Reducing the Carbon Footprint of Bituminous Composites Using Amine-Impregnated Zeolite

Moisture damage in bituminous composites is a major concern, reducing the service life of infrastructures. The use of lime and amine-based additives to mitigate the damage has been practiced in the industry. Lime production is an inherently carbon-intensive process involving large quantities of CO2 emissions. Amine-based additives lose their effectiveness over time when exposed to weathering and UV irradiation. Inspired by the technology of amine-functionalized porous materials, here, we study the merits of using amine-impregnated zeolite in bituminous composites to protect amines from weathering and UV. We hypothesize that some of the ethylenediamine molecules loaded into the zeolite's pores are gradually released into the matrix of bitumen, with the protection in the zeolite's pores allowing the ethylenediamine molecules to maintain their effectiveness as antistripping agents until their release. Our laboratory experiments used the moisture-induced shear-thinning index to compare the moisture resistance of samples with amine-impregnated zeolite to samples with amines and zeolite added separately. Initially (before aging), the samples with amines and zeolite added separately have higher resistance to moisture damage compared to amine-impregnated zeolite (in which 52% of the total amines are retained in the zeolite). The superior performance of amine-impregnated zeolite is shown after short-term laboratory aging. After short-term aging and throughout long-term aging in the laboratory, the comparative resistance to moisture damage increasingly favored the amine-impregnated zeolite owing to the slow release of retained amines under an external stimulus induced by aging. This slow release of retained amines extends the effectiveness of the amines as the amines retained within zeolite pores are protected from early aging. Our quantum-based calculations using density functional theory (DFT) show that some small polar compounds of bitumen take advantage of stronger interactions with zeolite's active sites to substitute for amine molecules; this could be a driving force for the gradual release of amine molecules from zeolite to the bitumen matrix. Our DFT studies on seven polar compounds of bitumen (quinoline, pyridine, benzofuran, benzoic acid, hexanal, 3-pentylthiophene, and hexanethiol) show that many of these compounds have stronger interactions with the zeolite model, particularly in the exterior walls of the intact sodalite cage ({111} face) and supercage-window sites, leading to deliver the loaded substance (ethylenediamine) into the medium. In contrast, amine molecules grafted onto the active sites of the broken sodalite cage ({100} cut) benefit from many H-bonding interactions at this active site, so these amine molecules are not easily substituted or released into the bitumen matrix.