Chemical structure analysis of chitosan-modified road bitumen after de-icing salt treatment

Abstract

Asphalt pavements are constantly exposed to many destructive environmental factors including de-icing salts. The problem of the negative effect of salt ions on the performance and consequently the durability of road pavements occurs mainly in temperate climates and regions directly neighboring saline water areas. The salt ions react chemically with the bitumen components, which consequently changes their electronic structure and results in a weakening of the intermolecular interactions occurring between them. Therefore, this study focused primarily on an investigation into the potential for inhibiting the destructive erosion process of bitumen by its modification with chitosan. Studies involving changes in the acidity of the eroding solution as well as chemical and surface properties of the eroded bitumen were carried out for three different salts (NaCl, MgCl₂, CaCl₂) at varying concentrations, i.e. 5%, 10%, 15% (w/w) after 7 and 28 days of erosion process. Main findings demonstrate that chitosan prevents negative changes in the bitumen physico-chemical properties occurring during the salt erosion process. This effect is especially visible for the bitumen eroded with a solution of MgCl₂ and CaCl₂. For these salts, chitosan biopolymer reduces the introduction of Cl⁻ ions into the bitumen-building hydrocarbon structures and formation of C–Cl bonds, which is demonstrated by a reduction in the pH changes of the eroding solutions. In addition, chitosan biopolymer inhibits leaching of organic matter from the bitumen, prevents C = O groups formation and reduces the negative effects of de-icing salts on the cohesion energy of the bitumen.