Fixing natural limestone tiles

If fixing on top of a concrete slab, it is essential to check that the concrete is completely dry, sound and free from significant cracks and that the surface is free from contamination. Poor quality substrates may need to be sealed. Dry concrete is a good foundation. The recommended minimum curing time is 6 weeks.^**

When preparing screeds, fixing mortars and grouts for jointing, great care should be taken to ensure that the materials are suitable for limestone. If using premixed proprietary brands, check with the manufacturer that the product is suitable for limestone (see below under ‘Staining’).

Materials used for preparation:

Sand: Sand can be used for the 10mm thick separation layer, screeds, the fixing mortar and the grouting compound. It must be clean (see below under ‘Staining’). The recommended size graduations should be respected.^**

Binding material (Matrix): For a base screed, use artificial cement (preferably high silica cement, relatively free from alkali) or hydraulic lime (see below under ‘Staining’). For fixing mortars and grouts for joints only white cement or hydraulic lime should be used. Please note that hydraulic lime mortar takes longer to reach maximum strength.

It may be advisable to lay an initial levelling screed. This can compensate for uneven concrete, incorporate under floor heating systems, plumbing and electrical conduits, cover flexible wooden floors or create falls. Tiles can later be fixed on a good levelling screed using tile cement or adhesive. The screed is composed of 1 part cement for 3 to 4 parts white or washed river sand. During cold or humid weather or if the sand is already damp the amount of water should be decreased. It should be laid on top of the separation barrier previously placed on the concrete slab. Before fixing the tiles the screed should be allowed to dry out. Allow a minimum of 3 weeks or 1 week for every 25mm of screed depth (longer if hydraulic lime is used). ^**  It is usually a minimum of 30mm thick, and 65mm if a piped water under floor heating system is installed. For large areas, or when laying on top of building expansion joints, it will be necessary to incorporate expansion joints (see below under ‘General Fixing Advice’). There are proprietary premixed screeds available.  For urgent work it is possible to reduce the curing time by replacing the regular cement with a quick setting cement such as BAL Quickset.  

Natural stone tiles may be fixed using a traditional mortar screed, made from sand and white cement or hydraulic lime. The thickness of the mortar varies between 20 and 50mm according to the type and dimensions of the tiles to be fixed. Fixing natural stone tiles or slabs with a liquid mortar is not recommended. The mortar should be mixed to obtain a homogeneous consistency according to building standards.^** It is recommended that the mortar is prepared with a mechanical mixer. Hydraulic lime based mortar or mixed hydraulic lime and white cement mortar is preferable. Lime makes the mortar more pliable and elastic and diminishes the risk of cracks and subsidence of the tiles. The presence of lime also creates a ‘self defence’ which increases the impermeability of the tiles and joints. Hydraulic lime is said to be more eco friendly, re-absorbing some of the carbon dioxide released during manufacture and facilitating stone reclamation. Alternatively flexible additives can be used, especially when fixing on top of flexible wooden floors or incorporating under floor heating.   

Typical composition of a lime/cement mortar: 175kg of white cement, 175kg of hydraulic lime to 1m³ of fine clean sand

The fixing mortar should be prepared with as little water as possible. Some fixers like to improve adherence to the mortar screed, particularly when fixing large size tiles or if the bottom face is smooth from sawing or calibrating, by buttering the bottom face of the stone with white cement slurry or sprinkling a small amount of white cement powder on the screed. 

Building regulations and new standards are starting to restrict on site mixing of screeds and mortars. There is a large variety of premixed tile fixing mortars available. It is essential to check with the manufacturer that the mortar is suitable for fixing limestone. Different formulations are available, for example flexible mortars to avoid cracking of tiles due to movement in the substrate and thick bed for when thicker mortars are required. Amongst the principal manufacturers are Mapei, Laticrete and Easipoint.  

Calibrated tiles may also be fixed using tile adhesives and cements. These must be suitable for limestone to avoid staining (see below under ‘Staining’). This is useful when the floor height is limited. The floor should be level (there should not be more than a 5mm gap under a 2m straight edge or a little more if a thick bed adhesive is used). Proprietary manufacturers of tile adhesives for natural stone include Mapei, Sika, Laticrete, BAL and Norcros. They supply a range of adhesives including products for thicknesses up to 35mm, and products with increased flexibility.

One can prepare one’s own mixture with fine sand and a binding material (white lime or white cement). Do not use grey cement. Too strong a cement mix will block evaporation through the joints. Ready mixed grouting products are available from builder’s merchants including Sika, Lafarge, and Weber & Broutin. They have many qualities: they do not shrink or crack when drying and they often contain additives that delay setting time and improve waterproofing. If adding colouring agents, do a trial on a dry piece of stone to check the result.

When possible chose mechanical expansion joints. When using flexible compounds, make sure that the product does not contain solvents that may cause discolouration (see below under ‘Staining’). There is a wide range of products, mainly silicon based, formulated specifically for stone including products by OttoSeal (S70) and General Chemical Engineering (Uniseal Silicon). Read more under ‘General fixing advice’.

Staining that could be difficult or impossible to remove may be caused by the incorrect use of materials for fixing or by allowing water to penetrate underneath the stone:

• White cement, hydraulic lime or a combination of both should be used in any screed, mortar or grout in contact with the stone. Grey cement should be avoided as it generally contains higher quantities of alkali than white cement, including sodium and potassium oxides. If grey cement is used these alkali will hydrate and migrate slowly through the stone, reacting with minerals in the stone and accumulating on the stone surface. This shows up as efflorescence and will often cause staining. Hydraulic lime is almost free from alkali and is widely used for stone restoration projects.
• The quality of any sand used in screeds and mortars is important.^** Use only white or washed river sand. Any impurities such as soil or clay particles and any dissolved salts will find their way to the surface of the stone and cause staining.
• When preparing mortar, cleanliness is important. Impurities such as cigarette butts and bits of wood can cause surface staining, particularly on lighter coloured limestone and marble. The mortar should be prepared with as little water as possible. Excessive water will evaporate through the stone increasing the risk of staining.
• A waterproof barrier laid over the concrete slab will prevent the transfer of salts from the slab which could later cause efflorescence on the tiles.
• For expansion joints, when possible prefer mechanical joints. When using compounds, make sure that the product does not contain solvents. These may leach into the stone and cause discolouration. There are specially formulated products safe to use on limestone and marble.
• When grouting, avoid too strong a cement mix. This will block evaporation through the joints and could encourage efflorescence around the edges of the tiles.

Additional stain removing tips:

• Make up a paste with plaster of Paris and clean water.
• Shape on top of the stain in the form of a crater and leave to dry
• 30 minutes later pour into the crater some solvent such as:
• For an oil or grease stain: acetone, ammonia or K2r stain remover
• For a coloured stain: bleach
• For a wine stain: alcohol
• For a fruit stain: acetone
• For a paint stain: the recommended paint thinner
• After two to three hours, remove all with a wooden spatula and rinse abundantly.
• If necessary repeat the operation several times until the stain is completely removed.

Cracks can occur, running through adjacent tiles, if basic rules are not followed:

• A separation layer (barrier) on top of a concrete slab, timber flooring or whatever substrate is used for outside paving will prevent differential movements and stresses in the substrate being transmitted to the stone. These movements may cause cracks to develop in the stone and to run through several adjacent tiles. If the joints are too narrow spalling can occur through the compression of two adjacent tiles against each other.
• The use of a flexible mortar or adhesive, either a proprietary product or a mortar containing hydraulic lime, will decrease the risk of cracks developing.
• The rules concerning the positioning of expansion joints must be closely followed (see above under ‘General Fixing Advice’).^**
• For outside paving, the dimensions and thickness of the paving and the quality of the substrate must correspond to the rules for each type of traffic.^** Cracks often occur when trucks are driven over areas designed for pedestrian traffic only.

Frost can cause damage to limestone used for outside paving. Check that the type of limestone chosen is suitable for the local climate where it is to be installed.

Remove any deposits of mortar remaining on the surface of the limestone tiles after fixing. Vacuum thoroughly to remove any dust, especially cement powder. Wash the floor with a damp floor cloth (water only) to remove as much of the fine dust still present as possible.

For the maintenance of natural limestone tiles you have two choices, using a water & oil repellent or the traditional method:

Only time will tell the long term effects of modern chemical treatments on stone. They have definite advantages when natural limestone is used for wet rooms and kitchens. For other areas the debate is open. 

Water and oil repellents or inhibitors protect porous stone from the penetration of water and oil by changing the surface tension, while allowing the stone to breath. Please note that they do not protect against acids such as lemon juice and cola based drinks, which should be wiped off immediately before they react with the limestone or marble. The inhibitors must be colourless and stable under ultraviolet radiation. Outside they also help prevent moss, lichens, bacteria and mould growth. The inhibitor covers the individual grains of the stone with a minute film without blocking the pores. The active compound is released by the solvent and hardens on the grain structure of the stone. The new range of inhibitors uses water based solvents, far more ecological than the previous chemical solvents. Efficiency varies with the type of stone and depth of penetration and tests should be made prior to application. Some well known manufacturers include Lithofin, HMK, Akemi, Industria Chimica Reggiana (ICR anti-drop L10), Federchemicals, Fila Industria Chimica, General Industria Chimica and Massimo Piraccini Treatment. The manufacturer’s instructions should be strictly followed. Repeat applications may be necessary at intervals indicated by the manufacturer. The use of these products does not replace tanking for wet rooms. These products should not be used without expert advice on stone that has been in place for years, or when water penetration from underneath the stone is a possibility.

Before applying the product, wait several days after fixing for the support to be thoroughly dry. Stop any under floor heating and wait for the floor temperature to return to ambient. Do not apply in rooms where the ambient temperature can descend below 5°c. Do not allow access to the treated room until the product has cured.

N.B. Inhibitors should not be confused with sealers, a chemically resistant coating, generally a urethane or acrylic resin. These can create a major undesirable change in appearance and prevent stone from breathing. We strongly advise against their use.

Many old buildings have beautiful limestone floors that have developed their patina over hundreds of years. They have mostly been maintained with soap such as Savon Noir or Savon de Marseille soap flakes. Look for traditional products based on olive oil or ask us to supply a suitable product with our stone. Those containing linseed oil should be avoided and using palm oil-based products may contribute to destroying tropical forests. When the floor is clean and dry carry out an initial treatment as follows:

Initial treatment
Take a large watering can (approx 10 liters) of very hot water (just off the boil) and add 8 to 10 tablespoons of liquid Savon Noir or Savon de Marseille soap flakes. Water the clean floor abundantly and evenly. The hot water will help the soap penetrate into the pores of the stone. Renew the operation several times always following the same procedure. Once the floor is saturated with soap (approximately 3 applications) the tiles will have a whitish appearance due to the presence of soap on the surface of the stone. Do not attempt to remove. The soap will nourish the stone and the whitish aspect will disappear after a few weeks.

Weekly maintenance
Use only water with some Savon Noir or Savon de Marseille following the manufacturer’s instructions. The formation of the skin of calcium carbonate, a self-defensive chemical reaction of the material, will safeguard the stone from staining and enhance the natural beauty of the stone. Do not use multipurpose floor cleaning products or any products containing free alkali (sodium, potassium, ammonia, bleach, synthetic detergents, etc), acid products even diluted, abrasives, waxes or encaustics with linseed oil. Use only cleaning products designed specifically for natural limestone. Outside grease stains, for example after a barbecue, will usually disappear over time through the actions of sun and rain