Matching Mortars for Pointing
|Mortar plays a key role in the appearance and durability of masonry, and must itself
withstand extreme weathering: correct specification is essential.
With so many uses as bedding, pointing and facing material, mortar plays a critical role in the structure and survival of historic buildings and their component materials and features. A pointing mortar fills the gaps between building materials, providing support at the surface and forming a continuous face to help protect against the weather. It needs to be strong enough to withstand weathering but its role is ultimately sacrificial – that is to say that, in the event of a problem such as movement occurring, it is the mortar that disintegrates, not the host material. So, in the vast majority of cases the mortar should be weaker than the host material.
Pointing needs to be repaired only when its condition or absence means it is failing to fulfil its protective function, and replaced where its presence is actively damaging surrounding materials.
Replacement and repair mortars should be as similar as possible to existing or surrounding mortars in chemical composition and physical appearance; this ensures the best material compatibility and visual continuity, and it encourages similar weathering. However, exceptions must be made where that means reinstating a poor or unsuitable mortar. It is therefore important to identify and understand why a mortar has failed. Mortar analysis can help establish the components of successful mortars and help understand the reasons for the failure of unsuccessful mortars so that inherent faults are not replicated.
A mortar can fail to perform its function for various reasons, and material composition is just one of a number of factors to be considered. A mortar which was originally well formulated and applied can still struggle if it is not able to fulfil what is now being asked of it; coping with the effects of poor maintenance, such as ivy growth or blocked and overflowing rainwater systems, or when its requirements have changed over the life of the building. When looking to devise a replacement or replicate mortar, it is important to look closely at a mortar’s suitability for its present role, the current condition of the building and its constituent materials. Further to this, the quality and longevity of a mortar is equally dependent upon the use of good quality materials, correct specification, skilled workmanship and sustained maintenance. Centuries of practical evolution have advanced the understanding of mortar technology and all surviving historic pointing is the product of the successful combination of all these elements: as the saying goes: ‘it’s not good because it’s old; it’s old because it’s good’.
A mortar is essentially composed of aggregate particles bound together with a binder which sets or hardens to create a sound matrix. The main bulk of a mortar is generally made up by the aggregate which is usually an inert component (although particles of some igneous rocks and certain ceramic materials may encourage a slightly faster, harder set due to their weak ‘pozzolanic’ effect). Aggregates found in historic mortars include pit, river and sea sand, grit, subsoil, crushed stone and stone dust, crushed brick, clinker, sea shells, kiln slag and old mortar.
|A selection of sands and other aggregates in a range of sizes and colours|
Constituting the majority volume, the choice of aggregate not only provides much of the visual character of a mortar but is also essential to its performance, almost equal to that of the binder, adding strength, assisting carbonation (setting) and reducing shrinkage. When looking to replicate a historic mortar, it is often necessary to use a mixture of aggregates to achieve a really good aesthetic match and a good range of particle sizes to ensure the mortar is strong. Although washed sand has not been widely available until recent times, it is generally preferable as its use ensures that impurities such as salts, clay, silt and loam (which inhibit the adhesion between aggregate and binder) are removed.
Locally sourced aggregates often provide the best geological and visual match but substitutes from elsewhere can usually be found for any aggregates which are no longer available from the original source. For any identified components which are unavailable, hard to reproduce or are likely to be detrimental to the performance of a mortar, an inert, visually similar substitute can be used.
In the past, the limited availability of good quality materials could be compensated for by clever workmanship. Galleting for example, which involves pushing shards of stone into joints to improve the durability of the mortar surface, is usually found in areas where the local lime is weak. Through identifying and understanding the available materials and their component role and effects it was also possible to make skilled adjustments to the binder/aggregate ratio, and with the addition of other additives, allowed effective mortars to be created with very limited resources.
Portland cement, which was first patented in 1824 by Joseph Aspdin and improved by his son in the mid 19th century, became a popular binder in the late 19th century, although it was not until the early 20th century that it became the predominant mortar for all new buildings. Prior to this, common binders included lime, clay, gypsum and vegetable oils, waxes and resins. Of these, lime is by far the most common binder found in historic mortars and the most widely used in building conservation. Lime is extremely compatible with the vast majority of historical building materials. Sharing many chemical properties, it adheres well to the host material and its vapour permeability does not restrict water movement within the fabric. It is also flexible and has some limited ability to self-repair. However, the use and specification of lime does not guarantee success and a great deal of consideration should be given to the type and strength of lime included. In conjunction with the information derived from analysis, the binder type and ratio should ultimately be determined by the requirements expected of the mortar and the condition of the surrounding materials. Some modification of the original specification may therefore be necessary.
|An example of galleting in Chichester, East Sussex: Slivers of
flint have been inserted into the joints, decreasing the
difference between the durability of the flint masonry and
surrounding mortar. (Photo: Colin Arnott)
The most common types of lime used in conservation today are non-hydraulic lime (most often as a putty lime), which hardens by carbonation when exposed to atmospheric carbon dioxide, and natural hydraulic lime (NHL), powdered lime which initially sets with water and develops strength as it carbonates.
Non-hydraulic lime is categorised as a ‘feeble’ strength lime and is compatible with very soft and weathered masonry. It cures slowly and requires extensive protection from frost and weather. It is best used in internal and sheltered areas but can also be used as sacrificial protection for very soft material in exposed locations. Its versatility can be increased with the inclusion of pozzolanic materials, discussed below.
Natural hydraulic lime is available in three classes; NHL 2, NHL 3.5 and NHL 5, which are commonly believed to equate to the classes of ‘feeble’, ‘moderately’ and ‘eminently’ hydraulic but in fact refer to their minimum compressive strength in N/mm2 at 28 days. It should also be borne in mind that the hydraulic set continues to strengthen for up to two years, making explicit rating difficult. There is also sufficient overlap in the boundaries of classification for some limes to fall into two classes, and the classified strengths can vary between manufacturers. It is advisable to check the strength of the mortar as recorded in the manufacturer’s data sheets.
NHL 2, which may be considered to be a moderately hydraulic lime, has many applications. It is compatible with moderately sound masonry and can survive in more exposed situations than a lime putty. NHL 3.5 can be used effectively in more challenging situations such as chimneys and copings and is compatible only with sound masonry. NHL 5 should only be used in the most exposed areas as its hardness is comparable to that of cement, although it is more permeable.
Other materials can be added to change the appearance, performance and properties of a mortar. Set-additives or pozzolans, (finely ground materials containing silica or alumina which have been heated to high temperatures, such as volcanic ash or by-products of manufacturing and burning of coal and clays) can be introduced to increase the versatility of a non-hydraulic lime as they react with the lime in the presence of water and produce a hydraulic set, which is both quicker and harder.
Many other organic materials are known to have been included in the past as they are mentioned in historic texts, but have left no physical trace. These include cheese, eggs, blood, urine and beer. Animal hair, although usually found in plaster, is less commonly found in historic pointing mortars. As its use has proved beneficial, the practice has not died out.
The process of matching a mortar begins with simple visual analysis of a sample of original mortar, if possible, in situ. There is of course a great deal more to matching historic pointing than simply formulating a compatible mortar, and any character elements such as joint profile, pointing style and tending finish are fundamental to the appearance and nature of pointing and should be carefully examined and recorded so that they too can be recreated.
With a good sample of unweathered mortar one can glean basic information through very simple tests. Surface scratching with a thumbnail and crumbling between the fingers gives an indication of the strength of the binder. Magnification with a hand lens (or electronic microscope for greater magnification) allows better examination of the distribution and types of aggregate.
To help with finding a good visual match, on-site analysis can be furthered with a chemical disaggregation test, dissolving the mortar in a weak solution of hydrochloric acid. This can usually provide basic information on binder/aggregate proportions by isolating acid-insoluble aggregates. (However, any lime-based material in the aggregate, such as limestone, chalk or shell, will also be lost in the process.) Sieving the residue using a set of British Standard mesh sieves will also give an idea of the particle size distribution. The level of solubility, the nature of the residue and the effervescent reaction seen during dissolution can also help to indicate the binder type.
More sophisticated laboratory testing can identify the age, composition, strength, moisture content, salinity and extent of carbonation in a sample of mortar; helping to understanding complex defects in mortars which have failed or deteriorated. These include more detailed visual analyses (such as more intricate chemical disaggregation analysis, and ‘sectioning’; the examination of slices of mortar which have been impregnated with resin and polished back to allow detailed analysis of physical structure) and instrumental techniques such as scanning electronic microscopy (SEM), X-ray microanalysis, and various forms of thermal analysis.
Although vital in the research of lime mortar technology, the results of such analyses taken in isolation are of limited practical use when formulating replacement mortars unless interpreted into accurate and practical suggestions by an experienced and wellinformed conservation scientist. Historic mortars may be poorly mixed (meaning that samples are not always reliably representative) and may also have undergone chemical changes which are largely impossible to differentiate from what may have been present at the point of its formulation. Such chemical changes may include environmental effects on the mortar itself, such as the introduction of salts and atmospheric pollutants, or changes occurring in individual elements. It is particularly difficult to differentiate between mortar used as an aggregate and the lime added specifically as the binder. Analysis cannot provide details about the quality or preparation of the original lime, which may also have performed differently to a modern substitute.
|A mortar ‘biscuit’ is compared to existing mortar. The correct size and distribution of aggregates is achieved by blending a number of sands and other aggregate inclusions.|
Broadly speaking, simple analysis can often be more practically and financially valuable, allowing resources to be better focussed on other equally important aspects, such as high quality workmanship and materials.
Like the results of analyses, the specific circumstances and requirements of the mortar and the structure as a whole (circumstantial factors) must also be fully understood and holistically considered when specifying a new mortar. As well as looking at how the original mortar has performed (and any causal factors), considerations must include the extent of the specific work, for instance whether an area of pointing is being repaired or replaced. The priority for a repair mortar is on achieving chemical and aesthetic match with the existing and presumably successful mortar, but where an area of missing or failed mortar is being entirely replaced, the emphasis is on remedying the failure. All elements and information must be borne in mind, especially where there is conflict between historical accuracy and performance requirements.
The performance of a mortar is of course the priority, if the rest of the fabric is to be saved. However, this need not be at the expense of a pleasing aesthetic match. With a good grasp of mortar technology and the specific requirements of the mortar, and a wide library of aggregates, there should be no need to compromise on either aspect. There may be various approaches to be considered in order to replicate the many visual aspects of a mortar. Any number of suitable ‘recipes’ can then be made up in small quantities as samples or ‘biscuits’ to see exactly how the components work and look together. Tended and cured properly, they can give a good idea of the size and distribution of aggregate in the binder and the overall workability, tone and texture of the mortar. The best of these can then be trialled in situ to see how each mortar will work with the surrounding material and in the specific joint sizes. These sample areas also provide an opportunity for further trials in recreating the profile, texture and character of the pointing style; those elements which alongside its ultimate protective function, make pointing mortars such an important element of our historic buildings in their own right.