# The Governing Role of Si/Al Ratio in the Structural Evolution and Mechanical Properties of N-A-S-H Gel

## TL;DR

This study shows how the Si/Al ratio controls the structure and strength of a green cement material called N-A-S-H gel.

## Contribution

The study reveals the atomic-scale mechanism by which the Si/Al ratio regulates the microstructure and mechanical properties of N-A-S-H gel.

## Key findings

- A Si/Al ratio of 1.8 leads to the highest silicate polymerization and densest 3D network structure.
- Materials with a Si/Al ratio of 1.8 show the highest compressive strength in experiments.
- Structural degradation occurs when the Si/Al ratio deviates from 1.8 due to charge imbalance or excessive polymerization.

## Abstract

Alkali-activated cementitious materials are environmentally friendly alternatives to traditional cement. The structure of their core product, sodium aluminosilicate hydrate (N-A-S-H) gel, is regulated by the silicon-to-aluminum (Si/Al) ratio; however, the atomic-scale mechanism underlying this influence remains unclear. Integrating reactive force field molecular dynamics simulations and experiments, this study systematically reveals the regulation mechanism of the Si/Al ratio (1.0–2.0) on the microstructure and macroscopic properties of N-A-S-H gels. Starting from well-defined PS and PSS oligomers, the simulation results demonstrate that the Si/Al ratio governs the polymerization pathway, aluminum coordination environment (especially the content of pentacoordinate aluminum), and evolution of nanoporosity. When the Si/Al ratio is approximately 1.8, the system exhibits the highest silicate polymerization degree, lowest nanoporosity, and densest three-dimensional (3D) network structure; deviation from this ratio leads to structural degradation due to charge imbalance or excessive polymerization. These computational findings are validated by experiments on fly ash-based geopolymers: the material achieves the highest compressive strength at a Si/Al ratio of 1.8. The consistency between simulations and experiments collectively reveals a cross-scale action mechanism: the Si/Al ratio determines the macroscopic mechanical properties by regulating the nanoscale packing density and defect distribution of the gel. This study provides critical atomic-scale insights for the rational design of high-performance geopolymers.