Utilising the binary in situ nanostructuring strategy of nanosilica/nanoalumina for long-term optimisation in high-dosage cement composites

The in situ growth of nanoparticles (NPs) on host surfaces has emerged as an advanced approach to enhance the performance of cementitious composites. However, the practical use of in situ grown NPs is limited by agglomeration and poor reactivity at dosages above 0.5%, restricting long-term reinforcement. That limitation was overcome in this work, enabling effective use of NPs at dosages of 1–2%. Highly dispersed in situ grown nanosilica (INS) and γ-nanoalumina (INA) were incorporated into cement composites (ICs) and compared with commercial NP composites (CCs) at identical dosages. The workability, hydration kinetics, mechanical properties, drying shrinkage behaviour, carbonation resistance and microstructural characteristics were evaluated up to 90 days. The IC samples exhibited excellent dispersion and minimal fluidity loss (–5.2% to –22.4%), whereas the CCs showed significant fluidity loss. The dual nucleation and dual pozzolanic activity of INS and INA accelerated hydration and sustained strength gain, with IC 1.0 (2 wt% NPs) achieving 14.1% higher 90-day compressive strength than the reference. The ICs also showed reduced early shrinkage, lower carbonation depth and refined pore structures (critical pore size of 6.8 nm for IC 0.5). These results indicate that binary in situ nanostructuring enables efficient, high-dosage NP utilisation, producing dense and high-performance cement composites with improved shrinkage behaviour and carbonation resistance.