Will they discolour or not?

Natural stone coverings from China, especially light to dark grey granites, do not always meet the developer’s standards, as large or partial stains cause the stone appearance to differ considerably from samples of the stone that do not come into contact with the mortar.

Known causes of colour changes in natural stone surfaces are the reaction of colouring minerals in the stone, accumulations of dirt on the surface of sandblasted, bush-hammered or lasered anti-slip coverings, insufficiently dried out foundations, large effect of moisture from outside and particularly attempts to clean the building for the first time with partly acid and partly alkaline cleaning agents, especially if the joints have not been made properly.

The rougher and coarser the surface, the larger the tendency to soiling and discoloration in the area near the surface. Irrespective of the stone type, discolorations are usually attributed to solution and transport symptoms of water-soluble, coloured components inside the stone. In the case of stone of volcanic origin, the visual property of the stone surface also primarily depends on the quantity and type of minerals and their colour. The colour of sediment stones is also affected by organic substances. Alkaline moisture like the tempering water of cement and adhesive mortars substantially increases the hazard potential here.

Especially stone types with translucent and colour-intensive, dark minerals such as light and grey hard stones from China frequently show surface colour changes after being laid with cement mortar. A stone often shows its original appearance again after some time, but an irreversible surface discolouration usually remains. The effects of using Chinese granite for unventilated flat roofs can be extremely problematic, as here the mortar bed – although made of very water-permeable single-grain concrete – is exposed to constant penetration of moisture. The surface then shows partial discolorations that can barely be explained. If the covering is then removed in these areas, it is apparent that the visual damage is caused by damp spots in the area of the damp-proofing or water backpressure and moisture transportation in the stone due to this. The intensity of the colour deviation varies from slight to very strong depending on the type of stone.
As part of a Mapei research project, mortar systems and the raw materials used for their formulation have now been examined for the first time for their effect on selected types of stone.
The examination showed that depending on the pore properties of the stones and the type and quality of the raw materials used for the mortar, visual changes can occur on the stone surface due to the deposit of microcomponents in the capillary pores. The following are especially significant for discolorations: plastics, binders, water and the type of stone. It is also known that the use of special adhesive mortars can reduce the potential effect of the moisture on the slabs.

The use of high-grade special adhesives helps to achieve a good result in many cases. This definitely calls for an expert to sensitively check the whole covering structure and point out if wrong or defective results are apparently to be expected. The stone is not necessarily always at fault, even if the defect first appears on the surface of the stone. A special adhesive together with a suitable check of the structure by an expert will produce a good result in many cases.
The peculiarities of Chinese granite materials must, however, be taken into consideration. Despite all commercial considerations, the expert must have the courage to say where and when Chinese granite can or cannot be used. If the priority is only on a cheap buying price and possibly a poor laying price, it should be no surprise if things go wrong. Perhaps people will think about using local granite despite lower prices elsewhere. This will often avoid unsatisfactory or disappointing results. Many a legal dispute shows that building cheaply is not necessarily always good and free of damage.