# Analysis of Hydration-Mechanical-Durability Properties of Metakaolin Blended Concrete
## 3 Property Evaluations of MK Blended Concrete
### Results and Discussion
#### 3.2.4 Chloride Penetrability  
[PERSON] et al. [7] measured the chloride penetrability of MK blended concrete using ASTM C1202. The chloride penetration resistance of concrete is determined through an electrical indicator: the total charge passed during a six-hour test. Chloride penetrability mainly depends on concrete conductivity, which relates to many factors such as capillary porosity in paste, aggregate content, binder composition (cement and mineral admixture contents), pore solution conductivity, and temperature effect [25]. For structural concrete, the variance of aggregate content is marginal and the average aggregate volume is about 65-75% of concrete volume [26; 27]. Capillary porosity in paste and binder compositions are the most influencing factors of chloride penetrability in concrete [7]. Using the MK blended hydration model, the capillary porosity in paste can be calculated. Figure 7 shows the relationship between the total charge passed and the capillary porosity of the paste. The water to powder ratios of concrete varied between 0.3-0.5, the MK contents ranged between 0% and 20%, and testing stages ranged from early to late stages (3, 7, 28, and 90 days). This figure shows that at the same capillary porosity, the MK blended concrete had a lower chloride penetrability than the control concrete. Therefore, as the MK content increased, chloride penetrability became much lower. This is because the MK addition lowers the average capillary pore diameter and increased the percentage of fine capillary pores in concrete [7]. In addition, the calcium silicate hydrate (CSH) gel produced from the pozzolanic reaction had a finer gel pore than that produced by cement hydration. This difference in CSH compound also contributed to the lower chloride penetrability [27]. [PERSON] and [PERSON] [28] proposed that the chloride penetrability of concrete could be calculated using a power function of capillary porosity of paste as follows:  
\[p_{cl}(t)=Q_{1}(V_{5})^{Q_{2}}\frac{2(1-V_{a})}{2+V_{a}} \tag{16}\]  
where \(p_{cl}\) is chloride penetrability; \(Q_{1}\) is intrinsic chloride penetrability; \(Q_{2}\) is the exponent of chloride penetrability; and \(V_{a}\) is the volume of aggregate. \(V_{5}\) is the volume of capillary porosity in paste which can be calculated from Equation (15). The item \(Q_{1}(V_{5})^{Q_{2}}\) considers the effect of binder hydration on chloride penetrability. The item \(\frac{2(1-V_{a})}{2+V_{a}}\) considers the dilution effect of the aggregate on chloride penetrability. The coefficients \(Q_{1}\) and \(Q_{2}\) are dependent on the MK replacement levels. We approximately assumed that the values of \(Q_{1}\) and \(Q_{2}\) were linearly dependent on weight fractions of cement and MK as follows:  
\[Q_{1}=Q_{1c}\frac{C_{0}}{C_{0}+P}+Q_{1 mk}\frac{P}{C_{0}+P} \tag{17}\]  
\[Q_{2}=Q_{2c}\frac{C_{0}}{C_{0}+P}+Q_{2 mk}\frac{P}{C_{0}+P} \tag{18}\]  
where \(Q_{1c}\) and \(Q_{1 mk}\) are the intrinsic chloride penetrability of cement and MK, respectively; and \(Q_{2c}\), and \(Q_{2 mk}\) are the penetrability exponent of cement and MK, respectively. The units for \(Q_{1c}\) and \(Q_{1 mk}\) are coulombs. For plain concrete without MK, the chloride penetrability only depends on \(Q_{1c}\) and \(Q_{2c}\). For the MK blended concrete, chloride penetrability is dependent on \(Q_{1c}\), \(Q_{2c}\), \(Q_{1 mk}\), and \(Q_{2 mk}\). For concrete with different mixing proportions at different stages, the values of \(Q_{1c}\), \(Q_{2c}\), \(Q_{1 mk}\), and \(Q_{2 mk}\) are constants. Based on the experimental results shown in Figure 7, the values of \(Q_{1c}\), \(Q_{1 mk}\), \(Q_{2c}\), and \(Q_{2 mk}\) were determined as 20,399, -44,460, 1.57, and 7.13, respectively. The intrinsic chloride penetrability of the control concrete, 10% MK concrete, and 20% MK concrete were 20,399, 15,953, and 11,507, respectively.