IntroductionSalt crystallization plays an important role in the physical weathering process of materials, and is generally considered to be one of the main problems affecting the durability of materials, and most of the weathering damage of porous materials is related to salt crystallization. Since the permeation and crystallization of salt solutions are unavoidable. Therefore, it is of great value to explore the crystallization process and mechanism of salt in pores for the study of material durability in the fields of construction engineering, historical building protection engineering, traffic engineering, underground and geotechnical engineering, etc, and is of great significance for improving the disaster prevention and mitigation capabilities of historical buildings and infrastructure, and reasonably predicting the service life of structures. At present, the supersaturated crystallization pressure hypothesis is generally accepted as the cause of salt crystallization failure of porous materials, which holds that the crystallization pressure exerted by crystal on the pore surface is the main cause of the failure of cement-based materials. However, existing study has found that the actual crystallization pressure produced by NaCl evaporation crystallization is about 0.46 MPa, which is much lower than the tensile strength of concrete, and the concrete material still subjected to damage. Therefore, in this paper, the finite element method is used to study the crystallization process of NaCl in the micro pore of hardened cement paste, and the mechanism of crystallization pressure during the crystallization process is revealed. In addition, the effects of pore and crystal geometry, crystallization pressure and filling degree on the maximum tensile stress inside the hardened cement paste are also discussed.MethodsThe micro pore model and NaCl crystal model of hardened cement paste are established respectively, and the NaCl crystal model is horizontally put into the micro pore model of hardened cement paste to obtain the NaCl crystallization process model in the micro pore of hardened cement paste. In order to accurately distinguish the crystallization process of NaCl crystal, a two-dimensional spatial filling degree is introduced. The model of NaCl crystallization process in micro pore of hardened cement paste is established in three steps. The first step is to determine the geometric properties of the crystal. The size change of the crystals in the NaCl crystallization process model is controlled by the filling degree, and the crystals without pore wall confinement are obtained, and the crystals in the pore confined state are obtained by Boolean operation. The second step is to split the model. The NaCl crystallization process model is divided into two parts: the hardened cement paste micro pore model and the NaCl crystal model. The third step is to apply crystallization pressure. The stress distribution diagram of hardened cement paste and NaCl crystal is obtained by applying crystallization pressure to the contact parts between the crystal and the pore wall in the micro pore model and the NaCl crystal model.Results and discussionDuring the crystallization process of NaCl in the micro pore of the hardened cement paste, the tensile stress region is distributed within 2 μm of the pore radius, and the maximum tensile stress in the crystallization process is about 3 times of the crystallization pressure, which appears at the apex of the long axis of the pore, while the NaCl crystal is mainly compressed and the tensile stress is small. NaCl crystal do not fail before hardened cement paste. The pore axial ratio, filling degree and crystallization pressure are the main factors influencing the maximum tensile stress in the cement-based material during the crystallization process. The maximum tensile stress of hardened cement paste increases with the increase of pore axial ratio, filling degree and crystallization pressure during the crystallization process, while the crystal axial ratio has no obvious effect on the maximum tensile stress.ConclusionsThere are free crystallization and crystallization restricted stages in the crystallization process of salt in micro pore. The crystallization pressure generated by the crystallization restricted stage is a major factor causing damage to cement-based materials, and the tensile stress concentration caused by crystallization pressure is the direct cause of the damage of hardened cement paste. The crystallization pressure leads to the tensile stress concentration in the hardened cement paste, and the maximum tensile stress is about 3 times of the crystallization pressure, which appears at the apex of the long axis end of the pore. NaCl crystals will not be damaged before hardened cement paste. In addition, the pore axial ratio, filling degree and crystallization pressure are the main factors affecting the maximum tensile stress in the hardened cement paste during the crystallization process.