## 3 Fundamental Properties of RP
### Specific Surface Area (SSA) and the Effect of Grinding Duration on RP's Physical Properties  
As stated in Section 3.2, for optimal reactivity and performance in cement-based materials, RP particles should be extremely fine. RP reactivity has been shown to increase with higher SSA and fineness, grinding time, and grinding type [7; 9; 38; 83; 114; 119; 120]. High reactivity is due to the larger specific surface area, which leads to a greater reaction area and the availability of numerous active atoms that efficiently connect with other atoms [9; 38; 119]. Figure 2 presents the SSA of RP reported in literature compared to PC, FA, and GGBS. There is a substantial variation in SSA values for RP, with an average of 454.25 m\({}^{2}\)/kg and atypical points from 281.7 m\({}^{2}\)/kg to 575 m\({}^{2}\)/kg. The maximum value is 1351 m\({}^{2}\)/kg [121] and the minimum value is 73.57 m\({}^{2}\)/kg [122]. In the case of cement, FA, and GGBS, SSA data have averages of 355.6 m\({}^{2}\)/kg, 390.05 m\({}^{2}\)/kg, and 458 m\({}^{2}\)/kg, respectively. The SSA data presented in Figure 2 are obtained using the Blaine method in accordance with GB/T8074-2008 [123] (equivalent to that stated in ASTM C204-18-1 [124]), and NBR 16372 [125]. The Blaine method relies on measuring the difference in pressure within a dry and compressed layer of powder [126]. Although the majority of the studies used the Blaine method, some recent studies used the Brunauer-Emmett-Teller (BET) method to measure the SSA [59; 80; 127; 128; 129; 130; 131; 132; 133]. Unlike air permeability, the BET approach is a precise measurement of specific surface area as it does not rely on any assumptions regarding the shape of the particles. The BET method relies on the adsorption of a gas onto the solid's surface, which includes any accessible surface pores and fissures. It quantifies the amount of adsorbed gas that corresponds to a monomolecular layer on the surface [134; 135]. Figure 3 shows the SSA (Blaine) of RP as a function of D50, including the dry grinding and the wet grinding of RP. The figure demonstrates that as the particle diameter of RP decreases, SSA increases. Figure 2: Specific surface area (m\({}^{2}\)/kg) for RP, cement, FA, and GGBS. However, the process of grinding to achieve fine RP requires a significant amount of energy, resulting in adverse environmental impacts and increased production costs for RP. Furthermore, excessive grinding duration leads to an agglomeration of the material and reduces the strength of the specimen. [5,102,160,161]. To minimize energy consumption during production and achieve the finest possible particle size of RP, it is crucial to determine the optimal grinding time [38,161,162,163,164,165]. Figure 4 presents the dry grinding time of RP as a function of D50, and Figure 5 presents the wet grinding time of RP as a function of D50. Based on the analysis of Figures 4 and 5, 20 to 30 min is an appropriate duration for dry grinding, whereas 60 to 80 min is ideal for wet grinding [9; 38; 45; 65; 168; 174; 175; 176; 177; 178; 182; 183], and the smallest RP particle sizes were produced by wet grinding [113; 114; 120; 170]. In their study, [PERSON] et al. [175] conducted a three-stage dry grinding procedure lasting 25, 50, and 75 min. Notably, there was a substantial alteration in the D50 value between the 25 and 50 min intervals. [PERSON] et al. [177] investigated the correlation between the duration of dry grinding and the size of particles. The researchers observed that as the grinding time increased, the average particle size of RCP initially decreased rapidly within the first 30 min. However, between 30 and 50 min, the average particle size decreased at a slower rate, indicating lower grinding efficiency. [PERSON] et al. [179] employed durations of 10, 20, and 30 min for dry grinding. The reduction in the particle size of RCP became more pronounced as the grinding duration increased up to 20 min, but then started to decline.