The deterioration of concrete microstructures in freeze–thaw (F–T) cycles is the primary reason for the reduction in the service life of concrete. This paper reviews recent progress in the theory of damage mechanisms and damage models of concrete in F–T cycles. It is a detailed review of the salt-freeze coupling condition, microstructure testing, and models for the evolution of concrete properties that are subjected to F–T damage. Summarized in this paper are the deterioration theory of water phase transition; the mechanism of chloride-F–T and sulfate-F–T damage; the microstructure testing of hydration products, pore structure, microcracks, and interfacial transition zones (ITZ). Furthermore, F–T damage models for the macrostructure are presented. Finally, the issues that are existing in the research and outlook of concrete F–T damage are highlighted and discussed. This paper is helpful in understanding the evolution of F–T damage, and also provides a comprehensive insight into possible future challenges for the sustainable design and specifications of concrete in cold environments.

Emerging nanomaterials provide an invaluable opportunity for the development of cementitious materials. Many scholars have explored the influence of graphene (GP) and graphene oxide (GO) on the performance of the cementitious materials. This article reviews the previous research on the effect of GP and GO on the properties of cementitious materials. Detailed review of the mechanical properties and durability of cementitious materials containing GP or GO nanofilms is presented, and the mechanism is discussed. The mechanical properties of GO-cementitious materials are significantly enhanced. The optimal improvement of GO-modified compressive, flexural, and tensile strengths is 77.3%, 78.3%, and 78.6%, respectively. The durability of GO- and GP-modified cementitious material is compared with the control group. The incorporation of GP or GO significantly improves the sulfate attack resistance, and the transport properties can be decreased, while the frost resistance of GO- and GP-modified cementitious materials needs further research. This literature review shows that the microstructure of GO- and GP-modified cementitious material is improved in three aspects: accelerating the cement hydration, refining the pore structure, and hindering the crack propagation.

Carbon nanotubes (CNTs) are very effective in improving the performance of cement-based materials. Mechanical properties and pore structure were investigated for cement mortar with CNTs. Meanwhile, the composite morphology of CNT–cement material and the evolution of hydration products were observed by scanning electron microscope (SEM), and the quantitative relationship between mechanical properties and pore structure was analyzed. The results indicated that the strength of mortar increased with the addition of 0.05% CNTs and decreased when the fraction of CNTs increased to 0.5%. The porosity of mortar with dispersed CNTs increased significantly, as these pores may be introduced by the dispersant. The quantitative relationship between porosity and strength proved that the increased porosity is the reason for the decreased strength of mortar with 0.5% CNT content, while mortar matrix strength with 0.05% and 0.5% CNTs increased by 44.03% and 71.18%, respectively. SEM images show that CNTs are dispersed uniformly in the mortar without obvious agglomeration and that the CNTs and hydration products form a meshwork structure, which is the mechanism by which CNTs can enhance the strength of the cement matrix.

Carbon nanotubes (CNTs) are considered among the ideal modifiers for cement-based materials. This is because CNTs can be used as a microfiber to compensate for the insufficient toughness of the cement matrix. However, the full dispersion of CNTs in cement paste is difficult to achieve, and the strength of cement material can be severely degraded by the high air-entraining property of CNT dispersion. To analyze the relationship between the gas entrainment by CNT dispersion and mortar strength, this study employed data obtained from strength and micropore structure tests of CNT dispersion-modified mortar. The fractal dimensions of the pore volume and pore surface, as well as the box-counting dimension of the pore structure, were determined according to the box-counting dimension method and Menger sponge model. The relationship between the fractal dimensions of the pore structure and mortar strength was investigated by gray correlation. The results showed that the complexity of the pore structure could be accurately reflected by fractal dimensions. The porosity values of mortar with 0.05% and 0.5% CNT content were 15.5% and 43.26%, respectively. Moreover, the gray correlation between the fractal dimension of the pore structure and strength of the CNT dispersion-modified mortar exceeded 0.95. This indicated that the pore volume distribution, roughness, and irregularity of the pore inner surface were the primary factors influencing the strength of CNT dispersion-modified mortar.

Nano-silica (NS) is an ideal modifier for mortar materials, and exploring the evolution of the fractal dimension of the pore structure in NS-modified mortar is crucial for elucidating the mechanism by which NS enhances mortar strength. In this study, NS reinforced mortar was prepared using an NS sol solution, which inhibited the aggregation of NS particles. The relationship between the strength and pore structure of NS-modified mortar was quantitatively analyzed based on fractal dimension theory and gray correlation degree. The experimental system evaluated the mortar strength, pore structure distribution, and micro-morphology. Based on this evaluation, the fractal dimension of the mortar pore volume was calculated in detail. Subsequently, models for mortar strength and NS content were further established using grey analysis. The results indicate that NS significantly enhances the strength of mortar while also increasing its porosity due to reduced fluidity. NS can improve the compressive strength of mortar by up to 35%. The curve fitting of volume fractal dimension and box dimension is effective and can accurately reflect the complexity of the pore structure. The calculation of the grey correlation analysis model shows that the impact of varying silica content on the mechanical properties of mortar specimens is not linear; the distribution and quantity of bubbles are the main factors affecting the strength of the specimen.