How lime mortar hardens
Carbonation of calcium hydroxide · Acid–base · Precipitation · Exothermic
Old brick walls set by slowly breathing in carbon dioxide and turning back into limestone.
Reactants approach
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ΔH = -113 kJ · exothermic, releases heat
ΔH per mol Ca(OH)2.
Meet the molecules
- Reactantsolid
Ca(OH)2
calcium hydroxide (slaked lime)
- Reactantgas
CO2
carbon dioxide
- Productsolid
CaCO3
calcium carbonate
- Productliquid
H2O
water
What you notice
Before Portland cement, builders bound bricks and stones with lime mortar, and it is still used to repair old buildings. It is a paste of slaked lime, calcium hydroxide, mixed with sand and water. Over weeks to months the mortar absorbs carbon dioxide from the air and slowly turns into calcium carbonate, the mineral in limestone, gaining strength as it does. The core of a thick wall can take years to carbonate fully. The same reaction turns limewater cloudy when you breathe into it and hardens a coat of whitewash.
Safety: Wet lime is strongly alkaline (pH about 12.5) and burns skin and eyes, so masons wear gloves and goggles.
Step by step
- 1
CO2 dissolves in pore water
CO2 from the air dissolves in the thin film of water in the mortar's pores, forming carbonic acid.
- 2
Neutralization
Hydroxide ions from Ca(OH)2 remove protons from carbonic acid, making carbonate ions and water.
- 3
Crystals grow
Ca2+ and CO3^2– ions precipitate as interlocking calcite crystals that glue the sand grains together.
- 4
Water leaves
The water produced evaporates. Masons keep new lime work slightly damp, not soaked, because carbonation needs a little water in the pores.