The technology group of The Linde Group has entered into a partnership with Skanska, the third largest construction services group in Europe, for the supply of Carbon Dioxide (CO2) to be used in a new construction project in Stockholm, aiming to neutralise the pH balance of the water in Lake Mälaren.

An environmentally sustainable solution using CO2 from the Linde Gases Division is being used to neutralise the pH balance of the water in Lake Mälaren, as the waterfront is transformed and previously unstable mud on the lakebed is being converted to firm ground.

In Autumn 2006, Skanska was granted planning permission to create a new beachfront for the town of Hornsberg, which is being expanded 14,000 square metres out in Ulvsundasjön to allow 400 apartments to be built in an area that had previously been a part of Lake Mälaren.

The challenge facing Skanska however, was that the lakebed in this part of Mälaren consists of 10-18 metres of deep mud, with firm ground beneath it. According to Sharyar Moeini Mostofi, project manager at Skanska, “Building on mud is unstable, and there is a high risk of subsidence. This was a problem Skanska had to deal with when the company got the job of preparing the ground.”

The solution was to build 11,000 pillars of lime and cement which are cast down through the 10–18 metres of mud, and 4-7 metres of water in some areas, into firm ground of the lakebed. The columns stabilise the lakebed in order to enable filling with building materials, such as gravel.

A new challenge to this project presented itself when Lime (calcium oxide) spillage from the concrete pillar castings came into contact with the surrounding water to form calcium hydroxide, thereby increasing the pH level to environmentally unsafe levels. “After a single day’s work the pH limit of 9.0 would be exceeded, forcing Skanska to stop production until the pH-value in the water would neutralise naturally. We needed a solution to reduce the pH level in the water quickly,” says Mostofi.

Linde met this requirement by devising an effective and surprisingly simple solution to neutralise the pH-value of the water by sinking perforated hoses to the bottom of Lake Mälaren, and pumping CO2 through the holes. When CO2 dissolves in water, it forms a very weak acid known as carbonic acid which neutralises the lime spillage from the pillars.

According to Svante Frid, Sales and Marketing Manager at the Linde Gases Division, “Using Linde’s SOLVOCARB B application, the pH-value of Lake Mälaren was kept at an even, environmentally safe level throughout this ambitious project. It also meant our partner avoided expensive production downtime, which was estimated at a cost of €7000 per day when work stood still and they waited for the pH level to drop naturally.”

Productivity is reported to have increased, allowing construction to take place 16 hours per day, 6 days a week and making up for the previously lost time.