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External Limestone Sealing

Published 20 Aug 2012


Why do mineral stains show up so prolifically on black limestone pavers laid outside?

We are often contacted about mineral stains on black limestone paving. This is often perceived as an issue with the supplied paving stones, laying process or sealing method. Below we explained how a combination of naturally occurring elements can cause this phenomenon. 

It is in fact not uncommon and occurs with most types of limestone; however the appearance of mineral salt deposits on a black or darker coloured stone is more prevalent. This is why it is possible to notice some marks appearing even before sealing. As a naturally occurring chemical reaction it is often difficult to avoid. However, if fully understood and considered when laying and protecting black limestone this potential issue can be effectively managed.

What causes white and grey mineral stains on black limestone pavers?

When rain water trickles through mortar it can dissolve and transport soluble alkaline compounds. Rain water percolating through sand-cement mortar screed, toppings and bedding under limestone paving can become moderately alkaline. As this rainwater evaporates, its alkalinity increases until the solution becomes saturated.

This solution can then travel via natural capillaries within the limestone to the surface where it starts to deposit solids such as white and grey encrustations of calcium carbonate commonly known as lime plumes and efflorescence as seen in the image of the black limestone pavers shown below.



Halo stains and bands of mineral deposits

Trace amounts of sodium and potassium oxides within limestone can also be activated by rain water. Even where rain is mildly acidic, the solutions of these salts in rainwater will gradually become alkaline. Surface wetting and evaporation can produce recurrent patches and bands of dampness known as accumulation zones (rainwater pooling on the surface where it cannot run off).

Evaporation concentrates the alkaline solutions until they become saturated at the outer limits or edge of accumulation zones. Further evaporation then causes precipitation, crystallisation and immobilisation of the salts at the surface and in pores immediately below the surface. The resulting bands of distinct pigmentation are halo stains as seen in the image of limestone pavers shown below.




Reducing the impact


Sealers used to enhance the appearance and prevent limestone pavers staining from organic matter outside should be breathable. LTP Mattstone, LTP Stone Oil and LTP Colour Intensifier are examples of breathable sealers suitable for external use. As well as protecting the stone from staining, breathable sealers will allow the microscopic transition of moisture assisting any trapped moisture to escape through the stone from the mortar and sub surfaces. However, moisture permeating through the stone and sealer to the surface can still dry to leave a calciferous deposit.


Wash the surface with a mild solution of acid. This will help disperse the mineral salt deposits from the surface. The downside of this approach is that the acid solution can have a slight etching effect on the surface of the stone causing it to dry much lighter.

LTP Grout Stain Remover is a product suitable for this task. Dilute 1 part LTP Grout Stain remover in 4 parts warm water and apply to the surface of the stone. Agitate using an emulsifying pad or similar abrasive and then rinse thoroughly with plenty of clean water. Repeat this process to remove all mineral deposits.


Once the surface is thoroughly dry and  mineral salt deposits on the surface are greatly reduced, re-seal the stone with an appropriate limestone sealer. LTP Mattstone is our most popular and highly effective natural stone sealing solution and is suitable for external use. However, many of our customers have successfully used LTP Stone Oil after acid washing their stone as it helps to re-enhance and deepen the colour.

Apply LTP Mattstone or Stone Oil impregnating sealer to the surface of the stone using a paint brush. Allow it to absorb into the stone and then remove any visible excess from the surface after 45 minutes using an absorbent dry towel. The aim is to completely saturate the stone with as many coats of sealer as necessary, but do not allow the sealer to dry and leave a film on the surface. Do not apply in direct sunlight.


Still unsure? Try our new product guide to help select the correct products.

Please note that this information is offered as general guidance only and without guarantee. Your specific circumstances may need an alternative approach. In case of doubt, any process should be tried out in an inconspicuous area before general application.

Resolving stain issues on natural stone can prove difficult. At LTP we believe in helping to offer solutions and this is why we offer a no obligation LTP TECHNICAL HELPLINE, so please give us a call on +44 (0) 1823 666213 and we will be happy to help you.