Mineral admixtures in shotcrete: A review of pozzolanic activity-based classification, micro-mechanisms and...

Shotcrete is widely used in tunnels, mining, and slope stabilization, yet its production is carbon-intensive. Under carbon peaking and neutrality goals, partially replacing cement with mineral admixtures offers a key pathway toward low-carbon shotcrete. This review establishes a classification framework based on pozzolanic activity, quantified by calcium hydroxide consumption via the modified Chapelle test. Accordingly, mineral admixtures are categorized into high-activity, medium-to-low-activity, and inert types, with silica fume, fly ash, and limestone powder as respective representatives. Their physicochemical properties and dominant micro-mechanisms-filling, nucleation, interfacial, and pozzolanic effects-are systematically elucidated. Their influences on workability, mechanical properties, volume stability, and durability are then comprehensively reviewed. Research indicate that silica fume, with ultrafine particles and high amorphous SiO₂ content, exhibits intensive micro-mechanisms, enhancing strength and durability optimally at a replacement ratio of 10%, but increases shrinkage. Fly ash, characterized by spherical morphology and glassy phases, provides balanced performance with optimal strength at a replacement ratio of 20% and improved volume stability. Limestone powder, lacking pozzolanic activity, primarily offers physical filling and nucleation effects; at a replacement ratio of approximately 10%, it moderately improves workability and early strength, but contributes very little to long-term durability. This activity-based framework links intrinsic material properties to macroscopic performance, offering theoretical guidance and practical dosage references for selecting mineral admixtures in low-carbon shotcrete tailored to specific engineering requirements.