Masonry Pore Structures

Lime putty for mortar has been in known use for over 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.

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1 LIME PUTTY TO 3 PARTS OF 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 have a tremendously high and desirable liquid and vapor permeability.

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1 NATURAL HYDRAULIC LIME (3.5) TO 3 SHARP SAND

Note the high porosity of this mortar formulation. The wall can breathe by allowing moisture to enter and exit the system, encourage the crystalline bridging phenomenon (also known as the autogenous or self-healing properties of lime mortar) by dissolving free lime in the mix and re-depositing it to close larger fissures. Excess moisture then quickly evaporates back into the atmosphere. The open pore structure allows the carbon dioxide, needed for carbonation during curing, to be delivered deeper into the mortar.

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1 NATURAL HYDRAULIC LIME (5) TO 3 SHARP SAND

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.

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1 PORTLAND CEMENT TO 3 PARTS SHARP SAND

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, Portland cement based mortars trap moisture within the wall system because their dense pore structure does not always allow it to escape through evaporation. The saturated wall of trapped moisture can lead to moisture being driven much further into the building when heat inside draws moisture through the wall, or trapped moisture evaporates out through the softer historic brick or stone accelerating its deterioration.

Petrographic thin section images courtesy of William Revie of The Construction Materials Consulting Group; Stirling, Scotland

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St. Astier Natural Hydraulic Lime for Historic Restoration, Conservation and New Build Projects

Prior to the 1940’s 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. All buildings move and 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.Portland-needle-lime-hexagonal

A contribution of the National Institute of Standards and Technology.

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.

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”. It 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 Aluminoferrites, (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.

For additional information or to purchase Natural Hydraulic Lime  click here

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Lime as a Green Build Material

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

St. Astier 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 a fraction of what is needed to produce cement. Consequently, the release of CO2 into the atmosphere is reduced considerably. Furthermore, contrary to cement, 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. Other attributes listed below, prove over and over NHL outperforms modern day Portland cement.

  • The absence 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.
  • Very rapid evaporation of moisture from NHL mortars ensures that the drying cycle is faster than cement mortars and subsequently the healing requirements are lower.
    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 to adjust pH levels.
  • Breathability, elasticity, plasticity, gradual development of strength, low shrinkage, longevity, CO2 absorption, self-healing through the presence of free (or available) lime in crystalline bridging to close minor fissures, are all highly desirable. These traits, with sustainability and “greenness”, are only some of the qualities of St. Astier Natural Hydraulic Lime.
  • 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 manufacturer’s warranty subsequently extends for 50 years.

 

CO2 Emissions Chart
CO2 Emissions Chart

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