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

(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.