Lime Plaster

Scientific

Lime-plaster chemistry:

1. Limestone (CaCO₃) is quarried and burned at high temperature (~900°C) to produce quicklime (CaO).
2. Quicklime is slaked with water to produce slaked lime (Ca(OH)₂) — a fine paste or powder.
3. Slaked lime + aggregate + water is the working plaster mix.
4. Curing — applied plaster cures by carbonation: Ca(OH)₂ + CO₂ → CaCO₃. The plaster reverts to limestone, slowly, over months to years. The reaction binds the aggregate and gives the finished plaster its hardness.

Two main lime types:

• Air lime (non-[[lime-binder|hydraulic lime]]) — pure CaO, cures only by atmospheric CO₂ absorption. Slower but produces softer, more vapor-permeable plaster.
• [[lime-binder|Hydraulic lime]] (NHL — [[lime-binder|Natural Hydraulic Lime]]) — limestone with natural clay impurities; cures partly by hydration (like cement) and partly by carbonation. Faster set, harder, but less vapor-permeable. NHL grades 2, 3.5, and 5 indicate increasing hydraulicity.

Properties of cured lime plaster:

• Vapor-permeable — buildings finished in lime plaster breathe; moisture moves through the wall in vapor phase.
• Mold-resistant — high pH (~12–13) of fresh plaster suppresses mold and bacterial growth; the alkaline reservoir persists in cured plaster.
• Self-healing — small cracks in lime plaster slowly close as continuing carbonation deposits new calcium carbonate at crack faces.
• Long-lived — properly applied lime plaster lasts decades to centuries; medieval European lime plasters are still functional.
• Reversible — lime plaster can be removed and replaced; not a permanent commitment to a substrate.

Practical

Standard lime-plaster application:

1. Wall preparation — substrate dampened to prevent moisture-suction from drying plaster too quickly.
2. Scratch coat — first coat (typically thick), worked into substrate keys or wall surface, scratched to provide tooth for next coat.
3. Brown coat — second coat, leveled.
4. Finish coat — final coat, typically thinner, trowel-finished, sometimes pigmented.
5. Curing — slow, mist-and-cover curing over weeks; protected from rain, freezing, and direct sun until cured.

Pigments and additives:

• Natural ochres, oxides, and earth pigments give traditional lime plaster colors.
• Casein (milk protein), tallow, or linseed oil can be added for water-repellence.
• Pozzolans (volcanic ash, brick dust, fly ash) make lime hydraulic.
• Hair, straw, or other fibers reinforce certain applications (especially when used over earth substrates).

Cultural

Roman concrete (the legendary durability of which has lasted 2,000+ years in marine environments like Pozzuoli and Herculaneum) is, in significant part, lime-plus-pozzolan chemistry. Recent research has revealed that ancient Roman lime concrete includes “lime clasts” — pockets of unmixed lime — that may be the source of its self-healing capacity, slowly reacting with infiltrated water to seal cracks over centuries.

Italian, Greek, Spanish, North African, and Middle Eastern building traditions are largely lime-based. The 19th-century rise of [[portland-or|Portland]] cement displaced lime in most industrial-scale construction, but lime never left traditional practice — Italian grassello lime, Indian suffi lime, Yemeni qadad lime, French chaux lime, and many regional traditions continue.

The 21st-century natural-building revival has reintegrated lime plasters with hempcrete, cob, straw-bale, and adobe. Specific regional traditions — tadelakt (Moroccan polished lime), Venetian marmorino, sgraffito (multi-color carved lime plaster) — have small revivals.

Lenses still to grow

• Roman concrete and the recent self-healing-mechanism research
• Tadelakt — Moroccan polished waterproof lime plaster
• Venetian marmorino and Italian fine-finish lime traditions
• The Lime Lab and U.S. lime-plaster supply network
• Pozzolan chemistry and the [[lime-binder|lime-pozzolan]]-cement gradient

See also

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• Enables: [[hempcrete]] · [[cob-building]]
• Shares approach with: [[clay-plaster]]
• Member of: [[practice]]

Sources

• Roy Ellsworth, Plastering Plain and Decorative
• Building Limes Forum (UK) publications
• Patrick McAfee, Stone Buildings: Conservation, Repair, Building
• The Lime Mortar Standard — modern practitioner resource
• Mike Wye & Associates (UK) educational resources

Rooted in life.