Multifunctional zeolite nanorods as wax carriers and acid scavengers in asphalt binder

Abstract

This paper investigates a dual role of wax-impregnated zeolite when used as an additive in warm-mix asphalt (WMA) technology. The dual role is reflected in the zeolite’s gradually releasing wax at high temperature and adsorbing dissolved organic acids at the locations vacated by wax. The latter role is referred to as acid scavenger. The study includes molecular-level approaches using density functional theory and atomic force microscopy (AFM). Images collected by AFM show that wax molecules are released from the zeolite and crystallize in the bitumen. The zeolite nanorods in bitumen are shown to adsorb part of the acidic content in the bitumen, as evidenced by a reduction of acid accumulation at the glass-bitumen interface. This was also confirmed by our computational modeling showing the surface adsorption energy of acid-zeolite is much higher than the energy of wax-zeolite; as shown by ΔE(wax-acid) = 5.1 kcal/mol for the adsorptions of wax on the broken sodalite cage, and ΔE(wax-acid) = 20.3 kcal/mol for the adsorptions of acid in the entrance supercage-window location. However, the penetration of acid into zeolite channels is not as easy as wax penetration. This is attributed to the electrostatic forces of zeolite’s sodium cations on polar acid molecules, which limit the free motion of acid molecules into the zeolite channels. The study outcomes show the merits of exploiting the dual role of wax-impregnated zeolite to enhance sustainability in construction.