Concrete has remarkable fire resistance properties. In the case of fire, it is found that the concrete affected by fire depends to a great extent on the intensity and duration of fire. Previous experience has shown that concrete structures are likely to have a good fire rating than structures made of other materials. Nevertheless, concrete undergoes important chemical and physical changes, starting at 400°C - 500°C. As calcium hydroxide and other hydration products start to decompose, concrete tends to lose its strength, typically around 600°C - 700°C. In order to improve the high temperature properties of concrete, admixtures can be used in concrete. In the following, a study on the effects of different admixtures on the properties of concrete at high temperature is presented.