Materials which enable lime mortars to set more rapidly include ash and brick dust. Known as 'pozzolans' after the volcanic additives used by the Romans, these materials are widely found in the lime mortars used in old buildings and monuments. Where conservation work is required, new mortars ought to match these mortars, not only to ensure continuity with the past, but also to ensure that the new work is both visually and physically compatible with the old. It is therefore important that we know more about the performance of these additives.

A simple everyday definition of 'pozzolan' could be 'a finely powdered material which can be added to lime mortar (or to Portland cement mortar) to increase durability and, in the case of lime mortars, to provide a positive set'. A more formal definition is given by ASTM(1) C618 as 'a siliceous or siliceous and aluminous material which, in itself, possesses little or no cementitious value but which will, in finely divided form in the presence of moisture, react chemically with calcium hydroxide at ordinary temperature to form compounds possessing cementitious properties'.

Simple non-hydraulic lime mortars harden by drying and carbonation, that is by the conversion of calcium hydroxide (slaked lime) to calcium carbonate by reaction with atmospheric carbon dioxide. In hydraulic mortars this hardening is supplemented by a more positive set brought about by chemical reactions between calcium hydroxide and reactive silicates and aluminates in the presence of water. In natural hydraulic limes the reactive silicates and aluminates are supplied by clay minerals in the limestone. Where a hydraulic set is required in a lime mortar and these minerals are not naturally present, or are not present in sufficient quantities, they can be added in the form of pozzolans.(2)

The addition of pozzolan to any lime mortar (hydraulic or non-hydraulic) will modify its characteristics. Pozzolanic materials can combine with uncarbonated lime (calcium hydroxide) to form stable compounds, thus reducing the risk of early leaching or frost damage and increasing the potential durability of the mortar. Depending on the type of pozzolan chosen, the density and compressive strength of the mortar may be increased and porosity reduced. In general, the softer pozzolanic materials (such as brick dust from clay bricks fired at less than 950°C) will produce more permeable and flexible mortars, whilst the hard-burned materials, such as PFA, will tend to produce a harder mortar, closer in its characteristics to cement.

In the past it has been argued that early setting and increased durability in lime mortars can be obtained by the addition of a small quantity of cement, but experience has shown that cement-gauged lime mortars are generally not durable unless the relative proportions of cement to lime are around 1:1. These proportions produce a mortar which is too hard and brittle for most conservation situations and which may introduce soluble sulphates into fragile stone or brickwork. Similar problems may also occur with some of the modern highly fired hydraulic limes.

HISTORY

The practice of modifying lime mortars by the addition of materials containing reactive silicates and aluminates was known to Roman builders, who utilised volcanic deposits from Pozzuoli near Naples, as well as crushed clay tiles. The extent to which this practice was continued throughout the middle ages is not clear, but interest in the use of both hydraulic limes and pozzolans had revived by the late 18th century, with the start of a long period of investigation and debate involving some of the great engineers, such as Vicat, Treussart and Smeaton.

These investigations involved the combined firing of suitable clay minerals with limestone as well as the addition of pozzolanic materials to various types of lime mortars and led, eventually, to the development of artificial hydraulic limes. It appears to have become generally accepted that, for engineering works, combined firing of the required materials could produce a more durable mortar than the separate addition of pozzolans after firing. To this end two methods of production, of what became known as artificial hydraulic limes, were developed. One method involved grinding limestone or chalk with clay before firing, to simulate the intimate mixture found in natural hydraulic limestones. An alternative method, involving the mixing of slaked high calcium lime with clay, drying and re-calcining the material, led eventually to the production of Portland cement.

At the same time, understanding was growing of the use of pozzolans for addition to mortars of non-hydraulic or hydraulic lime. Smeaton, particularly, was instrumental in developing specifications incorporating natural pozzolans with natural hydraulic lime to achieve exceptionally durable mortars for marine, and other engineering works.

The continuing search for a faster setting and more durable mortar led eventually to the development of modern cements and, for most of the 20th century, the increasing use of Portland cement has displaced the use of pure limes, hydraulic limes and pozzolans. With the current realisation that lime mortars of various types and properties do have a place in the construction industry, both for the conservation of traditional masonry or brickwork and for the construction of certain types of new building, attention is again being focused on the potential performance of non-hydraulic and hydraulic lime mortars, and on the use of pozzolans to modify these mortars. The use of pozzolans in lime mortars in current practice is based on historic practice and on pragmatic experience, supported by some preliminary scientific investigations, such as the work carried out by English Heritage and BRE under the Smeaton Project.(3) Published works on the effects of pozzolans on Portland cement concrete and mortar also contain useful information.

SOURCES AND TYPES OF POZZOLANIC MATERIAL

Pozzolanic materials can be divided into the categories listed below, according to their origin and properties.

Natural, very finely divided, highly reactive materials of volcanic origin. These materials were formed from a combination of minerals, (mainly consisting of silica and alumina with smaller and variable quantities of other minerals containing calcium, magnesium, iron, potassium, and sodium), ejected from volcanoes in the form of very finely divided vitreous material. Well known sources include puozzolana from Puozzoli in Italy, volvic pozzolan from South-east France, trass from the Rhinelands and tuff from the Aegean islands. Crushed pumice was also used.

These natural pozzolans were widely used in 19th century engineering works in conjunction with natural hydraulic limes. They were recognised as being particularly appropriate for marine engineering and other works in difficult wet conditions, and for civil engineering works generally. Imported Italian puozzolana and Rhineland trass are still available.

Local deposits of volcanic pozzolan are thought to have been utilised in the UK on a small scale: in the late 19th century a source was reported from 'near Edinburgh'.(4) Other vitreous volcanic material, such as basalt, may have mild pozzolanic properties if very finely ground.

Low temperature calcined clay products in various forms. Pozzolanic additives derived from lightly fired and finely crushed clay products, such as clay tile or brick, were used by the Romans and combinations of non-hydraulic lime and low temperature brick dusts have been used over a long period of time. Similar specifications are successfully employed in modern conservation practice where additional set and durability are required without seriously reducing the permeability and flexibility of the mortar. Historically such mixes were not thought to be as effective as hydraulic limes and volcanic pozzolans in producing a fast setting and durable mortar for use in continuously saturated conditions.

Bodies such as English Heritage have promoted the use, particularly for conservation work, of low temperature clay pozzolans in non-hydraulic mortars and research is ongoing. Current advice is that the material should be derived from clay fired at temperatures below 950°C, and ground to a range of particle sizes between 38 and 600 microns.(5) Modern sources of potentially suitable material include reject bricks and tiles from traditional producers, which can be crushed in a roller pan mill. Some manufacturers also produce low temperature purpose-made dusts for sale as pozzolans.(6)

Clay or kaolin products specifically manufactured as pozzolans.(7) These are produced primarily for use with Portland cement and all currently available technical and performance data relates to their use in that context. These materials are highly reactive and combine readily with calcium hydroxide to form calcium silicate hydrates and calcium alumino-silicate hydrates. Their effect on the performance and characteristics of lime mortars is not currently known but, subject to adequate investigation and trials, it is possible that their use could be extended into this field.

Also falling into the category of fired clays is the material known as HTI (high temperature insulation) powder. This was widely specified in the 1980s but has now largely been superseded by lower temperature materials which are thought to be more consistent in their performance.

Mineral slag, etc. Furnace slag is a vitrified material, produced as a by-product of processes such as smelting, and requires grinding to convert it to a reactive material. It contains silica, alumina, lime and other minerals in various proportions and, in modern practice, is more commonly used as an additive in Portland cement concretes. Historically, forge scale and iron-rich slag, known as minion, were also used.

Ashes of organic origin. Coal cinders generally have an acceptable balance of silica and alumina, and have been used historically as a pozzolanic additive, but their physical structure tends to weaken the mortar and to absorb excessive water. Coal ash is widely used, in the form of PFA (pulverised fuel ash) as an additive to cementitious mortars and in lime-based grouts. The use of coal-based products carries a risk of sulphate contamination and the materials should always be selected from low sulphate coals. The residue of fuels from lime burning, whether from coal-, coke-, or wood-fired kilns, known as lime-ash, is well known historically as a pozzolan and is still available.(8) Other vegetable ashes, such as rice husk ash, are used as pozzolans in other parts of the world. Bone ash is also known to have been used.

Certain natural sands and crushed rock products. Certain types of sand, such as argillaceous (clayey) sands containing high proportions of schist, basalt, feldspar and mica, can have mildly pozzolanic properties. Whilst these sands are not generally specified for modern lime-based mortars it may be useful to recognise that, historically, in certain localities, their use could have influenced the nature of local lime mortars.

Finely crushed rock products from sources containing an appropriate balance of minerals may also produce a mild pozzolanic effect.

Traditionally, mortars were often produced using techniques which brought the sand into contact with hot slaking lime, and it is possible that this heat would have encouraged any potential for a mild pozzolanic reaction between sand and lime.

SPECIFICATION OF POZZOLANIC MORTARS

Historically, in fat (non-hydraulic) lime mixes, the proportions of natural volcanic pozzolan to lime and sand were often in the region of one part of lime powder (or two parts of lime putty) to one of sand and one of pozzolan; or one part of lime powder (two parts of lime putty) to up to two-and-a-half of pozzolan or trass, without the addition of sand. For use with natural hydraulic limes the relative proportions of pozzolan were reduced. Brick dust is widely found, in varying proportions, in old lime mortars. Lime-ash is known to have been used, in combination with sand and gypsum, for floors, but the extent of its use as a pozzolanic additive in mortars is not known. Where there is a need to closely match historic materials an analysis of the original mortar should be commissioned.

The properties and behaviour of pozzolans should always be verified by trial samples before use. The choice of type and quantity of pozzolan to be used with a modern lime mortar will be determined partly by performance requirements and conditions of use, and partly by availability of materials and (especially in the case of modern manufactured pozzolans) of information and experience. Modern specifications tend to utilise soft-fired clay products for increased durability of non-hydraulic lime mortars, and pulverised fuel ash for structural grouts. Recently, the manufactured kaolin pozzolans have also been employed in hydraulic lime mortars. Although imported natural pozzolans and lime-ash (in the form of residue from coal-fired lime kilns) are available they are now rarely specified.

Where a soft permeable mortar is required to have increased frost resistance or durability in exposed locations, a gauging of brick dust is often specified. Proportions of the mix might be in the region of one part of lime putty to two or three parts sand, with the addition of up to one of fine brick dust. Fine, freshly ground brick dust is thought to be the most reactive as it has the maximum surface area due to the larger number of particles per weight. (The Smeaton Project found that brick dust with a range of particle sizes between 38 and 600 microns was the most successful, and that the smaller particles, probably those less than 75 microns, tended to be pozzolanic, whilst those greater than 300 microns tended to act as porous particulates.)

For severe exposure or other demanding situations, where a lime mortar may remain permanently wet or be subject to repeated cycles of wetting, drying and freezing, a natural hydraulic lime mortar or a more strongly pozzolanic mortar might be specified. In some locations, for example in marine environments, it may be appropriate to use a natural hydraulic lime mortar with a gauging of pozzolan.

Where pozzolanic materials are to be added to a non-hydraulic mortar this is done at the 'knocking up' stage, shortly before use. Brick dust should be wetted and thoroughly beaten into the mix. The gauged mortar can normally be knocked up again if not used immediately, but mixes not used within 24 hours should be discarded. The more hydraulic the mix the shorter will be the available working time before set becomes established.

Techniques for using pozzolanic mortars are similar to those normally recommended for other lime mortars. All the elements of good practice, such as thorough preparation of materials, adequate and appropriate preparation of backgrounds (including cleaning, dampening down to control suction and, where appropriate, making good and keying of surfaces), use of small volumes of mortar, slow curing and protection from sun, wind, rain and frost for new mortars, are all important. The use of pozzolans should never be regarded as a substitute for appropriate and careful site practice.

OTHER SPECIFICATIONS

Gauging with hydraulic lime, or even with Portland cement, is sometimes specified in order to modify the properties of non-hydraulic lime mortars. The pros and cons of these mixtures are outside the scope of this discussion, and strictly speaking neither material can be described as a pozzolan. It is generally agreed that proportions of the gauging should be as close as possible to those of the basic lime, and should be very thoroughly mixed in to achieve a uniform distribution throughout the mortar. Experience in Scotland over recent years has shown that mortars in which the total lime content comprises between 50 per cent and 75 per cent non-hydraulic lime, with between 50 per cent and 25 per cent moderately or eminently hydraulic lime, can successfully be used in situations of severe exposure, provided all the elements of good site practice are in place.

FUTURE DEVELOPMENTS

The use of pozzolans in Portland cement mortars and concretes is better understood than is their use in lime mortars. Recent work by Stafford Holmes and Michael Wingate has provided valuable information on the extent of their use historically.(9) What is now required is a programme of testing and performance monitoring for the various clay-based and proprietary pozzolans available on the market.

 

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Notes
(1) American Society for the Testing of Materials
(2) According to Eckel, in the 1920s American publication Cements, Lime and Plaster, natural pozzolans contain between 42 and 66 per cent silica and between 15 and 17 per cent alumina.
(3) The Smeaton Project, a joint research programme of English Heritage, ICCROM, Bournemouth University and the BRE, investigating the development of lime mortar formulations for use in historic buildings
(4) George Burnell, Rudimentary Treatise on Limes, Cements, Mortars, Concretes, Mastics, Plastering, etc, John Weale, London, 1850
(5) The Smeaton Project
(6) HR Supra, also known as Moler, a purpose made brick dust fired at 750°C, is available from Hepworth Refactories Ltd
(7) MetaStar pozzolanic materials, manufactured by English China Clay International, are available from:

• Bryn Gilby (01565 659040)
• The Cornish Lime Company (01208 79779)
• Masons Mortar (0131 555 5053)

(8) Lime-ash from coal-fired lime kilns is available from:

• The Lime Centre (01962 713636)
• Masons Mortar (0131 555 5053)
  

(9) Stafford Holmes and Michael Wingate, Building with Lime, Intermediate Technology Publications, London, 1977