Lime mortar and re-pointing
When the external walls of the property are constructed from natural stone it is very likely that they would have been bedded in lime mortar. As the existing pointing perishes, they are often re-pointed in modern sand cement mortar, which may lead to problems in the following areas.
Movement of walls
All buildings move for a variety of reasons, especially those constructed prior to 1900 which tend to have less solid foundations, and whilst lime mortars are able to accommodate such movement without much deterioration as they are flexible to a certain degree, modern sand cement mortars are rigid and crack under stress. If the cement mortar is stronger than the masonry, cracking can be transferred to the surrounding stonework. This not only causes deterioration of the stonework but also allows rainwater to penetrate into the body of the wall.
Dampness in walls
Cement mortars, due to their high density and crystal structure are in essence non porous. As mentioned above they will always crack eventually, allowing water to penetrate the wall with no ability for the water to evaporate, other than through the relatively soft sandstone. This can result in rapid spoiling of the stone leaving the mortar standing proud to collect more water, increasing the masonry’s deterioration.
If only the external elevation of a wall originally built with lime mortar has been re-pointed in sand cement mortar, there will also be a tendency for damp patches to form internally as the moisture evaporates through the more permeable internal finishes.
Pointing should be durable and offer protection from wind rain and frost. It should allow sufficient movement of the masonry without deterioration. It must be sufficiently impermeable to deny wind driven rain penetration, but permeable enough to allow free evaporation without a concentration of water within the masonry, which would then be vulnerable to accelerated decay and possible internal dampness. (It is for this reason, coupled with the ventilation provided by open fires and draughty window and door frames, that stone properties of this era, constructed without a cavity or damp proof course but using lime mortar to bind the stones together, remained dry internally.)
Until the beginning of this century most buildings were constructed with lime, often hydraulic. Many of those now remaining have been repaired in recent years using Portland cement based mortars and renders often with the following consequences:
1. All buildings move, especially pre 1900 which had less solid foundations. Cement mortars and renders are rigid and crack under stress. Often cement is stronger than the masonry; transferring fractures into surrounding brick or stonework.
2. Cement, due to its high density and crystal structure is 'non porous'. Cement renders will always crack eventually allowing water into the wall with no ability for the water to evaporate. In the case of cement mortars the evaporation can only take place through the masonry. This may result in rapid spoiling of soft stone and brickwork leaving the mortar standing proud like a shelf, to collect yet more water, increasing the masonry's deterioration.
3. Chemical action between brick or stonework and cement often takes place causing unsightly salt staining and in the case of some sandstone very rapid spoiling.
To some extent modern methods of construction have reduced, but not entirely eliminated, these problems.
With modern buildings therefore there are good reasons for using lime mortars since:
q Walls breath better and moisture can evaporate.
q Mortars and renders do not set too hard.
q Thermal movement can be accommodated without damage.
q Expansion Joints can be avoided.
q Insulation is improved and cold bridging reduced.
q Reduced risk of condensation.
q No risk of salt staining.
q Alterations can be effected easily and masonry revised.
q Masonry life is increased.
q Carbon Dioxide emissions in the manufacture of lime are 20% less than cement and during carbonation the mortars and renders reabsorb considerable quantities of Carbon Dioxide.
Hydraulic limes on the other hand provide a quick set permitting building work to proceed rapidly - on a par with cement and do have a significant degree of flexural strength, permitting its use on thin skinned buildings.
Buildings constructed with all but the strongest hydraulic limes can be altered easily and recovered masonry reused. Indeed the masonry can be reclaimed entirely if a building has completed its useful life.
Decayed pointing lets in damp; but hard cement pointing causes cracking in the stone and can also let in damp.
Pointing which stands out from the face of the wall traps water which then freezes. This destroys the pointing and can crack the stone.
It is important that new pointing is weaker than the stone; flexible enough to let the building move; does not provide any ledges to trap water; and blends well with the stone colour
Rake out loose material with a spike to a depth of 1" to 4". Do not use a grinder. Do not damage the corners of the stone. Brush or wash away dust. Wet the wall before pointing.
The constituents of good mortar are gritty sand, lime putty, a little cement and a perhaps a little water for the cement.
Use a clean neutral yellow or grey washed builders sand which is very gritty. Add to this a small quantity of red sand to prevent the final mortar being too grey. Crushed stone can give the right colour. Try to match mortar colour to the stone colour.
Lime does not set rock-hard like cement, and so gives the wall some movement flexibility. Either use bought lime putty in tubs from Bleaklow Industries, Hassop (tel: 024 658 2284), or make your own.
Making Lime Putty
Add dry powder lime (semi-hydraulic hydrated lime) to a tub of water, and mix to a thick cream. Leave for 24 hours to fatten up. It will then thicken to the consistency of toothpaste. Caution: Lime powder is caustic and dangerous. Wear gloves and goggles. Keep away from children and animals. Never add water to lime powder, only add powder to water.
Use ordinary Portland cement. Only small quantities are needed to help the initial set.
Use tap water. Much of the water will be in the lime putty. Small amounts may be added to the final mix if the putty is too stiff even after knocking-up.
parts parts parts parts
cement lime grit sand
Weak mix: 1 2 7 2
Strong mix: 1 1 4½ 1½
Use the strong mix for hard stone and exposed conditions such as chimneys. Use the weak mix for soft stone. Mixes somewhere in between may be appropriate. Consider the exposure, the strength of the stone, and the degree of joint flexibility required. If the mix is too strong (usually by having too much cement) it can damage the wall. A mix which is too weak will not do irreparable damage.
When mixing, vigorously stir the lime putty to improve workability. Only add more water if vigorous stirring does not make the mix sufficiently loose. Add the grit & sand. Lime putty or lime putty with sand (course stuff) will keep for weeks if covered with a damp sack, and will loosen-up if vigorously stirred (knocked-up). Add the cement immediately before use.
Wet the joint and ram the mortar well in. Do not build up the mortar proud of the stone. Do not smear the mortar onto the face of any stone. Leave the mortar slightly recessed behind the face of the stone. After about an hour, when the mortar has started to set, scrub the joints quite hard with a brush (a washing-up brush works well) to roughen the surface, recess the pointing, and expose a lot of the aggregate to give a very gritty appearance.