Choice of materials

sand

Must be clean river sand, washed and devoid of any clay substance.  The recommended size graduations should be respected.   It must be remembered that during curing of the fixing mortar the water content evaporates through the stone.   Any impurities such as soil or clay particles and any dissolved salts will find their way to the surface of the stone and rapidly crystallise on contact with the air causing efflorescence which will appear little by little on the surface where they accumulate.  

binding material (matrix)

For a base screed, use artificial cement such as “multibat” from Lafarge (high silica cement, relatively free from alkali) or hydraulic lime or white cement.   For materials in contact with the stone such as fixing mortars and grouts for joints only white cement or hydraulic lime should be used.   Do not use furnace cement or cement with a high alkali content as the alkali will migrate through the stone during curing and create brown stains on the surface. 

traditional fixing mortar

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.  The thickness of the mortar varies between 20 and 50mm according to the type and dimensions of the tiles to be fixed.   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. 

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 formulae 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. 

composition of a lime / cement mortar for the screed

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

impurities in stone

Natural stone is as nature provides, and even if one is not aware the ‘impurities’ that it contains is an integral part of its appearance, and contributes indirectly to the choice of one stone over another.    If one cannot influence the ‘impurities’ in the stone, the same is not true of the products on which it is fixed.   Sand can also contain organic materials and are also in contact with the alkalis in the cement; they can also produce soluble matters which will seek to migrate to the surface of the stone.   This is why the selection of the sand is so important (white or washed river) whether for the separation layer, the levelling screed or the mortar screed.    The recommended size graduations should be respected.. When preparing mortar, cleanliness is important.    Impurities such as cigarette butts, bits of wood, remains of paint etc. can cause surface staining, particularly on lighter coloured limestone and marble. 

alkali

Alkali include sodium and potassium oxides which are present in cements and which when mixed will hydrate with the water to produce the corresponding very soluble hydroxides (caustic soda and potassium).   Ciments contain more or less alkali according to the type or class to which they belong, the manufacturer and the method of production.   It should be noted that grey cement contains much more than white cement, and lime is practically exempt.   Please note that hydraulic lime mortar takes longer to reach maximum strength.   These alkalis are extremely damaging for natural stone, especially for light coloured or white stone.   The hydroxides can appear on the surface of light stone as efflorescence, or brown stains or rings.   They are almost impossible to eliminate and will leave a visible stain of lighter colour.