## 1 Introduction  
Mineral aggregates make up more than 80% by volume or 90% by mass of the commonly used paving asphalt mixtures and, according to the particle size, mineral aggregates can be divided into filler (<0.075 mm), fine aggregate (FA) (0.075 mm < 2.36 mm) and coarse aggregate (CA) (\(\geq\)2.36 mm). The aggregate geometry has long been recognized to have a significant influence on the volumetric, mechanical, skid-resistant and other properties of asphalt mixtures. Conceptually, the geometric characteristics of aggregate can be fully described by three independent properties, which are form, angularity, and texture [1, 2]. characteristics are determined by means of measuring the bulk properties of a group of aggregates. [PERSON] et al. adopted four kinds of indirect tests for CA geometry and compared and evaluated the test results [14]. Their research indicated that the particle index (Standard Test Method for Index of Aggregate Particle Shape and Texture, ASTM D-3398) and uncompacted void tests provide comparable measures of aggregate particle form, angularity and surface texture. [PERSON] et al. evaluated the particle form and texture properties of 18 FAs (eight natural sands and ten manufactured sands) by virtue of the ASTM D3398 test and results showed that the index value can distinguish these two kinds of FA [15]. On the other hand, the digital image processing (DIP) technique provides an alternative direct method to quantify the aggregate geometric characteristics [16; 17]. In the direct approach, the geometric characteristics (form, angularity, and texture) of aggregate are measured upon the individual aggregates. [PERSON] et al. proposed a new angularity index (AI) based on the University of Illinois Aggregate Image Analyzer (UIAIA) to characterize the angularity of CA. Their results demonstrated that this AI value is able to distinguish crushed stone from gravel [18]. [PERSON] et al. compared the DIP method with the manual method in measuring the CA of cement concrete [19]. It is concluded that the mean thickness/breadth and length/breadth ratios can be measured directly with the application of the DIP method. [PERSON] et al. evaluated aggregate geometry using the Aggregate Imaging System (AIMS) and results indicated that the level of anisotropy of an aggregate system is very sensitive to aggregate form, angularity, and textural properties [20]. [PERSON] et al. proposed several DIP techniques to characterize the microstructure of asphalt mixture [21; 22; 23; 24]. Moreover, they also investigated the relationship between FA shape properties and the performance of hot-mix asphalt mixture. [PERSON] et al. [25] investigated the particle shape index for fine aggregates based on the concept of volumetric coefficient, which has been recommended by a French standard (AFNOR XP P18-54). The optimal relation among these quantified indexes of FA and the resistance of rutting have also been observed by [PERSON] et al. [26] and [PERSON] et al. [27]. Even though numerous indicators have been proposed by previous researches in describing the geometric characteristics of aggregate [10; 28; 29], a comprehensive study on verifying their ability to distinguish different kinds of aggregate and view the correlations among different indicators is still lacking. Moreover, few researches have concerned the effect of FA geometry on the rheological property of asphalt mortar, while asphalt mortar is of great importance as it is a single characteristic-length scale smaller than asphalt mixture and is, therefore, closer in characteristic size of the damage that occurs with a mixture. This research focuses on the geometric characteristics of FA. In this research, 3 indirect indicators and 3 types of 7 direct indicators were adopted to evaluate the geometry of 13 different kinds of FAs. Based on the test results, the statistic method was used to verify the ability of the indicators to distinguish the FA type and analyze the correlations among different indicators.