Grinding up the vast amounts of waste concrete around the world and spreading it on fields could remove carbon dioxide from the atmosphere and potentially boost crop yields, with no harmful effects, according to a field trial in Ireland.
Doing so could partially compensate for the huge emissions from cement production. 鈥淚t鈥檚 sort of recapturing the CO2 that was lost in the first place,鈥� says at University College Dublin in Ireland, who presented his team鈥檚 initial findings at a meeting of the European Geosciences Union in Vienna on 15 April.
Cement is typically made by heating calcium carbonate, or limestone, to form calcium oxide, or lime, releasing CO2 in the process. The lime is then mixed with sand and gravel, known as aggregate, to make concrete.
Advertisement
The lime in concrete reacts with the CO2 in the atmosphere, forming bicarbonate ions that get washed into the ocean and eventually turn into calcium carbonate, locking away the CO2. But with solid concrete, this weathering process is very slow.
To speed it up and capture carbon on a bigger scale, a company called Silicate has been grinding up concrete into particles smaller than 1 millimetre and spreading them on fields in County Wexford, Ireland. Magee鈥檚 team has been testing water samples taken from pipes dug down into the fields to see what the results are.
The team found that bicarbonate levels rose in the treated areas and have remained higher than untreated areas of the same field for more than a year, showing that CO2 is being removed. However, the team cannot yet say how much CO2 is being removed.
Team member at Trinity College Dublin looked at the effects on the oat and barley plants growing in the fields. He found no difference in the oats but the seed mass of the barley was 35 per cent higher in the treated side of the field.
鈥淚t鈥檚 encouraging,鈥� he says. But the team cannot yet say what led to this increase, and why no effect was seen in oats. Silicate also has an ongoing trial in Illinois, which will provide more data.
Many other groups around the world are experimenting with adding ground-up rocks that react with CO2, such as basalt, to fields, known as enhanced weathering. This removes CO2 from the atmosphere in a similar way and can also have a fertilising effect.
Magee thinks applying ground-up concrete has a number of advantages. For one, there are already vast piles of waste concrete, so it doesn鈥檛 need to be quarried. Very little of this waste concrete gets reused, he says.
While the aggregate used in Ireland is limestone, in some countries basalt aggregate is used, says Magee. In these cases, the ground-up basalt aggregate will react with CO2, too.
Another advantage is that many farmers already apply lime to their fields to make them more alkaline, and applying powdered concrete has the same effect. 鈥淔armers are already familiar with this,鈥� says Magee.
But we know too little about the processes involved to start spreading ground-up concrete on fields around the world, says Magee. For instance, the team found that the application of nitrogen fertilisers greatly reduced the levels of bicarbonates formed, so this aspect will need to be optimised.
at the University of Antwerp in Belgium, who studies CO2 removal by enhanced weathering, says the use of concrete has promise because of the recycling aspect. 鈥淚 think it鈥檚 better in terms of circularity,鈥� she says.
