NATURAL STONE ISSUES

THE IMPORTANCE OF PREPARATION

It’s very important that prior to laying any paving stone, preparation of the paving area is carried out. Things to consider, do you need to install a weed control membrane? Depending on the ground soil we would advise to use one, if you have had problems in the past. This will also help reduce soil movement that can occur. We would ALWAYS recommend using a ‘full mortar bedorfull bedding layer‘, the right way! We strongly advise AGAINST this method of techniques ‘spot bedding, dab and dot or five point dabbing‘, this will cause problems later. Further consideration, we strongly advise to use an SBR (Styrene Butadiene Rubber) product which can be applied in two methods. SBR can be mixed into the mortar or cement mix before application or applied directly to the back of each flagstones. The backs of each slab must be covered completely with SBR before installation. These processes will help reduce the common issues with natural stone that can occur. Please refer to the our guidance notes which can be found by clicking here which can be downloaded for use. We must stress though please speak with your landscaper or paving contractor as they are the experts!

COMMON ISSUES WITH NATURAL STONE

The majority of natural stone, Basalt, Granite, Indian Sandstone and Slate are porous products, some more than others. As natural stone is porous it will absorb the moisture within the cement, which can result in surface changing. Some can be more harmful than others. There are some preventative measures to take but when to apply them is the key. Below is a list of common issues with natural stone that can occur with the type of natural stone. 

  • Blooming or white spotting
  • Efflorescence
  • Picture framing
  • Staining (brown/yellow in colour)
  • Tanning (leaf staining)

It is important to note natural stone is a wholly natural product. It will be inherent of variation in porosity, colour, texture and grain size. Each flagstone is unique and no one stone is the same, so it’s not uncommon a flagstone to discolour while its immediate neighbour remains completely unaffected. This does not mean there is a problem with the stone, it’s just the way things are with natural stone paving. 

Please refer to our article where we discuss sealants in more detail. 

BLOOMING

What is blooming or white spotting? Blooming referred to as white spotting are white deposits that can appear on slate stone. The white stains on your dark slate, it may be mineral (calcite) deposits from hard water, where by the salt dissolves. Calcite is a mineral composite found in slate stone which reacts with the natural elements of water and oxygen. As a result white markings can be visible.

How to remove blooming? Unfortunately this is natural occurrence some slate stones contain more minerals and salts than others. If you do see the gradually appearance of these white marks then try warm soapy water and scrub the area rigorously. I would recommend to try a stain remover if the stains still can’t be removed . Follow the read the instructions thoroughly. 

Solutions and Prevention’s? If you have laid your new slate paving stone and white marks appear try the above solutions first. As a preventative measure again to laying new paving, let the stone stone completely dry out. Again if it rains the area will need to dry out thoroughly as each slab will dry at different rates. If the stones white marks disappear I would then recommend to leave the stone to dry out completely before applying a sealant. 

EFFLORESENCE

What is efflorescence? Efflorescence or lime bloom is caused by vapor migrating through the slab bringing soluble salts to the surface, as a result leaving visible white markings. For the chemistry enthusiasts, calcium hydroxide (which is soluble in water) migrates to the surface as the material dries out. On reaching the surface it reacts with carbon dioxide from the atmosphere to produce a surface deposit of calcium carbonate. It is a natural occurrence which will gradually disappear over time.  Unfortunately there is no time limit, in some paving it dissipates quicker.  

How to remove efflorescence? Unfortunately this is natural occurrence the light sandstone, slate and granite stones contain more minerals and salts than others. If you do see the gradually appearance of these white marks then try warm soapy water and scrub the area rigorously. I would recommend to try a stain remover if the stains still can’t be removed . Follow and carefully read the instructions thoroughly. 

Solutions and Prevention’s? If you have laid your new slate paving stone and white marks appear then try above solutions first. Again if it rains the area will need to dry out thoroughly as each slab will dry at different rates. If the stones white marks disappear, I would then recommend to leave the stone to dry out completely before applying a sealant. 

PICTURE FRAMING

What is picture framing? This is more visible in granite (silver-grey and midnight grey) and light sandstone paving. The moisture in the grouting mortar permeates in the granite paving edges. This then evaporates to leave mineral deposits in the granite stones edges.

How to remove picture framing? Unfortunately picture framing cannot be removed. Over time this will gradually weather and the picture framing will become less apparent.

Solution and Prevention’s? It’s essential the paving stone is fixed properly to a solid bed. Another recommendation is to consider to prevent picture framing occurring is to apply SBR into the mortar or cement base mixture or cover the back of the granite stone. The SBR will improve the bond of the stone. The grouting to be applied using either a dry or wet based mortar depending on the area covered. I would also advise to make sure that the jointing grout is of similar colour to the stone you are using. If no picture framing is present, I would then advise to seal the paving stone, again after 6 weeks. The other option is to seal the stone prior to laying, covering all sides of the stone and edges.

BROWN OR WHITE STAINING

What is staining? Staining marks can be visible on lighter natural stone. They can appear as markings or sports on the stone. They can appear to be brown (rusty colour), white (efflorescence) and yellow markings. The rusty looking stains appear due to the iron rich composites of aggregates. When oxidation occurs this will leave a rust like stain. They can also appear when metal particles are deposited onto the paving stone. White staining (blooming and efflorescence) has been discussed above. Yellow staining arises from fertilizers which contain ‘Ferrous Sulphate’ which can leave iron traces on the paving stone. 

How to remove staining? A bucket of warm soapy water and some scrubbing should work. In extreme cases an acid based product can be considered. As a hint of caution this may damage the stone further.

Solution and Prevention’s? To prevent these staining issues, we would advise to use a sealant (impregnator). Again, the main thing to note is the preparation and treatment of the stone before and after use. We would advise to cut away any overhanging grass from the paving edges so that if you do use a fertilizer it doesn’t deposit any surface residue.

TANNING

What is Tanning? Simply caused by falling leaves, what really leafs you ask? When the leaves fall they degrade and leave a resin residue on the stones surface. The residue is more visible in lighter stone, granite and sandstone, will show up more. 

How to remove tanning? A bucket of warm soapy water and some scrubbing should work. In extreme cases an acid-based product can be considered. As a hint of caution this may damage the stone further.

Solution and Prevention’s? A simple and cost effective way is to sweep away any falling leaves, especially in the autumn months. We would advise to cut away any overhanging branches from the paving area, so the falling leaves don’t deposit any residue. If you have an area of garden where the stone will come in contact more with leave debris, then a more permanent preventative measure is to use a sealant (impregantor). There are a number of products on the market, chose one relevant to the stone type. We would always advise to test these sealants before use, samples are normally available. Read the instructions carefully or seek expert advice from your paving contractor or landscaper.

In Summary

In this article we have discussed a number of common issues with natural stone and how they can be prevented or reduced. The main things to check are;

  1. Ground preparation is key.
  2. Use a weed control membrane. 
  3. Full mortar bed or layer ALWAYS!
  4. SBR product
  5. Installation process guides
  6. Check with the landscaper or paving contractor who is doing your job! A professional will know what the issues are and how to treat these.
  7. Paving maintenance – brushing and cleaning away any debris.
  8. Use a sealant that is right for the type of stone you choose.

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