Carbon-reinforced cementitious nanocomposites represent a significant advancement in construction materials, combining the mechanical robustness of cement with the exceptional properties of carbon-based nanomaterials. These composites leverage carbon nanotubes (CNTs), carbon nanofibers (CNFs), or graphene derivatives to enhance strength, durability, and functionality, particularly in smart concrete applications. Key areas of focus include dispersion techniques, piezoresistive behavior for structural health monitoring, and addressing durability challenges such as alkali-silica reactions.

Dispersion of carbon nanomaterials in cementitious matrices is critical to achieving uniform reinforcement. Due to their high surface energy and van der Waals forces, carbon nanomaterials tend to agglomerate, reducing their effectiveness. Surfactants are commonly employed to improve dispersion. Sodium dodecyl sulfate (SDS) and sodium dodecylbenzene sulfonate (SDBS) are anionic surfactants that adsorb onto carbon surfaces, creating electrostatic repulsion to prevent agglomeration. Nonionic surfactants like polyvinylpyrrolidone (PVP) or Triton X-100 provide steric hindrance, further stabilizing the dispersion. Sonication is often used alongside surfactants, with optimal durations typically ranging from 30 to 60 minutes at power levels between 100 and 500 watts. Over-sonication can damage the nanomaterial structure, while under-sonication leaves agglomerates intact. Functionalization of carbon nanomaterials with carboxyl or hydroxyl groups also enhances dispersion by increasing hydrophilicity and compatibility with the cement matrix.

The piezoresistive properties of carbon-reinforced cementitious nanocomposites enable their use in smart concrete for structural health monitoring. When subjected to mechanical stress, the conductive network formed by carbon nanomaterials undergoes reversible changes in electrical resistance. This phenomenon allows real-time detection of strain, cracks, or deformations in concrete structures. For instance, composites with 0.5% by weight of CNTs exhibit gauge factors—the sensitivity of resistance change to strain—ranging from 50 to 200, depending on dispersion quality and nanomaterial type. The percolation threshold, the minimum concentration required to form a continuous conductive network, typically falls between 0.1% and 0.5% for CNTs and CNFs. Graphene oxide, due to its higher aspect ratio, may achieve percolation at lower concentrations. The piezoresistive response is influenced by factors such as moisture content, curing age, and temperature, necessitating calibration for accurate monitoring.

Structural health monitoring applications leverage these piezoresistive properties to assess the integrity of bridges, buildings, and other infrastructure. Embedded sensors made of carbon-reinforced nanocomposites can detect microcracks before they propagate, preventing catastrophic failures. The electrical resistance measurements are correlated with strain patterns, enabling early warning systems. For example, a bridge deck with integrated CNT-cement sensors can monitor traffic-induced stresses, with data transmitted wirelessly to maintenance systems. The durability of these sensors under environmental exposure—such as freeze-thaw cycles or chloride ingress—is critical, and research shows that well-dispersed carbon nanomaterials can mitigate degradation by reducing permeability and enhancing mechanical properties.

Durability issues, particularly alkali-silica reactions (ASR), pose significant challenges in cementitious materials. ASR occurs when reactive silica in aggregates reacts with alkali hydroxides in cement, forming a gel that expands and cracks the concrete. Carbon nanomaterials can mitigate ASR through multiple mechanisms. Their high surface area provides nucleation sites for the formation of calcium silicate hydrate (C-S-H) gel, reducing the availability of free alkalis. Additionally, the dense microstructure imparted by nanomaterials limits the diffusion of alkali ions and water, slowing the reaction kinetics. Studies indicate that adding 0.1% to 0.3% CNTs by weight of cement reduces ASR-induced expansion by up to 40% compared to plain cement. The nanomaterials also improve tensile strength and fracture toughness, counteracting the brittle failure associated with ASR.

Long-term performance of carbon-reinforced cementitious nanocomposites depends on the stability of the nanomaterial-cement interface. Alkaline environments in cement pores can degrade certain carbon nanomaterials over time, particularly those with functional groups. However, unfunctionalized CNTs and graphene exhibit superior chemical resistance. The interaction between nanomaterials and cement hydration products also affects durability. Well-dispersed nanomaterials accelerate early hydration by providing nucleation sites, but excessive concentrations may hinder hydration due to space-filling effects. Optimal concentrations balance mechanical enhancement with minimal disruption to cement chemistry.

The mechanical properties of carbon-reinforced cementitious nanocomposites are markedly improved compared to conventional concrete. Compressive strength increases of 10% to 30% are achievable with 0.1% to 0.5% carbon nanomaterial additions. Flexural strength and toughness improvements are even more pronounced, with enhancements up to 50% reported for CNF-reinforced composites. The bridging effect of nanomaterials across microcracks delays crack propagation, contributing to these gains. The modulus of elasticity also rises, though to a lesser extent, due to the high stiffness of carbon nanomaterials.

Challenges remain in scaling up production and ensuring consistent quality. Uniform dispersion at industrial scales requires optimized mixing protocols and surfactant selection. The cost of carbon nanomaterials, though decreasing, remains higher than traditional additives, limiting widespread adoption. However, the multifunctionality of these composites—combining mechanical reinforcement, smart sensing, and durability enhancements—justifies their use in critical infrastructure where long-term performance is paramount.

In summary, carbon-reinforced cementitious nanocomposites offer transformative potential for modern construction. Effective dispersion methods, leveraging surfactants and sonication, are essential to harness their full benefits. The piezoresistive properties enable innovative structural health monitoring applications, while their ability to mitigate alkali-silica reactions enhances durability. As research progresses, these materials are poised to play a pivotal role in building smarter, more resilient infrastructure.