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.

中文翻译：

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高活化火山灰材料开发可持续混凝土：光激发纳米 TiO2 的新视角

粉煤灰是燃煤的副产品，是一种火山灰固体废物，年产量为 16.3 亿吨，已被广泛用于替代水泥熟料以开发可持续混凝土。然而，由于其低火山灰活性，大量惰性粉煤灰的掺入会显著降低混凝土的早期机械强度。本研究提出了一种创新策略，通过利用羟基自由基 （•OH） 有效提高飞灰的反应性，羟基自由基 （•OH） 是纳米 TiO₂ 在紫外光激发下产生的高活性氧化物质。实验结果表明，光激发纳米 TiO₂ 显著促进了飞灰中惰性玻璃相的解聚，导致 Ca（OH）₂ 消耗量在 72 小时内增加了 28 %。因此，砂浆在 28 天时的抗压强度和抗弯强度分别提高了 37 % 和 16 %，强度活性指数达到 95.4 %。此外，由于增强的火山灰反应性驱动了更精细的孔结构，吸水率和氯离子扩散系数分别降低了 15 % 和 18 %。密度泛函理论 （DFT） 计算进一步表明，•OH 大大降低了 Si-O-Al 键裂解的能垒（从 OH− 的 22.93 kcal/mol 降低到 •OH 的 8.54 kcal/mol），证实了其卓越的催化效率和热力学优势。综上所述，研究结果证实了光激发纳米 TiO₂ 可以作为一种有效的活化剂，提高粉煤灰在可持续混凝土中的利用效率。