Influence of Nano-TiO ₂ Particle Size on Fresh, Physical, Mechanical, Photocatalytic, and Cost Performance of Cement Mortars

Nanosized additives have attracted significant attention for improving cement-based materials; however, the isolated effects of nano-TiO2 (NT) particle size and dosage on engineering performance and photocatalytic efficiency remain unclear. This study investigates NT with particle sizes of 15–35 nm and 55–75 nm incorporated at 1–3% replacement levels in cement mortars at a constant water-to-binder ratio of 0.5. NT reduced the flow diameter from 225 mm (control) to 209 mm at 3% (15–35 nm), confirming greater workability loss for finer particles. Water absorption and apparent porosity decreased from 8.62% and 10.12% to 8.12% and 9.53%, respectively. Compressive strength increased from 44.57 to 47.21 MPa at 2% NT (15–35 nm), while flexural strength remained stable (4.13–4.27 MPa). Photocatalytic activity improved markedly: ΔE values increased from 3.54 (control, 4 h) to 6.76 for 2% NT (15–35 nm), representing a 91% enhancement. Larger particles showed lower efficiency. The results indicate that 2% NT provides the optimal balance between mechanical improvement, photocatalytic performance, and cost efficiency.