Controversial steel slag is being found in more and more places: how do you recognize it?

Lately, there's been an increasing number of disturbing news reports about steel slag. This controversial byproduct is not only harmful to the environment but also to your health. But how can you identify steel slag in your garden or surroundings?

And what is the best thing to do with it?

As the name suggests, steel slag is created during steel production. When the steel is made, a liquid substance is also produced. This substance floats on top of the liquid steel. Once the steel is ready, the liquid substance cools and hardens. The resulting material is called steel slag. You can compare it to a type of stone, but made in a steel mill.

Steel slag is a very strong material: it's hard and can withstand high temperatures. That's why it's frequently used in construction. You'll find steel slag in roads, bridges, and buildings, or to raise areas or seal noise barriers.

Now you're probably wondering how to identify steel slag yourself, but it's not easy. It's very difficult to see with the naked eye whether a material contains steel slag. Because steel slag is often mixed with other materials, it looks like ordinary rubble or stones. However, there are a few characteristics that can indicate steel slag:

• Color: Often dark gray, blue-gray, or almost black. White, salty spots may appear on the snails when wet.
• Texture: The stones are usually angular, sharp, and irregularly shaped. They are not rounded, like gravel.
• Luster: The slag can have a glassy or metallic shiny surface, especially on fracture surfaces.
• Porosity: Some pieces are quite compact, others porous and airy. It then resembles pumice.
• Weight: Steel slag is often heavier than concrete rubble or natural stone.

Note: Keep in mind that there are many stony materials that resemble steel slag, but aren't. Think, for example, of basalt or blast furnace slag.

The use of steel slag is not (yet) prohibited. However, a temporary partial ban on its use has been in effect since July of this year. This regulation will last for at least one year (until July 2026). In the meantime, the government will conduct further research into the risks, after which it will decide whether strict regulations or a complete ban are necessary.

In recent years, it has become clear that steel slag can cause health problems . People who come into contact with steel slag report various health complaints, such as:

• Irritated eyes
• Irritated airways
• Nosebleeds
• Skin irritation

Steel slag is highly alkaline and has a high pH. This makes the material corrosive and can irritate your skin and the mucous membranes in your eyes, nose, and throat. This only occurs if you come into contact with steel slag dust.

According to toxicologist Paul Scheepers, exposure to steel slag can be dangerous. He is particularly concerned about steel slag lying in open spaces like parks, playgrounds, and courtyards. People can easily get nosebleeds from inhaling quicklime that can drift, for example, during dry weather or after it has just been laid. "It doesn't always cause immediate problems, but it can eventually lead to problems we don't yet know about. This mainly concerns metals that can accumulate in the body and don't break down themselves," he explains to Investico.

Last spring, research by NU.nl and Investico revealed that steel slag was found in more than a hundred locations in the Netherlands. The slag was found relatively frequently in locations in North Holland, the province where Tata Steel is also located. It was also frequently found on footpaths in parks such as the Bloemendaalse Bos and Zwanenburg Park in Hoofddorp, and under several pétanque courts and a cemetery in Geldermalsen.

It's now clear that many more locations are involved. NU.nl and Investico, following new tips from readers, discovered that steel slag was used in at least 216 locations in the Netherlands, almost twice as many as previously assumed.

If you suspect that steel slag is present in your area and is harmful to the environment or your health, it's best to contact your municipality or environmental agency. If the steel slag has been properly disposed of and covered, it poses no direct risk. However, it's a good idea to have it examined in a laboratory.