## 3 Results and Discussion
### Scanning Electron Microscope (SEM)  
In Figure 14, the Micrographs of specimens RS, C5S0, C4S510, and C15S2 at the magnification of 250, 500, and 1000 were scanned via scanning electron microscope (SEM) to evaluate the dispersion quality of nanomaterials as well as their interaction with the cementitious matrix.  
Three morphologies of FAC are observed: intact spherical FAC, broken FAC owing to the hollow shell structure that tends to be crushed under loading, and partially reacted FAC. According to [PERSON] et al. [69], FAC displays some extent of pozzolanic activity, attributed to the presence of amorphous silica, as well as lime, and can participate in the hydration reactions to produce CH and CSH. The use of FAC led to loose microstructure and highly porous structure, especially the agglomeration of FAC accounted for the propagation of large voids in the cement mortar, and the fraction of FAC can increase the porosity of the system. Due to the glassy surface and physicochemical characteristics of FAC, the bond between aggregates and cement matrix tend to be loose and microcracks take place in the mortar.  
Figure 13: Dehydration and dihydroxylation values in TG analysis.  
In contrast with the reference sample (Figure 14a), with the addition of NS and CNTs, the micro-cracks and voids in sample C15S2 (Figure 14b) were reduced. NS showed the filler effects, filling up the gaps as well as spaces between the cementitious matrix and FAC, leading to a denser microstructure and increasing the bond strength. Because of the impacts of NS to accelerate the hydration process, more homogeneous and compact CSH pastes were formed in LWCM, decreasing the porosity and improving the pore structure. This further contributed to the enhancement in compressive strength. The bridging and pull-out effects of CNTs were seen in the SEM graphs which effectively bridged the gaps and restrained the propagation of micro-cracks, increasing the bond strength of cementitious matrix and that between matrix and lightweight aggregates. Spaced CNTs filled the voids of cement matrix with an influence similar to fibres, impressively enhancing the flexural performance of LWCM. Similar effects of CNTs were observed for sample C5S0 in Figure 14c which showed a dense and homogeneously distributed porous cement matrix, greatly contributing to the growth of strength.  
In specimen C45S10 (Figure 14d) a growing amount of large voids and wide micro-cracks were observed compared to the control sample, resulting in an increasingly porous and uneven microstructure. This is in correspondence with the high water absorption value obtained in the tests. This can be ascribed to the exceeding amount of nano additives in LWCM which compete with cementitious materials for the water, adversely hindering the process of hydration reaction, and contributing to fewer hydration products. The effects of NS performing as nanofillers were witnessed but failed to considerably improve the microstructure. Additionally, packed CNTs were seen in the images, indicating that at high dosages, CNTs fail to evenly disperse in the cement matrix and form agglomeration, which results in less effective bridging impacts and negatively affects the microstructure of LWCM.