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

## TL;DR

This study examines how adding shell ash to cement affects early hydration processes, moisture distribution, and mechanical properties using nuclear magnetic resonance and kinetic models.

## Contribution

The novel use of LF-NMR and NG-I-D models to quantify hydration kinetics and microstructure evolution in shell ash-based cement is presented.

## Key findings

- Low shell ash content (≤8%) promotes C-S-H gel densification by consuming free water, while 10% SA reduces moisture in gel micropores and increases larger pores.
- Porosity initially decreases with SA content but increases at 10%, with minimum values observed at 3–5% and 8% SA.
- Hydration occurs in three stages (nucleation–growth, phase boundary reaction, diffusion control), with diffusion control dominating (>60% contribution).

## Abstract

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.