Concrete is made up of three main ingredients: water, Portland cement, and aggregates. The ratio of the ingredients changes the properties of the final product, which allows the engineer to design concrete that meets their specific needs. Admixtures are added to adjust the concrete mixture for specific performance criteria.
The water in the concrete mix should be clean and free of impurities. The amount of water relative to the amount of cement changes how easily the concrete flows, but also affects the final strength of the concrete. More water makes for easier flowing concrete, but also makes for lower strength concrete upon curing.
Cement hardens when mixed with water, which binds all of the ingredients together. Portland cement is the most common cement used and is composed of alumina, silica, lime, iron, and gypsum. Small amounts of other ingredients are also included.
The majority of a concrete mixture is made up of both coarse and fine aggregates, which help increase the strength of the concrete beyond what cement can provide on its own. Sand, gravel, and crushed stone are used as aggregates. Recycled materials, including blast furnace slag, glass (mostly for decorative purposes), and ground-up concrete are starting to be used as concrete aggregates.
Admixtures accomplish a variety of goals. This can be as simple as adding a pigment to color the concrete. Other admixtures are used for faster curing times in cold weather, creating extremely high-strength concrete, or for increasing the flowable nature of concrete without compromising the strength. Unfortunately, admixtures can generate unwanted results such as poor adhesion of finish-flooring. For this reason, many structural engineers and architects are hesitant to use admixtutres. We have an article that covers a number of different admixtures.