Cementitious composites are widely used as building materials due to their affordability, accessibility, and ease of forming. However, they have drawbacks such as less flexibility, brittleness, and low resistance to thermal stress. The use of fibers as reinforcing material is one way to alleviate these concerns. Carbon fibers have high elastic modulus and strength, resulting in composites with superior fatigue resistance, flexibility, and compressive strength. However, the surface inertness restricts the adhesion ability of carbon fibers to form a bond with the composite matrix and hence requires surface modifications to improve the surface characteristics. In this study, two strategies are used to improve the surface roughness of carbon fibers: first, an alkali treatment; second, a combination of an alkali and fly ash treatment. To determine the best strategy for enhancing the carbon fibers’ adhesive and mechanical characteristics, a comparative investigation is conducted. SEM microstructural analysis is used to examine the bonding between the carbon fibers and the matrix as well as to confirm the effects of the treatment on the fiber's surface. The sessile drop contact angle was used to study the surface wettability of the fiber and the four-probe electrical resistivity testing was used to analyze the resistivity of the fiber. The interfacial bond strength between the fiber and the matrix was obtained from pull-out testing. From the test, it was found that the alkali treatment weakens the mechanical strength and bonding propensity of the fiber, but the addition of the fly ash greatly compensates for the adverse effect and increases the bond strength when compared to the untreated fiber.