The Heritage Directory

                                                                                                                                 The Heritage Directory has provided the following articles as an informative guide to aspects of the historic environment. The articles are grouped into the following areas : General & Legal, Architectural History, Interiors, Exteriors, Gardens, Preventative and Remedial and Professionals and Contractors.  


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Lime, Lime Putty, Non-Hydraulic Lime, Hydraulic Lime and Pozzolans

Before the introduction of quick setting cement in the early to mid 19th century ( Portland cement was patented in 1824, with increased production from the 1850’s ), Lime had been universally used as a binder in mortars, renders, plasters and limewash. It is now seeing a revival, initially from the repair of historic buildings in the 1980’s but more recently as an environmentally less damaging alternative to cement. In Britain, Lime is made from Limestone  or Chalk (calcium carbonate) which when burnt becomes quicklime (calcium oxide)  also known as lump lime or unslaked lime) due to the removal of carbon dioxide . When quicklime is mixed or slaked with water it forms slaked lime ( calcium hydroxide), known as lime putty. By a process called carbonation, the lime putty combines with carbon dioxide from the atmosphere to gradually harden as calcium carbonate , mixed with sand to form a mortar.

Lime is of value today as it will match the mortar used in most historic buildings. It also allows a building to ‘breathe’ as it is porous and allows moisture to travel through the mortar rather than eroding the brick or stone. It acts as a sacrificial element by being softer than the walling material and its flexibility allows movement in a building. Cement by contrast, creates problems of damp by trapping moisture or forcing it through the walling material causing erosion of the stone or brick, and cracks form if the building moves due to the hardness of the mortar. Cement formulas have also increased in hardness and  impermeability in the 20th century,  particularly after first and second world wars. 

 Non-Hydraulic Lime & Lime Putty

Lime without clay or other additives is known as non- hydraulic lime as it will not set under water. It is sometimes know as fat lime. It sets when it absorbs carbon dioxide in the atmosphere (from carbonic acid in rain) . Whilst there is an initial set after a day, the process takes a long time and lime mortars can repair later hairline cracks by this carbonation process. Dry Slaking is where quicklime is slaked and immediately mixed with sand. Lime putty is better the more mature it is, so should have been stored for a minimum of 3 months.

 Hydrated Lime or Bag Lime

This is a dry powder, supplied in a bag, which by the addition of water forms lime putty. Care is needed to ensure that the powder is fresh as old powder will carbonate in the bag and lose its strength. 

Hydraulic Lime

Hydraulic lime derives its name from the ability to set under water. It does not need air to set unlike non-hydraulic lime (lime putty). The set is due to the presence of aluminates and silicates. Hydraulic limes were used by the Roman and revived for engineering works in the 18th century. They are also know as natural hydraulic limes  or NHL , graded according to compressive strength. ( NHL 2 = feeble, NHL 3.5 = moderate, NHL 5 =  eminent).  Historic hydraulic limes with clay content are often known as ’feebly hydraulic’. Hydraulic limes are preferable to cements in historic building work as they are more permeable.

Repointing

Only those joints that are weathered back by a joint thickness or are perished should be repointed. It is generally considered that if weathered less than 12mm, repointing is not necessary. When repointing, joints should be raked to at least twice the depth of the joint. As much should be removed without hammer and chisel as possible and the rest removed with a suitable plugging chisel by hand. Mechanical disc cutters should not be used as they have tendency to skid and damage the brick or stone. The joints should then be well wetted and filled using a pointing iron. A slight recessed joint tapped with a bristle brush before set is recommended for most vernacular work, particularly stonework, though regard should be had to any existing pointing pattern, for instance matching penny struck or tuck pointing for brickwork.

 Matching Mortar & Mortar Analysis

A good match in repointing needs the mortar to match in colour, texture and type of pointing. The use of lime putty is important in achieving a good texture and colour for traditional work. The careful selection of sand is also important and for the older  and more vernacular mortar, the use of a grit is important. A slight set back to take account of worn arrises (corners) is usually recommended. Tapping the mortar when green ie before final set, gives a more natural appearance. This avoids the visually over dominant ribbon or weatherstruck pointing  and oddly smoothly buttered appearance of much poor repointing. 

Simple analysis can be done by gently crushing a piece of mortar, mixing with water, and measuring after standing in a jar for half a day. More detailed analysis involves hydrochloric acid, sieves and weighing and a number of companies offer professional mortar analysis. It is also worth seeking advice on aggregates from specialist suppliers of traditional sands and grits. 

Sand should be well graded from fine sand to grit (one third soft sand, one third sharp sand, one third aggregate  for example), as this produces a stronger mortar as well as enhancing the appearance. However the size of aggregate will depend on the type of work and fine joints will have finer sand. Matching the existing mortar is not the only factor. An existing mortar may have been too hard to be sacrificial or too soft for an exposed location. 

Mortar Mix

For non-hydraulic lime, 1 part lime to 3 parts sand is traditionally the standard mix for lime mortar, though the lime can be increased to provide more strength according to location and exposure. Traditionally quicklime was also used so proportionally lime putty should be increased to achieve the same strength. ( The lime should be sufficient to cover the sand  and the 1 to 3 ratio has been demonstrated by adding water to a flask of dry sand to the top of the same and measure how much water was added, known as a surface area and void test used by the Lime Centre.) 

Pozzolanic Additives

Pozzolanic additives can be gauged with mortar to give a more rapid set. Types include;

Pozzolana, a volcanic dust from Naples.

Brick Powder or Brick Dust.

PFA (Pulverized Fuel Ash) also known as Fly Ash. (There is a need to avoid sulphates in the ash)

Metakaolin and HTI (High temperature Insulation) Fireclay ( but lower temperature sources preferred)

Trass: volcanic ash from Germany 

Adding Cement to Lime Mortar

The English Heritage Smeaton Project established that the common practice of adding a small amount of cement  to lime mortar  to achieve an initial set, affects the strength and integrity of the mortar, resulting in segregation. If more  cement than lime is used this will overcome the problem but result in a very hard mortar which may not be suitable for historic building work.  

Maturing the mortar and Knocking Up

Lime Putty  is best matured. Coarse stuff, if stored with air excluded, can also benefit from this maturing process. Mixing of the mortar is important before use, known as knocking up, as this improves the quality and workability of the mortar.

Frost

The use of lime mortar is best avoided during the winter, or protected if this cannot be avoided. Failure to to this will result in the mortar freezing and expanding, destroying the mortar. Even if not frosty, below 10 degree the set will be very slow and no work should done a month and half before the frosts. Equally if lime is used on a hot day in the sun or a very windy day, it can dry out too quick before setting. Protection in the form of sheeting is therefore recommended. 

Slumping

The height of wall that can be built in one day is less with lime mortar. Even with hydraulic lime this lis about a third to half the height achieved with cement ie said to be about 0.5m in a day. The collapse of part of Hampton Court Palace in the 17th century is perhaps the most  spectacular example of the danger of building too fast without letting the lime mortar set.  Non-hydraulic limes are not suitable for cavity wall single skin brick and where using hydraulic limes, the suitable strength of hydraulic lime for such work should be established with the manufacturer or supplier of the lime.  

Sustainability

The manufacture of lime consumes less energy and produces less greenhouse gases than cement. Bricks and stone can be salvaged in buildings made of lime mortar but little can be salvaged if cement has been used in terms of reuseable materials. Salvage saves the embodied energy in the manufacture of the brick or stone.  Lime also re- absorbs the carbon dioxide released in the process of its manufacture, unlike cement. 

Lime & Sandstone

Sandstone can be attacked by calcium sulphate (also known as Gypsum in its solid form) leached from limestone. Whilst the decay from mixing limestone and sandstone is well known, the use of lime mortar is a matter of debate as the use of a cement mortar could be more damaging due its hardness. Limewashing of sandstone is also consider to be problematic. 

British Standards 

Reference should always be made to the relevant current British Standard  for the specification of Building Limes.

 Health and Safety

Appropriate safety assessments and precautions should be taken when using lime, including protective clothing including overalls, boots, gloves and googles  as well as first aid. There should be a clean supply of water and clean water in an eye wash bottle. Quicklime is highly dangerous and boils when being slaked so all contact with your eyes or skin must be prevented. 

This is a living document, in the sense that it will be updated and added to over time.

This edition dated December 2010. Lime, Lime Putty, Non-Hydraulic Lime, Hydraulic Lime and Pozzolans

v1 Dec 2011