Concrete and Steel: Decarbonizing the Structure

The physical bulk of a data center is concrete and steel, and both are among the most carbon-intensive materials civilization produces. Cement manufacturing alone is responsible for roughly eight percent of global carbon dioxide emissions, and iron and steel add several percent more. When you pour the foundations, raise the structure, and build the equipment pads for a large facility, you are ordering thousands of tonnes of these materials, and the emissions from making them are booked to your project's Scope 3. Understanding where those emissions come from is the first step to cutting them.

Concrete's carbon problem lives in the cement, and specifically in the clinker. Clinker is the reactive binder made by heating limestone in a kiln to around 1,450 degrees Celsius, and it emits carbon dioxide twice over. First, the fuel burned to reach that temperature releases combustion emissions. Second, and unavoidably, the chemical reaction that turns limestone into clinker liberates carbon dioxide from the limestone itself; this is process emission, and it happens no matter how clean your kiln fuel is. Roughly 60 percent of cement's emissions are process emissions of this kind, which is why you cannot solve concrete's footprint simply by electrifying the kiln. You have to use less clinker.

That is exactly what low-carbon concrete does. The dominant lever is supplementary cementitious materials, SCMs, which replace a portion of the clinker with substances that provide similar binding properties at far lower emissions. The traditional SCMs are fly ash, a byproduct of coal power, and ground granulated blast-furnace slag, a byproduct of steelmaking; both are industrial wastes given a second life, which makes them circular as well as low-carbon. As coal plants close, fly ash grows scarcer, pushing the industry toward alternatives such as calcined clays, notably the limestone calcined clay cement known as LC3, which can cut the clinker fraction substantially using abundant raw materials. Mix design also matters: optimizing the recipe, allowing longer curing times, and specifying performance rather than prescriptive cement content can trim embodied carbon meaningfully without new technology. Further out, carbon-cured concrete injects captured carbon dioxide into the mix where it mineralizes and is permanently stored, though this remains a smaller lever today.

Steel's decarbonization follows a parallel logic of changing how the material is made. Conventional primary steel is produced in a blast furnace that uses coke, a coal derivative, to chemically reduce iron ore, and that reduction step is intrinsically carbon-emitting. There are two main paths to greener steel. The first is the electric arc furnace, which melts recycled scrap steel using electricity; if that electricity is clean, recycled steel carries a small fraction of the emissions of primary steel, which makes recycled content the single most accessible lever for a builder. The second, more transformative path is hydrogen direct reduction, which replaces coke with hydrogen as the reducing agent; when the hydrogen is made with clean electricity, the process emits water vapor rather than carbon dioxide. Commercial green-steel ventures in Sweden and elsewhere have delivered the first batches of hydrogen-reduced steel, and while volumes are still small and prices carry a premium, the trajectory is real.

There is a subtlety worth flagging about the byproduct-based SCMs, because it has real planning consequences. Fly ash and blast-furnace slag are wastes from coal power and traditional steelmaking, the very industries that decarbonization is designed to shrink. As coal plants retire and steelmakers move to hydrogen and electric-arc routes, the supply of these particular SCMs will tighten over the coming decades. That is a good outcome for the climate overall, but it means a builder cannot assume today's cheap slag cement will be available for the next project, and it is one reason the industry is investing in calcined clays and other SCMs that do not depend on dirty upstream industries. The prudent reading is to treat fly ash and slag as a transitional resource and to design supply relationships around the materials that will still exist once the industries that produce today's byproducts have themselves been transformed.

For a builder, the practical program is straightforward to state even if procurement is hard: specify high recycled content and electric-arc-furnace steel where structural requirements allow; specify low-clinker, high-SCM concrete mixes and give the schedule room for the longer curing they sometimes need; and demand Environmental Product Declarations, EPDs, from suppliers so the embodied carbon of each material is documented rather than assumed. The EPD is the receipt that lets the reported Scope 3 number rest on measured data instead of industry averages. My read is that the low-carbon versions of these materials are no longer exotic; the binding constraint is now regional supply and the willingness to specify them, which loops back to siting, because whether a green-steel mill or an SCM source sits within economic shipping distance depends, once again, on where you choose to build.