Molecular Dynamics Study On Microstructure,Mechanical And Interfacial Adsorption Properties Of Geopolymers

Geopolymer is a kind of environment friendly building materials with low energy consumption and low carbon emission.In the field of building materials,solid nuclear and waste materials,geopolymer has become a focusing topic due to its unique advantages of corrosion resistance,high temperature resistance,fire resistance and good mechanical properties.As the primary binding phase in geopolymer,sodium aluminosilicate hydrate(NASH)gel,determines the mechanical properties,durability and solid waste ability of geopolymer.However,due to the amorphous nature and the complexity of NASH gel structure,the fundamental properties of NASH gel have not been investigated comprehensively.In this thesis,adopting molecular dynamics simulation method,the molecular model of NASH gel was first constructed and then the mechanical properties and interface adsorption properties were investigated.The main research contents in this thesis were list as follows:(1)Adopting high temperature quenching simulation method combined with Grand Canonical Ensemble Monte Carlo method,the NASH gel molecular structure models with different Na/Al ratios were constructed.And the microstructure characteristics of the NASH gel was analyzed.On the nanometer scale,the NASH gel has irregular threedimensional network structure,in which silicon and aluminum are connected by bridging oxygen atoms and form silicon aluminum skeleton with tetrahedron as main structure.The sodium ions in NASH gel have around 4 ? 7 nearest neighbors,including the oxygen atoms and hydroxyl in silicon oxygen tetrahedron or aluminum oxygen tetrahedron.The sodium ions distributed in the cavity of the silicon aluminum skeleton and play a role of charge balance.With the increase of Na/Al ratio,the sodium ions around silicon aluminum skeleton increasing,resulting in the structure of NASH gel transformed from a complete network structure to a branching structure with partial defects.There are two existence forms of water molecules in NASH gel,physical adsorption and chemical decomposition.(2)The effect of Na/Al ratio on mechanical properties of NASH gels was studied by molecular reaction dynamics.The increase of sodium ions in NASH gel will lead to an increase of non-bridging oxygen atoms,which will further lead to the depolymerization of silicon aluminum skeleton.Hence,the mechanical properties of NASH gel will decrease gradually with increasing of Na/Al ratio.Water molecules adsorbed in the cavity of NASH gel are susceptible to hydrolysis with high reactive activity non-bridged oxygen atoms in the vicinity of defects.The newly generated hydroxyl group combines with aluminum atoms to form aluminum oxygen pentahedron structure.Subjected to external laod,the silicon aluminum skeleton was broken to withstand the tensile loading.Furthermore,the water molecules further hydrolyze near the stretched Si-O bond and AlO bond,which accelerates the decomposition of NASH gel.(3)Empirical field molecular dynamics method was employed to study the adsorption characteristics of water molecules and ions on NASH gel interface.Water molecules appear as orderly arrangement on the NASH gel interface,and some water molecules stay at the position of structural defects area.The silicon aluminum branch chain structure,formed by cracking on the surface of NASH gel,contains a large number of non-bridging oxygen sites.Theses sites combine easily with water molecules near the interface and form hydrogen bond network,making the movement of water molecules greatly restricted.Moreover,the oxygen atoms in the silicon aluminum skeleton also provide a large number of cationic adsorption sites,which have a strong adsorption effect on the cations.The analysis of local structure and dynamic characteristics of ions on NASH gel interface confirmed that,cations can form chemical bonds with the structural oxygen in NASH gel substrate and the order of stability of chemical bonds is Na~+-Os> K~+-Os> Cs~+-Os.The adsorption capacity of cations on NASH gel surface negatively correlated with the ion radius.The adsorption capacity of NASH gel surface to three cations follows the order Na~+ > K~+ > Cs~+.