Cement & Concrete Basics FAQs

Frequently Asked Questions

• What is the difference between cement and concrete?

  Although the terms cement and concrete often are 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.

• How is portland cement made?

  Cement manufacturers mine materials such as limestone, shale, iron ore, and clay, crushed and screened the rock, and place it 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.

  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). For each ton of material that goes into the feed end of the kiln, two thirds of a ton comes out the discharge end, called clinker. This clinker is in the form of marble sized pellets. The clinker is very finely ground to produce portland cement. Manufacturers often add gypsum and/or limestone during the grinding process. 

• Are there different types of portland cement?

  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 gray color.

  Some portland cements meet 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.

  See also:  What are blended cements?

   

• What are blended cements?

  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: 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 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 recognizes 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.

• What is portland-limestone cement?

  Portland-limestone cement (PLC) is a type of blended cement specified under ASTM C595 (or AASHTO M 240). In the US and Canada, PLCs are made with portland cement and between 5% and 15% fine limestone. Through particle packing and chemical effects, this type of cement has performance comparable to Type I portland cement with about 10% lower greenhouse gas emissions.

  PLCs with special properties like moderate heat of hydration or sulfate resistance are also available.

  This type of cement has been common internationally for decades but is relatively new to North America. Many state DOTs have adopted provisions to use PLC and they are accepted by ACI codes and specifications like ACI 301 and ACI 318, building codes of the International Codes Council (ICC) (which many local building codes are based on) as well as specifications of the Federal Aviation Administration (FAA), and the American Institute of Architects’ (AIA) MasterSpec.
  
  

  Status of acceptance of portland-limestone cement in state DOT specifications.
  
  Click here to download this map. 

  See SN3148 for more information

  
  
  
  

• Is there a universal international specification for portland cement?

  Each country has its own standard for portland cement, so there is no universal international standard. CEMBUREAU, the European Cement Association located in Brussels, Belgium, publishes a book titled "Cement Standards of the World." 

  The United States uses specifications developed by ASTM International, primarily ASTM C150 Standard Specification for Portland Cement (AASHTO M 85). Other standards used in the US include ASTM C595, Standard Specification for Blended Hydraulic Cements (AASHTO M 240), and ASTM C1157, Standard Performance Specification for Hydraulic Cement. There are other countries that also have adopted these as standards, however, there are countless other specifications. Unfortunately, they do not use the same criteria for measuring properties and defining physical characteristics, so they are virtually "non-translatable." Some cements conforming to other standards may meet US specifications, but they would have to be tested to confirm that with methods referenced in US standards.

• How do you control the strength of concrete?

  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. 

• What is alkali-silica reactivity (ASR)?

  Alkali-silica reactivity is an expansive reaction between reactive forms of silica in aggregates and potassium and sodium alkalis, mostly from cement, but also from aggregates, pozzolans, admixtures and mixing water. External sources of alkali from soil, deicers and industrial processes can also contribute to reactivity. The reaction forms an alkali-silica gel that swells as it draws water from the surrounding cement paste, thereby inducing pressure, expansion and cracking of the aggregate and surrounding paste. This often results in map-pattern cracks, sometimes referred to as alligator pattern cracking. ASR can be avoided through 1) proper aggregate selection, 2) use of blended cements, 3) use of proper pozzolanic materials and 4) contaminant-free mixing water. 

• How do you protect a concrete surface from aggressive materials like acids?

  Many materials have no effect on concrete. However, there are some aggressive materials, such as most acids, that can have a deteriorating effect on concrete. 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. 

• What are the decorative finishes that can be applied to concrete surfaces?

  Color may be added to concrete by adding pigments-before or after concrete is place-and using white cement rather than conventional gray cement, by using chemical stains, or by exposing colorful aggregates at the surface. Textured finishes can vary from a smooth polish to the roughness of gravel. Geometric patterns can be scored, stamped, rolled, or inlaid into the concrete to resemble stone, brick or tile paving. Other interesting patterns are obtained by using divider strips (commonly redwood) to form panels of various sizes and shapes ­ rectangular, square, circular or diamond. Special techniques are available to make concrete slip-resistant and sparkling. 

• How do you remove stains from concrete?

  Stains can be removed from concrete with dry or mechanical methods, or by wet methods using chemical or water.

  Common dry methods include sandblasting, flame cleaning and shotblasting, grinding, scabbing, planing and scouring. Steel-wire brushes should be used with care because they can leave metal particles on the surface that later may rust and stain the concrete.

  Wet methods involve the application of water or specific chemicals according to the nature of the stain. The chemical treatment either dissolves the staining substance so it can be blotted up from the surface of the concrete or bleaches the staining substance so it will not show. 
  
  
  

• What is 3,000 pound concrete?

  It is concrete that is strong enough to carry a compressive stress of 3,000 psi at 28 days. Concrete may be specified at other strengths as well. Conventional concrete has strengths of 7,000 psi or less; concrete with strengths between 7,000 and 14,500 psi is considered high-strength concrete. 

• What does 28-day strength mean?

  Concrete hardens and gains strength as it hydrates. The hydration process continues over a long period of time. It happens rapidly at first and slows down as time goes by. To measure the ultimate strength of concrete would require a wait of several years. This would be impractical, so a time period of 28 days was selected by specification writing authorities as the age that all concrete should be tested. At this age, a substantial percentage of the hydration has taken place. 
  
  

• Will concrete harden under water?

  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. 

• Why do concrete surfaces flake and spall?

  Concrete surfaces can flake or spall for one or more of the following reasons:

  • In areas of the country that are subjected to freezing and thawing the concrete should be air-entrained to resist flaking and scaling of the surface. If air-entrained concrete is not used, there will be subsequent damage to the surface
  • The water/cement ratio should be as low as possible to improve durability of the surface. Too much water in the mix will produce a weaker, less durable concrete that will contribute to early flaking and spalling of the surface.
  • The finishing operations should not begin until the water sheen on the surface is gone and excess bleed water on the surface has had a chance to evaporate. If this excess water is worked into the concrete because the finishing operations are begun too soon, the concrete on the surface will have too high a water content and will be weaker and less durable. 

• How can you tell if you're getting the amount of concrete you're paying for?

  The real indicator is the yield, or the actual volume produced based on the actual batch quantities of cement, water and aggregates. The unit weight test can be used to determine the yield of a sample of the ready mixed concrete as delivered. It's a simple calculation that requires the unit weight of all materials batched. The total weight information may be shown on the delivery ticket or it can be provided by the producer. Many concrete producers actually over yield by about ½ percent to make sure they aren't short-changing their customers. But other producers may not even realize that a mix designed for one cubic yard might only produce 26.5 cubic feet or 98 percent of what they designed. 
  
  

   

• Why does concrete crack?

  Concrete, like all other materials, will slightly change in volume when it dries out. In typical concrete this change amounts to about 500 millionths. Translated into dimensions-this is about 1/16 of an inch in 10 feet. The reason that contractors put joints in concrete pavements and floors is to allow the concrete to crack in a neat, straight line at the joint when the volume of the concrete changes due to shrinkage. 

• What are recommended mix proportions for good concrete?

  Good concrete can be obtained by using a wide variety of mix proportions if proper mix design procedures are used. A good general rule to use is the rule of 6's:

  • A minimum cement content of six bags per cubic yard of concrete,

  • A maximum water content of 6 gallons per bag of cement,

  • A curing period (keeping concrete moist) a minimum of six days, and

  • An air content of 6 percent (if concrete will be subject to freezing and thawing).