Introduction:
Due to its strength, durability and comparatively low cost, concrete is one of the most commonly used building materials today. It is used to produce objects as simple as bricks or as complex as hydroelectric dams. In its simplicity concrete consists of three materials: aggregate, cement and water. When sand and stones (aggregate) along with cement are mixed with water, a chemical process takes place between the water and cement, creating a crystal matrix. The crystals create strong bonds between each other resulting in a solid durable material. This chemical process is called hydration, the duration of which varies depending on the type of concrete and mix designation.
The Basics of Concrete:
Concrete has been used for thousands of years. Crude forms of concrete have been found in Bedouin structures dating as far back as 6500 BCE. The Bedouins found that sand, water and hydraulic lime allowed them to produce waterproof concretes that they used in the construction of stone walls and underground cisterns [1]. Later, circa 3000 BCE the Egyptians used locally sourced gypsum and lime-based concretes, with the Great Pyramid of Giza containing roughly 500,000 tons of concrete. At their technological height, the Romans were producing a form of concrete that is more similar to what is used today. They discovered that a naturally occurring volcanic sand called Pozzuolana, when combined with lime and water produced a water-resistant form of concrete that allowed them to construct docks, bridges and their famous aqueducts [1].
In the nineteenth century Joseph Aspdin started producing his own lime for cement by burning powdered clay and limestone in his kitchen stove. The concrete that he produced with this cement closely resembled a stone quarried in Portland, England which lead to the cement being labelled as Portland cement [2]. His simple recipe lead to a process that is now applied globally in the manufacturing of Portland cement. Today there are two main methods of manufacturing Portland cement, the dry method and wet method. The more common process, the dry method, begins with crushing dry limestone and clay into pieces with a maximum size of about three inches in diameter. While the wet method crushes and grinds these materials with water. Once the limestone has been crushed to its desired size the dry and wet methods follow the same procedure. At which point, for both methods the other necessary ingredients such as iron ore and fly ash are added to the crushed stone before being ground into finer particles. The mixture is then fed into large rotary kilns heated to roughly 1500 degrees Celsius [3]. While in the kiln, the extreme heat allows for chemical reactions to take place within the material, causing the formation of clinker. The rotary action of the kiln causes the clinker to collect into marble sized balls that are expelled out of the bottom of the kiln. After cooling, the clinker is ground into a fine powder and mixed with m...
