Scientific information of lime

Lime and lime products : production and application

Introduction

For centuries, lime has been one of the most important building materials. Lime can be used, like masonry mortars, as an elementary construction element, but nevertheless the decorative qualities of this product are unsurpassed. 

Due to the recent and quick rise of cement binders however, the basic knowledge of the production and application of lime mortars has largely been wasted. The renewed scientific interest and analyses, and the given of the living of historical buildings, prove the exceptional qualities of lime mortars.

This text explains, step by step, the know-how of the different lime products, starting with the raw materials, to properties and ending up with the application domains.

The relationship between lime and the other commercially available binders will also be discussed, to clarify the extensive possibilities of lime mortars.


Scientific background information

The lime cycles:

The binder lime is produced from limestone rocks. Limestone is mainly composed of the mineral calcite (with a chemical formula CaCO3). Consequently, other raw material composed of calcite, can be used to produce lime binders (e.g. shells, chalk). Under normal circumstances, calcite is a low to non reactive material, and therefore limestone is appropriate as a building material or as an aggregate. To obtain a reactive and workable material, limestone has to be burnt in a lime kiln at a temperature of ~900°C :
 

CaCO3  CaO + CO2



In this process, calcite (CaCO3) is transformed into calcium oxide (CaO) and carbon dioxide (CO2). Calcium oxide or quicklime is a very dangerous material, which will react violently in contact with water (risk of explosion). Under controlled circumstances, the addition of water will lead to the transformation of quicklime into slaked lime also called calcium hydroxide or portlandite :
 

CaO + H2 Ca(OH)2



Slaked lime consists of a white-coloured powder. This powder is without danger and is commercially available in trade. To increase the workability of this product a small excess of water is added during the slaking process. The result is a paste, called lime putty.

In contact with the atmosphere, lime putty will react following the carbonation reaction :
 

Ca(OH)2 + CO2  CaCO3 + H2O



The reaction product is calcite, with an equal chemical formula as the original host rock. Consequently, this sum of reactions is called the lime cycle.

Mostly, natural limestone not only contains the mineral calcite. The clay mineral content of limestones is at least 5% (Fig. 1). Limestones with clay contents between 10% and 30% are extremely useful for the production of natural hydraulic lime, which does not only harden with air but also under water. The reason of the hydraulic hardening is a reaction between calcite and clay minerals during the burning process.

Clay minerals are mainly composed of the components SiO2 and Al2O3. By burning clayey limestones at a temperature between 900°C and 1200°C, CO2will be liberated. A part of the calcium oxide will react with the components of the clay minerals :
 

2CaO + SiO2  C2S



In which :
C stands for calcium oxide (CaO) and
S stands for silicon dioxide (SiO2).

The reaction of aluminium oxide (A) gives further the components C2AS and C3A. As all these components can react with water, a minimum quantity of water is needed during the slaking process.

Two reaction mechanisms are important for the hardening of a hydraulic lime mortar : a carbonation reaction similar to air-hardening lime, and a more important hydration reaction. The hydraulic hardening or hydration takes place by the reaction with water, and results in a complex molecular structure with possible higher compressive strengths :
 

2C2S + 4H2 3CSH + Ca(OH)2



CSH is a needle-shaped mineral, providing the necessary strength during the hardening process. The newly formed portlandite becomes “free lime” and can, as described above, react with the surrounding air to form calcite. So, water is even required to harden a hydraulic lime mortar.

The water content during the slaking process is a very important factor. A high water content during slaking will result in a high free lime content in the dry binder. However, as the carbonation reaction is very slow compared to the hydration and as Ca(OH)2 is water-soluble, this will result in a high risk of efflorescence. On the other hand, if the water content during slaking is low, the initial free lime content is also lower. Although almost 40% free lime will be formed during the hydration, depending on the free lime already existing in the binder (dependent on the way of slaking), the distribution will be more diffuse. The scattered distribution of free lime and a high initial porosity of a lime mortar will allow CO2 to diffuse more swiftly into the lime mortar. This will result in an almost complete reaction of the calcium hydroxide into calcite.

Portland cement is burnt at even higher temperatures, namely >1450°C. Due to the elevated temperatures, C2S will not be formed, but C3SC3S will react with water at even higher speeds than C2S. As the amount of hydraulic components in cement is much higher (clay content, Fig. 1), the hardening is almost completely comprised of hydration. Due to the high hardening speed, cement has become very popular for industrial applications, in a very short time span. 

Hydraulic properties can also be obtained by the addition of certain volcanic rocks or crushed ceramics. This practice was already experienced in Roman times. The Vesuvian volcanic tuffs near the village of Pozzuoli were extremely suitable for addition in lime mortars, and became known as pozzolanas. In western European countries, pozzolanas of the Eiffel volcanic area were mainly used. This lime mortar type is still known as trass lime. 

More recently, the different types of building limes are classified according to their (CaO + MgO) content or, in the case
of hydraulic limes, to their compressive strength, according to the norm “European Pre-standard (ENV 459-1) Building lime – Part 1: Definitions, specifications and conformity criteria” (1994). The following terms are given:
 

Calcium lime 90 CL 90 dolomitic lime 80 DL 80
Calcium lime 80 CL 80 hydraulic lime 2 HL 2
Calcium lime 70 CL 70 hydraulic lime 3.5 HL 3.5
dolomitic lime 85 DL 85 hydraulic lime 5 HL 5


Table 1 : Classification of building limes, according ENV 459-1 (1994)



Properties of lime mortars, in relation with cement mortars

From binder to mortar:

Most binders (lime, cement, others) cannot be used alone. For most applications, the addition of an aggregate is required. The combination of binder and aggregate is defined as a mortar. The choice and the quantity of the aggregate is critical depending on the type of application. The aggregate will serve for: 1) counteracting shrinkage, 2) increase the cohesion and the strength of the mixture, 3) accelerate the crystallisation of the binder, 4) filler, which is cheaper than only binder.

The aggregate will be chosen in function of:

  • The purity: the aggregate has to be free of clay and organic matter. These components will degrade in time, and will form molecules, destructive for a mortar.
  • The angularity: angular aggregate fragments have a higher adherence capacity than rounded fragments.
  • The granulation: the grain size distribution of the aggregate has to be well-balanced between the different sizes. Each mortar type and application requires a specific grain size distribution. Usually, the ideal distribution has the shape of a Füller curve.


Generally, the higher the water content of a mortar, the lower the strength of the final product. Fig. 3 illustrates the development of the compressive strength during a period of 180 days, at different proportions H2O/binder.

 



 

Binder/aggregate ratio


It was already mentioned that the task of the aggregate is to increase the strength of the hardened end-product. A shortage of the binder amount will give negative consequences for the final strength. The ideal proportion between binder and aggregate is about 1/3. Practically, 600kg of binder will be required for each m³ of aggregate.
 

 


 

Granulometry of the aggregate


An increase in the maximal grain size of the aggregate will have a positive influence on the final strength of a lime mortar. (Fig. 5). Consequently, it is very important to use a grain size as large as possible, during the preparation of the mortar. The pores between the large grains can then be filled with grains of a smaller diameter (see Fig. 2 : Füller curve) and the binder. It is however evident that the maximal grain size will be dependent on the type of application. As an example : bedding and pointing mortars require larger grain size then the restoration of a fresco, which requires only very fine mortar layers.
 

 


 

Aggregate type


An increase in the hardness of the aggregate, will lead to higher compressive strength of a lime mortar (Fig. 6). Quartz sand (hardness 7) will have a much higher compressive strength then a dolomite sand (hardness 3). Not only is the initial hardness of the minerals of great importance. A rock, used as an aggregate, must not have harmful transformation products. Initially, a porphyric rock will have the same hardness as a quartz-rich rock. Porphyry however will transform into softer clay minerals, which will lead to lower final compressive strengths of the lime mortars (Fig. 6).
 

 


Evolution of the compressive strength of lime mortars in time (>180 days)

Generally, the evolution of the compressive strength of a lime mortar will be expressed in periods of 7, 28, 60, 90 and 180 days. These standard values are extremely suitable for mortars based on a cement binder. As the hardening of cement is very fast, the final strength of a cement mortar will be reached in several days. After 28 days, the hydraulic reaction of cement will be completed. Hereafter, cement will not be capable to have a further development of compressive strength.

The reaction of lime mortars (air-hardening and hydraulic) is much slower. Contrary to cement, lime will preserve its reactive capacities. To complete the reaction of lime mortars, hundreds to thousands of years are required. Consequently, the curve of the compressive strength will increase through years (Fig. 7). Furthermore, the standard curve between 7 and 180 day is actually (almost) useless for the study of lime mortars.

The long-lasting reactivity of lime mortars, provides a good response of a lime mortar, against changing pressure conditions. As a normal building will deform during its existence, lime mortar is an ideal product to withstand these strains, without cracking.
 

 


Environmental aspects

Lime mortars are burnt at temperatures between 900°C and 1200°C. To obtain cement however, temperatures above 1450°C are needed. Consequently, more energy is required for the production of cement. This is also translated into a higher fuel consumption, which is environmentally unfriendly.

Not only is the fuel a polluting agent, CO2 will be driven out during lime and cement production, and this gas is a main cause of the greenhouse effect. Looking to the formula of the lime cycle, indicates that as much CO2 is bound during the carbonation process as is released during the burning of limestone. Consequently, lime mortar is an ecologically sound product. 


Damp diffusion and hygroscopic salt content 

One of the most important properties of lime mortars is the damp diffusion capacity. Lime mortar, either air-hardening or hydraulic, has a damp-open structure. 

This means that gasses can migrate through a lime mortar, while liquids will be inhibited. Remark that during the hardening reaction, carbon dioxide and water will have to circulate in the mortar to obtain a good reaction. Fixation of CO2 in a lime mortar requires also dehumidification into the free air, to prevent water accumulation in the mortar. The presence of hydrated lime that has a natural porous structure, the crystallisation of a minor amount of CSH, also forming an open rack structure and the choice of well-balanced aggregate size (porosity 30%), will guarantee the damp diffusion capacities of a lime mortar.

In cement however, the hydraulic components are abundant. In this case, needle shaped CSH crystals will not form a rack structure, but a closed web, in which damp circulation is not possible. Consequently, water accumulation will occur in and behind a (hardened) cement mortar.

An open structure in which damp circulation occurs, will also have implications on the migration of salts in a lime mortar. Firstly, a lime mortar has a low salt content anyhow. Adding water to the dry lime mortar powder, the salts will dissolve partly. In a dry and porous medium, like pores in the lime mortar, part of the water will evaporate. This leads to a saturation of the dissolved salts, and a precipitation of salt in the pores of the mortar.

In practice, salts will be stored in a lime mortar. The storage capacity of a lime mortar is even that large, that externally supplied salt can be stored in the pores of a lime mortar.

In cements, the salt content is much higher. Furthermore, it was already mentioned that there was a low migration possibility for water and damp in a cement mortar.

The present moisture and the dissolved salt will try to migrate in porous structures.

As this is not possible in the cement mortar itself, water will migrate along cracks and weaknesses in the mortar and the adjacent wall (e.g. at the contact between the joint and the brick) Along these structures salt will be precipitated.

What does this mean concretely for constructions built or reinstated with lime mortar and cement mortar, respectively?


Constructions built with lime mortars

Imagine a construction, built with bricks and lime mortars, pointed also with a lime mortar. In this case, the damp diffusion capacity of the lime mortar is much higher than this of the brick. In dry weather conditions, the humidity level of the interior will by higher than that of the exterior. Vapour will migrate from the inside of the building to the outside. As the damp diffusion capacity of the lime mortar is higher than that of the brick, vapour will migrate preferably through the lime mortar joints. As a lime mortar is also waterproof, rainy conditions will not influence the migration pattern. Small amount of water will however penetrate into the bricks and the joints. In dryer conditions the migration pattern will again be turned from the bricks towards the joints and the moisture will be evaporated at the surface of the joints.

Suppose further a certain salt content in the rain drops and in the bricks. As the salts are soluble in water, the eventual migration of the salts will be from the interior to the exterior. As the porosity and thus the storage capacity of the lime mortars are very high, and the salt content is generally low, the damp circulation systems will be guaranteed for years. If the lime mortar should be saturated with salt, and in view of the expanding characteristics of these salts, the weakest spot in the lime mortar will be affected. Generally, this spot corresponds with the joint, which will be attacked. However, a lime joint is easy replaceable, leading to a cheap restoration.


Constructions built with cement mortars or buildings built with lime mortars, renovated with cement.

Suppose once more a brick building, bedded and pointed with a cement mortar. In this specific case, the porosity and the damp diffusion of the bricks are low, but higher than those of the cement mortar. In dry weather conditions, the humidity level inside will be higher than that outside, and the moisture will migrate to the exterior. Moisture will be barricaded against the cement mortar, and as bricks will not take up water, the moisture will be accumulated on the interior sides of the wall. Then, the brick and cement mortar will disintegrate.

The result of this was the development of a new building method: construction with cavity walls.

Historic buildings however are generally constructed with porous bricks, and the moisture will preferentially infiltrate in the pores or along weak zones in the wall (e.g. contact zone joint-brick). By evaporation, the salt will be stacked along these weak zones or in the bricks. Consequently, mainly the bricks will disintegrate and to lesser extent the cement mortars (Fig. 8). In restoration applications, it is much more expensive and difficult to repair this damage.
 


 

Applications with lime products

 

Possible applications with lime mortars are: bonding medium for weak substrates, injection, screed layers, bedding, pointing, plastering, and rendering. More specific applications are the decorative marble finishes, “Stucco Venetiano”, lime paints and the restoration of frescos. All these applications require specific demands to create a successful job. The different aspects of the lime mortars have to be adapted, depending on the application. These changes include the binder/aggregate ratio, the aggregate type and the amount of water to add to the mortar powder.

 

  • Bonding of weak substrates Bonding mortars are mortars based on natural hydraulic lime as binder, with addition of silica sand with an equilibrated grain size curve. Bonding mortars are applied when the background is unsuitable to guarantee an optimum bonding of other mortars on the substrate. Surfaces with a closed structure such as concrete slabs and very old masonry, or walls completely saturated with water will have to be treated with a bonding mortar.

 

  • Injection mortars Injection mortars are another type of bonding mortars, with a low plasticity, so they can be injected in cracks and pores. As the size of the opening increases, the grain size of the aggregate will also increase. For example, the static consolidation of crevices requires a maximal grain size of minimum 4mm. In more specific applications, like the consolidation of frescos, a very small-sized aggregate or just binder will be required.

 

  • Rendering Natural hydraulic limes are mostly used for rendering and plaster systems. Contrary to modern renders (in one layer), lime renders are composed of a well-defined layer system. If necessary, the first layer will be a bonding layer, to provide the necessary strength and an uniform suction. The next layer is an undercoat layer, consisting of a mortar, >1cm in thickness, and a coarse granulometry. The undercoat layer will be finished with a decorative finishing mortar, with a thickness of a few millimetres. When desired, the finishing coat can be completed with a compatible material like lime paints or wash, silicate paint, or marble finishes.

 

  • Bedding and pointing Through centuries, man has looked for different materials for the bedding of construction elements. The first-known bonding material was clay, which is hardened by drying. The Greeks developed the technology of burning lime at high temperatures. During Roman times, the knowledge of lime mortars reached a peak. Different types of aggregates were added to obtain the required properties of the lime mortar. In the Middle Ages, the practical technology of the Romans was preserved. From the Renaissance onwards, scientific research started on the lime mortars, leading to the understanding of the hydraulic reaction in the beginning of the 19th century. This knowledge was the direct reason for the invention of Portland cement.
     
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