FAQ

Although the terms cement and concrete are often used interchangeably, cement is actually an ingredient of concrete. Concrete is a mixture of aggregates and paste. The aggregates are sand and gravel or crushed stone; the paste is water and portland cement.

Cement comprises from 10 to 15 percent of the concrete mix, by volume. Through a process called hydration, the cement and water harden and bind the aggregates into a rocklike mass. This hardening process continues for years, meaning that concrete gets stronger as it gets older.

Portland cement is not a brand name, but the generic term for the type of cement used in virtually all concrete, just as stainless is a type of steel and sterling a type of silver. Therefore, there is no such thing as a cement sidewalk, or a cement mixer; the proper terms are concrete sidewalk and concrete mixer.

Portland cement is a hydraulic cement which means that it sets and hardens due to a chemical reaction with water. Consequently, it will harden under water.

Cement manufacturers mine materials such as limestone, shale, iron ore, and clay. The rock is then crushed and screened, and placed in a cement kiln. After being heated to extremely high temperatures, these materials form a small ball called “clinker” that is very finely grounded to produce portland cement.

For each tonne of material that goes into the feed end of the kiln, two thirds of a tonne comes out the discharge end as clinker. Manufacturers often add gypsum and/or limestone as part of the grinding process.

Lime and silica make up about 85 percent of the ingredients of cement. Other elements include alumina and iron oxide. The rotating kiln that cooks the materials resembles a large horizontal pipe with a diameter of 10 to 15 feet and a length of 300 feet or more. One end is raised slightly. The raw mix is placed in the high end and as the kiln rotates the materials move slowly toward the lower end. Flame jets at the lower end heat all the materials in the kiln to high temperatures that range between 2,700 and 3,000 degrees Fahrenheit.

This high heat drives off, or calcines, the chemically combined water and carbon dioxide from the raw materials and forms new compounds (tricalcium silicate, dicalcium silicate, tricalcium aluminate and tetracalcium aluminoferrite).

Though all portland cement is similar, eight types of cement are manufactured to meet different physical and chemical requirements for specific applications:

• Type I is a general purpose portland cement suitable for most uses.
• Type II is used for structures in water or soil containing moderate amounts of sulfate.
• Type II(MH) is a moderately sulfate resistant cement that also generates moderate heat during curing.
• Type III cement provides high strength at an early state, usually in a week or less.
• Type IV moderates heat generated by hydration that is used for massive concrete structures such as dams.
• Type V cement resists chemical attack by soil and water high in sulfates.
• Types IA, IIA, I(MH)A and IIIA are cements used to make air-entrained concrete. They have the same properties as Types I, II, II(MH), and III, except that they have small quantities of air-entraining materials combined with them.

White portland cement is made from the same raw materials as regular portland cement, but containing little or no iron or manganese, the substances that give conventional cement its grey color.

Some portland cements meet the requirements for multiple cement types. For example, some cements are sold as Type I/II cements, which means that those cements meet all of the specification requirement in ASTM C150 (or AASHTO M 85) for both Type I and Type II.

Blended cements are another type of hydraulic cement, like portland cement. Blended hydraulic cements are produced by intergrinding or blending two or more types of fine materials, often portland cement and one (or two) of the following: limestone, slag cement, or pozzolans like fly ash, silica fume, or calcined clay.

Blended hydraulic cements must conform to the requirements of ASTM C595 (or AASHTO M 240), Standard Specification for Blended Hydraulic Cements.

Blended cements are used in all aspects of concrete construction in the same manner as portland cements. Blended cements can also be used as the sole cementitious material in concrete or they can be used in combination with other supplementary cementitious materials added at the concrete plant.

ASTM C595 recognises four classes of blended cements:

• Type IL, Portland-limestone cement
• Type IP, Portland-pozzolan cement
• Type IS, Portland blast-furnace slag cement
• Type IT, Ternary blended cement

Blended cements can be tested to verify special properties like low or moderate heat development, and moderate or high sulfate resistance. If this is the case, suffixes are added to the cement type names:  LH, MH, MS, or HS. Air-entraining blended cement can also be produced.

The easiest way to add strength is to add cement. The factor that most predominantly influences concrete strength is the ratio of water to cement in the cement paste that binds the aggregates together. The higher this ratio is, the weaker the concrete will be and vice versa. Every desirable physical property that you can measure will be adversely affected by adding more water.

Many materials have no effect on concrete. However, there are some aggressive materials, such as most acids, that can have a deteriorating effect. The first line of defense against chemical attack is to use quality concrete with maximum chemical resistance, followed by the application of protective treatments to keep corrosive substances from contacting the concrete.

Principles and practices that improve the chemical resistance of concrete include using a low water-cement ratio, selecting a suitable cement type (such as sulfate-resistant cement to prevent sulfate attack), using suitable aggregates, water and air entrainment. A large number of chemical formulations are available as sealers and coatings to protect concrete from a variety of environments; detailed recommendations should be requested from manufacturers, formulators or material suppliers.