In 1997, Sweden used copper-smelting slag beneath a road near Luleå; after 10 years, copper, nickel and zinc leaching had declined below recommended thresholds

In modern times, the words ‘waste’ and ‘safe’ rarely sit comfortably in the same sentence. However, in 1997, researchers in northern Sweden decided to use some slag from copper smelting as a sub-base material for a public road outside Luleå. The question quickly arose as to how this would affect the area through rainfall, snowfall, and freezing over the next decade. The road was monitored for ten years, and the results were published a decade later, in 2017, in a study titled “Leaching behaviour of copper slag, construction and demolition waste and crushed rock used in a full-scale road construction,” in the Journal of Environmental Management, by researchers at Luleå University of Technology. The results were not as bad as expected. The researchers created three 100-meter-long test sections using different sub-base materials. These were fayalitic copper slag (glassy waste product from copper smelting), recycled concrete, and a control group with crushed rock. Researchers then tracked how much copper, nickel, zinc, chromium, and molybdenum leached from each section over the following ten years.

The uncomfortable part, and the good news

The news that may not make a ‘trust the science’ headline so appealing is that none of the materials were actually clean. All of the materials, including the control (crushed rock), showed high concentrations compared to the Swedish EPA guidelines on total element content for construction waste. This would be a damning statistic for any social media thread.

However, the fact that the study was looking at leaching means that the results are not just looking at total concentration. Leaching of chromium, molybdenum, nickel, and zinc peaked during the second year, while copper leaching peaked later, during the fourth year. The levels then steadily decreased, and after 10 years, copper, nickel, and zinc levels were below the recommended thresholds for Sweden. One issue was that the recycled concrete went through a chemical process called carbonation, causing a temporary increase in chromate released. However, this did settle. Overall, the study found that the environmental risks posed by this type of construction were largely short-term.

It’s not just a Swedish quirk

As found in a 2023 review article titled “Sustainable and Comprehensive Utilization of Copper Slag: A Review and Critical Analysis,” published in the journal Sustainable Metallurgy, similar results have been found. Copper, zinc, and nickel tend to have high concentrations in the first 5 years after the slag has been used. After 9-10 years, leaching usually reaches geochemical equilibrium, meaning the material becomes stable.

Copper slag isn’t only a Swedish or European concern, it’s used in road construction elsewhere too. In the U.S., roughly 3.6 million metric tons of copper slag is produced every year, and it’s classified as a standard granular aggregate in states such as Michigan. The FHWA’s “User Guidelines for Waste and Byproduct Materials in Pavement Construction” report notes that due to limited toxicity testing, no specific leachates of copper slag have been identified as hazardous via EPA testing. However, they do note that environmental suitability must still be assessed on a case-by-case basis.

Why should this matter

This single study isn’t grounds to greenlight every reused waste product in infrastructure. But it is a useful data point. Reusing industrial byproducts like slag is itself a form of the circular-economy thinking that “zero-waste” approaches promote, yet the long-term environmental behavior of these materials, such as how leaching changes over a decade, rarely gets the same attention as the headline idea of reuse itself. There was no disaster or scandal. However, the data does show that contamination levels decrease slowly over 10 years. Whilst this is not a reason to blindly trust every reused waste product being buried under U.S. infrastructure, it does help to highlight that just because something is a waste product does not mean it is dangerous forever. The data does show that sometimes waste products can be reused without causing issues.