Extended abstract Hyper plasticizer admixtures (HPA) are chemicals that improve the workability of the paste. They allow a lower water/cement ratio to be used for a given workability, resulting in higher-quality concrete. HPA affects not only the rheological behavior of cement paste, but also its hydration characteristic. The hydrated cement paste is composed of hydration products, pores and, some unhydrated clinker grains. Calcium hydroxide (CH) is one of the main hydration products. The others are calcium silicate hydrate (C–S–H) and calcium sulfoaluminates hydrates. CH constitutes 20 to 25 percent of the volume of solids in the hydrated cement paste (Mehta ve Monteiro, 2006). Microstructural observations reveal that concrete can be represented as a three-phase model: a bulk cement paste phase, an aggregate phase, and a phase linking these two, referred to as interfacial transition zone. This zone is up to 50 µm thick and surrounds the aggregate particles (Alexander and Mindess, 2005). Most important features that differentiate interfacial transition zone from bulk cement paste are larger pores and crystal dimensions; particularly of CH. The size of the crystals in the interfacial transition zone is a very important parameter that directly affects the macroscopic properties of concrete. Although many studies have been conducted on the effect of HPA dosage on the macroscopic properties of concrete, no studies have been conducted on the effect of HPA dosage on the size of CH crystals in the interfacial transition zone. In this study, the effect of HPA dosage on the size of CH crystals in the interfacial transition zone was investigated. In this context, nine concrete mixtures with three different HPA dosages (%1, %1.25 and %1.5) and three different Ac/A (coarse aggregate / whole aggregate) ratios (%40, %50 and %60) were prepared. The size of CH crystals in the interfacial transition zone of concrete was determined by scanning electron microscopy (SEM) images. It was determined that in all concrete specimens that have three different Ac/A ratios, increasing the dosage of HPA from %1 to %1.25 significantly decreased the size of CH crystals in the interfacial transition zone while increasing the dosage of HPA to %1.5 increased the size of CH crystals in the interfacial transition zone. On the other hand, the size of CH crystals in the interfacial transition zone was found to be increased when the Ac/A ratio of concrete was increased. The reason of this situation can be explained by the mechanism of cement hydration. One of the characteristics of concrete that make it such a useful material is the fact that there is almost no net change in volume as the cement paste hydrates. If this were not the case, it would be almost impossible to cast concrete structures in place since the size of the structure would change over time. Given the mechanism of cement hydration, which is that a layer of hydration product grows outward from the surface of each cement particle, one might expect that wherever two of these growing layers came into contact they would push the particles apart, causing the paste to expand. Fortunately, when the layers of hydration product from two adjacent cement particles come into contact, the CH and C–S–H phases stop growing in that particular direction. In this manner, CH forms as crystals with a wide range of shapes and sizes, depending primarily on the amount of space available for growth (Thomas and Jennings, 2009). In this sense, the cement particles in the interfacial transition zone are dispersed in such a way that less space is left between them by using HPA up to a certain dosage. The use of HPA over a given dosage is thought to increase the bleeding in the interfacial transition zone. The microstructure of a material depends on the way that it is processed, and the properties depend on the microstructure. Thus the microstructure provides a link between processing (how a material is made) and properties (how a material behaves). Large CH crystals form a more porous framework, and possess less adhesion capacity because of the lower surface area and correspondingly weak van der Waals forces of attraction. CH also is the most soluble of the hydration products, and thus is a weak link in concrete. By using the appropriate dosage of HPA, concrete with smaller CH crystals in its interfacial transition zone can be produced. This obviously makes a significant contribution to the physical, mechanical and durability properties of concrete.