Steel & Metals

Steel earns its place in almost every building because, per unit of strength, nothing else quite matches it: it is very strong in both tension and compression, stiff, ductile (it bends before it breaks, which is vital for safety), manufactured to precise and dependable properties, and it works in partnership with concrete - steel reinforcement gives concrete the tensile strength it lacks, making reinforced concrete possible. From rebar in every slab and foundation to the frames of towers, bridges and wide-span roofs, steel is woven through construction. In carbon terms, that ubiquity means steel is, alongside concrete, one of the two dominant material carbon flows in a typical building's structure.

The reason steel is high-carbon per tonne is the nature of making it. Iron does not occur as metal; it occurs as iron ore, an oxide, and turning that ore into metallic iron and then steel takes enormous heat and a chemical reducing agent to strip the oxygen away. Historically and still predominantly that reducing agent is carbon, from coke made of coal, and the heat too has come from fossil fuel - so primary steel-making emits large quantities of CO2 both from the chemistry of reduction and from the energy. A tonne of primary steel carries substantially more embodied carbon than a tonne of cement, which is why steel is often the highest-carbon material in a steel-framed building even though there is less of it by weight than concrete.

But - and this is the pivot the rest of the lesson turns on - steel has a property concrete's binder does not: it is endlessly and losslessly recyclable, and it can be made by a completely different, far lower-carbon route from scrap. Steel does not have to be born from ore. That single fact means steel's carbon is not one number but a range, set by how it was made, and it means the metal that is a big hitter today is also one of the more decarbonisable structural materials over time. So with steel the key question is never just 'how much steel' but 'how much, and made which way'.

There are two main ways to make steel, and the difference between them is the single most important fact about steel carbon. The primary route starts from iron ore: ore and coke (coal) go into a blast furnace, which reduces the ore to molten iron, and that iron is then refined into steel in a basic oxygen furnace (the BF-BOF route). It uses coal both as the chemical reducing agent and as fuel, and it is very carbon-intensive - this is the classic, high-carbon way steel has been made at scale. The recycled (or secondary) route starts from steel scrap: scrap is melted in an electric arc furnace (EAF) and reformed into new steel. Because it skips the ore-reduction chemistry entirely - the iron is already metal - and is powered by electricity, it is far lower-carbon, and its carbon falls further as the electricity that runs it comes from renewables.

The carbon gap between the two routes is large - recycled steel can carry a small fraction of the carbon of primary steel - which is why the recycled content and production route of specified steel is a genuine carbon lever, and why an EPD that states the route and recycled content matters. Reinforcing bar (rebar) is often made by the EAF route and can be relatively low-carbon; heavy structural sections vary. Specifying steel with high recycled content, made by the EAF route, is one of the clearer material-carbon wins available.

But honesty requires two caveats that the next lesson and Module 5 develop. First, there is not enough scrap in the world to make all our steel the recycled way - global steel demand outruns available scrap, so a large share of steel must still be made from ore for now, and 'just use recycled' cannot scale to everything. Recycled steel is genuinely low-carbon but its supply is limited by how much scrap exists. Second, recycled content figures and route claims must be verified, not assumed - which is what EPDs are for. And in India, where the steel industry has historically been coal-heavy and the route mix differs from Europe, the local carbon of steel is its own question. The principle holds everywhere: with steel, the route is most of the carbon - so ask which route, and get it verified.

Steel is not the only metal in a building, and one of the others deserves special attention because it is, per tonne, the highest-carbon common construction material of all: aluminium. Aluminium is everywhere in modern buildings - window and curtain-wall frames, cladding panels, louvres, trims, some structures - because it is light, strong, corrosion-resistant and easily formed. But making primary aluminium from its ore (bauxite, refined to alumina, then smelted) is astonishingly electricity-hungry: the smelting process uses electrolysis to tear aluminium out of its oxide, consuming huge amounts of electrical power. Aluminium is, in effect, solidified electricity - so its embodied carbon depends almost entirely on how clean the electricity used to smelt it was. Primary aluminium made with coal power is extremely carbon-intensive, well above steel per tonne; aluminium smelted with hydro or renewable power is far lower, and recycled aluminium - remelting scrap - is dramatically lower still, since it skips the smelting.

This gives aluminium the same shape of story as steel, only sharper: the route and the electricity source swing its carbon enormously, primary-versus-recycled matters hugely, and recycled aluminium is a strong low-carbon choice while primary coal-smelted aluminium is one of the worst. Because aluminium is used in relatively small quantities compared with structure, its total contribution to a building is usually smaller than steel or concrete - but its extreme per-tonne intensity means that lots of aluminium (a heavily aluminium-clad facade, say) can still be a meaningful carbon line, and specifying high-recycled-content aluminium matters.

The broader lesson generalises: for metals, embodied carbon is dominated by the energy and chemistry of winning the metal from its ore, so primary metal is high-carbon and recycled metal is far lower, and the cleaner the electricity, the lower both become. Copper, zinc and other building metals follow the same logic to varying degrees. The design responses are consistent - use less metal, specify high recycled content and clean-energy routes, and value the recyclability that lets metals be recovered at end of life (a theme for Module 7's circular thinking). Metals are high-carbon, but they are also, uniquely, the materials whose carbon most improves as the world's electricity cleans and as we recover and reuse what we have already made.

Pull the metals picture together into practice. Steel and metals are high-carbon per tonne, but their carbon is more movable than cement's because it depends on route and electricity, both of which are improving. The levers, in the order that usually matters: first, use less metal - a lean, efficient structure uses less steel just as it uses less concrete (Lesson 4.1, Module 6), and right-sizing sections and connections cuts tonnage directly. Second, specify the low-carbon route - high recycled content, EAF-made steel and recycled aluminium, verified by EPDs that state the route and recycled content. Third, design for recovery - steel and metals are highly recyclable, and detailing for eventual disassembly and reuse (bolted rather than welded, accessible connections) keeps that value alive for the future (Module 7.3). And fourth, at the whole-life scale, reusing existing steel structure avoids the carbon entirely.

A caution on recycled content, because it is widely misunderstood and greenwashed: because global scrap is limited, we cannot make all steel from scrap, and claiming a low number simply by pointing at recycled content can be misleading. What matters is the actual verified carbon of the actual product by its actual route, taken from an EPD - not a generic 'recycled steel is green' assumption. The most honest, robust move remains using less steel; route and recycled content are strong secondary levers, best confirmed with data.

And the firm boundary, as ever: this lesson gives you the principle - metals are high-carbon, the route (primary versus recycled) and the electricity dominate, aluminium is highest per tonne, and the levers are less metal, lower-carbon route, and recovery. It does not give you your building's numbers. The embodied carbon of your specified steel and aluminium depends entirely on the product, its route, its recycled content, the plant and the grid that powered it, and must come from verified EPDs for those actual products, applied by your engineer and a qualified LCA or carbon specialist against the recognised standards. The comparative figures here are illustrative and relative, meant to teach the shape - primary aluminium worst per tonne, primary steel high, recycled steel much lower, concrete low per tonne but used in vast volume - not to be quoted as your project's carbon. Ask which route, use less, and get the real numbers from the method.