## 4 Discussion of Findings  
Different paste-aggregate interfacial microstructures are created when different types of fine and coarse aggregates with different chemical composition, surface texture, angularity, and pore structures are used in concrete. The performance of concrete specimens was investigated in association with the thermal expansion behavior and compressive strength. The effects of aggregate type are discussed based on the microstructure, especially near the ITZ.  
Crushed sand is produced by crushing a large parent mass of rock. The shape depends on the nature and the degree of stratification of the rock deposit, the type of crushing equipment used, and the size reduction ratio. These properties have an important influence on the quality of fresh and hardened concrete [48; 49]. As the volume of cement paste depends on the shape, texture, grading, and fineness modulus of the crushed sand. The use of crushed sand is generally limited in quantity due to the high cement paste volume needed to obtain adequate concrete workability [1; 2; 4]. As shown in Figure 7a,b, the manufactured sand has a more angular shape and rougher surface than the natural sand and produced higher compressive strength and smaller CTE values. The strengthened ITZ between the paste and fine aggregate is attributed to the rough texture and shape of the manufactured sand. Compared with dolomite specimens, the specimens with granite had a slightly lower compressive strength and higher CTE. Zone 1 shown in SEM image of Figure 8 was assessed by EDX (see Figure 9b), and the result sufficiently supports that the granite used in this study contain mica. Thus, the presence of mica in granite rocks is believed to involve weak cleavage planes that initiate failure in the coarse aggregate, which is similar to previous findings [23].  
The CTE of concrete was simply correlated with the compressive strength. The concrete with lower CTE was expected to have higher compressive strength due to the lower thermal response, especially with low age, which would lead to fewer micro-cracks. This expected trend was observed with the fine aggregate, but the effect of the coarse aggregate type was not clear. The coarse granite and dolomite aggregates do not have distinct differences associated with surface morphology (surface texture, shape, surface pores, etc.). The mineralogy of the coarse aggregate in this study may rarely be a matter of concern unless the aggregate contains constituents such as opal, which is reactive silica mineral [21].  
In the normal strength concrete, the weakest components are the interface between hardened cement paste and aggregate. Microstructure images have been commonly used as a tool to characterize the ITZ of concrete; however, the image alone does not provide quantitative information. Therefore, in this study, the line scanning technique in SEM/EDX was employed to estimate the width of the ITZ, which can be one of the indicators to characterize the ITZ. The hypothesis is that that wider ITZ causes lower compressive strength. Figure 11 shows that the ITZ width matches well with the results of the compressive strength test. D-Ms-1 exhibits the smallest widest ITZ with the highest compressive strength while G-Ns-1 shows the largest ITZ with the lowest compressive strength. However, the effect of the coarse aggregate type is still not clear in the assessment of the ITZ width. Another possibility is that the fine aggregate is a predominant factor in controlling the formation of the microstructure of cement paste when two aggregate types involve a similar level of surface morphology and reactivity toward cement hydration.