Mark Atkins, LTP technical director, describes how to maintain and restore black limestone pavers.
A newly laid black limestone patio is an impressive sight but, unfortunately, the initial wow-factor often doesn’t last long. Black or very dark grey stone quickly fades, and you’re left with a wishy-washy grey aesthetic. So, why does this happen, and can anything be done to restore colour and avoid the process repeating?
A combination of UV light and normal rainfall causes this fade-to-grey. Limestone and many other dark stones are calcium-based, alkaline in nature and sensitive to acid. They contain a soluble mineral called pyrite; when this is combined with water and oxygen, it forms sulphuric acid, which dissolves the calcium and changes the colour of the stone. Meanwhile, as rain falls, it absorbs carbon dioxide from the air, which dissolves to form carbonic acid. This makes it slightly acidic – around 5.6 on the pH scale, rather than a neutral value of pH 7. Rainwater, humidity and UV-rays all contribute to the discolouration, as does erosion caused by the wind and general wear and tear. Acidic maintenance chemicals can also cause additional damage, as can jet washing.
How to protect new slabs – and restore an existing patio
Good quality water-resistant sealers go a long way in reducing discolouration – and many can enhance colour too – but rainfall also damages the protection they create. Topical sealers can be worn away and rainwater that seeps into the stone’s porosity will eventually compromise the effectiveness of an impregnator. For a long-term solution – which protects the stone and its colour – we recommend LTP Blackstone Sealer & Restorer. A water-based, impregnating sealer, Blackstone contains a permanent colour intensifying pigment. To maintain a very dark, pristine finish, annual treatment is recommended, although a treated stone will maintain a far darker aesthetic than an untreated stone for around four-five years.
Why good preparation is key
It’s very important to ensure surfaces are properly cleaned and prepared before applying a restorative treatment. Any solution applied will mix with residue and loose limestone particles; this can be caused by etching when the stone comes into contact with acidic rain.
The etching will create a sediment which will be carried into newly exposed capillaries and the sealer, in turn, will draw the sediment out of open pores. When this residue mixes with the sealer, it will create a smeared and patchy appearance. If you rub your hand across the surface before cleaning, you’ll see a dusty residue; that’s the limestone sediment. Cleaning the surface opens the pores and removes the sediment. We recommend a thorough conditioning clean.
When you come to apply the sealer, don’t be tempted to over-apply in order to achieve a darker finish or speed the process up. Less is more. It’s far better to apply several thin layers than one thick layer. If you apply too much product in one go it won’t be absorbed properly and the finish will be patchy.
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