While the terms are often used interchangeably, concrete and cement are not the same. Cement is an ingredient of concrete. It’s the fine grey powder that, when mixed with water, sand and gravel or crushed stone, forms the rock-like mass known as concrete. Cement typically accounts for 10% to 12% of the concrete mixture.
Cement acts as the binding agent or glue in concrete. When combined with water, cement forms a paste that coats the sand and gravel. Through a chemical reaction called hydration, the mixture sets and hardens into concrete.
When freshly mixed, concrete can take any shape or form; when hardened, it’s strong and durable. These qualities explain why one material, concrete, can build everything from skyscrapers and sidewalks to highways and dams.
Although natural cement dates back to the Romans, by the late 1800s builders were turning to its manufactured counterpart, portland cement, for higher strength and more consistent performance.
English bricklayer Joseph Aspdin is widely credited for the invention of modern portland cement. Aspdin received the first patent on portland cement in 1824. He named it “portland” cement because concrete made with it resembled a highly prized building stone quarried on the Isle of Portland.
The name endured. Today, portland cement is not a brand name, but the generic term for the type of cement used in most concrete.
Portland cement manufacturing is a three-step process:
- Cement-making requires calcium, silica and small amounts of alumina and iron. Raw materials rich in these elements include limestone, sand and clay. They are quarried at the cement manufacturing plant or shipped from nearby. Limestone, the source of calcium, makes up about 80% of the raw materials for cement.
- The materials are combined, blended and ground together, then processed in a huge industrial furnace called a kiln, which reaches temperatures of 1,450° C (2,650° F) or more. The intense heat triggers chemical reactions that form an intermediate product called clinker.
- The pellets of red-hot clinker are cooled and ground with a small amount of gypsum. The result is a fine powder that is now considered portland cement. Cement is ground so finely that it will pass through a sieve that is fine enough to hold water.
Most cement is used to produce ready mixed concrete, which is delivered to construction sites in the familiar trucks with revolving drums. Other concrete applications include precast products, pipe, masonry, oil and gas drilling and soil stabilization.
Engineers and construction professionals often specify a certain type of portland cement to meet special requirements of conditions.
Cement types are defined by a variety of standards and specifications. Portland cements conform to the American Society for Testing and Materials (ASTM) C150 Standard Specification for Portland Cement.
The standard addresses six types of portland cement:
Type I, Normal: General purpose portland cement suitable for concrete that does not require special properties.
Type II, Moderate Sulfate Resistance: Used when concrete is exposed to soil or water with higher-than-normal sulfate concentrations.
Type II (MH), Moderate Heat of Hydration: Sulfate-resistant cement that also reduces heat generation caused by hydration.
Type III, High Early Strength: Develops strength sooner than normal cements for early form removal, cold weather or when the concrete must be put into service quickly.
Type IV: Low Heat of Hydration: Minimizes heat generation when concrete is used in massive structures such as bridge supports or dams. However, other measures for controlling heat of hydration, such as blended cements, are more common.
Type V: High Sulfate Resistance: Used in concrete exposed to severe sulfate concentrations.
ASTM C150 Standard Specification for Portland Cement
Design and Control of Concrete Mixtures, Portland Cement Association’s reference on the fundamentals of concrete technology and construction.
Portland Cement Association