Assess the interaction of water reducers and accelerators on the rheological and early hydration properties of cement-based materials

Shotcrete is commonly used as the primary support material in tunnel construction, differing significantly from conventional cast-in-place concrete. The operational process of shotcrete construction involves two stages: pumping and spraying. From the perspective of admixtures, shotcrete requires water-reducing agents to ensure fluidity during pumping and accelerators to facilitate rapid hardening upon spraying. It is important to balance the performance of shotcrete in the two-stage process. This study utilized aluminum sulfate (AF) and sodium aluminate (AA) as the primary accelerators and polycarboxylate and naphthalene-based superplasticizers as water-reducing agents. A rheometer, isothermal calorimeter, low-field nuclear magnetic resonance (NMR), and scanning electron microscope (SEM) were used to systematically understand the early hydration process of cement paste. The setting time, rheological properties, hydration behavior and microstructure development were evaluated with the introduction of combination of various water-reducing agents and accelerators. The results indicate that polycarboxylate-based superplasticizers significantly enhance fluidity but delay hydration, particularly when combined with sodium aluminate accelerators, resulting in extended setting times and slower free water conversion into bound water. In contrast, aluminum sulfate accelerators induce higher early hydration heat, accelerating strength development but also increasing yield stress and plastic viscosity. The interaction between water reducers and accelerators creates a complex balance: while polycarboxylate superplasticizers improve workability, their presence alters hydration kinetics, influencing both hydration heat release and calcium hydroxide consumption. These findings quantitatively assess the interrelationship between rheology, hydration, and microstructure development in cement-based materials, providing a valuable reference for balancing workability and early performance in shotcrete applications.