## 4 Results and Discussion
### Study on the Heterogeneity of Aggregates Coated with Asphalt Mortar Based on ATR-FTIR Spectroscopy  
#### 4.2.1 Spectral Comparative Analysis of Asphalt and Aggregates Coated with Asphalt Mortar  
To better compare the spectra of asphalt and aggregates coated with asphalt mortar, as shown in Figure 5, the intensity ratio of the spectra needs to be adjusted to the same level. Therefore, spectral normalization pretreatment is required before spectral analysis. The spectrum preprocessing was carried out by Matlab software R2019b using the normalized \"mapmaxmin\" function. The normalized spectra of asphalt and aggregates coated with asphalt mortar after peak searching and peak marking are shown in Figure 6.  
In Figure 6, aggregates coated with asphalt mortar and asphalt exhibit similar spectral responses in the range of 3000-2500 cm\({}^{-1}\) but different spectral responses in the range of 2000-500 cm\({}^{-1}\) with obvious overlapping peaks at 1419 cm\({}^{-1}\) and 1018 cm\({}^{-1}\). The ATR-FTIR spectra of aggregates coated with asphalt mortar showed that the samples exhibited absorption peaks at around 2952 cm\({}^{-1}\), 2920 cm\({}^{-1}\), 2851 cm\({}^{-1}\), 1797 cm\({}^{-1}\), 1723 cm\({}^{-1}\), 1453 cm\({}^{-1}\), 1419 cm\({}^{-1}\), 1377 cm\({}^{-1}\), 1268 cm\({}^{-1}\), 1018 cm\({}^{-1}\), 873 cm\({}^{-1}\), 780 cm\({}^{-1}\), 729 cm\({}^{-1}\), and 712 cm\({}^{-1}\). According to the analysis in Section 4.1, the absorption peaks at 1797 cm\({}^{-1}\), 1419 cm\({}^{-1}\), 873 cm\({}^{-1}\), and 712 cm\({}^{-1}\) could be attributed to typical peaks of coarse limestone aggregate, while the absorption peaks at 1018 cm\({}^{-1}\) and 780 cm\({}^{-1}\) could be attributed to typical peaks of granite powder. And the absorption peaks at 2952 cm\({}^{-1}\), 2920 cm\({}^{-1}\), 2851 cm\({}^{-1}\), 1453 cm\({}^{-1}\), 1377 cm\({}^{-1}\), 729 cm\({}^{-1}\) could be attributed to typical peaks of asphalt. Furthermore, absorption peaks at 1723 cm\({}^{-1}\) (stretching vibration of C=O) and 1268 cm\({}^{-1}\) (ether) could be attributed to residual asphalt cleaner, which could be dismissed. Figure 4: Original spectra of aggregates and mineral powders. The vibration frequency is called group frequency and can be used to identify the presence of a specific group. By analyzing the position of the absorption peak in the infrared spectrum, we could know the types of groups that might be contained in the sample molecule. To further investigate the differences between asphalt and aggregates coated with asphalt mortar, spectral comparative analysis was conducted by analyzing the distribution of the characteristic peaks. The descriptions of peak positions are listed in Table 3, according to previous research on graded asphalt and aggregates [13, 24, 35, 38, 39, 40]. There are a total of 12 major characteristic peaks of the aggregates coated with asphalt mortar, with five concealed characteristic peaks of the asphalt, including the significant absorption peak at 1030 cm\({}^{-1}\). From the spectral analysis above, the characteristic absorption peaks of the aggregates coated with asphalt mortar are the superposition of the respective absorption peaks of the aggregates, mineral powders, and asphalt binders, which can all contribute to the spectrum of the coated aggregates. Figure 5: Original spectra of asphalt and aggregates coated with asphalt mortar. Figure 6: Normalized spectra of asphalt and aggregates coated with asphalt mortar. 2.2 Study on the Effect of the Heterogeneity of Aggregates Coated with Asphalt Mortar on Its Spectrum  
To further study the effect of the heterogeneity of aggregates coated with asphalt mortar on its spectrum, 15 spectra of the five samples were measured by ATR-FTIR technology as shown in Figure 7, which is drawn by Matlab software. It can be seen from Figure 7 that the spectral shapes and peak positions of the five samples obtained from the same batch of asphalt mixture fabricated at the same time across the entire mid-infrared region are very similar.