The present work addresses the aggregation/dispersion properties of two commercial titanias for application as photocatalysts in concrete technology. A microsized m-TiO2 (average particle size 153.7 +/- 48.1 nm) and a nanosized n-TiO2 (average particle size 18.4 +/- 5.0 nm) have been tested in different ionic media (Na+, K+, Ca2+, Cl-, SO(4)2-, synthetic cement pore solution) at different pHs and in real cement paste specimens. Results highlighted that ion-ion correlations play a fundamental role in TiO2 particles aggregation in the cement environment. A particle aggregation model derived from TiO2 surface chemistry is proposed here and used to justify such aggregation phenomena in real cement paste. Scanning electron microscopy-energy-dispersive X-ray spectroscopic investigations on hardened cement specimens completely confirmed the qualitative model based on titania surface chemistry. Experimental results also show how size and nature of TiO2 aggregates dramatically influence the overall photocatalytic activity of cementitious materials containing TiO2.