Concrete Basics: What is Portland Cement
You probably know the three components of concrete: cement, aggregates such as sand or stone, and water. However, the most critical and unique component, cement, is probably the least understood.
What is cement? How is it made, where does it come from, and why does it work? Knowing the basics are extremely important for anyone that works with concrete, either in small projects residentially or on a large scale industrially.
What is cement?
Portland cement is made largely from the most plentiful mineral on Earth, limestone. It is a precise mixture of lime, silica sand, iron oxide and alumina. Portland cement refers to a type of cement rather than a manufacturer. It was created by a bricklayer in the early 19th century and named for its similarity to a type of stone quarried on the Isle of Portland in England. A standard specification exists, ASTM C150, which defines the required mixture of components of portland cement. This ensures a stable base that will properly form concrete.
Types of portland cement
There are five types of portland cement that are commonly used in residential and industrial construction.
• Type I – The most common, standard form of cement. This is used to mix basic concrete that can be used for floor slabs of any size, columns or structural beams.
• Type II – Also common, this is most often used when a low heat of hydration (the heat given off as the concrete hardens) is required. Having a lower early strength than Type I cement, it is used commonly in foundations and large footings. Used frequently in areas where resistance to sulfate is not needed. Some cement is labeled Type I/II, meaning it has the qualities of both types.
• Type III – This type of cement has a very high early strength, which continues to accelerate as the concrete cures. After about a month, the strength evens out and will have about the same strength as concrete made with Type I cement. This concrete is generally used in pre-cast concrete, as well as in cold weather, due to its high heat of hydration.
• Type IV – This concrete retains its structural integrity incredibly well, making it very well suited for areas where cracking must be minimized. Generally known to be a low-heat cement, it is very popular in massive pour construction products.
• Type V – A concrete for problem areas, this cement works very well in areas that are exposed to water or have high sulfate contents, both of which generally deteriorate concrete structures.
How cement works
While aggregates are a critical component of concrete, it will actually harden with just cement and water. As soon as water is added to cement, hydration begins to take place. This is the chemical reaction that causes the molecules to bond and harden. Heat is released during this time, known as heat of hydration. It is critical to keep the water to cement ratio very low; excess water can make the cement porous, causing cracks and even weakening structural integrity over time.