## 1 Introduction
The structural change mainly occurs in pores with a pore size of less than 40 A. When the temperature reaches 600 \({}^{\circ}\)C, the total pore volume will increase. However, some researchers have explored the relationship between porosity and temperature for concrete with different properties and found a linear one [13]. The effect of high temperature not only causes the volume of concrete to change but also causes the surface to peel off or spall. In particular, when high-strength concrete members are exposed to a rapidly rising temperature environment, they are more likely to spall, which will cause problems in the safety of concrete structures. The spalling of concrete can be divided into the following categories [14, 15, 16, 17, 18]:  
* Explosive spalling, i.e., the most dangerous form;
* Destructive spalling, i.e., explosive in a few large pieces of concrete from the surface;
* Local spalling, i.e., a dislodging of minor pieces from some particular points of the surface;
* Sloughing off, i.e., a gradual reduction of a cross-section. There are many researchers investigating the spalling mechanisms of heated concrete including thermal reactions (heat conduction), chemical reactions (the release of water and carbon dioxide), hygral reactions (the transport of water vapor or water), and mechanical reactions (microstructure damage, cracking, and thermal expansion differences) [19, 20, 21, 22, 23, 24]. These reactions will result in the formation of dry/dehydrated, drying/dehydrating, and quasi-saturation regions in the concrete. The temperatures of these regions are also different, so the corresponding pore pressures are also different [18]. Some researchers believe that concrete may suffer from thermal stress-induced spalling or pore pressure-induced spalling [9]. Taking high-strength concrete as an example, during the heating process, pore vapor pressure is generated due to the evaporation and expansion of the internal water but cannot be quickly dissipated. Furthermore, the water on the concrete's surface will be evaporated, and the water vapor inside the concrete will flow more into the interior of the porous medium due to the temperature gradient. When the temperature continues to rise, more water vapor diffuses inward, and the vapor content of the change layer between the low temperature and high temperature parts inside the porous medium will be saturated, thereby forming a moisture clog that prevents any gas flow from going further into the medium [25]. If the temperature continues to rise, under the interaction of pore pressure and thermal stress (caused by temperature gradients and external restraint conditions), spalling will occur once the internal pore vapor pressure exceeds the tensile strength of the concrete [6]. This results in a substantial reduction in the cross-sectional area of the concrete member and direct exposure of the reinforcement to flames, thereby reducing its load-bearing capacity [26]. In the past two decades, Taiwan has achieved good results in the use of reservoir sediments and industrial waste to make lightweight aggregate (LWA) that not only meets the environmental protection requirements of resource recycling and reuse but also improves the water storage function of the reservoir and prolongs its service life [27; 28; 29; 30; 31; 32; 33]. The particle density of synthetic LWA using reservoir sediments is significantly lower than that of normal density aggregates [28]. This is attributed to the fact that synthetic LWA particles have porous cores. They also meet the requirements of ASTM C 330 with a bulk density of coarse aggregate less than 880 kg/m\({}^{3}\). In addition, the physical properties and crushing strength of the synthetic LWA are better than those of commercial LWA. This is due to the hard ceramic shell of synthetic LWA particles [28]. These results suggest that synthetic LWA can be used as an aggregate source for structural concrete.