## 4 Challenges and Solutions
### Environmental and Cost Considerations
#### 4.4.2 Aggregates  
[PERSON] et al. [2019] integrated recycled slag aggregates, known for their electrical conductivity, into carbon fiber-based multifunctional cement mortar to induce self-sensing feature. The results demonstrated enhanced mechanical and electromechanical performance in self-sensing cement mortar when traditional nonconductive fine aggregate was substituted with conductive recycled slag aggregate. This suggests a viable strategy for reducing the high costs associated with multifunctional cementitious composites. Correspondingly, [PERSON] et al. [2018] observed an increase in electrical conductivity in cement mortar upon replacing regular sand with recycled copper slag. [PERSON] et al. [130] identified that the content of steel slag aggregate (SSA) plays a pivotal role in governing the stress-sensing characteristics of a multifunctional cementitious composite incorporating MWCNTs and steel fiber. Furthermore, [PERSON] et al. [2019] conducted a study where waste glass coated with CNTs was used in part as a substitute for natural aggregate to produce self-sensitivity feature in multifunctional cementitious composites, yielding promising results. Similarly, research has shown that multifunctional cementitious composites containing conductive rubber crumbs exhibited a piezoresistive performance 44 times superior to that of conventional commercial strain gauges, highlighting their exceptional sensing capabilities [213]. Furthermore, it is noteworthy that integrating rubber into self-sensing cementitious composites and self-sensing asphalt concrete can improve the road pavement's ability to absorb sound, thereby enhancing its additional functionalities. Generally, to establish continuous electrical conductive pathways within the matrix material, conductive aggregates can be integrated into cementitious composites, leading to consistent electrical signals [58].  
The cost of conductive filler-coated matrix materials, a recently proposed technique for multifunctional cementitious composite fabrication, must be evaluated. In conductive filler-coated matrix materials, the growing process of carbon nanomaterials on the surface of matrix materials using resin may increase the cost of multifunctional cementitious composite. Nonetheless, the advantages of this technique featuring the high potential applicability of this method in in situ projects, the applicability of low-cost carbon derivatives, the required low percentage of carbon nanomaterials, and the reduced thickness of the multifunctional conductive layer could compensate for the additional cost caused by the growing process of carbon nanomaterials [214]. [PERSON] et al. [215] reported a USD 61.4 increase in the cost of 1 m\({}^{3}\) of conventional concrete after its modification into electrically conductive concrete using CNT-coated conductive aggregates. [PERSON] et al. [216] reported the 3.57 times and 52 times higher cost of the electrically conductive cementitious composite than the nonconductive cementitious composite, respectively, for sand-coated and direct mixing techniques, indicating the cost-effectiveness of the coating technique. The cost reduction in coating techniques over direct mixing originates from the low percentage of CNMs needed for fabrication.