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

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. (C) 2022 American Society of Civil Engineers.