## 4 Challenges and Solutions  
Multifunctional composites have been extensively explored in prior studies [7]. Nonetheless, they face various challenges, including dispersion difficulties, polarization effects during electromechanical testing, the inherently low ductility of cement-based materials, and environmental factors, all of which impact their performance [166]. A multitude of factors, such as curing duration, hydration time, temperature, water-to-cement (w/c) ratio, aggregate size, the ratio of cement to aggregate, and moisture content, significantly affect the electrical resistivity and its inverse, electrical conductivity, as depicted schematically in Figure 16a [115, 167, 168, 169, 170, 171]. For example, the quantity of hydration products escalates with prolonged curing and hydration processes, leading to the occlusion of pores. This phenomenon is demonstrated in Figure 16b,c for nonconductive and electrically conductive cementitious composites, respectively. The resultant occlusion of pores during cement hydration subsequently causes increased electrical resistivity or reduced electrical conductivity [172]. In essence, changes in electrical resistance during the curing process are contingent on the extent of hydration and the conductivity alterations in the pore solution. Initially, in the early hydration phase (0-0.7 h), the release of ions enhances the conductivity of the pore solution, accounting for the minimal changes and fluctuations in electrical resistivity observed during the initial dissolution stage [97]. Subsequently, as hydration progresses and hydration products accumulate, the increase in electrical resistivity occurs in distinct stages: competition, acceleration, and deceleration [173]. The impact and rate of influence of various factors on the electrical signals of nonconductive and electrically conductive cementitious composites can differ due to the variations in their conductivity mechanisms. In nonconductive cementitious composites, electrical conductivity relies solely on ionic conduction, as illustrated in Figure 16b. Consequently, the reduction in pore solution resulting from curing time, hydration, and other factors significantly affects the electrical signals of nonconductive cementitious composites. Conversely, as previously discussed, the conductivity mechanisms of electrically conductive cementitious composites involve both ionic conduction and electronic conduction (i.e., direct contact between conductive fillers and tunneling effects), as depicted in Figure 16c. Hence, the impact of influencing factors on the electrical signals of electrically conductive cementitious composites may be less pronounced than that observed in nonconductive cementitious composites. For instance, the increase in hydration products due to prolonged curing time occludes conductive pathways, as shown in Figure 16. However, the effects of these formed hydration products on the electrical signals of electrically conductive cementitious composites might be less significant compared to those observed in nonconductive counterparts, as tunneling effects and direct contact between conductive fillers exist, as depicted in Figure 16c [174]. Considering these considerations, electronic conduction experiences an upsurge with an increase in the concentration of conductive fillers, resulting in enhanced stability in the electrical signals of cementitious composites. For example, the rising electrical resistivity of a nonconductive cementitious composite diminished with the incorporation of CFs [175]. Similarly, it was noted that the rate of change in the electrical resistivity of the cementitious composite under various deterioration factors (such as freeze-thaw cycles, sulfate attack, and NaCl penetration) decreased with the increasing concentration of CNMs [175]. In this context, the synergistic effects of conductive fillers significantly enhance the performance and stability of electrical signals, primarily due to the improved electrical conductivity [176].