Sint Jacobskerk (Saint James Church) in Leuven, Belgium. Lime mortar use in many masonry structures has proved durable for hundreds of years. Photo: Koenraad Van Balen.

Project Objectives
Lime was and is one of the most geographically widespread building materials, extensively used where limestone or shells exist in sufficient quantities for the production. 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 BC in the Middle East and between 2,000–1,000 BC in Central America and Mexico.

Although high-calcium lime remains in use around the world as a primary construction material, more recent construction practices have incorporated hydraulic components in the 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 the wall thickness. These widely available materials may also have many properties (including high strength, low porosity, and high salts content) that are incompatible with historic high-calcium lime mortars, plasters, and renders. 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.

	This cement repair has caused damage to the brick units, while leaving the repair mortar itself unharmed and even more unsightly. Photo: John Fidler, © English Heritage.

The damage caused to original mortars and plasters has led to a search for more compatible conservation materials of similar composition and properties, sparking an 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 traditional techniques and the reasons and circumstances for the durability of ancient and historic architectural elements that incorporate high-calcium lime.

This project is identifying and interpreting current scientific knowledge, including the results of systematic laboratory studies carried out by the three partner institutions—the Getty Conservation Institute, the University of Granada, and Catholic University in Leuven, Belgium—in order to provide insights into the fundamental properties of high-calcium lime mortar that can be used to explain many previous failures of architectural conservation and to provide a wider basis for the appropriate choice of materials and methods in the conservation of high-calcium lime mortars and plasters.

Project Overview
In order to conduct meaningful and applicable studies of the technology of lime, a comprehensive, focused literature compilation was prepared and is available online, The Preservation of Lime Mortars and Plasters Project Bibliography. The bibliography is also subject indexed to provide a readily understood format for the professional architect, practicing architectural conservator, and student.

	The stages of the lime cycle indicated in red, with the fundamental properties that have been the focus of research projects indicated in blue. Drawing: Beril Bicer-Simsir.

The properties of lime vary greatly, depending upon variations in each stage of its production, commonly referred to as the lime cycle. The lime cycle was used as an organizing principle to isolate important issues in lime technology that needed to be addressed in laboratory studies.

Identifying the research components with individual stages of the lime cycle is important conceptually because the properties of different lime types may vary in one stage of the cycle, yet be similar in another stage of the cycle. For example, the working properties of slaked lime putty and hydrated lime putty vary in plasticity (ease of spreadability and application) but may vary little in the ultimate compressive strength of the mortar when a similar amount of binder and aggregate volume ratio is used.

The Lime Mortars and Plasters project involves a number of lines of research, based upon investigations of what occurs within individual stages of the lime cycle, and aims to increase the knowledge base of architectural conservators regarding the materials and application procedures that might be considered in the treatment of lime-based mortars and plasters.

The Inorganic Materials Research Laboratory of the Getty Conservation Institute investigated and compared the differences in 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 has to date established several important aspects of the traditional processes of slaking lime and the industrial processes of producing hydrated lime:

• the micro-textural (nanostructure) differences between calcium hydroxide in slaked lime putty and hydrated lime putty, and how these variations correlate with the behavior of the materials with regard to working properties of application and how the binders function in a mortar;
• the benefits of aging slaked lime (i.e., the time dependent changes upon long term storage under water);
• some of the differences in setting (hardening and carbonation) that are brought about by the use of slaked lime, hydrated lime, and hydraulic limes.

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

The competition between hydration of calcium silicates in hydraulic lime and the carbonation of calcium hydroxide is also being studied, as it has been noted that, unlike high-calcium lime, the strength of hydraulic mortars decreases after an initial increase in strength.

Also examined were the primary reasons as to why lime mortar, due to its weakness and deformability, has added to the durability of historic masonry.

The evolution of prisms into platelets as putty ages: a) fresh lime putty; b) putty aged two months, with corrosion on the prism side [arrow] as platelets begin to form; c) putty aged six months; d) putty aged two years. Photo: Carlos Navarro-Rodriguez.

FESEM photomicrographs of a) oven dried slaked lime putty showing agglomeration of platelike Ca(OH)2nanocrystals, and b) commercial dry hydrated lime also displaying extensive agglomeration. In both a) and b), randomly oriented and crystallographically oriented aggregates are present. Details of representative oriented aggregate of Ca(OH)2 nanoparticles attached by the (0001) planes are seen in c) and randomly oriented aggregates in freeze-dried slaked lime particles are seen in d). Photo: Carlos Navarro-Rodriguez.

The effects of a commonly used additive for lime mortars in the American Southwest and South America—the water extract of the nopal cactus (Opuntia ficus indica)—were also explored by the GCI.

	The nopal cactus (Opuntia ficus indica). The water extract of the nopal cactus is a common ingredient of plasters in the Southwestern United States. It is added to the slaking water to make slaked lime putty, or to the water to make the putty with hydrated lime. Photo: Eric Hansen.

In addition, the GCI is researching the short-term and long-term effects of organic and inorganic (pozzolans) additives on high-calcium lime properties, particularly in relation to rheological properties and strength development. A new line of research is the 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.

At the Department of Mineralogy and Petrology, Study and Conservation of Construction Materials of the Architectural Heritage Group of the University of Granada, Spain, further investigations of slaked lime putty through instrumental and physical testing were able to establish 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, among many other factors, the effects of organic compounds on the nanostructure and colloidal behavior of lime. This type of information is being used in applications for the design of restoration mortar mixes with the addition of natural and synthetic organic compounds.

At the Department of Civil Engineering, Building Materials and Research Group of the Catholic University of Leuven, Belgium, investigations of the carbonation reaction of lime and its kinetics at ambient temperature showed that carbonation speed seems to be dependent upon the specific surface of lime types.

	Detail of Sint Jacobskerk (Saint James Church) in Leuven, Belgium. The deformability of the masonry wall without cracking is due to the use of a weak lime mortar. Photo: Koenraad Van Balen.

The competition between carbonation and hydration on the hardening of calcium hydroxide and calcium silicate binders is now being studied. The Leuven group has previously shown, through tri-axial compression testing that simulates the stress state of mortar in masonry, how the weakness and deformability of lime mortars contribute to the durability of historic 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 (e.g., in areas where earthquakes are common or masonry is built upon a slowly shifting surface).

Last updated: October 2007