Early Hydration Kinetics of Shell Ash-Based Cementitious Materials: A Low-Field Nuclear Magnetic Resonance Study

This study systematically investigates the effects of shell ash (SA) content (0-10%) on early moisture evolution, pore structure, and hydration kinetics in cement paste using LF-NMR and NG-I-D hydration kinetic models. Key findings include the following: (1) Increased SA content significantly alters moisture phase distribution. Low contents (≤8%) consume free water through rapid CaO hydration, promoting C-S-H gel densification. However, 10% SA causes reduced moisture in 0.16-0.4 μm gel micropores (due to hindered ion diffusion) and abrupt increases in 0.63-2.5 μm pores. (2) Porosity first decreases then increases with SA content, reaching minimum values at 3-5% and 8%, respectively. The 10% content induces abnormal porosity growth from localized over-densification following polynomial fitting (R² = 0.966). (3) Krstulovic-Dabic model analysis reveals three consecutive hydration stages: nucleation-growth (NG), phase boundary reaction (I), and diffusion control (D). The NG stage shows the most intense reactions, while the D stage dominates (>60% contribution), with high model fitting accuracy (R² > 0.9). (4) SA delays nucleation/crystal growth, inducing needle-like crystals at 3% content. Mechanical properties exhibit quadratic relationships with SA content, achieving peak compressive strength (18.6% increase vs. control) at 5% SA. This research elucidates SA content thresholds governing hydration kinetics and microstructure evolution, providing theoretical support for low-carbon cementitious material design.