How many times have you heard someone say walk on the cement sidewalk? How about the concrete sidewalk? Which is correct? Actually, the correct phrase is "concrete sidewalk." Many people use the terms interchangeably, but actually they are different.
Concrete is a mixture of cement, water, sand, and gravel. By volume, 10 to 15% of concrete is cement. As concrete hardens, the mixture becomes stronger. The hardening process can take years. Concrete has the ability to withstand the pressure of heavy loads because it has high compression strength. It can also be molded into any shape, can be made porous or watertight, and is a relatively cheap material for use in construction projects.
Cement is powder and is one of the main ingredients in concrete. Cement and concrete have been used in construction since at least the Roman Empire. Modern cement is made of limestone, silicon, calcium, and often aluminum and iron.
The type of cement used in almost all concrete is Portland cement. Portland cement has been around since 1824. The name Portland does not refer to a brand name, as many might think. The original inventor, Joseph Aspdin, was a British bricklayer and named his new invention “Portland" because its color reminded him of the color of the natural limestone on the Isle of Portland which is a peninsula in the English Channel.
Although Portland cement is the main cement used in concrete, there are other types of cement. The three types that are often mentioned are Portland, blended, and hydraulic. All Portland and blended cements are actually hydraulic cement. What is hydraulic cement, though? Hydraulic cement is actually the generic term in the construction industry. It refers to any cement that will set and harden after it is combined with water. Most modern construction cements are hydraulic. There are six different types of hydraulic cement:
- Type GU: General Use
- Type HE: High Early Strength
- Type MS: Moderate Sulfate Resistance
- Type HS: High Sulfate Resistance
- Type MH: Moderate Heat of Hydration
- Type LH: Low Heat of Hydration
Portland cement is a particular type of hydraulic cement. Portland cement contains hydraulic calcium silicates. There are eight specific types of Portland cement that fall into categories ranging from Type I to Type V. Type I and Type IA are general purpose cements. Type II and Type IIA contain tricalcium aluminate, but no more than 8%. To compare to the hydraulic cement types, some of the Type II cements meeting the standard for the moderate heat of hydration type.
Type III and Type IIIA are similar to Type I cements. However, they have higher early strengths because they are ground finer. Type IV cements are used in special types of structures that require a small amount of heat to be generated from hydration. Type IV cements develop their strength over a longer period of time when compared to other types. Finally, Type V cement has a high sulfate resistance which means it contains no more than 5% tricalcium aluminate.
The third type of cement is blended cement. Blended cement is also hydraulic cement and is made by mixing two or more materials. Usually the primary materials used in blended cement are Portland cement and slag cement. Fly ash, silica fume, calcined clay, pozzolan, and hydrated lime are also used. There are two main types of blended cement:
- Type IS (X): Portland blast furnace slag cement
- Type IP (X): Portrland-pozzolan cement
The X represents the amount of the second material that is in the mixture.
The reason that there are different types of cements is not only required because of the different uses of the cement, but also because of the type of materials available differ by location. Many of the types described above actually cross-over between the different categories. This allows for flexibility in particular construction projects. For example, different pozzolans and slag are available in different regions. As long as the desired properties of the concrete can be achieved usually, there is flexibility in the final choice of cement that is used.