Understanding mortars

Choosing mortars

Resistance to salts

Protection and working practices

Repointing: preparation, application, finishing

Repointing dense masonry

Repointing ashlar masonry

NHL Mortar mixes

Re-Pointing Masonry Walls: Brick, Blocks and Natural Stone. 

Understanding mortars.

Before deciding to re-point a basic understanding of the function of mortars is required.   Mortar is used for jointing individual units in a mass of masonry. The final structure must have certain characteristics to function satisfactorily. It must carry the load for which it was designed, it must be durable and it must give protection against wind, rain, and frost. Mortar should develop sufficient strength and at such a rate as to be capable of withstanding the stresses to which it will be subjected during the construction of the building and subsequently when the structure is fully loaded.
It should not however set and harden so quickly that it becomes inflexible at any stage and cannot accommodate slight movement. There is normally no requirement for significant structural strength in the mortar of traditional masonry buildings, particularly in re-pointing work. Mortar should be permeable in itself, both so that the quantity of free water on the face of the building is reduced, thus reducing the possibility of wind-driven water penetration and so that moisture evaporation is not concentrated in the masonry, which may then be vulnerable to accelerated breakdown in the vicinity of the joints. Mortar should bond firmly to the units so that a tight joint is obtained through which rain will find it difficult to penetrate. Mortar should be workable, so that the material may be applied easily and to ensure that the vertical as well as the horizontal joints can be adequately filled.

Historic masonry buildings rely on their mass and the interlocking of individual units for their stability and the mortar in a masonry building serves in the main to provide a bedding for often very irregular components, filling the voids and maintaining the wind and watertight integrity of the building.

Choosing an appropriate re-pointing mortar.

Analysis of original bedding material is usually a good first step in determining the correct materials to use in the repair of historic masonry. Mortar can and does change with time, it is better that the analysis is carried out by a suitably experienced person or laboratory. It will not always be the case that simple matching will be sufficient. The exposure and condition of the building today may well be different from its original construction. Ruins and monuments in severe disrepair (i.e: lack of roofs, copings to wall heads etc…) might well require a repair mortar different from the original as the new mortar will have to cope with weather and exposure conditions for which the original mortar was not designed. Most important of all: the new mortar should be compatible with the old. Do not introduce mortars containing potentially damaging elements or that can constitute an impermeable barrier obstructing vapor exchange and retaining moisture. The consequences in a relatively short time could be disastrous and, in most cases, irreversible.

Resistance to salts

Soluble salts (sulphates, nitrates, chlorides) can be present in walls, they can be in the original mortar, in bricks and stones (often as a result of previous repair work), in ground water or from airborne pollutants. All St Astier NHL limes are resistant to salts. They do not contain the reactive components such as high levels of aluminium, potassium and sodium oxides and gypsum. Existing salts will therefore be allowed to migrate out of the structure without affecting the soundness of the mortar and, in time, be washed off.

Protection and good working practice.

For all lime mortar work, best practice requires proper curing and aftercare against the effects of drying winds, strong sunlight, rain and frost. Lime mortar may require slightly longer curing times but the methods and principles are the same. Where scaffolding is in place, fine mesh debris netting securely fixed to the outside of the scaffold gives basic protection to the working area slowing down strong wind while allowing good natural light for the works. In regards to external protection, the work should be covered with hessian sheets, polythene or both, depending upon conditions. Polythene should never come in contact with the work. To avoid rapid drying and consequent high shrinkage, especially in hot or windy weather conditions keep all work damp by repeatedly applying a fine mist of clean potable water, if necessary several times a day, until the mortar has hardened.

Preparation:

Starting from the top of the building, defective joints should be carefully cut out with appropriate tools and the joint thoroughly cleaned out, ensuring the inside upper and lower edges of the masonry are properly scraped clean of old mortar. In brickwork the joint should be raked out to 11/2 times the width of the joint or a minimum 3/4 of an inch. Ensure the back of the joint is square. The joints can be cleaned with a vacuum, low pressure compressed air and/or rinsed out with a garden hose to remove all loose materials: this is important as the mortar will adhere to dust which is left in the joints and deplete the bond. This is normally done form the top of the building down as the old lime in the mortar can stain the masonry if it is not properly washed of, working from the top down avoids unnecessary wetting of previously finished work. Prior to any re-pointing, controlling the absorption rate of the background is essential.

Application:

Mortar should be plastic and workable but as stiff as possible. It should be pushed into the back of the joints in layers, avoiding large volumes of deep filling at all times. On rubble elevations, pinning stones should be used to fill wide and deep joints in the same style as the original build. This will reduce the volume of mortar required and will assist the process of setting and final full carbonation. A good yardstick is to keep the joint thickness to no more than a “finger” thick, if the joints are wider than this they should be pinned with compatible matching masonry.

Finishing:

In natural stone masonry, to ensure good compaction and adhesion within the joint, the mortar can be tamped firmly back with a stiff bristle brush as it starts to firm up. The timing of this is critical. If it is carried out too soon after placing, fines in the mix will be drawn to the surface and will form a dense skin, inhibiting the proper curing of the mortar. Once the surface of the mortar is firm (usually the next day) lightly scraping the surface to expose the aggregate can improve the appearance of the mortar and make the joints less visible. This process should not be undertaken before the surface has stiffened or mortar will be smeared onto the face of the stone. A “well filled” joint is close to or flush with the surrounding masonry or to the weathered edge. Recessed joints define the masonry components and detract from the appearance of the wall, becoming a feature in themselves. Historically the common practice was to fully flush point and line out rubblework. Brickwork has a number of specific joint finishes too numerous to go into in this general guide, but the principles of timing the finishing of the joint still apply.

The fines in the mix will determine the finished color, therefore a wide range of natural colors is achievable without pigmentation. The whiteness of St Astier limes ensures the best color reproduction of the chosen aggregate.

Re-pointing dense impervious masonry.

Some masonries, such as granite, basalt, etc. and dense impervious bricks require special consideration. Due to their very nature of these materials have little if any, moisture absorption and therefore moisture is transferred to the joints. In these circumstances, the choice of mortar and method of application and finishing is very important. The joints are more vulnerable to the effects of wetting during placing and immediately afterward until a full set and carbonation have taken place. Using St. Astier NHL mortars will ensure setting without having to rely completely on carbonation. The stiffest mix possible should be used, avoiding free water in the joint cavity and consequent de-bonding effect.

The vapor permeability of NHL mortars will ensure moisture evaporation. Joints should be filled to flush, never recessed. Recessed joints will leave ledges for the accumulation of water that will keep the mortar joint wet for longer periods and accelerate the decay process. while feebly hydraulic limes were often used for the building of walls with impervious masonry the construction period usually left sufficient time for the joints to set up and cure before exposure to rain. Re-pointing is a much quicker process and more hydraulic materials are almost always a better option. Joints should be raked back to approximately 1″, thoroughly cleaned, including the top and bottom faces of the beds, ready for the new mortar. Pinning stones should not be removed, but if they are loose, they should be removed and put back during the re-pointing. Where a wall has previously been re-pointed and the pinning stones have been lost, suitable replacements should be used. The walls should be well washed to remove any dust and loose friable material making sure that the entire elevation is cleaned down to prevent staining on the walls. Impervious masonry should be dry when the work commences, however the original backing mortar should be kept damp.

Very badly worn or damaged edges may require surface repair (see stone
repair mortar sheet) and subsequent tuck pointing to reduce the
visual impact of the traditionally very white ashlar mortars, and new joint
lines struck and pointed.

This document is a guide only and is not intended to be a specification. Its purpose is to provide the reader with helpful information that may assist  in determining the correct choice of materials, methods of application and  determine the best working practice. The guidelines refer to our experience with St. Astier NHL binders and some recommendations might not be applicable to other products.

Traditional ashlar jointing and bedding mortars were made with lime and crushed chalk, usually with a small amount of crushed stone or sand added for bulk. This was mixed on a marble slab with just enough linseed oil to grease the tools, it was made up into a consistency similar to stiff glazing putty, wrapped in oiled cloths and stored until required.

Material. Supplied in dry powder form in 5 gallon plastic buckets with re-usable airtight lids.
Ready for site mixing with clean drinking water and linseed oil (optional).

Background. Generally, natural stone (Ashlar blocks) or rubbing bricks, for building, or existing masonry for re-pointing. The background should be clean and free from loose or friable material, well washed to remove dust. Dampen high suction units. (Do not dampen very low suction materials such as granite)

Application.
Laying sawn six sided blocks or rubbing bricks.
Butter on to beds and perpendicular ends a full bed of mortar, firmly pressed into place to slightly more than the desired bed thickness and lay next block firmly in place to line and level by tapping firmly and repeatedly. Excess mortar will squeeze out. Leave excess in place for several hours.

Laying single faced masonry units.
Lay a ribbon of ashlar mortar (approximately 1″ in width) on the leading edge of the ashlar faced unit and a bed or normal coarse stuff to bed the irregular meeting faces. Tap down the stone to line and level, excess ashlar mortar will squeeze out. Leave excess in place for several hours.

Mixing.
The materials are firstly mixed dry to fully combine all the ingredients and just sufficient water is added to make the material into a dough like consistency, a good double handful size lump of the mortar has a thimble full of boiled linseed oil added and kneaded into the mix until it leaves the hands clean and is fully plastic. (Disposable
or rubber gloves are usually worn for this process). The mortar should be used within 24 hours, if being stored for more than an hour it should be wrapped in polythene to keep it moist. Because it is feebly hydraulic the mortar sets slowly but positively and on final set and full carbonation resembles hard chalk, matching exactly traditional ashlar jointing.

Consumption:
3281 linear feet of bed joints, at 3/32″ thick, and 4″ on bed requires 52 Gallons of mortar. Re-pointing 3281 linear
metres of 3/32″ thick joints at a depth of 3/8″: 5 Gallons of mortar are required.

Re-pointing:
See Pointing with NHL (re-pointing ashlar joints)

St. Astier NHL products for use with masonry cladding.
For the purpose of this document Masonry Cladding means thin sections of masonry each one fixed back to a frame or substructure or to themselves. The masonry will generally be between 1 1/2″ to 2 3/4″ thick.

Its main function will be decorative. Many cladding designs have units fixed in such a manner that normal masonry coursing and jointing is not part of the design and continuous perpendicular jointing is common.

Cladding can be laid on a bed of mortar or retrospectively jointed.
In many instances it is simpler to retrospectively joint the cladding as the bed width does not offer sufficient stability for the masonry to stand one unit on top of another and depends on fixing by mechanical means.

The design of the fixings will determine whether or not the construction can be built without movement joints. Fixings that have horizontal and vertical movement slots but provide lateral restraint usually allow walls to be constructed without movement joints, provided a mortar with a low modulus of elasticity is used.

The need for compressive strength in this type of work is not significant with a mortar of 150 – 300 at 28 days being usually adequate for the purpose.
NHL 2 and NHL 3.5 are very suitable binders for the production of cladding mortars. Their elasticity moduli is adequate in most instances and their free lime content, mainly responsible for plasticity, is high (25% and 50% respectively).

Vibrant and beautiful, the St Astier lime paint range is the perfect compliment to lime plaster, render and harling, suitable for most backgrounds. Can be applied to gypsum plasters, lining paper, cement and lime backgrounds, bricks, blocks and untreated timber.

Mixing: The general ratio is 1 volume of powder to 2 volumes of clean water. Further dilution might be necessary on porous backgrounds. The lime paint is supplied in tubs as a powder for mixing on site with water. The tub has a fill line for normal use. Proper mixing is essential. When mixed it should be completely free of lumps or fine particles. Best to let sit with lid on for 24 hours before use. Check the bottom and sides of the mixing tub to ensure all the powder is fully combined. This ensures that no lumps or fine particles remain and are fully dissolved throughout the lime paint.

Application: Do not work in temperatures below 40^oF or above 85^oF or in a humid environment. Protect painted areas from direct sun, drying wind and rain for 3 to 4 days (longer in damp weather conditions). Do not use warm air dryers. All new rendered or plastered surfaces should be properly dry before applying lime paints.Apply with a good quality long hair brush in short strokes. Evenly and thinly spread the lime paint on the surface, work into any small hollows or cracks. Do not overload the brush, use only the bottom third of the bristles, do not apply unevenly. Stir regularly during application. Generally 2 coats are sufficient. Wait 12 – 24 hours between coats. On large areas of work, always keep the working edge damp, do not allow the lime paint to dry out and form a dry joint. Inadequate protection of the working area will exacerbate drying. Maintain a dampened background in advance of application with a light mist spray if necessary. Sufficient personnel should be used to complete large areas without forming dry joints. Always finish whole walls or elevations into details, angles, corners, down pipes or the like in one operation to avoid color banding. St. Astier Lime Paints can be sprayed: please consult your Supplier.

Background: Remove any loose or flaking paints and repair defective plasters or renders. Test the background for suction by lightly flicking clean water from a brush on to the surface of the wall.  Rapid suction into the background means that dampening with clean water will be required. In this case it may also be necessary to thin the first coat with 30% additional water. If thinning is required mix as normal then take out with a measured container the quantity you require to be thinned, place it in a separate container and add the extra water. A low suction background does not require pre-wetting or additional thinning for the first coat.

Very smooth or polished surfaces offering little or no suction might require sanding down with sand paper to create the necessary suction.

Ensure that background is dry, clean and sound. Do not use lime paint in areas subjected to abnormal condensation such as a kitchen or bathrooms, unless sufficient ventilation is available to prevent condensation.  Lime paints are not film-forming materials that will hide the background condition therefore salts and dampness will show through. Some surfaces may have variations in background materials (i.e.: brick, stone, cement, etc) and therefore care is required in controlling differing suction levels.Due to the additives content of some modern gypsum premixed plaster there may be some reaction with lime paints. Furthermore gypsum plasters tend to be extremely smooth and therefore would need to be lightly sanded down to provide a suitable surface before applying lime paint.Wallpaper joints are often smeared with adhesive during hanging. Make sure that the adhesive paste used is water-soluble. Clean the joint before applying lime paint, as excess adhesive residues on the face of the paper will cause the lime paint to dry out a different color.

Recommendation: If in doubt, always try on a small test area.

Unsuitable backgrounds: Not to be used on backgrounds with no suction or containing waterproofing agents, oil paints, grease, distempers or cement paints which have water repellent agents.

Protection: Do not use below 40^oF or above 85^oF. Avoid working in foggy/ damp weather. Protect from rain, strong direct sunlight and drying winds for at least 24 hours or longer if required.

Technical data: Made from pure high calcium hydrated lime. Acrylic addition: max. 5%. Pigments: mineral or earth oxides. Normal dilution ratio –  1 vol. powder: 2 vol. water (1kg. Powder: 4 litres water)

Shelf life: Powders: 12 months from purchase date if kept sealed in original container. Once mixed with water: 24 months if kept sealed in airtight container.

Health and Safety: Irritant to the eyes and skin. Wash affected areas abundantly with clean water (do not use soap).  Do not ingest or inhale dust. Wear adequate protection (mask/goggles/gloves) during handling and mixing.