## 3 Experimental Results
### Microstructural Evaluation
#### 3.4.2 Characterization of Interfacial Transition Zone (ITZ)  
The ITZ originates from the packing of small cement grains (with size ranging from 5 to 40 \(\upmu\)m) against aggregate particles. This size difference results in the wall effect, where small grains are accumulated in the zone closest to the aggregate with higher porosity with larger grains located farther away [44; 45]. Smaller cement grains are more prone to hydration, and fewer unhydrated cement grains are thus observed at the interface. The dimensions of the ITZ depend on the varying grain sizes of the cement matrix and the ITZ can involve various sizes with heterogeneity. Therefore, the ITZ is not commonly distinguishable through image analysis of the concrete microstructure and instead appears as a rapid change in chemical compositions most prominent in the region 10 to 20 \(\upmu\)m around the aggregate.  
SEM and EDX were conducted for microstructural and chemical analyses in the ITZ. For the characterization of ITZ, the line scanning technique in SEM/EDX was employed to estimate the width of the ITZ. The line scanning EDX results are shown in Figure 11 for D-Ms-1 and D-Ns-1 mixtures and Figure 12 for G-Ms-1 and G-Ns-1 mixtures. In the first 15 \(\upmu\)m of the line scanning for the dolomite specimens (D-Ms-1 and D-Ns-1), high calcium (Ca) content and very low silicon (Si) content were detected, which may indicate a large amount of calcium hydroxide (Ca(OH)\({}_{2}\)) in the ITZ [46; 47]. As the scanning distance increased, the chemical compositions varied drastically. The Ca and Si contents suddenly decreased while the magnesium (Mg) content sharply increased for both dolomite specimens. The calculated ratio of Ca:Si:Mg for both specimens was approximately 2:0:1, which is in good agreement with the chemical compositions of dolomite CaMg(CO\({}_{3}\))\({}_{2}\) detected by EDX and provided in Table 4. Larger variations in the chemical content were found in the ITZ due to its intrinsic non-homogeneous characteristic, but the detected chemical compositions were nearly constant when the dolomite was scanned. For the granite specimens, the G-Ms-1 specimen experienced the same trend observed in the dolomite specimens. However, G-Ns-1 had significant variation in chemical elements in the line scanning. Figure 12 shows darker areas involving higher porosity. Additionally, high and fluctuating concentrations of Si were detected approximately 20 \(\upmu\)m into the line scanning for the G-Ns-1 specimen. This phenomenon coupled with the high porosity may explain the increased CTE and lower compressive strength when compared to other mixtures examined in this study (see Figure 6).  
Figure 11: _Cont._  
The width of the \"core\" ITZ, the distance over which chemical compositions are dramatically changed, was determined by the EDX line scanning technique (see Figures 11 and 12). The ITZ is commonly considered to range from 20 to 50 \(\upmu\)m, but the width of the core ITZ can be associated with the degree of cement hydration in the ITZ and can be used as an indicator to characterize the  
Figure 11: Line scanning analyses (SEM and energy dispersive X-ray (EDX)) of ITZ regions for dolomite concrete mixtures.  
Figure 12: Line scanning analyses (SEM and EDX) of interfacial transition zone (ITZ) regions for granite concrete mixtures.  
microstructure of the ITZ. A narrower width of the core ITZ may cause a higher compressive strength of the concrete specimen. D-Ns-1 displayed the widest core ITZ, while G-Ms-1 had the narrowest. G-Ns-1 exhibited significant fluctuations in signal due to the high porosity and the width of the core ITZ could thus not be determined. The results of line scanning show a similar trend to those of the compressive strength, and a wider core ITZ corresponded to a lower compressive strength.