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The latest generation of dry mortars minimises water exposure.

Dry mortar key to perfect stone finish

Dr Michael Schmidt, Prof Dr Josef Fetixberger, Werner Glaser, Werner Stohr and Dr Steffen Wache discuss the specific properties to be addressed when selecting the right adhesive for natural stone.**

01 February 2009

Natural stone is probably the most fascinating floor finish – its unique character, giving each room, terrace or public area a special and luxurious atmosphere and a touch of exclusiveness.

No wonder natural stone is becoming increasingly popular, with countries like Brazil and China becoming important sources. This, in turn, has sparked a challenge for a perfect finish, leading to a new generation of dry mortar designed to avoid unsightly staining and discolouration.
The potential for discolouration and efflorescence varies from stone to stone. And so, knowledge about the history of natural stone helps understand its various properties.
Rocks, which are naturally occurring aggregate of minerals and/or mineraloids, are found in the Earth’s lithosphere. They are classified by mineral and chemical composition, texture of the constituent particles and the processes that formed them. They may also be classified according to particle size. In general, rocks are classified into three types – igneous, sedimentary and metamorphic.
• Igneous rocks – are formed from molten magma and divided into two main categories – plutonic and volcanic. Plutonic or intrusive rocks result when magma cools and crystallises slowly within the Earth’s crust (for example, granite), while volcanic or extrusive rocks result from magma reaching the surface either as lava or fragmental ejects (for example, pumice). Due to their composition and low porosity, plutonic rocks have no tendency for efflorescence, whereas volcanic rocks do have a tendency to discolour.
• Sedimentary rocks – are formed by the deposition of clastic sediments, organic matter, or chemical precipitates (evaporites), followed by compaction and cementation during lithification. Well-known types are chalk, limestone, dolomite, sandstone or shale. In general, sedimentary rocks show a tendency for efflorescence due to the presence of free lime.
• Metamorphic rocks – are the result of the transformation of a pre-existing rock type. The original rock is subject to heat and extreme pressure causing profound physical and/or chemical change. The most popular natural stones are quartzite, marbles, slate and serpentine.

Application problems
Porosity is the most important property of natural stones, having an impact on the bonding strength between the adhesive and the rear side, as well as discolouration, efflorescence and arching. In principle, all natural stones are porous. Depending on the type and history of the stone, the measurable porosity varies between 0.1 and 10 per cent. The table (left) indicates the porosity of some of the common natural stones.
If natural stone is exposed to water, water will be transported within, through the capillaries. Porosity determines the extent of the water transport, which has consequences regarding the stone’s application. Water absorption might lead to deformation of the stone called arching. Stones are subjected to tests for measuring arching. In general, natural stones can be categorised into three groups.
Group one consists of stones with a deformation of less than 0.3 mm after 24 hours. The second group consists of stones that are sensitive to water absorption. The deformation is between 0.3 mm and 0.6 mm after an hour of exposure to water. The application is more critical and the right products have to be chosen. The third group contains stones that are the most sensitive to water absorption. The slabs show deformation values of more than 0.6 mm after an hour of water exposure. In some cases, it might be necessary to switch to a non-water-based adhesive, such as an epoxy adhesive.
Efflorescence and discolouration are other problems that occur if coloured substances migrate to the stone’s surface together with the capillary water. Once the water evaporates, the coloured substances get concentrated and result in staining or discolouration. These substances are either located within the stone itself or migrate from the bedding mortar of the substrate. Typical critical substances are organic based like humic acid, bitumen or of an inorganic nature such as iron compounds.
Meanwhile, efflorescence is a process in which lime or limestone, as part of the dry mortar formulation, the substrate or the slab, migrates to the surface and reacts with carbon dioxide from the environment to produce calcium carbonate.
Both discolouration and efflorescence have an immense impact on the aesthetics of the natural stone. The surface will feature different colours, matt and rough.

Dry mortar technology
Developing a good adhesive for natural stone takes several aspects into consideration:
• Bonding between substrate and natural stone;
• Arching of natural stones;
• Discolouration of natural stones;
• Efflorescence; and
• Workability and application methods.
Fulfilling the bonding requirements is critical for every natural stone adhesive. Unlike ceramic tiles where the manufacturing procedure allows for profiling of the back side of the tile, natural stone tiles in general have a smooth rear surface, often with a fine layer of dust from grinding.
Beside the smooth reverse slab side, any substrate or stone slab movement has to be taken into account. Therefore, polymer-modified adhesives are mandatory. The adhesive for natural stones should meet the classification C2 following EN 12004.
All three problems – arching, discolouration and efflorescence – occur if water comes into contact with the natural stone. Since cementitious dry mortars require water for mixing, it seems to be a paradox. However, the latest generation of dry mortars for natural stones minimises the water exposure of the stone. Fast-setting mortar properties are the key to good performance. In addition to an enhanced water-retention system, the adhesive will slow down or even prevent water uptake by the natural stone.
The stone’s surface above the specially-formulated adhesive is dry and no water from the adhesive migrates to the surface. Traditionally formulated dry mortars do not have the capability to prevent water migration and there is a high potential for discolouration and efflorescence. Using the fast-setting technology will also reduce arching of the natural stone slabs.
Some natural stones are characterised by their very light colour and crystalline structure. Their attractive appearance is based an a high degree of light reflection. On the other hand, the thinner the slabs the more the light will pass through, making the slab appear transparent.
If the adhesive is applied on the rear side, the stone will lose its brilliance. Taking this effect into consideration, natural stone adhesives should be formulated with light-coloured raw materials like white cement. Additionally, from an application point of view, the reverse side of the slab should be completely covered with the adhesive.

Conclusion
Knowledge about the properties of natural stones helps overcome the challenge of attaining a perfect finish. Texture, porosity, mineral content, organic materials and lime vary with the mineral type, temperature, pressure, duration and environment of natural stones. In addition to natural stones, adhesives (and joint grouts) have gone through an evolution as well.
The latest generation of specially-formulated dry mortars (fast setting, polymer-modified with light-coloured raw materials) minimise the water exposure of natural stones during application to reduce or avoid arching, discolouration and efflorescence. At the same time, these dry mortars provide good bonding between substrate and stone stab and maintain the brilliance of the natural stone.

** Dr Michael Schmidt is regional business systems manager for BASF Construction Chemicals UAE in Dubai, while Prof Dr Josef Felixberger, Werner Glaser, Werner Stohr and Dr Steffen Wache are from PCI Augsburg of Germany – a part of BASF – which specialises in construction chemicals.
** This paper was earlier published in Medma’s Technical Bulletin.

Porosity of common natural stones