Calcination — Definition and GHG Accounting Context
Some industrial emissions come from burning fuel, and can in principle be cleaned up by switching to renewable energy. Others come from the raw material itself, and no change of fuel can touch them. The chemistry that produces this second, stubborn kind of emission is the same one that has been used to make lime and cement since antiquity — heating rock until it gives up its carbon.
That chemistry is calcination. Calcination is the heating of a carbonate mineral until it decomposes, driving off carbon dioxide from the rock itself — the origin of most industrial “process” emissions.
Calcination is the thermal decomposition of a mineral — most importantly a carbonate — that drives off a volatile component as gas. Its defining reaction is the breakdown of limestone: CaCO₃ → CaO + CO₂, in which about 44% of the limestone’s mass leaves as carbon dioxide. Because that CO₂ comes from the rock, not the fuel, it is a process emission that cannot be cut by switching to cleaner heat. Calcination releases the CO₂ in cement clinker, quicklime and glass, which is why these are “hard-to-abate” sectors.
Definition — Driving CO₂ Out of Rock
Calcination (or calcining) is the process of heating a solid to a high temperature — below its melting point — to bring about a chemical change, most commonly the decomposition of a mineral that drives off a volatile component. The archetypal case, and the one that matters for carbon accounting, is the thermal decomposition of a carbonate: heating calcium carbonate (limestone) breaks it into calcium oxide (lime) and carbon dioxide. The name comes from the Latin calx, lime — calcination is, literally, “lime-making”.
What makes calcination important for climate is where the CO₂ comes from. It is released from the mineral’s own chemical structure, not from the fuel used to heat the kiln. This makes it a process emission — distinct from the combustion emission of the fuel — and it is the single largest reason that cement, lime and glass manufacturing are so hard to decarbonise. You could heat the kiln with zero-carbon electricity and the calcination CO₂ would still be emitted.
Definition at a glance
| What it is | Heating a mineral (below melting) to decompose it and drive off a volatile — classically CO₂ from a carbonate |
|---|---|
| Defining reaction | CaCO₃ → CaO + CO₂ (limestone → quicklime + carbon dioxide) |
| CO₂ source | The mineral itself (geologic carbon) — not the fuel |
| Emission type | Process emission, not combustion |
| Where it occurs | Cement (clinker), lime, glass, ceramics, soda ash, some metallurgy |
| In accounting | Scope 1 process emission; embodied carbon downstream |
The Reaction and Its Chemistry
The calcination of limestone is a simple, fixed piece of stoichiometry. Calcium carbonate decomposes into calcium oxide and carbon dioxide, and the mass splits in a fixed ratio set by the molecular weights:
By mass, 100 tonnes of pure limestone (CaCO₃) yield about 56 tonnes of quicklime (CaO) and 44 tonnes of CO₂. So calcining a tonne of limestone releases about 0.44 t CO₂; and because lime is only the calcium-oxide part, a tonne of pure quicklime carries about 0.785 t CO₂ of calcination emissions. The ratio is chemistry — it does not change with technology.
This is why calcination emissions are described as unavoidable at the process level: short of using a different, non-carbonate raw material, the only ways to reduce them are to use less of the calcined product or to capture the CO₂ at the stack. The same reaction runs in reverse over time — cured lime and concrete slowly reabsorb some CO₂ from the air, a process called carbonation or recarbonation, which claws back a fraction of the original calcination emissions across the material’s life.
Worked micro-example
Feed 1 tonne of pure limestone (CaCO₃) into a kiln and heat it past ~900°C:
- Out comes ≈ 0.56 t quicklime (CaO)
- and ≈ 0.44 t CO₂ is released — from the rock, before counting any fuel
Pure stoichiometry (molar masses CaCO₃ 100.1, CaO 56.1, CO₂ 44.0). Real kilns also burn fuel to supply the heat, adding combustion CO₂ on top of this process figure — the two are reported separately.
Calcination Across Industries
Calcination is the chemical root of process emissions in several mineral industries. The live process factors below show how much CO₂ the calcination step alone releases per tonne of product — before any kiln fuel is counted:
| Product (calcination step) | Process CO₂ t CO₂ / t of product · live, IPCC default |
|---|---|
| Lime — dolomitic (per t lime) | 0.86 |
| Lime — high-calcium (per t lime) | 0.75 |
| Cement clinker (per t clinker) | 0.52 |
| Glass (per t glass) | 0.2 |
Lime sits highest because quicklime is almost pure CaO, so nearly the full stoichiometric ratio applies; dolomitic lime is higher still because it also calcines magnesium carbonate (MgCO₃ → MgO + CO₂). Clinker is lower per tonne because it is only about two-thirds lime by mass, and glass lower again because carbonates are a smaller part of the batch. In every case, this is the CO₂ that comes out of the rock — the fuel burned to reach calcination temperature adds more on top.
Calcination in GHG Accounting
In an emissions inventory, calcination CO₂ is a process emission — accounted for separately from the combustion of the kiln fuel, even though both happen in the same kiln. For the producer it is Scope 1, in the industrial-processes category rather than the stationary-combustion one. The cement & lime process calculator and the glass & ceramics process calculator compute it from the mass of carbonate and its purity. Downstream, the same CO₂ arrives as embodied carbon in the cement, lime or glass a buyer purchases. Because it is chemically fixed, the realistic routes to cutting it are carbon capture at the plant or reducing the amount of carbonate used — which is exactly why cement and lime are flagship targets for industrial CO₂ capture.
Common Confusions
- Confusing calcination with combustion. Combustion burns fuel; calcination decomposes a mineral. The calcination CO₂ comes from the rock and remains even with a zero-carbon fuel.
- Thinking clean energy solves it. Electrifying the kiln removes the fuel emissions but not the calcination emissions — the process CO₂ is chemical, so capture or material substitution is required.
- Applying a per-tonne-lime factor to limestone (or vice versa). A tonne of limestone releases ~0.44 t CO₂; a tonne of quicklime carries ~0.785 t. They differ because lime is the calcined product, not the feedstock.
- Forgetting recarbonation. Lime and concrete slowly reabsorb some CO₂ from the air over their life (carbonation), offsetting a fraction of the original calcination emissions — relevant to whole-life accounting.
- Assuming all cement/glass emissions are calcination. Calcination is the process share; the kiln fuel adds a separate combustion share. Both must be counted.
Dataset — the full factor set behind this term, versioned with source provenance and downloadable as CSV with a citable Zenodo DOI, is published as the IPCC industrial-process (IPPU) emission factors dataset.
Frequently Asked Questions
Calcination is the heating of a solid to a high temperature, below its melting point, to cause a chemical change — most often the thermal decomposition of a mineral that drives off a volatile component as gas. The most important example for carbon accounting is the breakdown of a carbonate: heating limestone (calcium carbonate) decomposes it into quicklime (calcium oxide) and carbon dioxide, CaCO₃ → CaO + CO₂. The word comes from the Latin for lime. Calcination is central to making cement, lime, glass and ceramics, and it releases carbon dioxide directly from the mineral being processed.
Because the CO₂ comes from the raw material, not the fuel. When limestone is calcined, the carbon dioxide is released from the carbonate mineral’s own chemical structure — CaCO₃ → CaO + CO₂ — as an inescapable product of the reaction. This is fundamentally different from combustion emissions, which come from burning fuel and can be reduced by switching to a cleaner energy source. You could heat a cement or lime kiln entirely with renewable electricity or biomass, and the calcination CO₂ — roughly 44% of the limestone’s mass — would still be emitted. That is why these are called “hard-to-abate” sectors, and why their deep decarbonisation depends on carbon capture or on using less carbonate, rather than on cleaner heat alone.
It depends on the mineral and product, but the chemistry sets fixed ratios. Calcining pure limestone releases about 0.44 tonnes of CO₂ per tonne of limestone, because 44% of calcium carbonate’s mass is CO₂. Per tonne of finished product the figure varies: high-calcium lime carries about 0.75 tonnes of CO₂ per tonne (live), dolomitic lime about 0.86 tonnes, cement clinker about 0.52 tonnes, and glass about 0.2 tonnes. Lime is highest because it is almost pure calcium oxide, while clinker and glass are lower because carbonates make up a smaller share of the product. These are process emissions only — the fuel burned to heat the kiln adds more.
No. Combustion is the burning of a fuel with oxygen to release energy, producing CO₂ from the fuel’s carbon. Calcination is the thermal decomposition of a mineral, releasing CO₂ from the mineral itself. Both often occur together in a kiln — fuel is burned to supply the heat that drives calcination — but they are accounted for separately. The combustion emissions depend on the fuel and can be reduced by fuel switching; the calcination process emissions depend on the mineral chemistry and cannot. Keeping the two apart is essential for correctly attributing and abating a cement or lime plant’s carbon.
Partly, and slowly. The calcination reaction runs in reverse when the calcium oxide reacts with atmospheric CO₂ to re-form a carbonate — a process called carbonation, or recarbonation. Lime mortars, concrete and other lime-based materials gradually reabsorb a portion of their original calcination CO₂ from the air over years and decades. This is a genuine, if modest, carbon sink, and whole-life carbon assessments for concrete increasingly account for it. It does not, however, undo the emissions at source: only a fraction is reabsorbed, and over a long period, so calcination remains a net emitter. Deliberately accelerated versions of this reaction are also being explored as a way to cure low-carbon concrete and to store captured CO₂.