Lime putty for mortar has been in known use for 6000 years. Modern portland cement lime and sand mortars have dominated for less than 100 years with many known failures. Here are what some modern scientific investigations have found about what worked so well about lime mortars and why so many modern mortars may not be appropriate for historic restoration. This information is offered so that you can make an informed choice on which binder to use for your projects.
	1 LIME PUTTY TO 3 PARTS OF SHARP SAND		1 NATURAL HYDRAULIC LIME (#5) TO 3 SHARP SAND
	This cross section cut away of a traditional lime putty mortar demonstrates via the blue dyed epoxy resin which fills the open pore structure that historic mortar based solely on lime and sand has a tremendously high and desirable liquid and vapor permeability.		Note that the pore structure is still open but finer and more dense than the Natural hydraulic lime #3.5. This mortar is suitable for copings, parging and pointing in extremely wet conditions including sea driven rain with high salt content.
	1 NATURAL HYDRAULIC LIME (3.5) TO 3 SHARP SAND		1 PORTLAND CEMENT TO 3 PARTS SHARP SAND
	Note the high porosity in this mortar fabric. The wall can breathe by allowing moisture to enter and deliver the carbon dioxide needed for carbonation during curing and to usefully encourage the crystalline bridging phenomenon,(also known as the autogenous or self-healing properties of lime mortar), moving about the free lime in the mix to close larger fissures. Excess moisture then quickly evaporates back into the atmosphere.		Note the dense fabric and the greatly reduced porosity with only the presence of shrinkage cracks. The shrinkage cracks are a portal for moisture to get into the absorptive bedding mortar (usually of lime and sand.) Normally, the sun will draw moisture back out of a lime joint. However, because of this portland cement based mortar, the inevitably supersaturated wall of trapped moisture can only have rain driven much further into the building and then to rooms when heat inside draws moisture, or out through the historic brick or stone accelerating its deterioration.
	Petrographic thin section images courtesy of William Revie of The Construction Materials Consulting Group; Striling, Scotland.
	© 2003 Pennsylvania Lime Works

PROPERTIES OF BINDERS	OPC Ordinary Portland Cement	Hydrated Lime	Lime Putties	St. Astier Natural Hydraulic Lime
Purity (Blended Material)
Elasticity
Permeability (Vapor Exchange Quality)
Resistance To Salt
Suitable Compressive Strength
Self Healing
Reworkable
Economic
Proficiency of Application
Resistance To Sulfate
Absence of Detrimental Chemicals
Slow Final Setting
Quick Initial Setting
Low Density

Why Use St. Astier Natural Hydraulic Lime For Historic Restoration, Conservation, and New Build Projects ?
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As late as 1940 many brick and stone buildings were constructed with lime and sand for mortar. Many times these limes were inadvertently hydraulic limes. The lime putty used may have had a hydraulic set because of impurities in the limestone when a limestone contained various degrees of reactive silica and was burned along with the pure calcium carbonate stone. Today it is unfortunate that many of these pre-1940 buildings have been repaired using Portland cement based mortars and stucco. There are some consequences with this remedy:
Because all buildings move, cracks develop in rigid Portland cement mortars and stucco. When a Portland cement mortar is stronger than the brick or stone laid up in the mortar, cracks that develop will transfer to the face of the exterior masonry allowing water penetration. Water can then be driven deeper into the masonry as it migrates to inside spaces.
	Because Portland cement has a dense pore structure and a needle-like crystal structure that has the same expansion and contraction coefficient as steel, the unyielding joints and stucco will eventually crack in various places. This is especially true with free standing church bell towers and the like. Water that does not migrate to the inside of the building may evaporate out of the more porous soft brick or sandstone and only accelerate its decay. The mortar on the other hand will remain proud as the masonry units will decay back and finally hollow out from their original face plane. *A contribution of the National Institute of Standards and Technology.
Tricalcium aluminates and Tricalcium silicates which form during the burning process of Portland cement have a detrimental chemical reaction when they come in contact with water that gets trapped in the bedding mortar and salts which are found in old buildings. This reaction results in what is called “The sulphation of cement” and is known to bulge once sound masonry walls as expansion occurs with this reaction sometimes causing even massive stone walls to topple over.Lime, however, has an open pore structure and a hexagonal crystal structure which allows the plates to shift between one another and yields flexibility and high vapor and liquid permeability. Some advantages of lime mortars are:
Walls breath better and moisture can escape Mortar and stucco does not set too hard Thermal movement can be accommodated without damage Expansion joints can be avoided Insulation is improved and cold bridging reduced There is a reduced risk of condensation There is little risk of salt staining because salts get flushed from wall surfaces Masonry life is increased CO2 emissions in the manufacture of lime are 20% less than cement and during carbonation of the lime, the mortar and stucco reabsorb considerable quantities of CO2 Natural Hydraulic lime gives an excellent reproduction of sand color
All St. Astier NHL mortars can be reworked (8-24 hours), reducing waste of material and increasing work rate due to its hydraulic set. St. Astier NHL contains no cement, gypsum, pozzolans, tetra calcium Aluminoferites, (high in Portland cement and contribute to expansion when reacting with gypsum.) St. Astier NHL does not have high aluminates, Sulphates, Alkalis making it suitable for marine environments.

Here's How St. Astier Natural Hydraulic Lime Plays An Important Role In Construction As A Green Build Material

St. Astier Natural Hydraulic Lime, or NHL, is a 100% natural product and does not contain any additives. It is one of the "greenest" materials used in construction. This is due to its purity, its calcium carbonate composition, its longevity and potential for allowing the materials to be reused or recycled, and the result of a low energy production process.

• The amount of energy used at the production stage is one-quarter of what is needed to produce cement. Consequently, the release of CO₂ into the atmosphere is reduced considerably. Furthermore, NHL reabsorbs most of the CO₂ during the curing process, while cement reabsorbs none.

• NHL has received the LABELVERT EXCELL, or "Green Label", in France. This label guarantees the total absence of contaminants and any risk of pollution. It also authorizes the use of this product in chemically sensitive areas such as living spaces, wine cellars, etc. The total absence of aromatic or polycyclic elements in NHL are proof of its purity.

• The absence or lower concentration of detrimental chemicals like tri-calcium aluminate, potassium and sodium oxides (which are ever-present in cement), protect NHL mortars from chemical reactions such as sulfate or alkali attacks.

• The total absence of Chromium VI eliminates the risk of skin cancer associated with clinker cement.

• Material used in construction with NHL may be reused or recycled. In addition, the NHL mortar itself may be recycled in a number of ways, such as an aggregate for new lime mortars, fertilizer (NHL is calcium carbonate), or it can be used for water purification or hazardous waste treatment.

• Breathability, elasticity, plasticity, gradual development of strength, low shrinkage, longevity, CO₂ absorption, self-healing, are all highly desirable. These traits, with sustainability and "greenness", are only some of the qualities of St. Astier Natural Hydraulic Lime (NHL).

• The change-of-use of older buildings through adaptation or preservation and restoration maximizes the need for the environmental recovery of materials. It is essential to ensure the long term survival of these structures with compatible materials. Some buildings have been in use for centuries; there is no logical reason that this cannot continue. Preservation, adaptation and restoration can have significant environmental advantages over new construction. Aside from the environmental impact, there is the aesthetic value in preservation. Natural Hydraulic Limes have a significant part to play in the process.
• Material longevity is unsurpassed when applied and maintained correctly and its life will span over several generations.

The bonding characteristics of pure NHL mortars, combined with good background preparation, correct suction control, appropriate binder : aggregate ratio and good workmanship will ensure the durability of an exterior stucco plaster. Bonding to the background and between subsequent coats is essential. This is provided by ensuring that the background offers a good key, by controlling the background suction in brick and stone and by casting on/harling at least the first coat on smooth or poorly keyed surfaces.
	First coat (bonding coat): This is normally rich in binder. It's functions are mainly to ensure a strong adhesion to the background, provide the anchor for the subsequent coats, and compliment the water resistance of the intermediate coats.						SUCTION CONTROL: SOME BRICKS ABSORB 1.5 LITERS OF WATER OR MORE
	Intermediate coats have insulation properties and protect against water penetration. Their thickness vary in relation to climate and exposure. The finishing coat is primarily decorative but can also serve as the first protective barrier, depending on the type of finish chosen.
			VAPOR EXCHANGE
			NHL mortars will combine the ability to resist water penetration while allowing the structure to breathe freely. Values above 0.5 grams of pair per m2 per hour are acceptable. NHL mortars have between 0.55 to 0.75 (the value of concrete is about 0.15). Allowing structures to breathe reduces or eliminates condensation and rot and enhances the living environment.
IF APPLIED PROPERLY, NHL MORTARS WILL ACHIEVE GREAT DURABILITY					NO CONSTRUCTION JOINTS!
					The elasticity of pure NHL mortars and the near zero expansion in NHL binders are such that joint free construction is possible. Expansion in all pure NHL binders is below 1mm (measured as per EU Norm EN 459.2/5.3.2.2)
					NHL LIMES GIVE THE BEST REPRODUCTION OF SAND COLOR
All St.Astier mortars can be reworked (8-24 hours), reducing wastage and increasing work rate. This is due to the absence of cement, gypsum, pozzolans, or high aluminates.