# Analysis of Hydration-Mechanical-Durability Properties of Metakaolin Blended Concrete
## 1 Introduction  
High reactive metakaolin is made through the calcination of a highly pure kaolinite and grinding the products to a fine particle size. Metakaolin presents various advantages in concrete, such as increased strength, increased resistance to chemical attack, enhanced concrete finishes, reduced shrinkage, and reduced permeability [1]. Due to these advantages, metakaolin is widely used in producing high performance and high strength concrete. Considerable experimental studies have been performed regarding material characteristics, hydration, and the mechanical and durability properties of metakaolin (MK) blended concrete [2; 3; 4; 5; 6; 7; 8; 9; 10]. [PERSON] et al. [2] reported that MK blended concrete had a lower slump than control concrete because MK has a higher specific surface than cement. [PERSON] et al. [3] performed experimental studies on the degree of reaction of MK for MK blended cement paste and showed that at early curing ages, the rate of reaction of MK in cement-MK blends was higher than silica fume in cement-silica fume blends. [PERSON] et al. [4] and [PERSON] [5] pointed out that the compressive strength was improved in MK blended concrete due to the pozzolanic reaction of MK. [PERSON] et al. [6] and [PERSON] et al. [7] stated that chloride diffusivity decreased as MK replacement levels increased. [PERSON] [8] and [PERSON] et al. [9] indicated that durability aspects, such as the resistances about sorptivity, water penetration, salt ponding, and carbonation, were improved due to MK additions. [PERSON] and [PERSON] [10] reported that MK could reduce concrete creep as MK blended concrete had a denser microstructure, a stronger binder matrix, and a modified interfacial transition zone. When compared with abundant experimental studies [2; 3; 4; 5; 6; 7; 8; 9; 10], the theoretical hydration-mechanical-durability models for MK blended concrete are relatively limited. [PERSON] et al. [11] suggested that the pozzolanic reaction of MK was controlled by diffusion. The MK reaction can be described using the Jander diffusion equation. [PERSON] and [PERSON] [12] evaluated the efficiencies of MK for concrete containing various MK contents and water to binder ratios at various curing periods. [PERSON] et al. [13] proposed three-parameter polynomial models to calculate the content of the superplasticizer, concrete strength, and efficiency factor of MK. [PERSON] et al. [14] proposed a diffusion-binding model for chloride ingress into MK blended concrete that considered the efficiency of MK. [PERSON] et al. [15] developed thermodynamic modeling of the hydration of cement-MK-limestone ternary blended concrete. These theoretical models [11; 12; 13; 14; 15] are useful for evaluating the properties of MK blended concrete; however, the current theoretical models [11; 12; 13; 14; 15] are not perfect as some of the points are not covered. The model provided by [PERSON] et al. [11] did not take into account the hydration of cement and was not valid for cement-MK binary blends. [PERSON] et al. [15]'s hydration model mainly focused on the chemical aspects of MK blended concrete. Furthermore, the effects of the MK reaction on concrete strength and durability were not fully considered [15]. Additionally, the studies undertaken by [PERSON] and [PERSON] [12], [PERSON] et al. [13], and [PERSON] et al. [14] mainly concentrated on the efficiency of MK. The efficiency factor of mineral admixtures is convenient for evaluating the properties of blended concrete; however, the efficiency factor shows some limitations as the efficiency factor changes with concrete mixing proportions and curing times [1]. For mechanical or durability aspects such as compressive strength or chloride ingress, the efficiency factors are also different. Moreover, using the efficiency factor is difficult when evaluating the compositions of reaction products, such as the calcium hydroxide content in cement-MK blends. Summarily, the efficiency factor is a macro indicator on the effect of mineral admixtures.