Molecular Simulations On Crystal Structures And Hydration Reactivity Of Cement Clinker Minerals

The crystal structure and hydration reactivity of clinker minerals have been studied for more than one hundred years.Yet,the relationship between properties of clinker and its electronic structures has not been revealed in most experimental researches.Hence,this work systematically investigates the crystal structures and hydration reactivity of C3 S,C2S,C3 A,and C4 AF based on first-principles calculations and molecular dynamics simulations.The fundamental relationship between the intrinsic hydration reactivity of clinker minerals and their crystal structures and electronic structures is revealed at the atomistic scale.The mechanism of doping effects on the reactivity of clinker minerals is discussed,and the thermodynamics-kinetics simulation of clinker hydration is implemented as a preliminary study.The main conclusions are as follows:(1)The occupancy mechanism of Fe and Al atoms in C4 AF crystalsIn C4 AF crystals,the occupation of Fe and Al atoms follows the “homogeneous layer principle”,i.e.,Fe and Al atoms do not randomly occupy octahedral and tetrahedral sites.They tend to be arranged alternately layer by layer in the b-axis direction with each layer containing only one kind of element(Fe or Al).Fe atoms preferentially occupy the octahedral sites yet not all of them.This is because while the enthalpy of the system is lower when Fe atoms occupy all the octahedral sites,the system features higher entropy increment when Fe atoms partially occupy the octahedral sites as the temperature increases and the contribution of entropy increase to the free energy exceeds that of enthalpy at room temperature.Thus the system is more stable when Fe atoms partially occupy the octahedral sites.(2)The reactive sites and hydration characteristics of C3 S,C2S,C3 A,and C4AFFor the four clinker crystals,the electrophilic reactive sites are mainly located around O ions.The electrophilic reactive sites in C3 S are mainly located on ionized O ions instead of O ions in the [Si O4] group,which indicates that the ionized O ions feature a higher electrophilic reactivity.For C3 S,C2S and C3 A,the nucleophilic reactive sites are mainly located on Ca ions,whereas the nucleophilic reactive sites for C4 AF are the Fe ions.This is due to that the conduction band minimum is mainly contributed by the Fe3 d orbitals for the C4 AF crystal while by the Ca3 d orbitals for the other three clinker crystals.In C4 AF,Al ions show higher reactivity than Fe ions.For both Al and Fe,their four-coordinated species feature higher reactivity than six-coordinated species.(3)The defect formation mechanism of Mn,Zn,and Cu ions in clinker minerals and the mechanism of their doping effects on clinker hydration reactivityC4AF is the most preferable one to incorporate Mn,Zn,and Cu ions among the four clinker phases,and the incorporation of these impurities stabilizes the C4 AF crystal.For the silicate and aluminate phases,Ca ion substitution is the most favorable defect structure of Mn,Zn,and Cu.The impurity ions introduced in the clinker crystals always tend to substitutes the atoms with the most similar electronic structure to the impurity element itself to keep the mismatch of the formed chemical bonds to a minimum.The degree of chemical bond mismatch can be evaluated by the bond order difference.In C3 S,C2S,and C3 A crystals,dopant ions introduce impurity energy levels in the energy band gap,resulting in the transfer of nucleophilic reactive sites from Ca ions to defect ions.For C4 AF,the introduced impurity energy level is covered by the intrinsic Fe3 d orbitals,thus the nucleophilic reactive sites in C4 AF cannot be changed.Cu and Zn ion doping causes the decrease of effective charges of Ca atoms in most cases but increases the effective charges of Fe atoms,which suggests that the impact of Cu and Zn doping on C4 AF is significantly different from that of the other three clinker phases.(4)The dissolution thermodynamics and kinetics of β-C2SBased on the classical Clay FF force field,a new force field Clinker FF is developed for simulating silicate hydration.The Ca dissolution process of β-C2 S includes four stages: kink structure,bidentate structure,inner-sphere adsorption,and outer-sphere adsorption.The free energy barrier of breaking the last chemical bond is the rate-controlling step of the entire Ca dissolution reaction.The calculated free energy barrier of β-C2 S dissolution is 33.8 k J/mol,which is in very good agreement with the experimental result of 32 k J/mol.Dissolved Ca ions can not move freely but oscillates within the interfacial water(about 2~3 water molecule layers).Ca ions need to overcome the diffusion free energy barrier(23 k J/mol)in the interfacial water to enter the bulk solution to become freely moving ions.Kinetics simulations yield the rate constants,equilibrium constants,and Ca ion activity of each step of the Ca dissolution.The calculated activity of Ca ions is 3.60×10-5,which is in the range of the experimental results of 2.44×10-5～8.75×10-5.