# The Efficiency of Calcium Oxide on Microbial Self-Healing Activity in Alkali-Activated Slag (AAS)
## 3 Results and Analysis  
### Mechanical Properties  
Incorporating calcium oxide was proposed by previous researchers [4; 37] to provide a highly alkaline environment that promoted the formation of C-S-H. Furthermore, it was expected that increasing the calcium ions in the matrix would enhance the biochemical reactions of the added bacteria, resulting in high calcium carbonate precipitation. The fact is that incorporating calcium hydroxide in the AAS mix had a negative influence on the compressive strength of the AAS mortar. A reduction in the compressive strength of 50.7% and 46.5% was observed after 7 and 28 days of curing, respectively, as presented in Figure 4a. These results are consistent with the findings of [PERSON] et al. [38], who tested the influence of calcium oxide on the behavior of alkali-activated concrete composed of various binders. The concrete binder with the highest CaO (6.1% of its raw constituents) recorded the shortest setting time when calcium oxide was incorporated into the alkali-activated mixture. The final setting time significantly decreased as the molarity of the activator solution increased. Hence, insufficient time was available for the solid dissolution of the composites, including CaO. Subsequently, a remarkable reduction was recorded for the 7-day compressive strength. The addition of lime causes an instant precipitation of metal hydroxide that consumes more localized hydroxyl ions. Therefore, a sudden drop was marked in the pH level, triggering the chemical reactions and the polymerization process. If these conditions occur at a saturated level of silicate concentration, rapid silicate gelation will take place, producing larger particles with less surface energy [39; 40]. This was confirmed by another study conducted by [PERSON] et al. [22] that reported a reduction of 66% in the compressive strength after adding lime to AAS systems. It is worth noting that the speed of gel formation mainly depends on the nature of the added solids, activator compositions, and the species of dissolved ions [38; 41]. Figure 3: (**a**) The Rapid Chloride Permeability Test (RCPT) under testing cylinder specimen (**b**) The electrical resistivity of the concrete surface while measuring using the four-probe Wenner electrodes. The influence of incorporating bacteria was not significant for this type of mixture composition. The negative effect of incorporating calcium oxide was more pronounced when compared with the microbial influence. One of the important steps required for microbiological enzymatic ureolysis is the production of hydroxide ions. Hydroxide ions increase the pH level, which adjusts the overall equilibrium for precipitating calcium carbonate, as explained in the following Equations (5)-(11) [5]. Nevertheless, upon adding lime to the composite, most of the hydroxyl ions were consumed by calcium ions, disrupting the biochemical reaction series and reducing the microbial activity. 1. Hydrolyzing urea to produce carbonate and ammonia in the presence of urease, followed by a series of biological reactions to produce bicarbonate and hydroxide ions. \[\text{CO(NH}_{2})_{2}+\text{H}_{2}\text{O}\rightarrow\text{NH}_{2}\text{COOH}+\text{NH}_{3}\] (5) \[\text{NH}_{2}\text{COOH}+\text{H}_{2}\text{O}\rightarrow\text{NH}_{3}+ \text{H}_{2}\text{CO}_{3}\] (6) \[\text{H}_{2}\text{CO}_{3}\leftrightarrow\text{HCO}^{3-}+\text{H}^{+}\] (7) \[2\text{NH}_{3}+2\text{H}_{2}\text{O}\leftrightarrow 2\text{NH}_{4} ^{+}+2\text{OH}^{-}\] (8)
2. Hydroxide ions elevate the pH value, promoting the formation of carbonate ions. \[\text{HCO}^{3-}+\text{H}^{+}+2\text{NH}_{4}^{+}+2\text{OH}^{-}\leftrightarrow \text{CO}_{3}^{2-}+2\text{NH}_{4}^{+}+2\text{H}_{2}\text{O}\] (9)
3. An attraction force takes place between the positively charged calcium ions (Ca\({}^{2+}\)) and the negatively charged carbonate ions (CO\({}_{3}^{2-}\)), precipitating calcium carbonate (CaCO\({}_{3}\)) at the cell surface.