The interfaces in cement-based materials and their structures(i.e., the cement paste-aggregate, mortar-aggregate, new-to-old concrete, and reinforcement-concrete interfaces) are widely recognized as the most critical and weakest zone governing overall mechanical properties and durability. Under the combined influence of wall, one-sided growth, and bleeding effects, these interfaces are typically characterized by high porosity, elevated calcium hydroxide (CH) crystal content, and a high degree of hydration, resulting in intrinsically low hardness and strength. As a result, cracks initiate, propagate, and interconnect within the interfaces under loading, leading to a macroscopic failure. Meanwhile, their loose and porous structure facilitates the ingress of water and harmful ions, reducing durability and accelerating deterioration.

For the pronounced multi-scale nature of cement-based materials, modifying microstructure in nanoscale provides an effective "bottom-up" approach to improving interfacial bond and overall performance. Compared with conventional top-down methods, this strategy exhibits effective modification for different types of interfaces. With the rapid development of nanotechnology, incorporating nanofillers demonstrates a great potential in interfacial modification. This review represents the effects and underlying mechanisms of nanofillers on the interfaces in cement-based materials and their structures, as well as discusses key challenges and future research directions.

Adding nanofillers can significantly enhance the bond performance of various interfaces in cement-based materials and their structures. The experimental studies show that the incorporation of nanofillers can increase the bond strength of cement paste-aggregate and mortar-aggregate interfaces by 65.3% and 42.8%, respectively. Also, nanofillers effectively enhance the bond of new-to-old cement-based materials, increasing interfacial strength by 39.0%. Their beneficial effects are evident in the interfaces between reinforcements and cement-based materials, where the addition of nanofillers improves bond strength and reduces slip behavior. These modifications contribute to a high stress transfer efficiency, as well as better load-bearing capacity and structural stability. The enhancement efficiency is closely related to the geometric dimensionality of nanofillers, generally following an order, i.e., two-dimensional (2D) > one-dimensional (1D) > zero-dimensional (0D).

The improved bond performance derives from the modified interfacial characteristics. Key effects include increased compactness, reduced porosity, refined CH crystal size, and decreased interfacial thickness. For instance, the thickness of the interfacial transition zone can be reduced from approximately 50 µm to 10 μm or less after nanomodification, as reflected by the reduced porosity and altered Ca/Si ratio of calcium silicate hydrate (C-S-H) gels. Meanwhile, nanofillers also suppress the formation and growth of large CH crystals, replacing them with finer and more uniformly distributed hydration products. Micromechanical characterization techniques such as nanoindentation and atomic force microscopy reveal significant increases in elastic modulus and hardness in the interfacial zone, indicating that new hydration products like superhard C-S-H gels induced by nanomodification are formed.

The mechanisms underlying nanofiller-induced interfacial modification can be categorized into two key aspects, i.e., the enrichment effect and the nano-core effect. The enrichment effect arises from the preferential accumulation of nanofillers in the interfacial region due to the wall effect, capillary-driven migration, and gravity effect. This leads to a higher local concentration of nanofillers, providing abundant nucleation sites for hydration products. The nano-core effect, which builds upon the intrinsic properties of nanofillers, involves several specific mechanisms, i.e., (1) Adsorption effect: Nanofillers adsorb cations such as Ca²⁺ and Mg²⁺, promoting the nucleation of C-S-H and leading to the formation of nano-core-shell elements. These structures inhibit the oriented growth of CH crystals and enhance interfacial densification; (2) Filling effect: Nanofillers fill interfacial pores through the formation of nano-core-shell elements, improving packing density and reducing porosity; (3) Short-range and long-range effect: Nanofillers can modify the nanostructure and properties of hydration products in a short range, and refine and optimize the hydration products near nanofillers in a long range; (4) Pozzolanic effect: Reactive nanofillers, such as nano-silica, react with CH to generate additional C-S-H, refining the microstructure and improving bonding; (5) Thermal conductivity effect: High-conductivity nanomaterials (e.g., carbon nanotubes, graphene) homogenize the temperature distribution during hydration, reducing thermal stress and early-age cracking; (6) Self-curing effect: Hydrophilic nanomaterials, such as nanocellulose, absorb and release water, mitigating early-age shrinkage and enhancing microstructural development; and (7) Pinning and bridging effects: Nanofillers inhibit crack initiation and propagation (pinning effect) and bridge microcracks (bridging effect), thereby improving interfacial toughness and strength.