Kalkboek. Het gebruik van kalk als bindmiddel voor metsel- en voegmortels in verleden en heden

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3 Summary

This publication deals with the application of a special and separate group of masonry mortars and pointing mortars: lime based mortars. Although attention will be paid to historical construction practices, the principal subject dealt with is contemporary restoration practices. In that context, the question is not only what the role of these mortars is now, but rather what their role could be.

Nowadays, bricklaying and pointing is done mainly with mortar in which Portland cement is the principal or even the only binder. As a result, much of the knowledge on lime mortars has been lost. Since the end of the nineteenth century, lime mortars have been replaced more and more by cement mortars. There are several different reasons for this phenomenon. The most important reason may be that a cement mortar achieves its final strength much more rapidly. Construction practice had to adapt to an ever-greater need for buildings, such as houses, factories, schools and civil engineering constructions. This was mainly the result of population growth and industrialisation. A milestone in this process was the explosive growth of construction activities immediately after the Second World War, when the restoration of wartime damage and the alleviation of the substantial housing shortage had to be dealt with simultaneously. Building had rapidly become a necessity, and it could only be achieved with cement mortars. Moreover, that is still possible. Nevertheless, a rediscovery of lime mortars is presently taking place, especially for use in restoration and renovation. This development is based on the principle of compatibility; mortars used for restoration must be highly compatible with historical materials. Nowadays, in the daily practice of restoration, mortars are sometimes used that are incompatible or insufficiently compatible. This leads to less durable construction and can cause damage in the long term. In many cases, lime mortars are compatible with historical brickwork, or at least more compatible than the cement mortars used in new construction. The examples of historical lime mortar that are still in good condition today demonstrate that lime mortar, provided it has the proper composition and was applied competently, can be extremely durable. This publication deals with the application of lime mortars for bricklaying and pointing and was motivated in part by an appreciation of this material.

The recipe for lime mortar

In lime mortars, the binder is either air-hardening lime (calcium hydroxide, Ca(OH)₂) or hydraulic lime. Hydraulic lime consists partly of calcium hydroxide, and, in greater quantity, calcium silicate, which reacts with water and then hardens. Slaking quicklime with water produces calcium hydroxide. Quicklime (CaO) is obtained by burning limestone or seashells (the raw materials for lime) in limekilns. An important constituent of limestone and seashells is calcium carbonate (CaCO₃).

Mortar is obtained by mixing a binder with aggregates. Mortar is usually composed mainly of aggregates. In the case of mortar for bricklaying or pointing, this is usually sand. The binder adheres to aggregate and brick. Sometimes, additives are also used. The nature and relative amounts of all these constituents determine the hardening process and the properties during use and after setting. The constituents of lime mortars are discussed in Chapter 3 (Section 2: The raw materials for lime mortar). However, the properties of a mortar are not determined purely by the properties of its constituents, but also by their relative amounts. The choice of a mix design for mortar must be made on the basis of the specific application for which the mortar is intended. This is discussed in Chapter 3, Section 6: The constituents and their proportions. However, this publication is not a book of recipes! Putting together a mortar is work for specialists and must therefore be left in the hands of such mortar technologists. Contractors and pointers are specialists in practical application of mortar and they must seek their satisfaction there, not in making small changes in mortar mixes or adhering stubbornly to their own recipes. As has been demonstrated all too often in practice, the monuments may then be the victims.

Hardening

Following the discussion of the composition of mortar, attention is given in Section 7: The hardening of lime mortar to the hardening process.

The hardening of air-hardening lime mortars can be summarised in the lime cycle: the process from stone to stone. Pieces of limestone or seashells (calcium carbonate) are transformed into calcium oxide by burning. After slaking with water, this calcium oxide becomes calcium hydroxide. The use of such mortar again results in a petrified lime: by reacting with carbon dioxide from the air, calcium hydroxide is transformed into calcium carbonate and the cycle is complete.

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Depending on the mineral composition of the raw material, lime that is made from limestone (lump lime) sometimes contains not only lime but also so-called hydraulic secondary constituents. They react with water and lime to form hardening gels. These gels are responsible for the primary hardening of the binder. If there is still free lime (calcium hydroxide) present in the mortar after this primary hardening, then the primary hardening is followed by hardening due to the reaction between calcium hydroxide and carbon dioxide (provided of course that air is not excluded from the construction, for example, because the construction is under water). The last reaction is thus the same as during the hardening of air-hardening lime. This lump lime may, therefore, be looked upon as a mixture of a hydraulic constituent and air-hardening lime. The first hardens relatively rapidly, even under water, while the second hardens relatively slowly and only if carbon dioxide from the air can reach the lime. The degree to which the two constituents are present determines whether the lime is strongly or weakly hydraulic.

In the absence of (sufficient) hydraulic constituents, pozzolanic components may be added to the lime mortar in order to increase its hydraulicity and final strength. Together with these pozzolanic components, lime can harden more or less in the same way as hydraulic lime. The pozzolanic components may be either ground natural materials, such as trass, or artificial products such as crushed roofing tiles or other weakly burned ceramic materials.

A long history

Chapter 2 looks back at the traditional use of lime mortars. The use of lime for making construction mortar has a long and rich history. This is reviewed in detail in Section 2: ‘The use of lime over the centuries’. Section 2 is preceded by a section on the other binding agents that were and are used for the manufacture of mortar. From ancient times to the recent past, construction with mortars based on, for example, loam or gypsum was also common. Outside Western Europe and especially in developing countries, a binding agent such as loam, which has been totally forgotten in our region, is still often used.

Research

Chapter 5 starts with a section on research. When we investigate historical lime mortars, we see that a great diversity of lime mortars was used in the past. The principal differences are in the proportions of binder and sand, the type of lime used (air-hardening lime or hydraulic lime), and the presence or absence of additives, for example in the form of trass. These differences can be explained, on the one hand, by the exploitation of local and regional sites for obtaining the raw materials (limestone, seashells). On the other hand, a role is played by the increase in technical expertise over time. Thanks to this increase in expertise, the composition of mortars could be increasingly finetuned to the circumstances in which and for which the mortar was used (rising brickwork, dams and dikes, etc.).

If we wish to use lime mortars in restoration or renovation work, we must have an understanding of the stone and the historical mortar to which we must connect. The weather, wind and other environmental influences are also important. Chapter 4 deals with environmental influences and provides an insight into weathering and degradation and the related properties such as durability. This Chapter constitutes an important basis for the last part of Chapter 5, which deals with the use and composition of lime mortars (Section 3.2) and discusses some examples from daily practice (Section 4).

State of the art

The intention of this book is to describe the state of the art. Based on their own expertise, from both old and recent publications and from the contributions from the working group, the authors have attempted to draw as complete a picture as possible of limebound mortars for pointing and bricklaying. The incentive behind this was not only the insight that equally good, if not better, mortars can often be made with lime as with the cement that is normally used. It also sprang from the important conviction that the conservation of monuments would benefit from the re-introduction of this material in the spectrum of products that are available for restoration work. If one gets to know the material better again, it will also be used more often.