## 3 Properties of Multifunctional Cemertitious Composites
### Electrical Conductivity  
Conventional cementitious composites exhibit semiconducting properties, rendering them weakly electrically conductive when wet due to ion conduction. However, once dried, they become electrically isolated, as they lack ionic conduction. Consequently, enhancing the electrical conductivity of conventional cementitious composites necessitates the incorporation of conductive fillers. By incorporating these fillers, conventional cementitious composites are enhanced to become multifunctional variants. These multifunctional composites, containing conductive or functional fillers, comprise three phases with distinct characteristics, including various levels of electrical conductivity, matrix materials, fillers, and pores. Thus, the electrical conductivity of multifunctional cementitious composites relies on the condition and properties of each of these phases. Electrical conductivity plays a crucial role in enabling functionalities such as self-sensing, self-heating, electromagnetic interference shielding, energy harvesting, grounding, and cathodic protection in cementitious composites. This conductivity is facilitated by electron hopping and conductive networks induced by the conductive fillers [119]. Enhancing these mechanisms necessitates increasing the percentage of conductive fillers in the range of the percolation threshold. Furthermore, the electrical conductivity of multifunctional cementitious composites can be enhanced by incorporating various types of conductive fillers. For example, the inclusion of long carbon fibers (LCFs) stabilizes the electrical conductivity, which is vital for the functionality of electrically conductive cementitious composites [120]. Notably, the electrical conductivity of cementitious composites incorporating CFs demonstrates two distinct regions concerning the percentage of CFs: an ionic dominant region up to 0.5 vol.% content and an electronic dominant region beyond that [121]. Stability in the electrical conductivity of multifunctional cementitious composites increases with higher percentages of conductive fillers, resulting in reliable functionality features. However, it is crucial to ensure that the percentage of conductive fillers remains within the percolation threshold to achieve effective electrical conductivity in multifunctional cementitious composites [122]. Additionally, the characteristics of the conductive fillers also influence the electrical conductivity of multifunctional cementitious composites. Studies have shown that the electrical conductivity increases with the aspect ratio of CFs [45] and that MWCNTs are more effective than graphite (G) and carbon black (CB) in improving the electrical conductivity of cementitious composites without compromising the mechanical strength [123]. For instance, while the incorporation of graphene (G) into cementitious composites significantly enhances electrical conductivity, it also leads to a reduction in mechanical properties [124]. This decline in mechanical strength can be attributed to the high dosage of graphene required to achieve a substantial reduction in electrical resistance [67]. For example, the upper boundary of the percolation zone in cementitious composites was reached with more than 40 wt.% graphene [125], whereas in composites containing multi-walled carbon nanotubes (MWCNTs), the percolation threshold was achieved with only 0.5 wt.% MWCNTs [126]. This stark contrast in percolation thresholds highlights the superior efficiency of MWCNTs in reducing electrical resistance compared to graphene. Furthermore, incorporating 1.1 vol.% CNTs into cement resulted in a gauge factor of 220 [123], whereas the addition of 7 wt.% carbon black (CB) yielded a gauge factor of only 30 [127], further demonstrating the higher sensing efficiency of CNTs. Consequently, MWCNTs are widely used among various types of conductive fillers. For example, [PERSON] et al.