| Potassium magnesium phosphate cement MKPC is a new type of civil engineering material.It has many excellent properties such as fast setting speed and high strength.Its main cementitious product is magnesium potassium phosphate hexahydrate（Mg KPO4·6H2O）,commonly known as K-Struvite.Some unreacted magnesium oxide（MgO）particles in the curing process will act as fine aggregate and fill between K-Struvite cementite substrates as particle reinforcing phase.These two components work together cementation form MKPC cement solid substrate,its structure and properties of cement matrix strength,stability,water resistance and curing metal ions has an important influence,many ways,such as the mechanical properties of both in the homogeneous model of potassium magnesium phosphate cement and has important guiding significance to multi-scale modeling.However,the physical and chemical properties of K-Struvite are still not well understood,especially the mechanical properties of K-Struvite,and the results of young’s modulus measured by nano indentation experiments are inconsistent.Therefore,in this paper,the first-principles calculation method based on density functional theory was used to study the microstructure of K-Struvite and MgO,the main components of potassium magnesium phosphate cement hydration products and calculate their mechanical properties and related properties.The main research work and conclusions are as follows:（1）In this study,the main components of K-Struvite and MgO crystal models of hydration products of potassium magnesium phosphate cement were theoretically constructed and optimized.Their crystal structure,charge density,Bader charge and electron state density were calculated and analyzed.The calculated results of K-Struvite lattice constants are a0=6.939?,b0=6.142?,c0=11.215?,which are in good agreement with experimental parameters a=6.174?,b=6.903?,c=11.146?,and the error is less than 1%.The calculated result of the MgO lattice constant is a0=4.247?,which is in good agreement with the experimental parameter a=4.214?,and the error is only 0.7%.Electronic performance analysis shows that MgO and K-Struvite are typical wide band gap insulators.K-Struvite is vulnerable to electrophilic attack,showing stable chemical properties under alkaline conditions and unstable under acidic conditions,and MgO has high ionic properties.The thermal stability of K-Struvite and MgO,the main components of potassium magnesium phosphate cement hydration products,was analyzed by the molecular dynamics method.The results showed that both still maintained good thermal stability at 400 K.（2）The calculated values of K-Struvite and MgO elastic constants are consistent with all the mechanical stability criteria.The calculated value of K-Struvite’s young’s modulus is 36.5 GPa（close to the experimental value of 37.3 GPa）,the calculated value of volume modulus is 48.1 GPa,the calculated value of shear modulus is 13.3 GPa,the Debye temperature is 399.0 K,and K-Struvite shows elastic anisotropy.The mechanical properties of potassium magnesium phosphate cement are closely related to the mechanical properties of K-Struvite,the main hydration product.Theoretically,the more K-Struvite the cement is formed,the closer the elastic modulus of K-Struvite is（36.5 GPa）.The calculated value of MgO’s Young’s modulus,volume modulus and shear modulus are287.6 GPa,153.6 GPa and 121.0 GPa,which are all close to the experimental value.Debye temperature is 909.0K,and MgO tends to elastic isotropy.The unreacted MgO particles are equivalent to fine aggregate in the MKPC cement matrix and play a particle strengthening role. |
