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The Lime Mortars and Plasters (LMP) project, conducted in partnership with the Catholic University of Leuven, Belgium and the University of Granada, Spain, identified and interpreted current scientific knowledge on high-calcium lime mortar, including the results from systematic laboratory studies carried out by the LMP project, to provide insights into the fundamental properties of this ubiquitous building material.

The components of the project included:

The insights obtained through this project explain many previous failures of architectural conservation methods and provide a wider basis for the appropriate choice of materials and methods in the conservation of high-calcium lime mortars and plasters.

Lime is one of the most geographically widespread building materials, used extensively in areas where limestone or shells exist in sufficient quantities. High-calcium lime is also one of the oldest components in ancient and historic floors, masonry, wall paintings, renders, and architectural sculptures. Its use dates to at least 8,000 BCE in the Middle East and between 2,000–1,000 BCE in Central America and Mexico.

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Although high-calcium lime remains in use around the world as a primary construction material, more recent construction practices have incorporated hydraulic components in mortar mixes: pozzolana, natural hydraulic (hydrated) limes, and, since the end of the nineteenth century, ordinary Portland cement (OPC). These materials produce mortars that are comparatively faster setting, stronger, and stiffer. Their use shortens construction time and reduces wall thickness. These widely available materials may also have properties that are incompatible with historic high-calcium lime mortars, plasters, and renders, including high strength, low porosity, and high salts content. Many mistakes have been made with their use in architectural conservation as repair or replacement material. Complicating the inappropriate use of cement and cement blends is the difficulty of removing or replacing them without causing major damage to existing mortars, brick, and stone.

The damage caused to original high-calcium lime mortars and plasters by the inappropriate use of mortar mixes has led to a search for more compatible conservation materials of similar composition and properties and sparked interest in the revival and use of traditional technologies of high-calcium lime production and application. However, there is a lack of modern scientific knowledge and little relevant technical information about these traditional techniques and the reasons and circumstances for the durability of ancient and historic architectural elements incorporating high-calcium lime.

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In order to conduct applicable studies of the technology of lime, the project team researched and prepared a comprehensive bibliography, which is now searchable within AATA Online. Once this was completed, the project conducted a number of lines of research, based upon investigations of what occurs within individual stages of the lime cycle, and focused to increase the knowledge regarding the materials and application procedures that might be considered in the treatment of lime-based mortars and plasters.

The project investigated and compared the physical properties of putties and mortars made with slaked lime and hydrated lime. Initial studies carried out at the GCI also determined the micro-structural changes that occur in the aging of lime putty. The project established several important aspects of the traditional processes of slaking lime and the industrial processes of producing hydrated lime:

  • micro-textural (nanostructure) differences between calcium hydroxide in slaked lime putty and hydrated lime putty, and how these variations correlate with behavior of the materials with regard to working properties of application and how the binders function in a mortar
  • benefits of aging slaked lime (i.e., the time-dependent changes upon long-term storage under water);
  • differences in setting (hardening and carbonation) brought about by the use of slaked lime, hydrated lime, and hydraulic limes
  • effects of a commonly used additive for lime mortars in the American Southwest and in South America—water extract of the nopal cactus (Opuntia ficus indica) on rheological properties and strength development.

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The results of these studies, along with exploration of the effects of the traditional practice of adding organic materials to lime, were used to develop and identify the most appropriate remedial materials and mixtures for case specific use.

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Regarding hydraulic lime mortars, the GCI researched the short-term and long-term effects of inorganic (pozzolans) additives on high-calcium lime properties, particularly in relation to strength development. The competition between hydration of calcium silicates in hydraulic lime and the carbonation of calcium hydroxide was also studied—unlike high-calcium lime, the strength of hydraulic mortars decreases after an initial increase in strength. Another line of research developed by the GCI is use of X-ray diffraction and thermal-gravimetric analysis to characterize the composition of natural hydraulic limes (NHL), which is often unclear or in doubt.

Investigations carried out by LMP project partners at the University of Granada, Spain, established the nanostructure and colloidal nature of calcium hydroxide in putties. The colloidal nature profoundly affects the rheology of the putties, including plasticity, and other properties affected by particle size and shape, such as viscosity and water retention. This data can be used to explain, the effects of organic compounds on the nanostructure and colloidal behavior of lime. This type of information is being used in the design of restoration mortar mixes with the addition of natural and synthetic organic compounds.

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Project partners at the Catholic University of Leuven, Belgium, investigated the carbonation reaction of lime and its kinetics at ambient temperature to show that carbonation speed seems to be dependent upon the specific surface of lime types. Also examined were the primary reasons why lime mortar, due to its weakness and deformability, has added to the durability of historic masonry. They demonstrated, through tri-axial compression testing simulating the stress state of mortar in masonry. While those mortar properties have minor effect on the strength of the masonry, they increase the ability of masonry to withstand deformability and shock.

Results from the Lime Mortars and Plasters project have been published extensively. A complete list of articles can be found on the Related Materials page.


Page updated: March 2011