Study on the Physical and Mechanical Properties of Mortar with Added Silica Fume, Nano-CaCO₃, and Epoxy Resin under the Action of Salt and Freeze–Thaw

Abstract

Many engineering problems are caused by the destruction of cement-based materials through the combination of salt mixtures and freezing/thawing. To improve the performance of cement-based materials under harsh environmental conditions, mortar was selected as a research object, and its mechanical and phase transformation characteristics were investigated by adding nano-CaCO3, silica fume, epoxy resin, and mixtures of these components. Laboratory tests were conducted on salt mixtures and freeze/thaw cycles, and the compressive strength and mass loss rate of the mortar recorded before and after freezing/thawing were compared. Subsequently, the effects of the type and amount of additive on the compressive strength and mass loss rate of the mortar were analyzed. The microstructural characteristics of the modified mortar were investigated using scanning electron microscopy, and the mechanisms of the physical and mechanical properties of the mortar modified by additives were revealed. Furthermore, the phase transition properties of water and salt in the modified mortar were studied using differential scanning calorimetry, and the crystallization amount, unfrozen water content, supersaturation ratio, and crystallization pressure in the mortar were calculated on the basis of heat balance and mass balance. The results indicate that during the cooling process, the solution in the internal pores of the mortar undergoes a crystallization phase transition, and the maximum crystallization pressure can reach a value of 30.1 MPa. The additive forms a compact structure inside the mortar to prevent harmful ions from infiltrating the mortar, thereby mitigating the damage caused to the mortar. The apparent deterioration reduced and the compressive strength of the modified mortar improved after several salt and freeze/thaw cycles, and an improvement rate of 15% for the epoxy resin was the most prominent result. The variation in temperature in this study was defined to model cold regions in northwest China and there are similar saline soils in cold regions in other parts of the world, such as the Mediterranean Basin, California, Southeast Asia, Arctic coast, and Central Siberia (Hivon and Sego 1993; Serrano and Gaxiola 1994; Bohren and Albrecht 1998; Shaterian et al. 2005). Therefore, cement-based materials worldwide suffer damage from salt erosion and freeze/thaw cycles. Although there are differences in temperature and salt concentration in different regions of the world, this research has practical reference value for improving the quality of mortar. For the additives considered in this study, all mass loss ratios of the improved mortar significantly reduced. In soil containing sodium chloride, a mixture of silica fume and epoxy resin is suggested to improve the durability of mortar based on reduced crystallization inside pores. In soil containing sodium sulfate, adding a mixture of na-CaCO3 and epoxy resin can improve the physical and mechanical properties of mortar by reducing salt and ice crystallization in pores. Although the improvement of mortar under the action of salt and freeze/thaw cycles was investigated in this study and the internal mechanisms of mass loss rate and compressive strength enhancement were discussed, the relationship between freeze/thaw cycles and actual environmental temperature changes has not yet been established, and phase changes in porous materials are closely related to the speed of temperature variation. The manner in which freeze/thaw cycles correspond to an actual environmental cycle and the improvement and phase transformation of mortar under ultralow temperatures (below −20°C) are worth studying in the future. Furthermore, the volume of a specimen has a significant influence on the transition temperature, in which supersaturation and supercooling decrease as the volume of soil increases (Wan et al., 2021). Furthermore, the effects of volume on the law of phase transition in mortar remain to be addressed in future studies.