an estimated CO2 storage potential of 2.9 to 8.5 billion tonnes per year by 2100 [4]. Feeding carbonated residues back into concrete closes a loop that otherwise ends in a landfill.

What it does not solve

Utilisation in concrete does not by itself decarbonise cement. The strategies assessed as deliverable within the next decade are unglamorous and supply-chain-wide: supplementary cementitious materials, optimising clinker content, and similar marginal gains whose combined effect could cut greenhouse gas emissions by up to 50 percent if all stakeholders engage [2]. CO2 utilisation is one contributor alongside those, not a substitute for them.

Open challenges

• Additionality. Distinguishing accelerated carbonation from the uptake that would have occurred naturally is the central accounting problem, and the natural sink is large [3].
• Quantity per unit of concrete. The CO2 taken up is small relative to the emissions of the cement in the same product, so the route improves a material rather than neutralising it.
• Standards and acceptance. Construction is slow-moving and risk-averse, which is a real constraint on how fast any change to a structural material propagates [2].
• Residue logistics. Alkaline residues are abundant but dispersed, and matching them to CO2 sources and concrete plants is a transport problem as much as a chemical one [4].