5.1.2. Strength Activity Index (SAI)

The RCM's ability to function as an active addition was assessed on the basis of its pozzolanic properties. The SAI index was calculated according EN 450-1:2012 [33] as mentioned in Section 3.3.1. The SAI of the tested material should be ≥0.75 after 28 days of maturation. Figure 5 presents the strength activity index (SAI) test results for composites with 25% addition of RCM as a cement supplement (compositions according to Table 2) after 28 days of curing for different calcination temperatures and the same time of calcination, which was 60 min. The results were compared to control mortar made of Portland cement (series 11, Table 2).

**Figure 5.** Compressive strength of the composites with and without RCM after 28 days of curing with the strength activity index (SAI).

As seen in Figure 5 the highest compressive strength was obtained for the composite containing 25% of the addition in the form of the RCM calcined at the temperature 650 ◦C for 60 min (SAI reached the highest level, SAI = 1.07). The increase in SAI value for these samples compared to the non-calcined RCM (series 12, SAI = 0.59) was 79%, and it was 66% compared to RCM calcined in 350 ◦C (SAI = 0.67). The composites with RCM calcined at temperature 500 ◦C also confirmed the requirements of standards [34], (SAI = 0.80), but the compressive strength reached 25 and 20% lower than the strength of the composites calcined at 650 ◦C and the control series, respectively.

#### 5.1.3. X-ray Diffractometry

In order to determine the effect of thermal treatment on the phase composition of RCM, the X-ray diffractometry test was performed. The X-ray patterns for non-calcined RCM and thermally treated specimen are shown in Figures 6 and 7, respectively.

**Figure 6.** XRD pattern for RCM without thermo-mechanical treatment (Q—quartz, P—portlandite, C—calcium silicate hydrate, S—belite, K—calcite, E—ettringite).

**Figure 7.** XRD pattern for RCM after thermo-mechanical treatment at temperature 650 ◦C (Q—quartz, P—portlandite, S—belite, K—calcite, O—calcium oxide).

The main crystalline phases found in RCM without thermo-mechanical treatment (Figure 6) are C–S–H gel, belite (C2S), portlandite (CH), ettringite, and calcite (CaCO3). The XRD pattern of the RCM after thermo-mechanical treatment indicates that some diffraction peaks of the preheated samples gradually disappear, such as ettringite and C–S–H gel. When the heating temperature is raised to 650 ◦C, it is mainly composed of belite (C2S), CaO, partial CH and non-crystalline dehydrated phases.

As expected, in case of material not subjected to the calcination process (Figure 6) the peaks associated with the presence of portlandite were quite intensive and frequent. In case of RCM after thermal treatment, only a single peak was observed. This indicated a properly selected treatment temperature, allowing for almost complete Ca(OH)2 decomposition. The small peaks from portlandite in the specimen subjected to calcination (Figure 7) can be explained by high hygroscopicity of disintegrated RCM. The finely ground and calcined RCM contains active CaO, which may react with moisture contained in the air. This phenomenon could not be avoided during sample preparation for XRD, hence the presence of a secondary portlandite in phase composition. Moreover, the calcined sample revealed higher and more frequent peaks, indicating the presence of Portland clinker components (belite), which are responsible for reactivity with water and for the hydration process. This could explain the applicability of calcined RCM as a pozzolanic additive and active filler [35,36]. The above-mentioned components of RCM facilitate further hydration and reaction with new cement paste, which cause the improvement of physical properties of cement composites. This is confirmed by the observations of other authors, who have noticed the content of non-hydrated cement, calcium hydroxide (CH), and dicalcium silicate (C2S) in RCM, which are capable of hydration and creation of rehydration products [19,20,22].
