Synergistic mechanism of mechanical strength, photocatalysis and carbonation resistance in cementitious matrix doped with nanosilica and nanotitania

The dosage-dependent synergistic effects of nano-silicon dioxide (NS) and nano-titanium dioxide (NT) on the compressive strength, photocatalytic activity and carbonation resistance of cement pastes were investigated. Compressive strength, methyl orange degradation, accelerated carbonation, mercury intrusion porosimetry and X-ray diffraction were used to correlate macroscopic performance with microstructural evolution. The hybrid mixture containing 2 wt% NT and 1 wt% NS (T20S10) achieved a 28-day compressive strength of 67.1 MPa, 21.6% higher than that of the plain paste. Photocatalytic activity was mainly governed by NT content, while NS affected nanoparticle dispersion and titania active-site accessibility. T20S10 exhibited the lowest carbonation depth among the hybrid mixtures, corresponding to a 34.6% reduction relative to the control, although it did not outperform the best single-NS mixture. Microstructural analyses indicated that the pozzolanic effect of NS and the nucleation/filling effects of NT promoted hydration, reduced portlandite content and refined the pore structure. Overall, multi-functional performance depended on balancing synergistic modification against nanoparticle agglomeration rather than simply increasing the nanomaterial dosage.