Material Insights · Understanding the Material World

Using energy: food, materials, mobility and buildings: Materials: steel, cement, plastics and aluminium

Understanding Energy / Module 11: Using energy: food, materials, mobility and buildings Part 3 of 5

Materials: steel, cement, plastics and aluminium

Industry is the largest user of final energy, and a few materials dominate it. Smil names four pillars of modern civilisation: steel, cement, plastics and ammonia. Add aluminium and silicon and you have most of industry's energy bill and a large share of its CO2. This part looks at what each costs in energy and why.

3.1 A few bulk materials carry most of industry's energy bill

History names its ages after materials: stone, bronze, iron. Smil argues that we still live in a material age, just a much bigger one. By mass, the largest flows are construction minerals: sand, gravel and crushed stone, tens of billions of tonnes a year, most of it bound into concrete and roads. After that come cement, steel, plastics and ammonia. In recent years the world has made about 3.8 to 3.9 billion tonnes of cement a year, about 1.85 billion tonnes of steel, around 430 million tonnes of plastics, about 185 million tonnes of ammonia and about 73 million tonnes of primary aluminium. Global material extraction rose roughly tenfold during the twentieth century.

Energy cost per tonne varies over a huge range. Lumber and cement need under 5 GJ per tonne. Steel needs about 20. Copper about 45. Plastics need 80 to 120, much of it as the oil or gas that becomes the polymer itself. Primary aluminium needs over 150, and the industry's own mine-to-metal figure is about 186, mostly as electricity.

Multiply cost by volume and you see where the energy goes. Cement is cheap per tonne but made in such quantities that it adds up to many exajoules. Aluminium is made in far smaller amounts, but its cost per tonne is so high that it is one of the world's largest single industrial users of electricity. Steel combines high volume with moderate intensity and is the largest of all.

Per person use rises steeply as a country builds its housing, roads, rail and power grids, then levels off once the stock exists. Rich countries hold enormous material stocks. India is still in the building phase. Its per capita use of steel and cement is a fraction of China's, which means its industrial energy demand will keep rising for decades whatever happens to efficiency.

Data The four pillars and two more: output and energy cost
MaterialRecent world outputEnergy per tonneMain energy form
Cementabout 3.9 billion t (2024)3 to 5 GJcoal, petcoke, gas in kilns
Steelabout 1.85 billion t (2025)about 20 GJ (ore route)coking coal, electricity for scrap
Plasticsabout 430 million t (2024)80 to 120 GJoil and gas, mostly as feedstock
Ammoniaabout 185 million t (2020)28 GJ best, about 41 GJ averagenatural gas or coal
Primary aluminiumabout 73 million t (2025)about 150 to 190 GJelectricity for smelting
Copperover 20 million tabout 45 GJdiesel for mining, electricity

Output figures from worldsteel, USGS, Plastics Europe, IEA and IAI; energy figures are typical ranges from Smil, with ammonia from the IEA and aluminium from IAI.

Source: US Geological Survey, Mineral Commodity Summaries 2026: Cement, https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-cement.pdf (world about 3,900 Mt in 2024 and 3,800 Mt in 2025 estimated; China 1,800 and 1,700 Mt)

3.2 Steel and cement are energy-hungry because chemistry, not just heat, is involved

Steel starts as iron ore, which is iron bonded to oxygen. To get the metal, the oxygen must be pulled off. In a blast furnace, coke made from coking coal does two jobs: it burns to give heat above 1,500 degrees Celsius, and its carbon grabs the oxygen from the ore, leaving molten iron and releasing CO2. A basic oxygen furnace then blows oxygen through the iron to cut its carbon content and make steel. This chain, still the source of about 70% of world steel, uses roughly 20 GJ per tonne of finished steel in a good integrated mill. That is about a tenth of the 200 GJ per tonne of pig iron needed around 1800, one of the great efficiency stories of industry. Steel still accounts for roughly 7 to 9% of global CO2 emissions.

The main alternative is the electric arc furnace, which melts scrap with electricity and needs only a fraction of the energy, because the oxygen was removed long ago. Smil estimates that of about 51 billion tonnes of steel made between 1850 and 2015, about 15 billion tonnes was still in use. That stock is a future scrap mine.

Cement has a similar double burden. Limestone (calcium carbonate) is heated to about 1,450 degrees in a kiln to make clinker. The heat comes from fuel, but the reaction also splits CO2 straight out of the limestone. About 60% of cement's emissions come from this chemistry, not from combustion, so clean heat alone cannot fix it. Cement makes up about 7 to 8% of global CO2.

The scale is set by China. At its peak in the late 2010s it made close to 60% of the world's cement, and it still made about 45% in 2024. In just two years it used about 4.7 billion tonnes, more than the United States used in the whole twentieth century.

Process flowchart From iron ore to steel in an integrated mill
Inputs Iron oreabout 1.5 t Coking coalabout 0.7 to 0.8 t, made into coke Limestone and oxygenfluxes and refining gas Process steps 1 Coke oven

coal baked into strong porous coke; gas recovered

2 Blast furnace

coke burns for heat and strips oxygen from the ore; molten iron and CO2

3 Basic oxygen furnace

oxygen blown in to cut carbon; scrap added

Outputs Crude steel1 tonne, about 20 GJ in total CO2about 2 tonnes

Rounded typical values for a good blast furnace mill. A scrap-fed electric arc furnace needs a fraction of this energy.

Source: Smil, Still the Iron Age: Iron and Steel in the Modern World (2016), via Smil Mastery Course, M11 'Steel today: energy, stocks and the future'
  • Smil, Still the Iron Age: Iron and Steel in the Modern World (2016) · via Smil Mastery Course, M11 'Steel today: energy, stocks and the future'
  • Smil, Numbers Don't Lie (2020) · via Smil Mastery Course, M11 'Cement and concrete'
  • Smil, Making the Modern World (2014) / Materials and Dematerialization (2nd ed., 2023) · via Smil Mastery Course, M11 'Cement and concrete'
  • Global Efficiency Intelligence, 'Global cement industry's GHG emissions', 2021 · web
  • US Geological Survey, Mineral Commodity Summaries 2026: Cement web ↗ · web

3.3 Plastics, aluminium and silicon trade small mass for very high energy per tonne

Plastics are long chains of molecules built mostly from oil and gas: polyethylene, polypropylene, PVC, PET and polystyrene among them. Output rose from almost nothing in 1950 to several hundred million tonnes a year. A large share of their energy cost is the hydrocarbon that becomes the material itself, so in a sense the energy is still there, locked in the product, and can be released by burning it or saved by recycling it. Smil is careful to stress that plastics are not only waste. Syringes, tubing, gloves, blood bags and sterile packaging made modern medicine safer, as the COVID-19 pandemic showed. But plastic is also hard to recycle well, and much of it ends up in landfill, incinerators or the sea.

Aluminium is the opposite case. Its ore, bauxite, is plentiful, but the metal is bonded so tightly to oxygen that only electrolysis can free it. The Hall-Heroult process, invented in 1886, passes enormous currents through molten alumina. A modern smelter needs roughly 14 to 15 megawatt-hours of electricity per tonne of metal at the potline, which is why smelters cluster next to cheap hydro or coal power. Recycling aluminium needs only about 5% of that energy, so every scrap can is valuable.

Silicon is tiny in mass but extreme in energy per kilogram. Making the ultra-pure polysilicon for solar cells and chips takes large amounts of electricity, and chips themselves are among the most energy-intensive products per gram.

Smil adds a warning about the energy transition. Wind turbines need steel, concrete, copper and composites. Batteries need lithium, nickel, cobalt and graphite. Solar panels need silicon, glass, aluminium and silver. Building clean energy will raise demand for mining and materials, not end it.

Process flowchart From bauxite to aluminium
Inputs Bauxiteabout 4 to 5 t Electricityabout 14 to 15 MWh, roughly 50 GJ, at the smelter Process steps 1 Bayer refinery

bauxite dissolved and roasted to alumina, using steam and heat

2 Hall-Heroult smelter

alumina dissolved in molten salt at about 960 degrees Celsius and split by electric current

Outputs Primary aluminium1 tonne If recycled insteadabout 5% of the energy

Counting alumina refining and the fuel burned to generate the smelter's electricity raises the total to about 186 GJ per tonne on IAI's global 2019 figures.

Source: Smil, Making the Modern World (2014) / Materials and Dematerialization (2nd ed., 2023), via Smil Mastery Course, M11 'Plastics, aluminium, silicon and transition materials'

See it in the table

Kc Coking coalCu CopperAl AluminiumNi NickelFe Iron oreSt SteelSi SiliconAg SilverLi LithiumCo CobaltGr GraphiteCem CementStn Crushed stoneO2 Oxygen

Check your understanding

Why can clean heat alone not remove most of cement's CO2?

Heating calcium carbonate releases CO2 from the rock itself. That process emission remains even with zero-carbon fuel, which is why cement needs carbon capture or new chemistries.

Why does an electric arc furnace using scrap need far less energy than a blast furnace?

The costly step in steelmaking is reducing iron oxide. Scrap has already been reduced, so the furnace mainly supplies melting heat.

Which material has the highest energy cost per tonne in primary production?

Primary aluminium needs about 150 to 190 GJ per tonne, mostly as smelter electricity, against about 20 for steel, 45 for copper and under 5 for cement.
← Part 2: Food: nitrogen, ammonia and fossil-fuelled farmsMark read · Part 4: Moving people and goods →