The mutual effect and reaction mechanism of bitumen and de-icing salt solution

Abstract The two most used de-icing salts, sodium chloride and calcium chloride are detrimental to asphalt pavements during winter maintenance. However, limited research focused on the effect and mechanism of de-icing salt on bitumen. To fill the gap, this research aims to investigate the mutual effect and reaction mechanism between bitumen and de-icing salt solutions. 10% NaCl and 10% CaCl2 solutions were prepared to immerse neat bitumen and aged bitumen for 7, 28, and 90 days, respectively. The pH value of the salt solution, the morphology and element analysis, chemical structure, and rheological properties of the bitumen were evaluated by pH meter, scanning electron microscope with energy dispersive spectrometer (SEM-EDS), Fourier transform infrared (FTIR) radiation spectrometer with an attenuated total reflectance (ATR), and dynamic shear rheometer (DSR), respectively. After the immersion process, the pH value of the salt solution can decrease to 4.33; the bitumen surface became rougher, and O content of bitumen increased by 2%, around 1% Cl and Na/Ca element appeared; S = O, C = O, C-OH, C-Cl functional groups increased by up to 60%; and the bitumen showed sufficient deformation resistance and worse viscous behaviour. According to the test results, the reaction mechanism between bitumen and de-icing salt solutions was summarized into three sub-mechanisms: the release of hydrogen ions from the bitumen into the solution; the substitution reaction of Cl provided by de-icing salt; and the oxidative reaction between bitumen and oxygen.