## 3 Fundamental Properties of RP
### Activity Index  
Various test methods for evaluating the pozzolanic activity have been documented in the literature. These methods can be classified as either direct or indirect [161; 193; 196; 236; 237]. Direct methods employ analytical methods such as XRD, TGA, or traditional chemical titration to observe the presence of CH and its gradual decrease over time as the pozzolanic reaction progresses [236; 237]. The Frattini test is a widely employed direct technique that utilizes chemical titration to ascertain the amounts of dissolved Ca2+ and OH- in a solution containing CEM-I and the test pozzolan [236]. The saturated lime method is a simplified variation of the Frattini test, where the pozzolan is combined with a solution of saturated lime instead of CEM-I and water [236; 237]. Indirect testing methods assess a specific physical characteristic of a test sample that determines the degree of pozzolanic activity [236; 237]. This may entail the assessment of characteristics such as compressive strength, electrical conductivity [236], or heat generation by conduction calorimetry [236; 238]. Direct tests are frequently used to verify the occurrence of pozzolanic reactions and confirm the results obtained from an indirect pozzolanic activity test [236; 239]. Generally, indirect testing methods such as the strength activity index (SAI) are commonly used to assess the activity of RP [52; 115; 153; 183; 193; 202; 203; 204; 209]. In literature, the most used standards for this purpose are GB/T 1596-2017 [240], BS EN 450-1 [241], NBR 12653 [242], and ASTM C618 [230]. In addition to these standards, which relate activity index (AI) to mechanical resistance, standards such as GB/T2847-2005 [243] and BS EN 196-5:2011 [244] relate AI to the chemical nature of the powder. Regarding the standards, the majority of the studies measured the AI according to GB/T 1596 [164, 168, 173, 176, 178, 183, 195, 201, 209, 245], BS EN450-1 [116, 245], or JG/T 573-2020 [202]. According to these standards, the AI is calculated by dividing the 28 d compressive strength of mortar with 30% SCM by the 28 d compressive strength of plain mix without SCM, and the AI of SCM should be above 70% [164]. In general, the AI of RP fulfills the standard requirement (GB/T 1596; BS EN450-1; JG/T 573-2020) as presented in Figure 10, and therefore, it is possible to utilize RP as a substitute for SCM in cement-based materials [164, 176, 178, 183, 195, 201, 209, 245]. On the other hand, [PERSON] et al. [168] calculated the AI of WCP as 57%. The particle size of the WCP milled for 100 min was within the desired range, although its activity was quite low. They explained that the WCP utilized in the test was produced through the demolition of the previous brick-concrete construction, and its initial strength was not substantial. In addition, they stated that the cement particles have undergone complete hydration over time, resulting in a low level of activity induced by physical grinding. [PERSON] et al. [116] argued that RCP consists of both inert components and hydrated substances, which reduces its reactivity. Some other studies used SAI to evaluate the strength-gaining process of pozzolans at different ages according to ASTM C311-2007 [193, 194, 203, 204]. The following is the ASTM C618 specification regarding the reactivity of SCMs: The compressive strength of mortar containing 20% SCMs should be at least 75% of the compressive strength of the mortar without SCMs, both at 7 and 28 d. In all of the studies, the AI of RP fulfills the standard condition [193, 194, 203, 204]. [PERSON] et al. [203] stated that the SAI for both the 7 d and 28 d periods has shown an increase due to the larger size of the particles in RCP, their dispersion, and the formation of reactive spots on the surface of the RCP. They concluded that 25% RCP replacement with OPC is the ideal level for achieving suitable SAI. [PERSON] et al. [85] also observed that the small particle size of RCP led to an increase in SAI.