Mixing bacteria into concrete can prevent damage to roads from de-icing salt, a new study suggests. Calcium chloride is put on roads to stop ice formation and snow accumulation, but can later cause potholes and cracks in concrete.
This happens when the calcium chloride reacts with calcium hydroxide in the presence of water. Calcium hydroxide is created when Portland cement reacts with water, so is usually present in concrete. The reaction forms calcium oxychloride, which takes up a greater volume than the original calcium hydroxide and surrounding pores, causing the concrete to crack as it expands.
Researchers in the US dealt with this problem by adding Sporosarcina pasteurii bacteria and nutrients to a common type of cement used in roads. This concrete experienced almost no deterioration after calcium chloride salts were added, and levels of calcium oxychloride were found to be much lower in these samples than those with no bacteria (Construction and Building Materials, 2019, 195, 1).
The bacteria work by attracting positively charged calcium ions, and by harvesting carbon dioxide, which forms bicarbonate ions in solution. When brought together, the calcium ions and bicarbonate ions react to form calcium carbonate, a constituent of limestone. Its presence suggests that the bacteria could be used to strengthen road surfaces, though this would require more research, according to the team at Drexel University in Philadelphia, Pennsylvania, US.
‘They have added live bacteria to the concrete. They’ve assumed that the bacteria survive the mixing, hydration/hardening and conditions in the concrete,’ says Kevin Paine, a civil engineer at the University of Bath, UK, who has worked on research projects that add spores of bacteria to concrete to fix cracks as they occur.
He suggests there are easier ways to prevent calcium oxychloride forming, such as adding aluminosilicate materials that will react with calcium hydroxide to form calcium silicate hydrates within the first 56 days or so. ‘Consequently, when calcium chloride is added to these concretes there is no calcium hydroxide available,’ he explains.
The UK does not suffer from damage to roads from calcium chloride to the same extent as Canada and the US.
‘In the UK, I don’t think we use calcium chloride in de-icing salts. Nearly all UK de-icing salt is rock salt [sodium chloride], from Northern Ireland,’ Paine explains. Rock salt does not generate calcium oxychloride.