During construction of the 9.5km Filder Tunnel, part of the Stuttgart 21 rail line, Porr and German company MC-Bauchemie developed and used an annular gap mortar which replaces cement with granulated slag, a waste product of steel manufacturing.

“The nature of the soil meant that a cement-based construction material would not have been suitable,” said Porr CEO Karl-Heinz Strauss. “This gave us the opportunity to use this brand new construction material.”

Porr says 8% of global greenhouse gas emissions come from cement production. “This is why we are increasingly looking at new, innovative solutions,” said Strauss, adding that the Filder pilot project provided a template for future projects.

“Apart from allowing us to contribute to reducing CO2, this construction material has two fundamental advantages: it is less sensitive to environmental factors than concrete containing cement; and it can be transported for long periods without any problems before it is processed as it needs an activator to fully harden,” he said.

The new annular gap mortar has been patented and Porr is now looking for more partners willing to work with the material.

“Clients are often very conservative in their tender conditions and demand a certain cement content. Work is still needed here to convince them,” said Strauss.

Porr is also testing other recycled products to assess their suitability as binders in construction materials, including brick sand. Across the Group, Porr recycles 2.2 million tonnes of construction materials per year and, of that, 1.7 million tonnes is used in place of primary raw materials.

The Filder Tunnel is the longest tunnel on the high-speed rail line between Stuttgart and Ulm. Porr was part of the Arge Atcost 21 joint venture with G Hinteregger & Söhne Baugesellschaft, Östu-Stettin Hoch- und Tiefbau GmbH, and Swietelsky Baugesellschaft.