# The Efficiency of Calcium Oxide on Microbial Self-Healing Activity in Alkali-Activated Slag (AAS)
## 3 Results and Analysis
### Durability Properties
#### 3.2.2 Rapid Chloride Permeability Test (RCPT)  
It is important to highlight the factors influencing the results of the RCPT. RCPT is significantly affected by the microstructural properties of the concrete matrix in terms of pore structure, pore solution, chloride binding capacity, and phase evolution [13,44]. Pore distribution, pore continuity, and pore size have a pivotal role in transporting chloride ions. For instance, a smaller pore-size matrix is known to have higher chloride resistance [13,45]. The typical value of the pH level of AAS is 13.6, which is similar to that  
Figure 5: Physical properties of AAS specimens in terms of (**a**) water absorption and (**b**) porosity.  
of the OPC systems (13.28). Nevertheless, the pore solution of AAS completely differs from the OPC solution due to the excessive presence of Na\({}^{+}\), OH\({}^{-}\), and HS\({}^{-}\). The presence of high OH\({}^{-}\) along with a constant pH value enhances the stability and formation of C-A-S-H, reducing the Cl\({}^{-}\)/OH\({}^{-}\) ratio [44; 45]. This reduced Cl\({}^{-}\)/OH\({}^{-}\) ratio improves the chloride resistance. However, it is worth mentioning that the high availability of mobile ions could reflect incorrect chloride resistance results due to the increase in charge conductivity. AAS systems have been reported to have lower binding chloride capacity, which was confirmed by the absence of chloro-aluminates (reaction between Cl\({}^{-}\) and C\({}_{3}\)A) in the XRD [46; 47]. Based on the aforementioned factors, the automatic stoppage of the RCPT when applied to AAS mixtures could be explained. The increase in the ionic concentration by chloride penetration increases the mobility of the available free ions such as Na\({}^{+}\), OH\({}^{-}\), and HS\({}^{-}\), reflecting false results about the passage of high electric charges.  
The presence of calcium oxide in the bio-AAS specimens prolonged the testing time and increased the specimen resistance, as described in Table 6. The increase in the resistance time of Ca-5 compared to the mix incorporating bacteria only was mainly associated with the leaching and dissolving of more Ca\({}^{2+}\) ions. Various reactions took place in the Ca-5 mixtures, consuming the mobile ions that might possess conductivity through the capillary pore structures of the AAS matrix. One of these reactions was the formation of hexagonal portlandite crystals [41]. In addition, anorthoclase (Na, K)(SiAlO\({}_{5}\)) and albite (NaAlSi\({}_{3}\)O\({}_{8}\)) were detected in the XRD, as illustrated in Section 3.3. One more important reaction was the biomineralization reaction that occurred due to the enzymatic ureolysis effect of _Bacillus subtilis_[5]. Hence, the intensity of the mobile ions concerning C5 was reduced due to the reactions performed in the presence of Ca\({}^{2+}\) ions. This finding is well supported by the results reported by [PERSON] et al. [4] about observing high electrical conductivity in RCPT testing of specimens incorporating lime. The increase in porosity in C and C-5 confronted the behavior postulated here, as the existence of high porosity allows the existence of alkali ions as Na\({}^{+}\) ions, which conduct electricity easily. Similarly, in Ca-5, however, some of these pores were plugged by the formation of calcium carbonate or more calcite.