Mechanisms of NOx entrapment into hydrated cement paste containing activated carbon — Influences of the temperature and carbonation

• Ca(OH)2 and C–S–H degrade NO2 in nitrate and nitrite via a disproportionate reaction.
• The high surface area of activated carbon boosts the NO2 fixation in cement paste.
• Disproportionate reaction competes with adsorption in presence of activated carbon.
• The NO2 abatement by the activated carbon material remains stable between 20 and 50 °C.
• The NO2 abatement by the activated carbon material is not affected by carbonation.

The NOx pollution produced by road traffic in confined volumes, such as tunnels, is an issue for public health. This work focuses on the mechanisms of NOx removal by modified cement pastes. The samples (made of pure synthetic powders or cement paste cylinders) are continuously exposed to 220 ppbv of NO and/or 110 ppbv of NO2 gas. The ability for the main hydrates (such as Ca(OH)2 and C–S–H) to trap NO2 is then quantified. After the leaching of samples, the chromatography experiments show that the adsorbed NO2 is transformed in nitrate and nitrite ions by a disproportionate mechanism. The addition of activated carbon into cement paste enhances the NO2 abatement, which is not influenced by carbonation. The NO2 abatement by the activated carbon materials is also stable between 20 and 50 °C, revealing a competition between the disproportionate reaction and the gas adsorption.