| CL90 air lime — compressive strength | 1–5 N/mm² (after 28 days in air) |
|---|---|
| NHL5 hydraulic lime — compressive strength | 10–20 N/mm² (after 28 days) |
| Calcination temperature (CaCO₃ → CaO) | 900–1,000 °C (air lime) |
| CO₂ emissions from lime calcination | ~0.75 kg CO₂/kg CaO |
| CO₂ reabsorbed during carbonation | ~0.50–0.65 kg CO₂/kg CaO (full cycle) |
| Vapor permeability (sd) | 0.01–0.10 m (breathable) |
| EU reference standard (limes) | EN 459-1 (building limes) |
| EU reference standard (renders) | EN 998-1 (rendering and plastering mortars) |
The Court of the Lions at the Alhambra in Granada was completed in 1391 under Sultan Muhammad V. The 124 white marble columns surrounding the fountain are set off by walls covered in lime-stucco muqarnas — decorative stalactites executed in air lime mixed with gypsum and very fine sand, carved with metal tools while the material was still plastic, smoothed and sometimes painted. Seven centuries later, that thin shell — average thickness 8–15 mm — is still substantially intact in the areas sheltered from rainwater.
The secret isn't a miracle additive but the chemistry of the carbonation cycle: air lime hardened by carbonation (CaCO₃) has the same mineralogical structure as the limestone it came from. It's a material that "remembers" its parent rock and that, kept dry, has no biological expiry date. Problems arise when the cycle is broken: rising damp, soluble salts, frost — not the lime itself.
The chemistry of lime: the carbonation cycle
Air lime production unfolds in three reversible stages. Calcination: CaCO₃ → CaO + CO₂ at 900°C (the limestone loses 44% of its weight as CO₂, and the product is quicklime). Slaking (hydration): CaO + H₂O → Ca(OH)₂ — a strongly exothermic reaction (65 kJ/mol), producing heat and a 100% volumetric expansion. Carbonation (hardening): Ca(OH)₂ + CO₂ → CaCO₃ + H₂O — the slaked lime, in contact with atmospheric CO₂, slowly transforms back into calcium carbonate. Carbonation speed depends on the air's relative humidity (optimal between 50–75%), CO₂ concentration (400 ppm in outdoor air), and the plaster's porosity. In 10 mm layers, full carbonation takes 3–6 months. Hence the traditional-trade rule: don't apply the second coat over the scratch coat before the first has carbonated — otherwise CO₂ can't penetrate and the plaster stays plastic underneath.
The variants
CL90 air lime (hardens with CO₂)
CL90 air lime (EN 459-1) contains at least 90% CaO + MgO. It's the product of firing pure limestone — free of clay impurities — at 900–1,000°C. It hardens exclusively by carbonation in air: it doesn't set underwater. Low mechanical strength (1–5 N/mm²): unsuitable as a structural binder, ideal as a binder for flexible plasters compatible with weak historic masonry (tuff, old fired brick). Its slow set and controlled shrinkage make it the binder of choice for the conservative restoration of historic buildings — where mechanical compatibility with the existing masonry matters more than absolute strength.
NHL natural hydraulic lime (hardens with water)
Natural hydraulic lime (NHL) is obtained by firing marly limestones — rocks with a variable clay (silicate) content. During firing, the silicates react with the lime, forming small quantities of hydrated calcium silicates (C₂S, C₃S): these hydraulic compounds allow hardening in the presence of water, even without CO₂. EN 459-1 distinguishes three classes: NHL2 (strength 2–7 N/mm², low hydraulicity, behavior similar to air lime), NHL3.5 (3.5–10 N/mm², balanced), NHL5 (10–20 N/mm², high hydraulicity, usable in wet and submerged environments). Choosing the right NHL grade is critical: an NHL5 on low-strength tuff masonry creates a matrix stiffer than the substrate — at the first thermal cycle, the plaster cracks and delaminates.
Hydrated lime powder
Hydrated lime powder (Ca(OH)₂) is quicklime industrially pre-slaked with controlled water, then dried and ground. It has a very high specific surface area (15–20 m²/g) that speeds up carbonation compared to lime putty. It's the mass-market product for sites where slaking lime on the bag isn't practical: more convenient, but with lower performance than lime putty for high-end applications. In standard civil plasters, hydrated lime powder is the main component — it's mixed with silica or carbonate sand at a 1:3 ratio (lime:sand) to obtain rendering mortars with strength of 2–4 N/mm².
Lime putty (water-slaked)
Lime putty is quicklime slaked by immersion in an excess of water: a soft, plastic paste forms, with a very high water content. The putty is left to mature in pits for at least 6 months — ideally 2–3 years: maturation reduces particles of unhydrated CaO that could cause surface popping, increases plasticity, and improves adhesion. Mature putty is one of the most plastic plasters available and allows for smooth, mirror-like finishes that hydrated lime powder can't replicate. Venetian marmorino, Roman stucco, Venetian polished plaster — all require mature putty as their base.
Venetian marmorino
Marmorino is a finishing plaster made of mature lime putty and finely ground marble (marble powder, micro-ground calcium carbonate). The mix is applied in two thin coats (2–3 mm each) and polished while still fresh with a heated iron (the polishing closes the pores and creates a semi-translucent surface layer of calcite). The result: a hard, glossy, slightly translucent surface, water-repellent through very low capillarity. It isn't waterproof — and it shouldn't be: vapor permeability is preserved. Venice uses it systematically in Palladian villas, in the palazzi along the Grand Canal, and in church interior decoration. The work requires specialized craftsmen: the training takes years, and it can't be learned from a video.
Cocciopesto (Roman pozzolanic mortar)
Cocciopesto — the Romans' opus signinum — is a pozzolanic plaster made by adding ground fired clay (crushed brick, crushed ceramic) to air lime at a 1:2 ratio (lime:fired clay). The fired terracotta fragments contain reactive silicates in an amorphous state (meta-kaolin activated by firing at 600–800°C) that react with the lime in the presence of water, forming hydrated calcium silicates: the same chemical process as natural volcanic pozzolans. The result is a low-tech but highly durable hydraulic binder: waterproof, resistant to standing water. Used in Roman baths, aqueducts, and the floors of villas and thermae. Today it's returning to use in restoration and natural-material construction, with powdered antique brick or with modern ceramic systems (products such as Saint-Gobain's Tradical).
Lime-hemp plaster
Lime-hemp plaster (hempcrete plaster) is a mix of air lime or NHL2 with industrial hemp fibers (the waste from processing hemp shiv). The hemp adds thermal insulation (thermal conductivity of the mix: 0.10–0.15 W/(m·K)), resistance to shrinkage cracking, and hygroscopic regulation (the hemp absorbs and releases vapor). It isn't a load-bearing plaster (strength 0.5–2 N/mm²): it's a thermally regulating finishing system for straw walls, rammed earth, and existing masonry undergoing restoration. Widespread in natural building and European bio-based architecture.
Roman and Venetian stucco
Venetian stucco (or Venetian polished plaster) is technically a marmorino with a beeswax or carnauba wax finish applied hot over the last smoothed coat. The wax fills the remaining pores and increases the mirror-like reflection. Roman stucco, by contrast, is historically richer in aggregate and less plasticized: used for moldings, cornices, friezes — three-dimensional forms executed by dragging metal profiles across the fresh material. Both require lime putty matured for at least a year. The main difference: Venetian marmorino is flat (imitating marble); Roman stucco is three-dimensional (imitating carved stone).
Behavior over time
An air-lime plaster correctly applied and carbonated is chemically stable for practically indefinite periods in the absence of excessive moisture. The degradation mechanisms are: salt crystallization (soluble salts dissolved in rising damp or in the brick crystallize near the surface during evaporation, breaking down the porous structure); frost spalling (absorbed water freezes and expands by 9%, fracturing the micropores); incompatibility with Portland cement (a cement-mortar patch on a historic lime wall creates a difference in stiffness and porosity that causes detachment at the joints).
The fundamental principle of restoration is compatibility: a repair plaster must have mechanical strength equal to or lower than the underlying masonry, a compatible coefficient of thermal expansion, and vapor permeability equal to or higher. A cement plaster on antique masonry is a serious technical error: its impermeability traps the moisture that then destroys the masonry from within.
| Dimension | Score | Assessment |
|---|---|---|
| 🌿 Environmental | 7/10 | CaO production emits about 0.75 kg CO₂/kg (decarbonation of limestone at 900°C). However the cycle is partly closed: carbonation reabsorbs 0.50–0.65 kg CO₂/kg over the course of hardening and throughout the service life. Net balance: about 0.15–0.25 kg CO₂/kg — significantly lower than Portland clinker (0.9 kg CO₂/kg without recovery). Lime requires no aggressive chemical additives and degrades naturally with no toxic residue. |
| ⚖️ Ethics | 9/10 | Entirely local production: limestone is present throughout Italy, in small or medium-sized quarries, with a short supply chain. No documented ethical controversy over working conditions. Lime is a traditional material with a low degree of industrial processing. The one critical note: handling quicklime requires appropriate PPE (gloves, goggles, mask) due to its high causticity (pH > 12). |
| 🏘️ Social | 9/10 | Lime is the material of skilled craftsmanship par excellence: every square meter of Venetian marmorino is a unique piece that can't be industrially replicated. It allows the transmission of endangered artisan skills. The material is breathable and hypoallergenic — it contributes to indoor air quality (no VOC emissions). Lime is the binder of choice for restoring Italy's historic building stock, which accounts for roughly 70% of the country's buildings. |
| 💶 Economic | 6/10 | Low raw-material cost: lime powder €0.20–0.40/kg; mature lime putty €0.80–1.50/kg. Application is slow and requires skilled labor: a three-coat lime plaster needs 3–5 days of waiting between coats (carbonation). The final installed cost for a traditional lime plaster is €25–60/m², against €10–20/m² for a pre-mixed single-coat cement-based render. For restoration and natural building, the cost is justified; for ordinary new construction, it's competitive only when paired with other performance advantages. |
| Average score | 7.75/10 |
Historical uses
Roman antiquity and opus signinum (3rd century BC – 5th century AD): The Romans knew air lime and used it systematically for plasters, bedding mortars, and floors. Cocciopesto (opus signinum) is the hydraulic system par excellence of baths, pools, and aqueducts — like the cocciopesto base of the Pantheon. Pozzolanic mortar — lime plus volcanic pozzolan from the Campi Flegrei — is the binder of opus caementicium: the pozzolan confers hydraulicity and strength even in marine environments, as documented in the concrete hulls found in the ports of Caesarea Maritima.
The Middle Ages and Romanesque architecture (6th – 12th centuries): With the fall of the Roman Empire, lime-firing techniques were simplified and supplies of volcanic pozzolan ceased. Italian Romanesque architecture uses air-lime mortars with local sand of highly variable quality — the cathedrals of Modena, Parma, and Piacenza have mortars weaker than the Roman average, but better proportioned to the brick masonry they sit on. The frescoed plasters of Romanesque basilicas (Aquileia, Sant'Ambrogio in Milan) are examples of three-coat lime plaster — scratch coat, brown coat, finish coat — executed with mature lime putty and river sand.
The Renaissance and Baroque: marmorino and stucco (15th – 18th centuries): The Italian Renaissance rediscovers scagliola and Venetian marmorino as economical alternatives to carved marble. Andrea Palladio uses pigmented lime plasters to simulate the architectural orders in Veneto villas where the budget didn't allow for real stone. In the Baroque, Roman stucco reaches its peak of plastic complexity: Borromini's ceilings, Guarini's domes, and the pediments of Sicilian villas are all executed by craftsmen specialized in lime work using putty matured for years.
Reference numbers for the project
| CL90 air lime — Rc (28 days in air) | 1–5 N/mm² |
|---|---|
| NHL2 — Rc (28 days) | 2–7 N/mm² |
| NHL3.5 — Rc (28 days) | 3,5–10 N/mm² |
| NHL5 — Rc (28 days) | 10–20 N/mm² |
| Cocciopesto NHL + fired clay — Rc | 5–15 N/mm² |
| Vapor permeability (sd, CL90 plaster) | 0,01–0,05 m |
| CO₂ emitted (calcination) — air lime | ~0,75 kg CO₂/kg CaO |
| CO₂ reabsorbed (carbonation) | 0,50–0,65 kg CO₂/kg CaO |
| Installed cost, lime plaster (3 coats) | €25–60/m² |
Research and the contemporary frontier
The most interesting lime research is focused on two fronts. The first is lime-hemp (hempcrete): a mix of hemp shiv (the woody core of industrial hemp, Agrochanvre, Isohemp) with NHL2 lime and water to obtain a bio-based composite with thermal conductivity of 0.10–0.15 W/(m·K), density of 300–500 kg/m³, and a CO₂ sequestration capacity in the shiv of about 1.5 kg CO₂/kg of material. The European HempCrete 2030 project (Horizon Europe) is developing EN standardization for lime-hemp blocks as a non-structural building material with thermal insulation and hygroscopic regulation properties.
The second front is accelerated carbonation of concrete: researchers at EPFL Lausanne have shown that exposing demolished concrete, ground to powder, to concentrated CO₂ (30–60%), carbonation happens in hours instead of decades, sequestering 0.10–0.20 kg CO₂/kg of material and producing a CaCO₃ aggregate usable as a sand substitute in new concrete. It's a permanent CO₂-capture system applicable at industrial scale, with a potential abatement of 100–200 Mt CO₂/year globally if applied to all demolished concrete.