天然火山灰作为潜在的矿物掺合料，具备降低水泥用量、减少碳排放的环保优势，但因其活性较低，单掺常导致水泥基材料早期性能下降。为提升其工程适用性，本文以川藏地区低活性火山灰为基础，复掺少量硅灰，构建复合胶凝体系，系统研究其水化行为与微观结构演变。采用XRD、SEM、TG-DTG和压汞法（MIP）等手段，结合力学性能试验，对不同龄期试样的水化产物、孔隙结构及强度发展规律进行表征与分析。结果表明：单掺火山灰体系早期水化反应迟缓，活性指数与强度较低；硅灰的引入显著促进C-S-H凝胶生成，细化孔径分布，提高结构致密性，有效提升早期和后期强度表现。28 d龄期下，掺27%火山灰与3%硅灰的复合体系较单掺试件抗压强度增加了64%，较纯水泥基准试件增长了15%，活性指数提升明显。

Natural volcanic ash is a promising supplementary cementitious material due to its potential to reduce cement consumption and carbon emissions. However, its inherently low pozzolanic activity often leads to compromised early-age performance when used alone. To enhance its engineering applicability, this study investigates a composite cementitious system based on low-reactivity volcanic ash sourced from the Sichuan-Tibet region, supplemented with a small dosage of silica fume. The hydration characteristics and microstructural evolution of both single and blended systems were systematically evaluated using XRD, SEM, TG-DTG, and mercury intrusion porosimetry (MIP), in conjunction with mechanical strength testing at various curing ages. Results indicate that the single-blended volcanic ash system exhibits sluggish early hydration, resulting in lower strength and activity index. In contrast, the incorporation of 3% silica fume significantly promotes the formation of C-S-H gel, refines pore structure, enhances matrix densification, and improves both early and long-term mechanical properties. At 28 days, the composite system containing 27% volcanic ash and 3% silica fume exhibited a 64% increase in compressive strength compared to the VA-only specimen, and a 15% improvement relative to the plain cement reference, with a notably enhanced strength activity index..