# Durability of Mortars with Partial Cement Replacement by Recycled Brick Powder
## 3 Results  
### Compressive Strength  
Figure 4 presents the average compressive strength of the specimens analysed for each manufactured mix.  
As shown in Figure 3, the partial replacement of OPC with RBP has a notable impact on the compressive strength of the mortars. For replacement levels of up to 35%, strength progressively increases with the substitution percentage, reaching a 51% improvement in mortar C35 compared to the reference mortar C00 (61.7 MPa vs. 40.8 MPa, respectively). However, for higher replacement levels, this trend reverses, and compressive strength declines. Nevertheless, the strength of the mortar with 45% replacement (mix C45) remains 25% higher than that of C00 (51 MPa vs. 40.8 MPa, respectively).  
The increase in compressive strength observed in mortars with partial cement replacement by RBP is attributed to the pozzolanic reaction of the brick powder. The pozzolanic reactivity of RBP was assessed by measuring the electrical conductivity and pH of calcium hydroxide/RBP aqueous suspensions. The results are presented in Figure 5 (brown line). The suspension, initially saturated with calcium hydroxide, showed a notable reduction in electrical conductivity within the first few hours of testing, decreasing by 60% after 7 days (168 h). According to [PERSON] et al. [83], when the conductivity loss is higher than 30%, it means that unsaturation of the suspension has been reached, which in turn indicates that a pozzolanic reaction has taken place. The silica in RBP reacts with ions in the solution, resulting in the formation of insoluble reaction products that precipitate. Consequently, as the ion concentration declines, electrical conductivity also decreases. Simultaneously, a notable change in pH values occurs due to the reaction of OH\({}^{-}\) groups (from calcium hydroxide) with RBP (Figure 5, right). After 7 days of testing, the pH decreased by 0.8 units, which is significantly greater than the 0.15-unit threshold proposed by [PERSON] et al. as an indicator of pozzolanic activity [83]. The significant decrease in pH recorded in the solution confirms the results obtained in the electrical conductivity loss test.  
Figure 4: Compressive strength.  
In summary, due to the pozzolanic nature of recycled brick powder (RBP), its high silica dioxide (SiO\({}_{2}\)) content reacts with the calcium hydroxide (Ca(OH)\({}_{2}\)) released during cement hydration, promoting the formation of C-S-H gels and, consequently, enhancing compressive strength. However, at higher cement replacement levels, the availability of Ca(OH)\({}_{2}\) becomes limited, thereby restricting the pozzolanic reaction of RBP. Consequently, when cement replacement with RBP exceeds 35%, the increasing trend in compressive strength reverses and begins to decline (Figure 4).  
Electrical conductivity and pH measurements were also taken in a calcium hydroxide solution containing crushed brick with a larger particle size, ranging from 100 to 250 \(\upmu\)m (Figure 5, grey line). In this solution, no significant changes were observed in the electrical conductivity or pH after 7 days of testing. This suggests that coarser brick particles are considerably less reactive than finely ground ones, which may explain the substantial differences in mechanical performance reported by various authors in mortars and concretes incorporating partial cement replacement with RBP [83; 84; 85].  
Figure 6 presents the average flexural strength of the specimens analysed for each manufactured mix. As observed, unlike the compressive strength study, no clear trend was identified. In all cases, the flexural strength ranged between 3.93 MPa and 4.83 MPa.  
Figure 5: Loss of electrical conductivity (**left**) and pH (**right**) values for suspensions made with recycled brick powder and coarser brick particles, tested during 7 days at 60 \({}^{\circ}\)C.  
Figure 6: Flexural strength.