## 3 Composite Matrices and Applications
### Metals
In addition, graphene oxide and CNTs are used to add functional properties, such as conductivity and thermal stability [120, 121, 122, 123]. [PERSON] et al. investigated the properties of MWCNT-reinforced cement-based composites. The study successfully used \(N\),\(N\)-dimethylformamide (DMF) as a dispersant for MWCNTs, gradually adding it to the MWCNT solution with mechanical stirring for 10 min [124]. Another MWCNT-cement solution without DMF was prepared with mechanical stirring for comparison. The mixture underwent 60 min of ultrasonication in a probe sonicator. Various concentrations of MWCNTs and defoamer suspensions were mixed [124]. Mechanical strength increased gradually with increasing MWCNT dosages, peaking at 0.04 wt% before declining. The maximum compressive strength and flexural strength reached 77.2 MPa and 11.2 MPa, respectively, representing a 2.9% and 21.7% increase compared to the control sample. Samples with dispersed MWCNTs (with DMF) exhibited better mechanical properties compared to those with individual MWCNT dosages (without DMF). MWCNTs enhanced flexural strength while minimally affecting compressive strength. The ratio of compressive strength to flexural strength decreased by 15.9% with 0.04 wt% MWCNTs, indicating improved fracture resistance [109]. The X-ray diffraction (XRD) patterns show that no new phases were generated, indicating that the structure of the final hydration product remained unchanged in MWCNT/cement composites. The ratio of the integral areas of the CH (18 degrees) and C3S peaks (31.4 degrees) was used as an indicator of hydration degree. The integral peak area ratio of MWCNT/cement composites increased gradually with higher MWCNT dosages, indicating an accelerated cement hydration reaction (Figure 13). [PERSON] et al. also showed that the addition of MWCNTs led to a reduction in the porosity of the cement composites, as confirmed by mercury intrusion porosimetry. This was a result of the MWCNTs inhibiting the extension of cracks and enhancing the compatibility of the composites. Furthermore, SEM revealed that MWCNTs were well-integrated into the cement matrix, acting as bridges across microcracks, thus improving the microstructure and overall durability of the composites [124]. [PERSON] et al. also investigated the enhancement of mechanical properties of cement composites using CNTs and titania nanoparticles (TiO\({}_{2}\)) as nano-reinforcements [111]. To synthesize titania nanoparticles, titanium tetrachloride and ethanol were mixed. The gel formed was dried at 120 \({}^{\circ}\)C for 6 h and then heated at 400 \({}^{\circ}\)C for 2 h to obtain titania powders. MWCNTs were produced by chemical vapor deposition and were subsequently functionalized with carboxyl groups via oxidation with nitric acid, washed, and dried to obtain MWCNTs-COOH [125]. MWCNTs and TiO\({}_{2}\) nanoparticles were simultaneously incorporated in cement slurries at numerous weight percentages [125]. Mechanical characterization of the composites indicated that the compressive strength ranged from 36.5 to 39.5 MPa, and flexural strength ranged from 7.8 to 10.3 MPa, showing greater scatter for compressive strength. The addition of nano-reinforcing agents significantly increased both the flexural and compressive strength of hardened cement, aligning with prior studies on nano additives that included MWCNTs and nano titania. Among tested specimens compared to the control, the highest increase in compressive strength (32.5%) was observed in the specimen where 1.5% of Portland cement was replaced with titanium nanoparticles, MWCNTs, and surfactants. The highest increase in flexural strength (41.8%) was seen in the specimen where 1.0% of Portland cement was replaced with titania nanoparticles, MWCNTs, and surfactants. This indicates that replacing 1-2% of cement with nanofillers significantly improves mechanical properties [125].