## 2 Influential Factors on the Performance of GSP  
### Microstructure  
Raw materials and structures of GSP are the essential elements affecting laboratory performance and also yielding technical problems. In a microscopic view, the relationships among asphalt, aggregate, and cement particles build the macro-strength of GSP. Indeed, the micro-analysis method is most commonly used in laboratory tests to demonstrate the intuitive influential factors of micromorphology on the behavior of GSP.  
In recent years, resolution of micromorphology techniques, especially the Scanning Electron Microscope (SEM), has reached to the nanometer level, which can capture the microstructure views between different particles to analyze their relationships [34; 35]. The GSP samples are observed in various instruments, resolutions, and observing ranges. The methods and results are shown in Table 1 and Figure 1.  
GSF has higher density and solidity than traditional AC, which contributes to great complex shear modulus and compressive strength [36]. The strength of GSP is reinforced with increase of age (at ages of 7 d, 28 d, and 90 d), and the inner hydrated cement forms a fibrous-like network that penetrates through asphalt films, as shown in Figure 1[39]. This exclusive phenomenon, refered to as three-dimensional lattices or a mosaic spatial network, bridges asphalt and cement, which increases the thickness of interface transition zones and enhances the bonding force between asphalt and cement [36]. Meanwhile, the un-hydrated cement plays a role as mineral powder to strengthen the adhesion between asphalt and aggregate.  
The propagation of micro-cracks in GSP can be also investigated by SEM (Figure 1). The observation results indicate cracking emerges from the bottoms of samples and expands along with the interface between asphalt and cement [38; 40]. This reveals that the asphalt-cement interface is the weak interface existing through the whole depth of GSP structure, approved by the Heavy Vehicle Simulator test and the Dynamic Cone Penetrometer test [41; 42]. Additionally, the reaction of internal among raw materials is mainly physical, because no chemical or asphalt aging processes are identified at the interfaces by the Fourier Infrared Spectrum Analyzer (FISA) (2005 XVFT-IR) [37].  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Authors** & **Observation Methods** & **Zoom Ratios** & **Observing Ranges** \\ \hline [PERSON] et al. [36] & SEM & 500 times & Interfaces \\ [PERSON] et al. [37] & SEM (2005 XVFT-IR), Fourier & 5000 times & Interface transition zone, Chemical \\  & Infrared Spectrum Analyzer & & or asphalt aging reaction \\ [PERSON] et al. [38] & SEM (Model) SM-6390 LV, JEOL) & 10,000 times & Microcracking, Fiber-like network \\ \hline \hline \end{tabular}
\end{table}
Table 1: Methods of microscopic observation for GSP.  
Figure 1: The GSP microscopic images observed by SEM: (**a**) Microstructure at 90-day age; (**b**) Interface and fibrous structures; (**c**) Wrapping phenomenon of asphalt film; (**d**) Generation of micro-cracking and bridging effect of the fiber-like network.  
In general, these characteristics of micro-structure determine the macro properties of GSP, which has high rutting resistance and low cracking resistance. Moreover, the micro-analysis method can effectively and intuitively gain the relationships of each of these components and raw materials. On the other hand, this method is also limited, owing to the subjectivity of corresponding researchers and the complexity of GSP's structure. For example, some different fibrous structures can be observed from GSP samples, but which has a better coherent strength between materials is unknown because it is hard to get quantitative analysis from SEM images.