Highly activated pozzolanic materials to develop sustainable concrete: a new perspective from photoexcited nano-TiO2

Abstract

Fly ash, a by-product of coal combustion, is a pozzolanic solid waste with annual production of 1.63 billion tonnes, which has been widely used to replace cement clinker to develop sustainable concrete. However, the incorporation of high volumes of inert fly ash significantly reduces the early mechanical strength of concrete due to its low pozzolanic activity. This study presents an innovative strategy to effectively enhance the reactivity of fly ash by utilizing hydroxyl free radicals (•OH), highly reactive oxidative species generated by nano-TiO₂ under UV light excitation. Experimental results demonstrated that photoexcited nano-TiO₂ significantly promoted the depolymerization of inert glassy phases in fly ash, resulting in a 28 % increase in Ca(OH)₂ consumption within 72 h. Consequently, the compressive and flexural strengths of mortar at 28 days increased by 37 % and 16 %, respectively, with a strength activity index reaching 95.4 %. In addition, the water absorption and chloride ion diffusion coefficient were reduced by 15 % and 18 %, respectively, due to a more refined pore structure driven by enhanced pozzolanic reactivity. Density Functional Theory (DFT) calculations further revealed that •OH substantially lowered the energy barrier for Si-O-Al bonds cleavage (from 22.93 kcal/mol for OH⁻ to 8.54 kcal/mol for •OH), confirming its superior catalytic efficiency and thermodynamic advantage. In conclusion, the findings confirm that photoexcited nano-TiO₂ can serve as an effective activator for enhancing the utilization efficiency of fly ash in sustainable concrete.