# Transport Properties of Solutions in γ–FeOOH/CSH Pores of Steel Fiber-Reinforced Concrete (SFRC) Derived Using Molecular Dynamics

## TL;DR

This study uses molecular dynamics to explore how water and ions move through pores in steel fiber-reinforced concrete, showing that narrower pores and certain chemical interactions slow down transport.

## Contribution

The paper introduces a molecular dynamics investigation of solution transport in γ–FeOOH/CSH pores of steel fiber-reinforced concrete, revealing ion-specific interactions and pore size effects.

## Key findings

- Water penetration depth in NaCl solution increases parabolically with time.
- Narrower pores reduce transport rates and cause ion clustering at inlets.
- Solutions move faster along the CSH side than the γ–FeOOH side, slowing ion and water transport.

## Abstract

Steel fiber-reinforced concrete structures designed for marine environments can become compromised by the ingress of water and ions. Water and ion transport through the pores between steel fibers and concrete gels significantly affects the durability of such structures, but the mechanisms of this transport are not sufficiently understood. Reported here is a molecular dynamics-based investigation of the transport of water, NaCl, Na2SO4, and mixed solutions of NaCl and Na2SO4 through γ–FeOOH/CSH pores. The effect of pore width on the capillary transport of NaCl + Na2SO4 solutions was also investigated and reported. It is shown that the depth of water penetration in NaCl solution increases parabolically with time. It is further shown that the CSH surface forms bonds with different ions to form Na–OCSH, Cl–CaCSH, and S–CaCSH compounds, which results in reduced rates of solution transport. The mixed NaCl + Na2SO4 solution was found to have the lowest transport rate. A reduction in pore width was found to reduce the transport rate of water molecules and diminish the transport of ions. In pores smaller than 2.5 nm in width, the immobilized ions aggregate into clusters, occupying pore inlets and blocking more ions from entering the channels. Compared with the matrix on both sides, solutions are transported significantly faster along the CSH side than along the γ–FeOOH side, indicating that the addition of steel fibers can effectively slow down the transport of water molecules and ions in concrete. These data on the difference in the transport of solutions along the two sides of the matrix may provide molecular-level insights to support studies on the durability of concrete materials.