Local charge compensation method and atomic-scale mechanisms of tensile deformation failure in polycrystalline and multiphase calcium carbonate

The products from microbially induced calcium carbonate precipitation are polycrystalline and multiphase calcium carbonate (PMCC). When constructing models of PMCC, local charge imbalances and non-stoichiometric issues may arise near grain boundaries and phase boundaries due to molecular truncation. A local charge compensation method was proposed, in which the O atoms in the carbonate group were selected to bear the compensating charges. A PMCC model containing calcite and aragonite grains was constructed. The uniaxial tensile behaviour was simulated by molecular dynamics method. By comparing with polycrystalline calcite (PC) and polycrystalline aragonite (PA), the mechanism of deformation and failure of PMCC was analysed, and the influence of environmental temperature was discussed. Results show that: (1) Local charge compensation can eliminate the excessive stress, abnormal Ca coordination, and local structural disorder caused by long-range electrostatic background errors. (2) PMCC has the smallest stiffness and a strength slightly higher than that of PA but much lower than that of PC. (3) The grain boundary densification dominates the elastic deformation of PMCC, while the deformation and failure after yield are characterized by the formation and development of nanoscale cracks and voids in the grain boundaries, accompanied by the migration of the grain boundaries. Unlike PC and PA, no new phase is formed inside the grains of PMCC. (4) An increase in temperature promotes the development of pores and cracks, leading to a decrease in elastic modulus, tensile strength, and peak strain of PMCC. These results were obtained under conditions free of water and impurities.