## 1 Introduction  
Asphalt pavement is extensively used in road construction because of its excellent performance on the road, including attributes such as excellent resistance, comfortable driving, and convenient maintenance [1, 2]. The conditions of asphalt pavement are important for driving safety, speed, and comfort [3]. Therefore, the rapid and accurate measurement of an asphalt mixture is meaningful for real-time intelligent monitoring of pavement conditions. Fourier-transform infrared spectroscopy (FTIR) is a rapid testing method to characterize the molecular properties of organic and inorganic materials in heterogeneous and complex mixtures [4], such as geological samples [5], archeological materials [6], and painting materials [7]. In the past few decades, FTIR, including attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy, has also been widely used in measuring asphalt materials [8, 9, 10]. These studies have achieved a lot of results, such as the analysis of oil sources [11, 12], asphalt aging mechanism [13, 14, 15], modifier types and dosage of asphalt performance and structure of the degree of influence [16, 17, 18, 19], and recycled asphalt [20, 21], among others. However, it seems that in the past, FTIR spectroscopy was mainly used for measuring asphalt binders, but not directly for asphalt mixtures. Asphalt mixture is a heterogeneous material [22; 23] due to its various components with different properties and characteristics, such as aggregates, asphalt mortar, and air voids, which are combined to form a complex three-dimensional structure with diverse interactions [24]. The gradation and content of aggregates, the content of asphalt, and the three-dimensional geometry features of aggregates all contribute to the heterogeneity of asphalt mixtures [25], which affects the mechanical strength, adhesion properties, and resistance to environmental factors such as water and temperature. The above studies show that the heterogeneity of asphalt mixture has become an important research field in road engineering. Asphalt mortar is also highly heterogeneous as an important component of the asphalt mixture [26], which directly affects the performance of asphalt mixtures [27]. In recent years, some researchers also have paid attention to the research of asphalt mortar (asphalt + filler) with FTIR spectroscopy. [PERSON] et al. [28] found that the infrared characteristic absorption peaks of asphalt mortar are the superposition of absorption peaks of mineral powder and asphalt. [PERSON] et al. [29] observed that asphalt mortar had all functional groups of its component materials. [PERSON] et al. [30] used FTIR to characterize the functional groups and differentiate the interaction between asphalt and limestone mineral filler. Specifically, [PERSON] et al. [31] used the obtained spectral images of functional compounds to study the heterogeneous mortar micro-regions using ATR-FTIR. However, it seems that past studies on the heterogeneity of the asphalt mixture using FTIR spectroscopy mainly focused on asphalt mortar, ignoring aggregates. As aggregates have an indispensable contribution to the heterogeneity of asphalt mixture, research on aggregates coated with asphalt mortar with FTIR spectroscopy is comprehensive and crucial for future rigorous studies. Therefore, research on the effect of the heterogeneity of aggregates coated with asphalt mortar on their spectra is meaningful for accurate measurement with FTIR spectroscopy. This study mainly focused on the impact of the heterogeneity of aggregates coated with asphalt mortar on its FTIR spectra and spectral reproducibility. Firstly, spectra of fine aggregates, coarse aggregates, and mineral powders were collected by FTIR spectroscopy in transmission mode via a KBr squashing method, followed by analysis of the characteristic peaks.