Influence Of Aluminum On The Hydration Behavior Of Silicate Cements: Molecular Simulation

Under the dual carbon background,supplementary cementitious materials are widely used to replace cement to reduce carbon dioxide emissions and energy consumption.The components with high pozzolanic activity in auxiliary cementitious materials are often rich in aluminum phases.These aluminum phase can significantly change the hydration process of cement and the microstructure of hydration products during the cement hydration,thereby affecting the macroscopic properties of cement concrete.However,the studies are still unclear about the hydration mechanism of cement with the participation of aluminum phase at the atomic scale,which greatly limits the wide application of supplementary cementitious materials.Based on the above problems,this paper combined first-principles and molecular dynamics simulations to study the influence mechanism of aluminum phase on cement hydration behavior in cement mineral dissolution,silicon chain polymerization and liquid phase nucleation.The specific research contents and conclusions are as follows:(1)The mechanism of the influence of Al on the initial hydration of cement was probed at the atomic scale.It was found that Al delays the initial hydration of cement,and this delayed effect originates from two main reasons.On the one hand,the hydration of aluminate ions in solution forms a water shell structure around them,which disrupts the original H-bonding network,resulting in a higher energy required for water molecules to attack from water shell to the surface than for water molecules to overcome the H-bonding between water molecules.On the other hand,aluminates close to the surface are adsorbed on the cement surface to form aluminates-silicate-calcium complexes,which hinder the attack of water molecules and the dissolution of cement ions.Besides,new proton migration modes driven by hydroxyl groups in aluminates were discovered.(2)The reactant structure,reaction pathways,energy changes and electronic structure changes of aluminosilicates in the early nucleation stage were probed at the atomic scale.It is found that aluminosilicates monomers exist mainly in the form of Ca atoms as central adsorptions.the Q0 dimer reaction is mainly driven by Ca-O bonds and hydrogen bonds between aluminates and silicates,and the mechanism of this action is a mutual matching of the local electrostatic potential values of minima and maxima.(3)The dimerization reaction path of calcium aluminosilicate can be divided into three stages: Mutual proximity of Q0 monomers,distortion of the dimer structure,and bonding and stable Q1 dimer formation.The aluminosilicate dimerization reaction consumes more energy than the silicates dimerization reaction,revealing the mechanism of the initial nucleation process in which the aluminum phase delays the hydration of the cement.As the reaction proceeds,the interactions between the atomic pairs’ orbitals are gradually excited,and the interactions between paired atoms transform from weak van der Waals forces to bonding interactions.(4)The effect of aluminum phase on the nucleation of cement polymorphs was studied at the molecular scale.The results show that the addition of aluminum phase accelerates the nucleation process of hydration product polymorphs.The aluminates do not affect the polymerization of silicates and acts as a cross-link,causing a higher number of bridging atoms in the system and promoting the generation of higher Qn structures.The polymorph nucleation process of cement coincides with the formation of amorphous nanoclusters(PNC)mechanism.Aluminosilicate clusters are larger but dispersed,while the multimer silicate clusters are slightly smaller but integral.