The construction industry faces a dual-challenge of achieving rapid early-age strength development in cement-based materials, while ensuring a long-term durability, which is further complicated due to the demand for reduced carbon emissions. Conventional approaches (i.e., using chloride-based accelerators or steam curing) often compromise later-age performance or incur high energy costs. Calcium silicate hydrate (C-S-H) as a primary binding phase in cement dictates the evolution of both microstructure and macroscopic properties. Consequently, controlling its early formation is a key to enhancing performance. Nanotechnology offers a promising solution, with C-S-H based nano-seeds gaining significant attention due to their compositional and structural similarity to the C-S-H gel. These seeds provide a low-energy nucleation substrate, effectively shifting the nucleation pathway from the surface of cement grains to the pore solution, thereby accelerating hydration without the negative effects associated with conventional methods.

However, the practical application of C-S-H nano-seeds is hindered by their high surface energy, leading to severe agglomeration and reduced efficiency. To overcome this, polycarboxylate ether (PCE) superplasticizers are employed as dispersants. The resulting C-S-H/PCE nanocomposites combine a nucleation effect of C-S-H with a dispersing ability of PCE, forming a stable suspension that represents a significant advancement in the field. Although the existing reviews discuss C-S-H nano-seeds, a comprehensive overview dedicated specifically to the design, mechanisms, and performance of C-S-H/PCE nanocomposites and their recently developed derivatives is lacking.

This review systematically summarizes recent progress on C-S-H/PCE nanocomposites. The interaction between C-S-H and PCE is primarily governed by surface adsorption, where carboxylate groups on the PCE backbone coordinate with calcium ions on the C-S-H surface. This interaction, combined with the steric hindrance of PCE side chains, stabilizes the nanoparticles and affects the non-classical nucleation pathway of C-S-H, leading to the formation of smaller, more uniform particles.

The synthesis of these nanocomposites, typically via co-precipitation, is critically examined. Key parameters such as pH value, reactant concentration, and calcium-to-silicon ratio are identified as crucial for controlling particle size, morphology, and subsequent seeding efficiency. Furthermore, the molecular structure of the PCE dispersant (i.e., functional groups, side-chain length and density, and topology) can be tailored to optimize the nanocomposite's properties. Furthermore, novel nanocomposites incorporating aluminum, magnesium, or iron into the C-S-H structure, such as C-A-S-H/PCE and C-M-S-H/PCE, are developed. These next-generation materials demonstrate enhanced stability and superior nucleation seeding effects, paving a way for high-performance and sustainable applications.

The mechanism by which these nanocomposites accelerate cement hydration is elaborated as a dual-action. They adsorb calcium and silicate ions from the dissolving clinker, lowering the supersaturation threshold for nucleation, and they provide a multitude of low-energy sites for the precipitation of new C-S-H gel directly within the pore space. This process significantly shortens the induction period, accelerates early hydration, and leads to a more uniform and densified microstructure. Importantly, this seeding effect does not compromise a long-term strength development. Instead, it can enhance durability via refining the pore structure. The efficiency of this process is affected by both intrinsic factors (i.e., the specific surface area of the nano-seeds), and extrinsic factors (i.e., dosage, water-to-cement ratio, and curing temperature).

The synergistic effects of combining C-S-H/PCE nanocomposites with other chemical admixtures are also discussed. The combination with sodium sulfate exhibits a pronounced "1+1>2" effect in low-cement systems, where the sulfate accelerates the dissolution of supplementary cementitious materials, while the seeds provide nucleation sites for the hydration products. The similar synergistic mechanisms are analyzed with organic accelerators like triethanolamine, though a careful optimization is required to avoid negative interactions.

Finally, the application of C-S-H/PCE nanocomposites across various cementitious systems is reviewed. They prove effective in ordinary Portland cement and in blended systems with fly ash, slag, and calcined clay. Their ability to provide a high early strength without sacrificing later-age performance makes them suitable for a range of applications, including low-temperature construction, emergency repairs, non-steam-cured precast concrete, and high-volume solid-waste binders, contributing to reduced energy consumption and carbon footprint in the construction sector.