Silicate Dissolution Mechanism from Metakaolinite Using Density Functional Theory

Abstract

Metakaolin (MK) is a high-quality, reactive nanomaterial that holds promising potential for large-scale use in improving the sustainability of cement and concrete production. It can replace cement due to its pozzolanic reaction with calcium hydroxide and water to form cementitious compounds. Therefore, understanding the dissolution mechanism is crucial to fully comprehending its pozzolanic reactivity. In this study, we present an approach for computing the activation energies required for the dissolution of metakaolin (MK) silicate units at far-from-equilibrium conditions using the improved dimer method (IDM) and the transition-state theory (TST) within density functional theory (DFT). Four different models were prepared to calculate the activation energies required for breaking oxo-bridging bonds between silicate or aluminate units. Our results showed that the activation energy for breaking the oxo-bridging bond to a silicate neighbor is higher than that to an aluminate neighbor due to the ionic interaction. However, for complete silicate tetrahedra dissolution, a higher activation energy is required for breaking the oxo-bridging bond to the aluminate neighbor compared to the silicate neighbor. The findings provide methodology for missing input data to predict the mesoscopic dissolution rate, e.g., by the atomistic kinetic Monte Carlo (KMC) upscaling approach.

Keywords: activation energy; density functional theory (DFT); dissolution; improved dimer method; metakaolinite.

Figure 1

(A) The initial periodic 1 × 1 × 3 ideal kaolinite supercell, Al₁₂Si₁₂O₃₀(OH)₂₄. (B) Partially dehydroxylated ideal kaolinite, Al₁₂Si₁₂O₃₉(OH)₆. (C) Metakaolinite, Al₁₂Si₁₂O₄₂. Silicate tetrahedrons are depicted in dark blue, aluminate octahedrons in light blue, oxygen atoms in red, and protons in white.

Figure 2

Illustration of the initial scenario for dissolution of silicate tetrahedra (Si(OH)₄) depending on the bridging of oxygen to the silicate and aluminate units. Silicate atoms are shown in blue; aluminum in light blue; oxygen in red; and hydrogen protons in white.

Figure 3

Model 1: (A) Ground state obtained as an optimized geometric structure of reactant, including water molecule as absorbent on MK surface (00$\stackrel{-}{1}$) for attacking and breaking the bridging oxygen, bonded to the silicate neighbor (Si1). (B) Transition state at the saddle point using improved dimer method. (C) Ground state obtained as an optimized geometric structure of product after protonation of the bridging oxygen to the silicate neighbor. The structural building units are described in caption of Figure 2. E_reactant, E_transition state, and E_product have been collected in Table 1.

Figure 4

Model 2: (A) Ground state obtained as an optimized geometric structure of reactant, including water molecule as absorbent on MK surface (001) for breaking the bridging oxygen, bonded to the aluminum neighbor (Al1). (B) Transition state (TS) at the saddle point using improved dimer method. (C) Ground state obtained as an optimized geometric structure of product after protonation of the bridging oxygen, bonded to the aluminum neighbor. The structural building units are described in the caption of Figure 2. E_reactant, E_transition state, and E_product have been collected in Table 1.

Figure 5

Model 3: (A) Ground state obtained as an optimized geometric structure of reactant, including water molecule as absorbent on MK surface (00$\stackrel{-}{1}$) obtained from model 2(C) for breaking the bridging oxygen, bonded to the aluminum neighbor (Al1). (B) TS at the saddle point using improved dimer method. (C) Ground state obtained as an optimized geometric structure of product after protonation of the bridging oxygen, bonded to the aluminum neighbor. The structural building units are described in caption of Figure 2. E_reactant, E_transition state, and E_product have been collected in Table 1.

Figure 6

Model 4: (A) Ground state obtained as an optimized geometric structure of reactant including water molecule as absorbent on MK surface (00$1)$ obtained from model 1(C) for breaking the bridging oxygen, bonded to the silicate neighbor (Si1). (B) TS at the saddle point using improved dimer method. (C) Ground state obtained as an optimized geometric structure of product after protonation of the bridging oxygen, bonded to the silicate neighbor. The structure building units are described in caption of Figure 2. E_reactant, E_transition state, and E_product have been collected in Table 1.