A novel nanocomposite of Ca(OH)2-incorporated zeolite as an additive to reduce atmospheric emissions of PM and VOCs during asphalt production

Abstract

An investigation is conducted into the emission reduction characteristics of atmospheric pollutants produced from an asphalt concrete (AC) mix using a Ca(OH)₂-incorporated zeolite nanocomposite (Ca(OH)₂@zeolite NC) synthesized as a recycling additive. As the synthesized additive was added into the AC mix, the water vapor produced from the thermal decomposition of Ca(OH)₂@zeolite NC, even at low temperature, greatly reduced the asphalt viscosity and hence the emission of air pollutants such as volatile organic compounds (VOCs) and particulate matter (PM). The presence of water vapors in the feed suppressed the VOC adsorption capacity of zeolite due to the competitive adsorption of water vapors on the active sites. In addition, the enhanced catalytic activity of Ca(OH)₂@zeolite was attributed to the synergistic catalysis between the super acidic and basic Brønsted sites of aldehydes and Lewis acid–base sites of Ca(OH)₂@zeolite, which are the bridging OH groups in SiO₄⁴⁻ and AlO₂⁻, due to thermal degradation of Ca(OH)₂. The relatively less viscous asphalt is effectively mixed with coarse aggregates to coat or bind the particles, which greatly reduces the PM emissions compared to zeolite. Compared to the hot mix asphalt (HMA) with no additive at 180 °C, the addition of Ca(OH)₂@zeolite NC at a concentration up to 6 wt% of the asphalt fraction into the warm mix asphalt (WMA) at 120 °C greatly reduced the atmospheric emissions (by 61–98% for VOCs, 52–92% for aldehydes, and 85% for PM_2.5). The resulting large reduction of the working temperature during AC production and atmospheric emissions of hazardous chemicals will reduce workers' health risks.