## 1 Introduction  
Concrete is one of the most widely used building materials today mainly because of its many advantages [1]. Concrete can be used in a variety of structures including high-rise buildings, tunnels, petrochemical plants, offshore platforms, bridges, nuclear plants, and other concrete structures. In essence, concrete has good fire resistance, and even if it is exposed to high temperatures or encounters fire damage, it still has a certain fire resistance [2]. However, concrete is a porous material filled with moisture and air. Once it is exposed to an intense heat flux incident, its mechanical properties deteriorate rapidly [3]. In view of this, many researchers have engaged in exploring the effect of high temperatures on the performance of various concretes [4; 5; 6; 7; 8; 9; 10; 11]. Generally speaking, the composition of concrete can be divided into an aggregate, an interface transition zone (ITZ), and a cement matrix [1; 2]. The properties of an aggregate are closely related to the unit weight, elastic modulus, and stability of concrete. In particular, the thermal properties of the aggregate play a key role in changing the volume of concrete that is subjected to high temperatures. Due to the presence of factors such as cement particle accumulation, bleeding, unidirectional growth, wall effects, and ion diffusion, the ITZ has properties different from the cement matrix. From a microscopic point of view, the cement matrix can be subdivided into solid and nonsolid microstructures, of which the solid microstructure mainly includes C-S-H gels, Ca(OH)\({}_{2}\), monosulfoaluminate, ettrinigite, unhydrated residual cement particles, chemically bound water, and gel pore water; nonsolid microstructures are mainly pores (capillary pores) and water (including capillary water, adsorbed water, and interlayer water) [1,2]. When concrete is subjected to fire damage and high temperatures, the microstructure and properties of its cement matrix will change with the increase in temperature [2,3]. As the temperature is between room temperature and 105 \({}^{\circ}\)C, the capillary water and adsorbed water are gradually lost. When the temperature is between 105 and 200 \({}^{\circ}\)C, the C-S-H gels begin to lose bound water and undergo chemical changes. As the temperatures increases to 250-350 \({}^{\circ}\)C, most of the bound water in the hydrate containing Al\({}_{2}\)O\({}_{3}\) or Fe\({}_{2}\)O\({}_{3}\) will be lost; approximately 20% of the bound water of the C-S-H gels will also be lost. At temperature between 400 and 700 \({}^{\circ}\)C, approximately 80% of the bound water in the C-S-H gels will be completely decomposed at this stage; however, at approximately 500 \({}^{\circ}\)C, the water in the matrix has been decomposed completely. In addition, the Ca(OH)\({}_{2}\) in the cement matrix begins to decompose at around 440-580 \({}^{\circ}\)C, but the reaction is reversible. As the temperature reaches 750 \({}^{\circ}\)C, the calcium carbonate in the calcareous aggregate begins to decompose and releases carbon dioxide; the quicklime (CaO) produced by the high temperature endotherm expands after absorbing water, which may cause cracks in the concrete. At temperatures between 800 and 1000 \({}^{\circ}\)C, the hydrates in the cement will be resintered to form the main components of cement such as C\({}_{2}\)S, C\({}_{3}\)A, and C\({}_{4}\)AF. Once the temperature reaches 1425 \({}^{\circ}\)C, the remaining cement hydrate may be further sintered to form C\({}_{3}\)S [2,3]. When concrete is exposed to high temperatures, the loss of the stability of the cement matrix and aggregate affects its pore structure, and the physical and chemical changes that occur in the solid phase alter its pore size distribution and total porosity. After heating, the concrete's total pore volume does not increase linearly with temperature [12]. When the temperature reaches 300 \({}^{\circ}\)C, the increase in pore volume is smaller than the weight loss. Some scholars believe that it is caused by drying and hydrolysis processes.