Thermal Behavior and Degradation Mechanism of Calcium Aluminosilicate Hydrate: The Role of the Calcium:Silicon Ratio Revealed by Molecular Dynamics

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This study investigates the influence of the calcium:silicon ratio (C:S) on the high-temperature resistance of calcium aluminosilicate hydrate (C-A-S-H) using molecular dynamics simulations. The effects of different C:S ratios (0.8:1.2) on the normalized deformation, volume and density, molecular bonds, and elastic properties of C-A-S-H were investigated. A "bond-mass ratio" hypothesis was introduced to predict the thermal stability of molecular bonds in C-A-S-H, offering a quantitative method to assess bond stability during thermal degradation. The bond-mass ratio effectively ranks the thermal sensitivity of different bonds (H-O, Al-O, Ca-O, and Si-O), providing a perspective on the atomic-level behavior of C-A-S-H under high-temperature conditions. Furthermore, in combination with the previous test results, the thermal degradation mechanism of the alkali-activated slag cement containing powdery bottom ash (ASB) was also clarified, indicating that the deterioration of Al-O and Ca-O bonds led to the structural reorganization and amorphous transformation of C-A-S-H, ultimately affecting the stiffness and residual strength of ASB. The C:S ratio is identified as a critical factor influencing the high-temperature resistance of alkali-activated cement (AAC), with an intermediate C:S ratio (∼1.0) optimizing density and bonding configurations, resulting in enhanced stiffness and thermal stability. These findings provide valuable insights for developing more resilient alkali-activated materials for high-temperature applications.