Molecular simulation of water and chloride ion diffusion in nanopores of alkali-activated aluminosilicate structures

Abstract

In this paper, a molecular model is proposed to investigate sorption of water molecules and chloride ions in nanopores of alkali-activated binders (geopolymer) gels. The structure of water molecule and geopolymer matrix was optimised by molecular dynamics simulation, and sorption of water molecules and chloride ions was performed by Monte Carlo approach. Totally, four systems containing nanoscale geopolymer structure, 50 molecules of water and 0, 2, 5 or 7 chloride ions were investigated. The results showed that water molecules can be absorbed by nanopores and remain in the system where this absorption releases heat. Thermodynamic analysis showed that the water absorption is temperature-independent. Absorption of chloride ions was suggested to depend on chloride ion concentration. As concentration of chloride ions raises, more absorption occurs and as a result, heat release of absorption and possible degradation of geopolymer structure increases. The thermodynamic analysis showed that higher concentration of chloride ions may decrease Gibbs free energy and raise configurational entropy of the systems. It is proposed that by increasing the temperature, absorption of chloride ion by the geopolymer gel nanopores diminishes and more corrosion may occur in other parts/structures of the matrix.