The word "carbonation" almost always carries a negative connotation in structural engineering: in reinforced concrete, carbonation of the cover concrete is the main cause of corrosion in steel reinforcement, a process that engineers try to slow with adequate cover thickness and low-permeability concrete. It's therefore counterintuitive to discover that, for air lime, the exact same chemical reaction — the transformation of calcium hydroxide into calcium carbonate by absorbing atmospheric CO₂ — isn't a pathology to be prevented, but the very mechanism through which the plaster hardens and gains strength. It's the same reaction, with the same reagent, but with a completely opposite role depending on the material it occurs in.
The full cycle: calcination, slaking, carbonation
The chemical cycle of air lime unfolds in three steps. The first, calcination, converts the calcium carbonate of limestone into calcium oxide at around 900–1,000°C, releasing CO₂ into the atmosphere (CaCO₃ → CaO + CO₂) and causing the rock to lose about 44% of its weight. The second, slaking, hydrates the calcium oxide into calcium hydroxide (CaO + H₂O → Ca(OH)₂) — a strongly exothermic reaction, with a volumetric expansion that can double the initial volume. The third step, carbonation, is the one that matters here: the calcium hydroxide, applied in place as plaster, reacts slowly with the CO₂ present in the air to transform back into calcium carbonate (Ca(OH)₂ + CO₂ → CaCO₃ + H₂O) — exactly the mineral it started as. Lime, in a sense, "remembers" the limestone it came from and returns to it — that's not a poetic analogy, it's the literal description of the reaction.
Why it's slow, and why the slowness is deliberate
Carbonation speed depends on three factors: the air's relative humidity (optimal in a mid-range interval, around 50–75%, because the reaction needs water as a medium but excess saturation hinders CO₂ diffusion through the pores), the concentration of carbon dioxide in the environment (about 400 ppm in outdoor air), and the porosity of the plaster itself, which determines how deep the CO₂ can penetrate. In a plaster layer of about 10 mm, full carbonation typically takes 3–6 months — a timeframe that seems inconvenient for modern construction schedules, but is precisely why traditional craftsmen never applied a second coat of plaster over the first before it had carbonated: if the first coat remains plastic and uncarbonated under an already-closed second coat, CO₂ can no longer penetrate deeply, and the interior stays permanently uncarbonated — a hidden defect that compromises long-term durability while remaining invisible to surface inspection.
The net CO₂ balance
The figure that makes this chemistry relevant to the debate over decarbonizing building materials is the net emissions balance. Firing lime emits about 0.75 kg of CO₂ for every kg of CaO produced — a value comparable, per unit of weight, to producing Portland clinker. But unlike clinker, which once hydrated in concrete no longer reabsorbs significant amounts of CO₂ in the short-to-medium term (concrete carbonation does proceed, but it's viewed as a problem for the rebar, not as an intended environmental offset), air lime reabsorbs a substantial share of that same CO₂ emitted during calcination over the course of its service life — estimated at about 0.50–0.65 kg of CO₂ per kg of CaO over the full carbonation cycle. The resulting net balance, about 0.15–0.25 kg of CO₂ per kg of material, is significantly lower than that of Portland clinker without capture mechanisms, which stays around 0.9 kg CO₂/kg.
The flip side: why it's a problem in cement
In reinforced concrete, the same chemical reaction has an unwanted effect because it happens in a different context: the carbonation front advances from the surface inward, progressively lowering the pH of the cementitious matrix. As long as the front doesn't reach the steel reinforcement, the steel stays passivated (protected from corrosion) by the strongly alkaline environment of uncarbonated concrete. When the carbonation front reaches the depth of the reinforcement — an event the design cover thickness is sized to delay beyond the required service life — passivation is lost and corrosion can begin. It's the same chemical mechanism as in lime, but applied to a material that contains a vulnerable element — steel — which lime, with no reinforcement, simply doesn't have.