## 3 Results and Discussion  
### Mechanical Properties  
#### 3.1.1 Compressive Strength  
Compressive strength is an important parameter of cement-based materials. Many durability and strength parameters of concrete can be predicted accurately using its compressive strength [53; 54]. The compressive strength of each mix at 7, 28, and 180-days is presented in Figure 2. The general trend in results indicates that an increasing level of mechanically activated bentonite (MAB) reduces the compressive strength. The use of 10% MAB gives optimum results after 28 and 180-days compared to the control mix (CON). Compressive strength at all ages is badly affected by increasing levels of MAB. After 180 days, mixes with 10%-20% MAB gave better compressive strength than the strength of the control mix at day 28. The improvements in the compressive strength of concrete with the inclusion of 10% MAB can be ascribed to (1) its smaller particle size than OPC, which improves the particle size distribution in the binder matrix and (2) the reaction of bentonite particles with free CH (a primary hydration product) to produce secondary CSH-gels [21]. The reduction in compressive strength with an increasing incorporation level of MAB can be ascribed to reduction in overall CH content in the binder owing to lower calcium oxide percentage of bentonite which reduces the chance of MAB consumption in pozzolanic reaction. Naturally, bentonite is found in consolidated form; therefore, without a suitable treatment/activation method, it cannot be used directly in \"as is\" condition as a supplement for OPC. [PERSON] et al., [13] reported that an MAB replacement of up to 21% does not affect the compressive strength of concrete compared to the control mix. Concrete mixes with thermo-mechanically activated bentonite (TMAB) showed higher compressive strength than MAB mixes for a given age and replacement level. After 28-days, 10% incorporation of MAB and TMAB showed 1% and 4.3% higher compressive strengths than the control (CON), respectively, whereas, after 180-days, incorporation of 10% MAB and TMAB resulted in 3.5% and 9.8% higher compressive strengths than CON, respectively. Using the thermo-mechanical activation process, the replacement level of bentonite can be enhanced from 10% to 20% to produce a higher compressive strength than CON at 28-days. Clays may consist of some fixed crystalline structures and many of them possess pozzolanic properties. Calcination of a clay mineral alters its crystalline structure into an amorphous one and increases its pozzolanic reactivity considerably [55]. The heating of clay, shale or other mineral changes their structures to alumina or quasi-amorphous silica, which have good pozzolanic reactivities with CH [56]. Strength activity indices (SAIs) of all mixes at days 7 and 28 are displayed in Figure 3. ASTM C618 [33] states that for any material to be considered pozzolanic, the SAI should be at least 75% of the control blend at both day 7 and day 28. From the SAI values, it can be seen that MAB can be used up to 15% with a SAI higher than 75%. Whereas for TMAB up to a 25% level, SAI is well above 75% for both days 7 and 28. It can be concluded that thermo-mechanical activation produces mixes with more compressive strength than mechanical activation, which can help to maximize the level of bentonite in concrete production. A relative analysis of compressive strength is also illustrated in the Figure 4. Compressive strength of the control mix (B0 (CON)) at day 28 was taken as the reference. It can be observed from Figure 4, that the day 28 compressive strength of concrete with 10%-20% MAB almost overlaps with the compressive strength of the reference mix and a notable decrease is noted for 20%-35% MAB. Similarly, for day 28's compressive strength of concrete modified with 10%-25% TMAB, it overlaps or nearly coincides with the compressive strength of the reference mix and a decline was observed for 25%-35% MAB.