The use of admixtures or additives that can improve either the ‘plastic’ working properties or the final performance of mortars, renders and screeds has taken place for over 2,000 years in the UK, thanks to the Romans who brought with them their sophisticated understanding of adding blood, milk and fats to lime mortars. This knowledge allowed them to build all manner of structures and buildings, from using water-proofed mortars for aqueducts and baths to making self-levelling (lime) concrete screeds using ‘super plasticisers’.
The use of admixtures by our ancestors was based on both practical experience and observation not necessarily on an understanding of chemical theory. However, it is clear from his Ten Books on Architecture that the Roman architect and engineer Vitruvius understood the processing and use of all manner of materials that went into mortars, screeds and plasters including admixtures or additives.
Thomas Telford, engineer extraordinaire, used bull’s blood in the construction mortar for building the Pontcysyllte Aqueduct completed in 1805. The addition of blood would have afforded some plasticising or water repelling properties, eminently sensible on an unprotected structure with weathering on all sides of the pillars. I have this awful vision of the site with vats of warm, dark red blood being poured into mortar!
Now much of this knowledge has been all but lost, at least as far as lime mortars are concerned. However, additives for cement mortars and cement-bound concrete have been developed in the 20th century which were based on past traditions of using proteins and natural sugars, albeit in more user friendly forms. On the other hand some materials that have been developed for cement bound mortars and concrete are totally unsuitable for using with lime mortars, such as the calcium chlorides used as accelerators to increase strength development and early resistance to frost.
Today, many (by no means all) conservationists often shun the idea of ‘polluting’ lime mortars with admixtures and additives without even stopping to think. Additives and admixtures of the right types can be used very effectively in new lime mortars (and remember all work is new work anyway) and are being developed into much more user friendly forms. These materials can promote earlier freeze-thaw resistance, repel water droplets, allow mortars to cure more slowly, make mortars and plasters much more plastic and workable off a trowel (no down side to that for the applicator but little understood by the specifier). They can also reduce the overall water content in a mortar or plaster thus significantly reducing problems of drying shrinkage (probably the principal defect we find in failed renders and pointing mortars).
Most admixtures or additives used in lime mortars are surface active or surfactants, which are generally split into two components, one positively charged and the other negatively charged and they react with the air, water and the solid material interface within the mortar resulting in ‘orientation’ (which means the particles all face or point the same way) and adsorption (which means the adhesion or binding of molecules on the surface of a solid material).
The reader will probably be most aware of air entraining admixtures, notably the practice of adding washing up liquid to a cement-bound mortar to counteract the leanness and harshness of cement mortars, which is not to be recommended. At the Scottish Lime Centre’s training facility at Merryhill, Charlestown there is a block work wall bedded in an over air entrained cement mortar. The act of over air entraining the mortar has significantly reduced its strength to the point where it is possible to easily scrape away the mortar with a car key. The brickie who constructed the wall had absolutely no idea that he had produced a weak friable mortar despite the fact he was using a mortar bound with a cement which has the minimum stated compressive strength of 55 Newtons (and probably more like 65 Newtons), over 12 times the strength of our strongest natural hydraulic limes.
The danger of over air entraining a lime mortar cannot be emphasised enough. As a rule of thumb, for every one per cent of entrained air there is a five per cent drop in strength. (There is an absolute correlation between strength, vapour permeability and brittleness in mortars: weaker mortars are more vapour permeable and more flexible, while stronger mortars are less vapour permeable and more prone to cracking.) Although principally we are not looking for high strength mortars when using lime mortars, as lime mortars are so much weaker than their cement equivalents, over air entrainment could have disastrous effects. Used in renders and harls, over air entrained mortars would have less contact points, akin to sticking an Aero bar to a wall.
This article looks at just three types of additive or admixture that are compatible with hydraulic lime mortars if specified correctly: air entrainers, water repellents and water retainers. (Historical precedent shows there were many, many more.)
AIR ENTRAINERS AND WATER REDUCING/PLASTICISING ADMIXTURES
The most commonly used air entrainer is sodium sulphonate. It comes from coconut oil, made into a soap. The soap acts as a surfactant at the air-water interface in the lime paste, resulting in the formation of stable entrapped air during the mixing process in the form of very small separate air bubbles. The addition of the air entrainer to the mix lowers the surface tension of the water thereby assisting in the formation of bubbles, just like adding washing up liquid in the sink bowl. This ‘lubricates’ the mix making it more workable or more ‘plastic’.
An additional effect of these admixtures is to disperse or deflocculate the lime particles within the mix. The more uniform distribution of the lime particles throughout the mix results in freeing up some of the mix water, thereby reducing the water demand and, importantly, avoiding potential problems of drying shrinkage (right).
Lower water content also improves the compaction of the mortar, its density, its mechanical performance and ultimately increases its compressive strength by offsetting the loss in strength resulting from the entrained air.
Another advantage is the reduction of capillarity. High capillarity is not necessarily a feature we would want in a lime mortar on buildings with no damp-proof courses. The bubbles will create air pockets that will impede water circulation and discourage water to be drawn into and up the walls of historic buildings, which might endanger their ability to handle moisture adequately. However, over air entrainment of lime mortars could result in decreased vapour permeability, particularly if used with water repellents.
The third advantage is the increase in the frost resistance of a mortar. Well distributed entrained air provides two conditions that inhibit the effects of freeze/thaw in mortar. Firstly, this increases the air space and pore structure to allow water to expand harmlessly into the free voids preventing stress. Secondly, the presence of the entrained air bubbles (typically 0.05mm in size) which are larger than capillary pores, disrupt and break the capillary action, thereby reducing water absorption due to capillarity.
The most common water repellents (as opposed to water proofers) are stearates (magnesium, sodium or calcium) which are derived from animal fats such as tallow or vegetable fats such as linseed or olive oil. The effect of the stearate is to reduce the surface tension of a mortar inducing the ‘lotus effect’ (below) where water droplets ‘bead’ and roll off the surface of the mortar, and if dosed correctly will still allow water vapour to escape to atmosphere. (It is worth Googling the lotus effect for a more detailed explanation.) Waxing a jacket or polishing leather shoes produces similar effects. A secondary effect of this additive is a mild air entraining as a soap is formed producing bubbles. The word ‘stear’ is the Greek word for tallow (animal fat or lard).
If animal fats are introduced into the production of ‘hot lime mixes’ (those made by slaking quicklime, sand and water together in one operation), the resultant exothermic reaction produced by quicklime slaking will melt them and disperse them throughout the mix, and the high PH of the quicklime will cause saponification of the fat, turning it into a soap and calcium stearate. It is calcium stearate that gives the mortar water repelling properties.
Water repellents are often incorrectly called water proofers. Water proofers would not allow either water droplets or water vapour to pass through them, like a plastic mac with no ventilation or 'shrink wrap', resulting in a very sweaty building. Bear in mind if you are excluding water from a wall, it cannot succumb to the effects of water freezing.
We frequently specify tallow limewash for the last coat on new lime render or harling as the effects of wind-driven rain are mitigated by the tallow; when rain water hits the wall, it ‘beads’ like water off a duck’s back.
Again, overdosing with water repellents could be dangerous in producing a hydrophobic mortar and markedly decreasing its strength, and can also have a long term effect on the performance of any repair work carried out on these materials in the future.
The principal water retainer used in mortars is methyl cellulose, based on cellulose, one of the most common chemical compounds in organic matter. Cellulose is a long chain of linked sugar molecules that gives wood its remarkable strength as it is is the main component of plant cell walls. Methyl cellulose particles are long sugar chain fibres which have been finely divided and dried in a powder form. These form a gel when mixed with water in a mortar. The water is retained within the gel, slowing down evaporation and increasing drying time.
This effect is particularly useful in lime mortars for preventing over rapid drying and consequent shrinkage cracks. A hydraulic lime mortar will acquire strength slowly by avoiding desiccation. It is of particular note that hydraulic lime mortars should be kept damp for at least the first 72 hours after placing to ensure the hydraulic set forms. If a mortar is allowed to dry out within this 72 hour period, rewetting will not reinitiate the hydraulic set, and the work will have to be removed and re applied. Water retainers are particularly useful on high suction backgrounds or where protection of the new work is particularly difficult to achieve.
In Mexico, cactus juice is used as a water retainer in mortars to mitigate the effects of intense dry heat.
A WORD OF CAUTION
Although there is a place for the forms of admixtures discussed above, and their appropriate use could have alleviated many of the failures that have been investigated by us in the past, they need to be used in the knowledge of their impact on the mortars in which they are included and how their inclusion can affect the mixing, use and aftercare of the mortars.
Doing so should increase the success and thereby the confidence of both specifiers and practitioners in the use of lime-based mortars and renders.
The Building Conservation Directory, 2014
ROSAMOND (ROZ) ARTIS MRICS DipBldgCons(RICS) IHBC is director of the Scottish Lime Centre Trust, one of its primary course tutors and the head of its building advisory service. A conservation-accredited surveyor, Roz is also director of Dundee Historic Environment Trust and a member of the Scottish Churches Trust grants committee. She has written widely on the practical use of lime and other mortars.
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