Natural Hydraulic Limes

NHL 2 Feebly Hydraulic lime is used for applications such as the consolidation of frail historic masonry. It can be used where the highest amount of "free lime" is desired along with the greatest modulus of elasticity for applications such as when fine sand is added to make interior finish coats of lime plaster. It can be used to make very soft pointing mortars for repairing around friable historic masonry where the wicking away of water from the brick or stone to the lime mortar joint and out to the atmosphere is greatly encouraged. NHL 2 closely duplicates the material used for the majority of all building history as it is like lime putty in many of its properties. Traditional lime putty sets with air to fully cure over a six week period and it is referred to as an "Air Lime." NHL 2 sets with air and water and cures in about four days and is referred to as a "Water Lime."                            Cured NHL 2 is about as hard as a piece of chalkboard chalk. Mixes containing NHL 2 and 1-2% casein powder can make an injection grout that will flow into every interstice in cavities behind plaster if you are considering exterior historic stucco re-attachment to re-establish the bond of the back of the stucco to the masonry substrate. Grout injections with NHL 2 can also be used to stabilize masonry before repointing. Consider using casein as the catalyst to increase flow rather than pulverized fly ash (PFA) since PFA is a pozzolan. Tests done on lime mortars modified with PFA raise the question as to whether run-away strengths may continually increase with on going densification of the lime/PFA grout. Increased densification may possibly trap moisture in cavities filled with lime and PFA.

NHL 3.5 Moderately Hydraulic lime is used for applications such as laying or repointing brick, stone and terra cotta units even in extreme freeze-thaw climate cycles. It can be used to make scratch, brown and finish coats of exterior stucco or interior plaster. It has a significantly high "free lime" content that is responsible for self-healing mortars. The available free lime goes into solution during subsequent rains and then re-crystallizes across open fissures as is characteristic of historic pure lime mortars. NHL maintains a hexagonal plate crystal structure which allows the plates to go in between each other for greater flexural ability. Portland cement has a needle-like crystal structure which is brittle and  must incorporate many control joints to strategically control cracking. NHL 3.5 is designated as a moderately hydraulic lime and will reach over 750 p.s.i. in six months when                            blended with 2-1/2 parts clean, sharp and well-graded sand. Most all above grade interior and exterior brickwork and stonework built with modern cement is recommended to be built using a Type N cement mortar, which reaches at least 750 p.s.i. in 28 days. Since modern designs call for structurally supporting steel or retaining walls made of concrete or concrete blocks laid with strong cement mortars, NHL mortars can be used for all of the veneer work, stucco and repointing where the old-world look, feel and properties of lime mortar is desired.

NHL 5 Eminently Hydraulic lime is used for applications such as building or repairing wall head copings, pointing and parging the water table or foundation of a building and making lime concrete. Sea-bearing walls can be built and repaired with NHL 5 since it has only trace amounts of soluble salts such as Potassium Oxide, Sodium Oxide and tri-calcium aluminates. This diminishes sulphate attack which normally happens in mortars prepared with Portland cement or where white cement has been introduced into synthetically created hydraulic lime at the time of manufacturing to ensure a hydraulic set. When Portland cement is present, you will find a high percentage of tri-calcium aluminates which will react with the sulphates thus producing an overall expansion and the eventual disintegration of the mortar or concrete especially in marine environments.
                           St. Astier's Pure and Natural Hydraulic Lime is formed naturally by the presence of reactive silica which is caught up into the limestone deposit making it a truly pure naturally occurring siliceous hydraulic lime. Portland cement is an extreme form of "Hydraulic Lime", whereby it will also set with water and under water. Portland cement however is known to have many of the adverse chemical reactions as noted above, a brittle nature and releases tremendous amounts of Carbon Dioxide into the environment in its production over the equal amount of NHL 5. Specifiers should be aware of Argillaceous Hydraulic Limes where clay is naturally occurring or added to make these limes become hydraulic and what consequences may result from their inappropriate applications.

St. Astier Natural Hydraulic Lime Mortars

In 1833, Louis Vicat surveyed the limestone of the St. Astier basin in Dordogne, France, (near Bordeaux), where building limes have possibly been produced as early as the Roman occupation of the region. Vicat concluded that this material was suitable and appropriate for the production of Natural Hydraulic Lime.

Industrial production began in 1851. Today, St. Astier Company's production capacity is over 100,000 tons per year. For over 150 years the St. Astier Company has supplied the required material that scientists, conservators, and practitioners have specified for repair, restoration, and new build work at some of Europe's most impressive historic buildings. Many of the buildings in Europe are much older than that in the United States and techniques in appropriate historic restoration have long been discovered and applied, such as the use of Natural Hydraulic Lime and sand to repoint over the bedding joints of buildings built with only lime and sand for mortar.

Thousands of years of the history of lime and sand mortar have proven its effectiveness and the use has largely laid dormant in the United states for the last fifty years. Irreversible damage and the premature need for re-repointing of buildings has caused building owners and conservators in our country to seek alternatives to modern repointing mortars that are not working or don't have a long track record of success. The cost of mortar for repointing is inconsequential compared to the labor to do a job again if it is not done correctly. Many historic buildings over 100 years old in the United States, originally built with lime and sand mortar, are just now receiving their first repointing work. This helps to prove that the right lime and sand mixture can be very durable and cost effective to repeat the success of service that the historically proven mortar has given.

What is Natural Hydraulic Lime ?

Lime comes basically in two forms, Hydrated and Hydraulic.

Hydrated Lime (or calcium lime, air lime, high calcium lime) is the most commonly used and known lime. It comes in powder form, like Type N, NA, S or SA, or in a paste form, known as Lime Putty. Although it has wonderful physical and aesthetic qualities, hydrated lime has very strong limitations in the construction industry. Hydrated Lime sets only by carbonation (the re-absorption of CO2) and thus, thicknesses are very limited. It is not suitable or practical for scratch or brown coats in a plaster, but can do wonders as a finish coat but requires blending most of the time. The application requires very highly skilled labor and that special care must be taken to avoid physical harm, such as burns when in the form of quicklime. Consequently, it becomes a specialty with associated costs.

Hydraulic Lime, although basically of the same chemical composition as hydrated lime, hydraulic lime has its initial set with water, much like cement, and a second set by absorption of CO2. This allows for simplicity of application although basic care is required, such as minimizing the amount of water, good sand and tarping. The cost of labor is comparable to normal stucco installation. Hydraulicity for St. Astier lime is achieved by the nature of the raw material.

Hydraulicity of Hydrated lime may be obtained by the addition of cement, pozzolanes, etc. In such a case, the introduction of foreign chemical elements may have immediate or long range inauspicious consequences. It is not advised to use hybrid products with unknown performances.

Documents:

5 Most Important Steps (English)

5 Most Important Steps (Spanish)

Material Safety Data Sheet

St. Astier NHL 2 Data Sheet

St. Astier NHL 3.5 Data Sheet

St. Astier NHL 5 Data Sheet

Masonry Repointing and Reconstruction Specifications

St.Astier Information Packet - provides an overview of some of the properties and advantages of using St. Astier Natural Hydraulic Lime Products.

Visit the St.Astier Web Site for more techinical information and application guidelines.