Calcination is the thermal decomposition of limestone (CaCO₃) into lime (CaO) and carbon dioxide (CO₂), expressed by the reaction CaCO₃ → CaO + CO₂. In cement manufacturing, this reaction takes place in the preheater and rotary kiln system before clinkering and is a mandatory step in producing calcium oxide, the main reactive component of Portland cement clinker.
Process Conditions
Decomposition of CaCO₃ begins at around 600°C and is essentially complete by approximately 900°C at atmospheric pressure. In industrial cement kilns, calcination typically occurs in the precalciner and lower cyclone stages of the preheater tower before material enters the rotary kiln, where temperatures reach up to 1450°C for clinkering reactions.
CO₂ Release and Stoichiometry
The molar masses of CaCO₃ (100 g/mol), CaO (56 g/mol) and CO₂ (44 g/mol) determine the mass balance of the reaction. Decomposing one tonne of pure CaCO₃ yields approximately 560 kg of CaO and releases approximately 440 kg of CO₂. This CO₂ is generated by the chemical transformation of the raw material itself, independent of the energy source used to heat the kiln.
Share of Total Process Emissions
In conventional Portland cement production, CO₂ emissions arise from two distinct sources: fuel combustion to reach process temperatures, and calcination of the limestone feedstock. Calcination-related CO₂ typically accounts for roughly 60% of total CO₂ emissions from clinker production, with the remainder attributable to fuel combustion in the kiln system.
Relevance for Decarbonisation Strategies
Because calcination emissions originate from the chemical composition of the raw material rather than from the energy source, they cannot be eliminated through fuel switching, electrification, or the use of alternative fuels. Reducing or eliminating this emission source requires either carbon capture technology, a reduction in clinker content through supplementary cementitious materials, or the use of alternative binder chemistries that do not rely on limestone calcination. This distinction is central to industry discussions about which decarbonisation measures can address combustion emissions versus the structurally unavoidable process emissions from calcination.
Calcination in Our Coverage
In Padeswood CCS: How Heidelberg Materials' Track-2 Decision Advances Full Cement Decarbonisation, calcination is identified as one of the two emission sources that the Padeswood carbon capture system is designed to address at near-complete capture efficiency, alongside fuel combustion emissions. In Wienerberger UK: How Robust Are Its Sustainability Governance Claims?, calcination is discussed in the context of clay ceramics manufacturing, where process emissions from this reaction are lower than in lime production and fuel combustion remains the dominant emission source for the sector.