layers 0 £ 0
Diggers working a natural stone quarry

How to source the best natural stone

Mike Plaster
Tuesday 21st March, 2017

When choosing natural stone for your project, it’s important to make sure the stone you specify is up to the job.

It may be one of the most versatile and durable materials on the market – but there are tell-tale signs that the stone you are looking to work with won’t have the quality to last the course.

The present checks and balances in place such as CE marking and the British Standard only go so far. With such a wide selection of natural stone available from sources across the globe, it is important to remember than some may not be suitable for every project. It’s always worth seeking advice from a specialist with a proven track record, such as Marshalls.

We would recommend visiting a similar scheme to your own project after six months to physically observe how well the stone has weathered. If a stone will fail, it will usually start to show signs within the first 6 months.

Taking sandstone as an example, Marshalls’ natural stone expert and geologist Rory Kendrick here examines the potential problems caused by poorly sourced stone – and how you can quickly spot them.

Frost damage

The expansion and contraction of water within stone can lead to nasty cracks and structural weakening.

How do you spot it?

A classic sign of frost damage is when the product starts to degrade and return back to the individual particles or grains that form the stone. This usually starts at the edges, which change from being sharp to rounded and irregular.

What causes it?

Natural stone is made up of grains or crystals that are either stuck together by a natural ‘cement’ or interlock. Sometimes there are small gaps which water can get into – it then freezes during winter and causes expansion, followed by contraction. This repeated freezing and thawing puts pressure on the grains within the stone, causing them to gradually come apart.

How do you avoid it?

Rory says: “The best way to avoid this problem is to choose a natural stone product with few of these gaps between the grains. Every stone has what’s known as a water absorption value – the lower the percentage, the less susceptible the product is to frost damage.

“Check the test data from the supplier or do your own testing. A product with a water absorption of less than 1.5% won’t be affected by frost damage.

“Some suppliers recommend applying an impregnator or sealant to natural stone. These can be effective but need to be frequently reapplied, and could simply mask the fact that the stone isn’t frost resistant and is therefore unfit for purpose.”

High porosity

The high quality natural look of stone can quickly deteriorate if the material is too porous.

How do you spot it?

You won’t spot it – not until it’s installed anyway. All types of natural stone display their natural colour and texture at first, but those with a high porosity quickly develop a dirty or damp appearance. In particularly severe cases the paving can go black or green, as dirt and algae accumulate on the stone’s surface.

What causes it?

Generally the higher the porosity, the higher the water absorption. Water is drawn into the body of the stone rather than staying on or close to the surface, and can take longer to dry.

The water often contains soluble minerals and dirt, which are drawn into the stone and stain the surface. The constant dampness also provides the ideal conditions for the formation and growth of algae on the surface of the stone.

How do you avoid it?

Rory says: “Source a stone with low porosity and water absorption. You’ll find details in the test report from the supplier, or by doing your own testing. A porosity under 10% is recommended.

“The effects of high porosity and water absorption can also be reduced by regular cleaning. Other things to consider include only using the product where it is not exposed to rainfall or is south-facing – more direct sunlight helps to dry out the stone.”

Delamination

Even sedimentary rocks formed over millions of years aren’t guaranteed to last.

How do you spot it?

Large amounts of the top of the stone start to become detached from the main body of the stone. Effects vary; it can happen in small areas, or it can occur across the whole surface of the slab.

What causes it?

Slates and sandstones are the most common types of sedimentary rocks used for paving - formed from layers of sediment compacted together over millions of years in the oceans. In poor quality natural stone paving, these layers allow water to move along them which, combined with the removal of the pressure from above, causes the layers to open up and sheets of stone to come away. This process is often accelerated where there’s a lot of frost or snow.

How do you avoid it?

Rory says: “To entirely avoid the risk of delamination choose a ‘free stone’ paving which isn’t sedimentary so doesn’t have layers, like granite or marble. However, most natural stone used for paving is sedimentary.

“Choose a natural stone that’s been used for many years as paving, and is known not to delaminate even in the most hostile environments. Study the test reports – a low density stone of less than 2500kg/m3 might delaminate. You’ll also get an idea by looking at the geological description of the stone, and how compacted/crystalline the particles are.”

Risk of slipping

Worn down or wet stone means users are more likely to slip on the surface if the cut of the stone isn’t appropriate.

How do you spot it?

When the paving is wet, you don’t get the grip you would expect – it’s at best an inconvenience, at worst a potential accident waiting to happen.

What causes it?

Sometimes paving isn’t given a suitable surface texture (such as flaming, shot blast or water jet) to give it adequate grip. Most sawn natural stones have a smooth surface, and adding a suitable non-slip texture can be costly. Heavy footfall can also abrade the surface, making it smooth and giving it a glass-like sheen.

How do you avoid it?

Rory says: “Always choose a surface texture that’s suitable for the application – a flamed texture on granite gives a rough surface, whereas a diamond sawn or honed surface might be very slippery.”

“Also choose a stone that has suitable technical qualities – testing should be done to determine the abrasion resistance (you should look for a value less than 20mm – the lower the number, the better), and to assess how slippery the surface is after a lot of use.”

29f034e6-e963-4e67-b8f5-f8006113cbbf

Chemical composition and risk of staining

Mineral deposits within the stone can cause unsightly stains on the surface.

How do you spot it?

Certain chemicals and minerals in a natural stone can cause staining on the surface which is then hard to remove. It can be difficult to spot unless you know what you’re looking for.

What causes it?

Most natural stones are made of minerals that are inert (very stable) such as silica, calcium carbonate and feldspar. However, it only needs a small amount of other minerals to cause a chemical reaction that can rapidly alter the surface appearance.

Most commonly, iron-based minerals tend to change to a brown colour when exposed to water and air, in a process similar to rusting.

Other minerals are partially soluble, so when a paving unit is installed and exposed to rainfall moisture moves up from the bedding mortar, causing these minerals to dissolve. When the water later evaporates on the surface it leaves a residue which is often dark in colour and tough to remove.

How do you avoid it?

Rory says: “This is a complex area but there are some basic steps you can take. Use a product that’s been used before – experience will tell you it should be okay. Also be sure to confirm the quarry source; some products look the same, but come from different quarries –this can make all the difference.

“The only way to be sure you’re using stone with a suitable chemical composition is to get the testing results from the supplier. This should include a ‘petrographic assessment’ which should flag any potential issues.

“The method of installation can also have an effect on whether staining occurs – always use a fully proven method of installation, proper bedding and jointing materials, and ensure that the subbase is free from any soluble minerals.”

Surface cracking

Stone of a weaker flexural strength can break and crack on the surface.

How do you spot it?

The paving slabs crack, which is clearly visible across the surface.

What causes it?

All natural stone has a flexural strength. When loading on the surface exceeds this threshold, the slab cracks. The load can be from vehicles, or point loading – even a person’s weight can be enough to cause damage over time.

How do you avoid it?

Rory says: “Part of the rigorous assessment of a stone’s suitability for paving is to do flexural strength testing in a laboratory. This is a requirement of CE marking, and test reports should be readily available from any professional supplier of natural stone.

“Once the flexural strength is determined you can calculate a suitable plan size and thickness – the greater the thickness and the smaller the plan size, the more resistant to breaking a slab can be. However, all natural stone paving should have a minimum flexural strength of at least 12 MPa.

“It’s also worth considering that if you use a stone with a good flexural strength you can use large plan sizes and reduce the thickness of the paving – achieving considerable cost savings and creating a better look.”

Sign up to receive the latest news in your inbox daily.