Triple synergistic control mechanism of pre-assembled nano-SiO ₂ /CNT composites on cement hydration and nanostructure

The pre-assembly of nano-silica (NS) and carbon nanotubes (CNTs) into structured hybrid modifiers offers a potential route to improve nanomaterial dispersion and interfacial activity in cementitious systems. However, the relationship between pre-assembled NS/CNTs architecture, cement hydration kinetics, and nanostructure development remains insufficiently quantified. In this study, ellipsoidal pre-assembled NS/CNTs composites were prepared through a chemical activation route and incorporated into Portland cement paste. Their effects on hydration evolution, phase assemblage, pore structure, and microstructure were investigated using isothermal calorimetry, X-ray diffraction, thermogravimetric analysis, nitrogen adsorption, scanning electron microscopy, and Krstulović-Dabić kinetic modeling. Under the present mix design and test conditions, the optimized pre-assembled NS/CNTs composite shortened the induction period and increased the main hydration peak intensity by approximately 100% relative to the plain reference group, indicating enhanced heterogeneous nucleation. Thermogravimetric analysis, interpreted using the adopted dilution-correction framework, indicated an apparent CH-consumption rate of 55.5% relative to the plain reference group, which is consistent with the occurrence of a pozzolanic reaction. Nitrogen adsorption further indicated pore refinement, with total porosity reduced by 52.4% relative to the plain reference Group 5 and the most probable pore diameter shifting from the capillary-pore region toward the gel-pore region. These results support a triple synergistic mechanism involving heterogeneous nucleation, pozzolanic reaction, and multi-scale filling. The findings provide evidence supporting the hydration-regulation role of pre-assembled NS/CNTs composites and offer a mechanistic basis for designing nano-engineered cementitious materials with improved microstructural density.