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,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和压汞法等手段,结合力学性能试验,对不同龄期试件的水化产物、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。28 d龄期下,复掺27%火山灰与3%硅灰的胶凝体系的抗压强度较单掺试件增长了64%,较纯水泥基准试件增长了15%,活性指数提升明显。

[Objective] Natural volcanic ash has environmental advantages in reducing cement consumption and lowering carbon emissions. However, due to its low reactivity, the use of volcanic ash alone often leads to reduced early-age performance of cement-based materials. This study aims to develop a volcanic ash-silica fume composite cementitious system based on low-reactivity volcanic ash from the Sichuan-Xizang region, and to systematically investigate its hydration behavior, microstructural evolution, and mechanical properties, thereby providing theoretical and technical support for the engineering application of volcanic ash. [Methods] Multiple characterization methods were used to systematically analyze the hydration characteristics and microstructure of the composite cementitious system. First, X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to qualitatively and quantitatively analyze hydration products at different curing ages to investigate the formation of C-S-H gel and its microstructural characteristics in the volcanic ash-silica fume composite system. Second, thermogravimetric and derivative thermogravimetric analysis (TG-DTG) was conducted to examine the thermal decomposition behavior of the materials, thereby revealing the composite effect of volcanic ash and silica fume from a thermodynamic perspective. To further evaluate the influence of pore structure on strength, mercury intrusion porosimetry (MIP) was used to analyze the pore size distribution of samples at different curing ages. Meanwhile, mechanical performance tests, such as compressive strength tests, were conducted to evaluate the mechanical properties of systems with different mix proportions. [Results] The single volcanic ash-blended system exhibited relatively sluggish early hydration, resulting in a lower strength activity index and mechanical strength. In contrast, the incorporation of silica fume significantly promoted the formation of C-S-H gel, refined the pore size distribution, enhanced the densification of cement-based materials, and consequently improved both the early- and late-age strength of the composite system. At 28 days, the composite system containing 27% volcanic ash and 3% silica fume showed a significant advantage, with its compressive strength approximately 28% higher than that of the single volcanic ash-blended system, and with a markedly increased strength activity index. Microstructural analysis further indicated that the C-S-H gel in the composite system was dense and uniformly distributed, and the pore structure was effectively optimized. [Conclusion] The combined use of low-reactivity volcanic ash and silica fume significantly enhances hydration activity and mechanical performance, particularly strength. The novelty of this study lies in optimizing the combined dosage of volcanic ash and silica fume, which not only improves mechanical performance but also optimizes the microstructure, resulting in a denser structural system. Therefore, the volcanic ash-silica fume composite cementitious system shows strong engineering applicability and provides an important reference for the development of low-carbon and environmentally friendly cement replacement materials.