## 3 Test Results and Discussion
### Mortar Properties
#### 3.2.2 Microstructure and Porosity  
The microstructure (2D slice surface) of the mortars (OPC, EXW, BFN, MFN) was investigated under SEM + EDS. The SEM micrographs are mainly intended for qualitative analysis, and the EDS analyses of selected areas (labelled \"Spectrum\") are used to adjudge the chemical composition within the spectrum. The quantitative 3D void microstructure of the mortar was investigated by XCT. These results were at 28 days of the mortar age. * 2D microstructural analysis  
Figure 8a,b show the SEM micrographs for the PC mortar. In this micrograph and others, \"S\" indicates a sand particle, \"P\" indicates a pore, and \"C\" indicates cement paste. The white particles in the cement paste areas surrounded by circles are unhydrated cement grains. In Figure 8b, the \"Spectrum\" labels represent areas where EDS points were taken, and the elemental results are shown in the Appendix A (Figure A2). EDS analysis on Spectrum 175 showed major elements of Ca, Si, O, and Al, which implies that calcium aluminate silica hydrates (C-A-S-H) gels are formed as hydration products. Spectrum 176 identified major Ca, Al, O, and Fe elements, confirming that the white patches are tetracalcium aluminonferrite (C\({}_{4}\)AF) of unhydrated cement paste. Spectrum 173 contains only Si and O, representing silica sand particles. Figure 9a-c shows the SEM results of the EXW mortar. From the magnified image (Figure 9b), it can be observed that the cement paste between the sand particles (S) is significantly denser with less porosity than that of the control (PC) in Figure 8. In addition, major elements in Spectrum 112 (within the pore, see Appendix A Figure A3) are Ca, O, and Al, indicating that the plate structures formed inside the pore are hemi/mono carbonaceous (C\({}_{4}\)AC\({}^{-}\)\({}_{0.5}\)H\({}_{12}\), H\({}_{\rm c}\) or C\({}_{4}\)AC\({}^{-}\)H\({}_{11}\), M\({}_{\rm c}\)) produced from the reaction of limestone with aluminate (Al\({}_{2}\)O\({}_{3}\)) phases and Portlandite (CH). This shows that the limestone and metakaoin, forming carbo-aluminates, do fill up the microstructure. EDS Spectrum 114 has major elements Ca, Si, and O, which suggest the hydration of alite (C\({}_{3}\)S) and belite (C\({}_{2}\)S) to produce calcium silicate hydrate (C-S-H). A higher magnification micrograph (\(\times\)5) (Figure 9c) shows cracked fibril structures (Spectrum 131 and 133) that look similar to those of the clay morphology in Section 3.1.4. The EDS analysis confirms the major elements as Si, O, and Al, suggesting metakaoin (Al\({}_{2}\)O\({}_{3}\)2 Si\({}_{2}\)); that is, after 28 days, not all the calcined clay particles are fully hydrated. It should be noted that these metakaoin particles and EDS spectrum analysis results were observed for the other LC\({}^{3}\) mortars, as shown later. Figure 8: (a) SEM image showing the different particles in the OPC cement matrix where ‘S’ is sand, ‘P’ is a pore, ‘C’ is cement paste, and black circles showing unhydrated cement grains, (b) white blocks indicating the spectrums analysed with SEM-EDS. Figures 10 and 11 show the 2D microstructure of the BFN and MFN LC\({}^{3}\) mortars, respectively. Similar to EXW, a denser microstructure (compared to OPC) can be observed. Spectrum 88 and 154 in the ITZ of the BFN and paste of MFN mortars, respectively, show the metakaolin morphology observed earlier for EXW; the EDS analysis also shows major elements of Si, O, and Al, indicating metakaolin (Appendix A Figure A4). Spectrum 159 EDS analysis of MFN mortar (Appendix A Figure A5) shows major elements Ca, C, and O, suggesting limestone (CaCO\({}_{3}\)). This highlights the pure filler effect of the limestone. Figure 10: (**a**) SEM image showing the different particles in the BFN mortar matrix and white circles showing unhydrated cement grains, (**b**) SEM image of metakaolin particle in the inter transitional zone (ITZ) of the mortar paste between the aggregates.