## 3 Results and Discussion
### Water Absorption  
Water absorption of concrete indicates the durability of cement materials and is correlated to pore structure as well as the microvoids existing in the hardened concrete [63; 64]. The original data from the water absorption tests are recorded in Appendix A (Tables A5-A7). In Figure 9, the characteristics of water absorption of LWCM are assessed via the weight of absorbed water, as well as the percentage of absorbed water, which is the percentage ratio of the weight difference to the dry weight [38]. The weight difference is the value between the sample weight after submerging into the water and the weight after drying up. As can be seen in the bar graph, in contrast with the referencing specimen, with 17.2 g of water absorbed in the test, the addition of nanomaterials contributed to the decline of water absorbed by the LWCM on different levels. The specimen C45S10 exhibited the lowest decrease in water absorption, of approximately 1.2 g, showing that at high dosage, the synergic incorporation of CNTs and NS failed to reduce the number of microvoids in the cement mortar. This also addresses the high weight of absorbed water (11.5 g) in the sample including only 0.45% of CNTs. Another reason can be ascribed to the nature of nanomaterials that require a high amount of water. Specim C5S10 shows the lowest percentage of water absorption, 4.3%, demonstrating that the synergic presence of nano additives at a proper amount is effective for improving the pore structure of cement mortar and decreasing porosity. No obvious trend was observed between the amount of NS or CNTs and the water absorption capacity in the sample. Only when the CNTs content is constant at 0.05% and 0.15% were average greater synergic effects measured.  
Figure 10 illustrates the change in the percentage water absorbed in comparison with the reference sample. On average, specimens with 0.05% and 0.15% of CNTs displayed comparatively higher loss in percentage water absorption, with the most remarkable value observed for samples C5S2 and C5S10, with 11.40% and 11.52%, respectively. This can be associated with the increasingly dense microstructure due to the incorporation of nanofillers at a proper dosage. The changing values varied within the range from 7% to 12%, except specimens C45S0 and C45S10, which exhibited a high percentage of water absorption, with only 5.21% and 0.30% of loss, respectively. This can be related to the loose morphology of the two specimens, as is proven in the SEM images.  
Figure 11 exhibits the amount of water absorbed by the samples along with the time. As can be seen in the graph, for all the samples, within 24 h, the same sharp increment of the amount of water absorbed was observed, followed by a slight enhancement in the upcoming days. This indicates that the samples have reached their saturation to absorb water at the early stage. After that, the capacity of water absorption of samples is limited. Other than the reference sample, regardless of the NS content, specimens containing 0.45% of CNTs all exhibited greater absorbing velocity compared to others with 0.05% and 0.15% CNTs and absorbed a higher amount of water at saturation. This indicates that 0.45% dosage of CNTs is less effective to improve the porosity of LWCM in contrast with other CNT contents.