Preserving our built heritage: St. Astier NHL mortars

Lime mortars have been used since time immemorial. With lime mortars all our buildings, prior to the advent of modern cement after 1915, have been built. The survival of most of these buildings for hundred and even thousands of years is a testimony to the durability of lime mortars.

Lime is produced by burning limestone (CaCO3)at temperatures below 1250oC traditionally in vertical kilns loaded from above and for a period of up to 48 hours. The material extracted from the furnaces is Quick Lime (CaO), typically in lumps. These, when in contact with water, exothermically disintegrate into smaller pieces varying from friable chunks to very fine particles. The process of water addition is called Slaking, hence a slaked lime is quick lime that has been hydrated or Hydrated Lime. If necessary the slaked lime is ground, once cooled down, into fine powder and bagged.

During the burning the limestone (CaCO3) loses CO2, hence the quick lime has the symbol of CaO. When exposed to air, some CO2 is reabsorbed, reconstituting CaCO3 or limestone. This process is called carbonation and its result is the hardening of the lime. In a mortar the lime has bound together the aggregates and by hardening produced the hardening of the mortar, normally expressed as Compressive Strength. Carbonation is a slow process and in damp or wet environments, is impeded by the moisture patina which forms over lime mortar surfaces obstructing the contact with air.
Lime mortars which are not hardened are very susceptible to adverse weather conditions and, in cold climates, can be easily damaged by frost.

If the limestone being burned contains hydraulic components (hydraulicity is the property of hardening in contact with water) such as silica (SiO2), alumina (Al2O3) and ferrites (Fe2O3) these will in total or part combine with the CaO forming Calcium Silicates, Calcium Aluminates and Calcium Ferrites. These combined elements harden in contact with water, producing Hydraulic Limes.
Mortar made with hydraulic limes will harden quicker that air lime mortars and will have a better performance in cold climates.

The current Standards classify lime in various categories:

  1. Calcium/Dolomitic limes (CL/DL): these are the result of burning and slaking (hydration) of limestone that does not contain hydraulic components (silicates, aluminates and ferrites). Their hardening process is solely due to carbonation, know also as “air set”. They are classified in accordance with their Calcium content as CL90,CL80 etc..... Their setting is slow (weeks and longer, depending on climatic conditions). They are available as Putty or dry powders in bags.
  2. Natural Hydraulic Limes (NHL): they are produced by burning and slaking limestone containing silica, alumina and ferrite. Their main set is hydraulic (in contact with water) and their secondary set is by carbonation (in all hydraulic limes there is a percentage of air lime that will harden only by carbonation). No addition of any sort is allowed. They are classified in accordance with their minimum compressive strength at 28 days as follows:
    NHL 5 – 5N/mm2
    NHL 3.5 – 3.5 N/mm2
    NHL 2 - 2N/mm2
    The initial setting of these limes is measured in hours.
  3. Hydraulic Limes (HL): these are binders obtained by blending different materials, typically cement, some hydrated lime, fillers and additives. There is no burning involved, only blending. Their adoption in Conservation, Restoration and repairs of old structures is not recommendable.

For a number of years the use of lime mortars in conservation and restoration is accepted and understood. Indeed lime mortars are also being adopted in new build, especially because of their breathability and elasticity. The category that is most adopted, especially in cold climates, is the NHLs as these provide the setting and durability properties required.

The use of NHL mortars in Restoration work

Today we are charged with the responsibility of ensuring the survival of our historical and vernacular built heritage through careful and considered conservation, restoration and renovation. All too often buildings are repaired with inappropriate materials and poor skills. Many are the subject of interventions that in time lead to their total loss. Others are simply changed out of all recognition. In most cases, the mortar used for repairs, restoration or conservation contains cement. In these buildings, the use of cement mortars promote condensation build up, salt reactions and, generally, drastically affects the long term survival of the building.

St. Astier natural hydraulic Lime mortars provide a valid alternative to cement mortars and, if applied properly, they will have a much longer life. Environmentally they offer far greater efficiency than cementitous mortars by re-absorbing CO2 and not compromising the recyclability of the building materials.

The following is a brief illustration of the qualities of St. Astier natural hydraulic lime mortars that explains their extensive use:


Cement mortars are dense and therefore promote the accumulation of moisture in the building elements (bricks, stone, timber).

Permeability of cement concrete = 0.15 grams of air per m2 per hour
Permeability of 1:1:6 cement/hydrated lime/sand = 0.23 grams
Permeability of a St. Astier NHL 3.5 mortar @ 1: 3 = 0.72 grams


NHL mortars have much better elasticity than cement mortars. Especially in the first 6 to 12 months small movements will be accommodated. Elasticity is one of the main reasons why, when building with NHL mortars, there is no need for construction joints.

Example (the lower the value the better):
Elasticity moduli of a cement/lime/sand mortar at 28 days (1:1:6) 16200MPa
Elasticity moduli of an NHL 3.5 mortar at 1:3 at 28 days 8970MPa


Cement mortars are too strong and therefore brittle. St. Astier NHL mortars have more than sufficient strength to satisfy the building requirements but will not achieve 30-40 N/mm2 as this will be a disadvantage in the long term. If a joint or a render is too brittle and cracks, moisture (especially in marine and damp environments) will penetrate with the obvious consequences.


Due to the absence of cement or gypsum all St. Astier NHL mortars are reworkable, from 4 to 24 hours depending on the type and weather conditions. This means less waste and more productivity.


The presence of free lime (from 20% in NHL 5 to over 50% in NHL 2) allows small airline shrinkage cracks to heal if cured in time.


Important for the builder at work. It is due to the presence of free lime (or hydrated lime) in the St. Astier binders (between 25 and 55%, depending on the type).


All St. Astier limes contain ZERO or only traces of soluble salts. They will therefore NOT promote sulphate attack and alkali-silica reactions which cause in so many occasions cracking and delamination. In the case of cementitious mortars, there will be a high presence of sulphates and aluminates. The sulphates are due to the addition of gypsum (CaSO4) to the cement in order to delay the extremely quick set given by the aluminates. Sulphates + aluminates + rain water cause sulphate attack. The qualities above give St. Astier NHL mortars a very high DURABILITY. If properly applied their life can be measured in hundreds of years.


NHL products are bought by weight but mixed by volume. Their low density in comparison with other binders means that with the same weight one will produce over 2 times the amount of mortar.

Binder consumption for 1 m3 of mortar
Volume/weight comparison with sand density of 1500 kg/m3
    Volume of Mixes/kg x m3
Binder Density kg/m3   1:2 1:2.5
NHL 5 666   333 266
NHL 3.5 610   305 244
NHL 2 560   280 224
Lime Putty 1350   675 540
OPC W.Cement 1450   725 580


Due to the very low presence of aluminates and other components (gypsum, alkalis), St. Astier limes are suitable to be used in marine climates. The sea salt or salty air attack cementicious products (including mortars made with cement+hydrated lime). The result is very visible in all areas subject to marine climates where the durability of cement or cementitious mortars is very poor. St. Astier mortars are very successfully used in marine environments, including intervention in harbours and lighthouses.


In cold climates such as Canada and Scandinavia, St. Astier lime mortars are widely employed for their performance in these conditions. This is mainly due to their setting properties and their void structure that allows water to freeze and subsequently thaw without excessive heave.


Pozzolans are used to try and make non hydraulic lime into hydraulic lime. There are dangers in the use of pozzolans. The best ones are expensive but, most of all, some modern Pozzolans can make mortars become too dense and therefore affecting their breathability and elasticity. The hydraulic set of the St. Astier NHL mortars is guaranteed by the hydraulic nature of their binder component and no pozzolanic addition is required.


St. Astier natural hydraulic limes are produced very efficiently. The result is that the ENERGY consumed is far below the one consumed in the production of cement and hydrated limes whilst a high re-absorption of CO2 is maintained.

The following figures are based on industry standard calculations for the production efficiency of kiln outputs for Non-hydraulic lime with a 90%+ Calcium Carbonate level, St Astier Limes (actual) and Ordinary Portland Cement (OPC).

Energy used in Production of Binder
Material Therm’s per Ton Kilo Watts per Ton
NHL 2 16 460
NHL 3.5 17 505
NHL 5 18 550
Non-Hydraulic Limes (CL) 34 - 40 1000 - 1200
OPC (Clinker) 32 - 40 900 - 1200

Environmental Impact in Kg per ton of Binder produced or used

Units: kg of CO2 per ton CO2 Emissions during Production During Usage
Material Emitted
NET Emission
NET CO2 emission
MORTAR ***1 : 2 ratio
NHL 2 753 350 403 57
NHL 3.5 606 270 336 55
NHL 5 635 220 415 77
Non-Hydraulic Limes (CL) 872 535 337 132
OPC (Clinker) 819 0 819 205

*** measured in relation to the weight of the binder per Ton of mortar produced.

Clearly there is a substantial gain in CO2 re-absorption with Non-Hydraulic limes, but their initial CO2 emission is the highest.


Modern production in St. Astier started in 1851. The basic product, NHL, has not changed and the worldwide experience gained over so many years in the most varied climatic conditions is a treasure at the disposal of all users.

Ugo Spano 31.3.2009