## 3 Materials and Methods
### Mix Designs  
Normal and doped mortar was achieved through the combination of Portland cement type 42.5R, quarry aggregates with grains in the dimensional range of 0 \(\div\) 3 mm, and 6-mm chopped CMFs (SIGRAFIL(r) type C C6-4.0/240-G100) supplied by SGL Carbon. The sand was added in the ratio of 3.5 with respect to the volume of the cement, as is typical for mortars produced for construction. The specific microfibers were selected due to their compatibility with water-based systems. Such carbon fibers are obtained through the cutting of continuous carbon fiber tos, obtaining a short reinforcing filler for functional addition. Their primary application lies in the production of compounds for the enhancement of strength, stiffness, and conductivity of various types of binders. The main properties of the fibers are reported in Table 1. Microfibers were incorporated at relative amounts of 0, 0.01, 0.05, 0.1, 0.25, 0.5, and 1% with respect to the weight of the cement. Greater amounts of CMF were considered in the investigation for identifying the maximum theoretical doping level of this type of composites. A consistent water-to-cement ratio of 0.7 was employed across all mixes. Table 2 reports the composition of the mix designs adopted in the research.  
\begin{table}
\begin{tabular}{c c c} \hline
**Property** & **Value** & **Units** \\ \hline Fiber density & 1.80 & g/cm\({}^{3}\) \\ Fiber length & 6 & mm \\ Fiber diameter & 7 & \(\mu\)m \\ Tensile strength & 4.0 & GPa \\ Tensile modulus & 240 & GPa \\ Bulk density & 1.7 & g/L \\ Single filament resistivity & 15 & \(\mu\)m \\ Sizing type & glycerin & - \\ \hline \end{tabular}
\end{table}
Table 1: Physical and performance properties of carbon microfibers.  
Figure 1 shows the main steps for the preparation of the mortar samples doped with carbon fibers. The procedure chosen for the preparation of all the mix designs adheres to typical standards used for masonry constructions. The use of chemical dispersants or procedures is not envisaged, since their effect could affect physical, mechanical, and electrical, performance of the composites. The aim is to produce comparable and replicable composites. First, the powder components were homogenized through manual mixing (Figure 1a). In particular, the carbon microfibers were first manually separated and then added to avoid, as much as possible, agglomerations during the pouring and mix. Then, the water was slowly introduced, and the compound mixed for five minutes until it became smooth (Figure 1b). The material was subsequently tested for density and for workability, and then poured into standard prismatic molds for mortar, namely 40 \(\times\) 40 \(\times\) 160 mm\({}^{3}\) (Figure 1c). The samples were unmolded after 24 h and cured in a laboratory environment for 28 days. After the curing time, permeability and mechanical tests were performed. For each mix, i.e., for the reference one and the six doped ones, six samples were produced. The seven types of investigated mortars, with different amounts of carbon doping, were labeled with three numbers that defined the percentage of microfibers, with the precision note in the second digit after comma (e.g., N005C is referred to a mortar with 0.05% of CMF with respect to the weight of the cement).  
All tests followed the indications of the European standards, as described in the next sections. Tests were performed on both the fresh and hardened material. In the fresh state, density and workability were carried out, while hardened samples were subjected to flexural, compressive, and permeability tests.