## 3 Composite Matrices and Applications
### Metals
In the results shown in the Pareto charts in Figure 14, the most significant effects on compressive strength were the incorporation of titania nanoparticles and the interaction between titania nanoparticles and CNTs. These graphs showed how compressive strength varied with different combinations of titania nanoparticles, CNTs, and surfactant concentrations. Optimal combinations for maximum compressive strength depended on fixed values of one of these components while varying the others [125]. Another study by [PERSON] et al. explored the use of modified MWCNTs to enhance the properties of cement-based composites [126]. The experiment involved the incorporation of pristine MWCNTs (p-CNTs), carboxyl functionalized MWCNTs (c-CNTs), and low-temperature plasma-modified MWCNTs (m-CNTs) into ordinary Portland cement (P.O 42.5R) and standard sand. A polycarbonate superplasticizer (SP) was used to disperse each of the MWCNT types and ensure proper fluidity of the mixtures. To ensure proper dispersion, MWCNTs were first dispersed in water with SP using ultrasonic vibration for 10 min at 400 W, with intervals to prevent overheating. The mixture was then combined with cement and sand in a standard mixer, poured into molds, compacted, and left to cure for 24 h before removal. Finally, specimens were cured under standard conditions for an additional 27 days before testing [126]. Figure 13: Integral peak area ratio (A) for CH and C3S as a function of MWCNT content (%) in cement composites [124]. Further analysis of the surface characteristics was conducted using Fourier Transform Infrared Spectroscopy (FTIR) and Raman Spectroscopy of all three types of MWCNTs. The FTIR spectra of m-CNTs revealed characteristic peaks indicating the presence of oxygen-containing functional groups generated during plasma modification. Raman spectra showed an increase in defect size and amount after functionalization (carboxyl or plasma), with m-CNTs exhibiting the highest level of defects. Thermal stability tests showed that p-CNTs were highly stable up to 790 \({}^{\circ}\)C, while c-CNTs and m-CNTs experienced weight loss of around 3.6% and 7.9%, respectively, due to the elimination of oxygen-containing functional groups. MWCNTs were found to have a positive impact on cement matrix strength compared to the reference group, validating the nano-reinforcement effect. Specifically, samples containing 0.10 wt% m-CNTs exhibited the highest flexural strength (9.6 MPa), representing a 37.1% increase over the reference cement sample and a 17.1% increase over c-CNT at the same percentage. Moreover, the highest compressive strength was observed in m-CNT20 (57.2 MPa), showing a 24.3% increase compared to the reference sample. c-CNTs demonstrated similar but weaker reinforcement effects than m-CNTs. The addition of  
Figure 14: Three-dimensional surface plots and 2D contour plots showing the influence of two process variables on compressive strength, while the third process variable is fixed: (**a**) Surfactant (S) is fixed at 0.012 wt%; (**b**) Titania (T) is fixed at 1.0 wt%; and (**c**) MWCNTs (C) are fixed at 0.06 wt% [125]. In samples without any nano-additives, loose stack structures of hydration products with numerous pores and microcracks were observed, facilitating crack initiation and propagation. Cement composites with p-CNTs showed agglomerates of carbon nanotubes, as demonstrated by SEM in Figure 16, acting as micro-defects and adversely affecting mechanical properties. However, in composites with c-CNTs, a crack-bridging mechanism was observed, where c-CNTs effectively hindered crack propagation, leading to improved mechanical behavior. The above studies collectively demonstrate the substantial potential of nano-reinforcern, particularly CNTs, in enhancing the mechanical properties of cement composites.