## 3 Properties of Multifunctional Cemertitious Composites  
### Conductive Fillers  
To transform conventional cementitious composites into multifunctional cementitious composites, conductive fillers must be added, as schematically illustrated in Figure 2a [29]. These fillers are mainly categorized into two groups: metal-based and carbon-based conductive fillers [30]. [PERSON] and [PERSON] [31], for example, incorporated up to 40% metal-based conductive fillers into a cementitious composite to detect damage using electrical resistance tomography. In another study, a microstructure-based finite element (FE) model was developed for multifunctional cementitious composites containing metal-based conductive fillers [32]. However, carbon-based conductive fillers are more commonly used due to better harmonization with matrix materials, lower percolation thresholds, and higher piezoresistive performance [33]. For these reasons, carbonaceous materials including carbon fibers (CFs), carbon nanotubes (CNTs), graphene nanoplatelets (GNPs), graphene nanoribbons  
Figure 1: Keywords co-occurrence bibliometric map.  
(GNRs), and other types have been widely utilized in previous studies [34]. For instance, CFs have been incorporated into cement to evaluate the piezoresistive properties of multifunctional cementitious composites under flexural loading [35]. The findings indicated the capability of CF-based multifunctional cement paste to detect strain and cracks under such loading. Similarly, the effective performance of multifunctional cementitious composites containing CNTs has been reported in other studies [14]. However, the cost of CNTs presents a significant challenge for their use in cementitious composites [36]. To address this, GNPs have been used in self-sensing cementitious composites. It should be noted, however, that the performance of GNP-containing composites is not as robust as that of CNT-based ones due to their differing bridging effects [37]. CNTs effectively bridge microcracks in cementitious composites due to their one-dimensional (1D) structure, significantly enhancing mechanical performance. In contrast, GNPs, with their two-dimensional (2D) layered structure, exhibit a relatively lower efficiency in microcrack bridging compared to CNTs.  
To balance the high cost of expensive conductive fillers and the weaker piezoresistive performance of others, evidence from previous studies suggests that hybrid conductive fillers are more effective in developing multifunctional cementitious composites [38]. Consequently, composites containing two or more types of conductive fillers have been widely explored [38]. For example, a hybrid of multi-walled carbon nanotubes (MWCNTs) and graphene nanoplatelets (GNPs) has been successfully incorporated into 10% cement-stabilized sand to detect strain and damage, benefiting from the filling effects of GNPs and the bridging effects of MWCNTs [11]. Some of the widely used carbon-based conductive fillers are briefly discussed in the following subsections.  
Figure 2: (**a**) Difference in conventional and multi-functional cementitious composites; (**b**) microstructure of carbon fiber-based self-sensing cementitious composite [39], published by Elsevier (2022).