# Leaching Behavior of Cesium, Strontium, Cobalt, and Europium from Immobilized Cement Matrix
## 3 Results  
### Leading of the Solidified Cement Specim  
#### 3.1.1 Temporal Variations in pH, EC, and TOC  
The pH values of the leachates leached from the solidified cement specimens for up to 90 days are shown in Figure 2. The pH value of the ultrapure water used in the test was about 5.6 because the ultrapure water was exposed to carbon dioxide in the atmosphere. The pH of the leachate ranged between about 10.5 and 12.0 during the leaching test. These high-alkaline conditions in the leachate were caused by dissociation of the cement material. The main component of OPC, lime (CaO), reacts with water to form portlandite [Ca(OH)\({}_{2}\)], producing an alkaline solution with a pH of about 12.4 because of the relatively high dissociation constant of Ca(OH)\({}_{2}\) (Reactions (1) and (2)) [17; 46; 47]. In addition, the porewater chemistry in cement paste is influenced by the cement's alkali content, the alkali availability of the solution, the water/cement ratio, and the presence of soluble waste components [48].  
\[\text{CaO}+\text{H}_{2}\text{O}\rightarrow\text{Ca(OH)}_{2} \tag{1}\]  
\[\text{Ca(OH)}_{2}\rightarrow\text{Ca}^{2+}+2\text{(OH)}^{-} \tag{2}\]  
All leachates, except for the leachate from the cement specimen doped with Cs, showed similar temporal pH trends. The pH slightly increased to 12.0 until 19 days and decreased to approximately 11.0 by the end of testing (Figure 2). The decrease in pH after 19 days  
Figure 2: Leachate pH from the cement specimens up to 90 days of testing.  
might have resulted from the consumption of hydroxides during the dissociation process of cementitious material.  
The correlation between pH and EC of the leachate is shown in Figure 3. The pH of the leachates was positively correlated with EC. The EC was proportional to pH due to the dissociation and diffusion of the cement material. Two distinct linear correlations between pH and EC in two pH ranges (11.0-11.7 and 11.4-12.0) were observed because these values were determined using non-time-sequential data from the tests. The EC linearly increased from approximately 300 to 900 \(\mu\)S/cm as the pH increased from 11.0 to 11.7 in the early period of testing, whereas the EC increased linearly from approximately 900 to 2100 \(\mu\)S/cm as the pH increased from 11.4 to 12.0 in the later period of testing. Due to the increase in reaction time during the later period, the release of cementitious material increased, and, therefore, EC in the earlier period was lower than that in the later period.  
The TOC in the leachates was measured periodically during the leaching tests (Figure 4). Organic materials in immobilized wastes are physicochemically decomposed by microorganisms, which can accelerate the migration of radionuclides and impair the physical stability of the disposal facility [46; 47]. Therefore, the temporal trends in TOC were similar, regardless of specimen type. The TOC in the leachate was approximately 1 mg/L for up to 5 days and then increased slightly to approximately 2.5 mg/L at 90 days. This increase in TOC may have been caused by the dissociation and diffusion of organic materials in the cement specimens. These TOC results are comparable to the results of the increase in EC in the later period (Figure 3).  
Figure 4: TOC in the leachates as a function of elapsed time.  
Figure 3: Correlation of pH and electrical conductivity (EC) for each leachate.