Accelerating mechanism of cement hydration by hydroxyl free radicals: new perspectives from photoexcited nano-TiO2

Abstract

Previous studies primarily considered TiO₂ as a nano-filler to accelerate cement hydration due to the nucleation site effect. However, hydroxyl free radicals (•OH), released from TiO₂ under UV irradiation, also exhibit great potential to accelerate cement hydration owing to their high nucleophilicity arising from unpaired electrons. This study is the first to reveal the accelerating mechanism of •OH generated by photoexcited nano-TiO₂ on cement hydration. Current experimental results indicated that 5.0 wt% addition of photoexcited nano-TiO₂ (under 6 h of UV irradiation during the early curing stage) improved the dissolution rate of tricalcium silicate (C₃S) by 30 %, followed by a 35 % increase in the hydration degree of cement paste. More importantly, the polymerization degree and high-density content of C–S–H gels were also improved by 26 % and 13 %, accompanied by a 46 % reduction in porosity of the composites. All the aforementioned improvements were attributed to the presence of •OH generated by photoexcited nano-TiO₂, which significantly accelerated cement hydration by accelerating C₃S dissolution, facilitating faster nucleation and growth of C–S–H gels. In addition, the Density Functional Theory (DFT) calculations revealed that the accelerating effect of •OH on cement hydration may stem from the stronger interaction between •OH and the Ca ion on the C₃S surface compared to the water molecule, and the increased surface nucleophilicity due to the dispersion of unpaired elections from •OH to all the O ions in the surface layer. These findings provide a high-efficiency approach to accelerate cement hydration by photoexcited nano-TiO₂, thereby paving the way for the development of advanced and sustainable cement-based materials.