Preserving our built heritage

with Saint-Astier® NHL mortars

General information
Lime mortars have been used since time immemorial. All our structures were built with lime mortars, prior to the advent of modern cement in 1915. The survival of most of these buildings for hundreds, even thousands of years is a testimony to the durability of lime mortars.

Lime is produced by burning limestone (CaCO3) at temperatures below 1250°C for a period of up to 48 hours, traditionally in vertical kilns loaded from above. 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 (or Hydrated Lime) is quick lime that has been hydrated. If necessary, once cooled, the slaked lime is ground 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.

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 has produced the hardening of the mortar, normally referred to as Compressive Strength. Carbonation is a slow process and in damp or wet environments, it 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 than air lime mortars and will perform better 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, also known as air set. They are classified according to 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 recommended.

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

The use of NHL mortars in restoration work
Today it is our responsibility to ensure 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 have been subject to 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.

Saint-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.

Qualities of Saint-Astier® NHL
The following is a brief illustration of the qualities of Saint-Astier® Natural Hydraulic Lime mortars that helps explain their extensive use:

1) Vapour permeability
Cement mortars are dense and therefore promote the accumulation of moisture in the building elements (bricks, stone, timber).
See the comparison below:
– 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 Saint-Astier® NHL 3.5 mortar @ 1: 3 = 0.72 grams

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

3) Brittleness
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 reach 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 obvious consequences.

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

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

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

7) Free of soluble salts
All Saint-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. There is a high presence of sulphates and aluminates in cementitious mortars. The sulphates are due to the gypsum (CaSO4) which is added to the cement in order to delay the extremely quick set caused by the aluminates. Sulphates + aluminates + rain water cause sulphate attack. The qualities above give Saint-Astier® NHL mortars a very high durability. If properly applied, their life can be measured in hundreds of years.

8) Low bulk density
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.

9) Suitable in marine environment
Due to the very low presence of aluminates and other components (gypsum, alkalis), Saint-Astier® limes perform well in marine climates. The sea salt or salty air will attack cementicious products (including mortars made with cement+hydrated lime). The result is very visible in marine climate structures where the durability of cement or cementitious mortars is very poor. Saint-Astier® mortars have been very successfully used in marine environments, including harbours and lighthouses.

10) Resistance to frost
In cold climates such as Canada and Scandinavia, Saint-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.

11) No need for pozzolanic additions
Pozzolans are used to try and make non hydraulic lime into hydraulic lime. There are risks to using pozzolans. The best ones are expensive but, most of all, some modern Pozzolans can make mortars too dense and this affects their breathability and elasticity. With Saint-Astier® NHL mortars no pozzolanic addition is required, its hydraulic set is guaranteed by the hydraulic nature of their binder component.

12) Environment-friendly
Saint-Astier® NHL are produced very efficiently: far less energy is consumed than in the production of cement and hydrated lime and at the same time 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, Saint-Astier® Limes (actual) and Ordinary Portland Cement (OPC).

The qualities above give Saint-Astier® NHL mortars a very high durability. If properly applied, their life can be measured in hundreds of years.

Environmental impact in kg per ton of binder produced or used
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
Units: kg of CO2 per ton
CO2 Emissions during Production During Usage
Material
Emitted
Re-absorbed
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.

Binder consumption for 1m² of mortar
Volume/weight comparison with sand density of 1500 kg/m²
Volume of Mixes/kg x m²
Binder
Density kg/m²
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