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 basically a mixture of aggregates and paste. The aggregates are sand and gravel or crushed stone; the paste is water and portland cement. 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. 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.
So, 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?
Materials that contain appropriate amounts of calcium compounds, silica, alumina and iron oxide are crushed and screened and placed in a rotating cement kiln. Ingredients used in this process are typically materials such as limestone, marl, shale, iron ore, clay, and fly ash.
The kiln resembles a large horizontal pipe with a diameter of 10 to 15 feet (3 to 4.1 meters) and a length of 300 feet (90 meters) 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 are at the lower end and all the materials in the kiln are heated to high temperatures that range between 2700 and 3000 Fahrenheit (1480 and 1650 Celsius). 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 the 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. A small amount of gypsum is added during the grinding process to control the cement's set or rate of hardening.
What does it mean to "cure" concrete?
Curing is one of the most important steps in concrete construction, because proper curing greatly increases concrete strength and durability. Concrete hardens as a result of hydration: the chemical reaction between cement and water. However, hydration occurs only if water is available and if the concrete's temperature stays within a suitable range. During the curing period-from five to seven days after placement for conventional concrete-the concrete surface needs to be kept moist to permit the hydration process. new concrete can be wet with soaking hoses, sprinklers or covered with wet burlap, or can be coated with commercially available curing compounds, which seal in moisture.
Flyash
 Enhancing Concrete Workability
The difference between fly ash and portland cement is apparent under a microscope. Fly ash particles are smaller and almost totally spherical in shape, allowing them to fill voids, flow easily, and blend freely in mixtures.
Additionally, when water is added to portland cement, it creates two products: a durable binder that glues concrete aggregates together and free lime. Fly ash reacts with this free lime to create more of the desirable binder.
| The “ball-bearing” effect of fly ash particles creates a lubricating action when concrete is in its plastic state. This creates benefits in: |
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Workability. Concrete is easier to place with less effort, responding better to vibration to fill forms more completely. |
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Ease of Pumping. Pumping requires less energy and longer pumping distances are possible. |
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Improved Finishing. Sharp, clear architectural definition is easier to achieve, with less worry about in-place integrity. |
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Reduced Bleeding. Fewer bleed channels decrease permeability and chemical attack. Bleed streaking is reduced for architectural finishes. |
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Reduced Segregation. Improved cohesiveness of fly ash concrete reduces segregation that can lead to rock pockets and blemishes. |
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Increasing Concrete Performance
In its hardened state, fly ash creates additional benefits for concrete, including:
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Higher Strength. Fly ash continues to combine with free lime, increasing compressive strength over time. |
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Decreased Permeability. Increased density and long term pozzolanic action of fly ash, which ties up free lime, results in fewer bleed channels and decreases permeability. |
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Increased Durability. Dense fly ash concrete helps keep aggressive compounds on the surface, where destructive action is lessened. Fly ash concrete is also more resistant to attack by sulfate, mild acid, soft (lime hungry) water, and seawater. |
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Reduced Sulfate Attack. Fly ash ties up free lime that can combine with sulfates to create destructive expansion. |
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Reduced Efflorescence. Fly ash chemically binds free lime and salts that can create efflorescence, and dense concrete holds efflorescence producing compounds on the inside. |
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Reduced Shrinkage. The largest contributor to drying shrinkage is water content. The lubricating action of fly ash reduces water content and drying shrinkage. |
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Reduced Heat of Hydration. The pozzolanic reaction between fly ash and lime generates less heat, resulting in reduced thermal cracking when fly ash is used to replace portland cement. |
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Reduced Alkali Silica Reactivity. Fly ash combines with alkalis from cement that might otherwise combine with silica from aggregates, causing destructive expansion. |
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Why test concrete?
Concrete is tested to ensure that the material that was specified and bought is the same material delivered to the job site. There are a dozen different test methods for freshly mixed concrete and at least another dozen tests for hardened concrete, not including test methods unique to organizations like the Army Corps of Engineers, the Federal Highway Administration, and state departments of transportation.
What are the most common tests for fresh concrete?
Slump, air content, unit weight and compressive strength tests are the most common tests.
- Slump is a measure of consistency, or relative ability of the concrete to flow. If the concrete can't flow because the consistency or slump is too low, there are potential problems with proper consolidation. If the concrete won't stop flowing because the slump is too high, there are potential problems with mortar loss through the formwork, excessive formwork pressures, finishing delays and segregation.
- Air content measures the total air content in a sample of fresh concrete, but does not indicate what the final in-place air content will be, because a certain amount of air is lost in transportation, consolidating, placement and finishing. Three field tests are widely specified: the pressure meter and volumetric method are ASTM standards and the Chace Indicator is an AASHTO procedure.
- Unit weight measures the weight of a known volume of fresh concrete.
Compressive strength is tested by pouring cylinders of fresh concrete and measuring the force needed to break the concrete cylinders at proscribed intervals as they harden. According to Building Code Requirements for Reinforced Concrete (ACI 318), as long as no single test is more than 500 psi below the design strength and the average of three consecutive tests equals or exceed the design strength then the concrete is acceptable. If the strength tests don't meet these criteria, steps must be taken to raise the average.
Can it be too hot or too cold to place new concrete?
Temperature extremes make it difficult to properly cure concrete. On hot days, too much water is lost by evaporation from newly placed concrete. If the temperature drops too close to freezing, hydration slows to nearly a standstill. Under these conditions, concrete ceases to gain strength and other desirable properties. In general, the temperature of new concrete should not be allowed to fall below 50 Fahrenheit (10 Celsius) during the curing period.
What is air-entrained concrete?
Air-entrained concrete contains billions of microscopic air cells per cubic foot. These air pockets relieve internal pressure on the concrete by providing tiny chambers for water to expand into when it freezes. Air-entrained concrete is produced through the use of air-entraining portland cement, or by the introduction of air-entraining agents, under careful engineering supervision as the concrete is mixed on the job. The amount of entrained air is usually between 4 percent and 7 percent of the volume of the concrete, but may be varied as required by special conditions.
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 6 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 6 days, and
- An air content of 6 percent (if concrete will be subject to freezing and thawing).
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 (.4 cm in 3 meters). 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.
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 1/2 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 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.
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.
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.
What is 3,000 pound concrete?
It is concrete that is strong enough to carry a compressive stress of 3,000 psi (20.7 MPa) 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.
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 effected by adding more water.
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.
To remove blood stains, for example, wet the stains with water and cover them with a layer of sodium peroxide powder; let stand for a few minutes, rinse with water and scrub vigorously. Follow with the application of a 5 percent solution of vinegar to neutralize any remaining sodium peroxide.
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 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.
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. The United States uses the specification prepared by the American Society for Testing and Materials-ASTM C-150 Standard Specification for Portland Cement. There are a few other countries that also have adopted this as their standard, 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." The European Cement Association located in Brussels, Belgium, publishes a book titled "Cement Standards of the World."
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.
Are there different types of portland cement?
Though all portland cement is basically the same, 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, or when heat build-up is a concern.
- 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 and IIIA are cements used to make air-entrained concrete. They have the same properties as types I, II, and III, except that they have small quantities of air-entrained materials combined with them.
White portland cement is made from raw materials containing little or no iron or manganese, the substances that give conventional cement its gray color.
What is natural cement?
Natural cements are hydraulic cements
produced by mining natural deposits of limestone and clay with a specific
chemical composition within a narrow range. When heated in a kiln,
and ground to a fine powder, a type of cement is produced, that, like
portland cement, sets and hardens when mixed with water through chemical
reactions. Strength and uniformity of natural cements are lower than
for portland cements, but these are much more historically accurate
materials for use on many historic restoration projects, which is
their primary application. Natural cements were extensively used in
19th and early 20th century construction, and many historic structures
were built with these materials (see Table below).
However, with improved technology for producing portland cements,
sales of natural cements began to decline in the late 1800s (see
Figure below), stopping entirely by the mid-1970s.
Brief History The first national specification
for cements was issued in 1904 by the American Society for Testing
and Materials, now ASTM International. It included requirements
for both portland cement and natural cement. By 1909, specifications
for natural cement (C 10) and portland cement (C 9) had been separated
and continued to be developed to serve the construction industry.
ASTM C 9 was replaced in 1941 by C 150, but C 10 continued to be
updated and revised until 1976. In 1978, the specification was discontinued,
as natural cements were no longer being produced in the U.S.
ASTM Committee C01 began reevaluating the withdrawn standard in
2004 to meet the demand for historically accurate cement for reconstruction
purposes. A task group updated the specification to meet modern
standards formats and submitted the item for ballot through ASTM’s
consensus standards development process. In October 2006, the standard
was reissued as C 10-06. Architects and engineers working on historic
preservation projects have a new specification to refer to when
natural cement is desired.
Natural Cement Requirements
Like specifications for other cements, C 10 sets chemical and physical
requirements, although with some modifications compared to modern
standards like C 150, C 595, or C 1157. For example, insoluble residue
is limited to a minimum of 2% in C 10, compared to a maximum of
0.75% in C 150. This requirement assures that the natural cement
is not over-processed in the kiln.
Other differences in requirements include a modified C 109 mortar
composition for specimens tested for compressive strength, and a
modified composition for autoclave expansion (soundness) specimens.
These compositions follow the historic compositions traditionally
used in the specification (in 1976 and prior), allowing for easier
comparison with properties of cements used when the historic structures
were originally built.
Another unique feature of the specification is the requirement for
the purchaser and the manufacturer to agree on the fineness requirement.
This allows the fineness to be chosen to match the cement used in
the structure under renovation—an important consideration
in historic preservation.
A copy of ASTM C 10 can be obtained at ASTM
International’s Web site.
Natural cement was used in the construction of thousands
of historic architectural and engineering structures. The table
below lists a few of the better-known examples.
Historic Structures
Made with Natural Cement |
| Canals |
Capitols |
| Over 150 canal systems, including: |
• California |
| • Erie |
• Colorado |
| • Chesapeake |
• Georgia |
| • Ohio, Delaware and Hudson |
• Iowa |
| • Ohio River |
• Illinois |
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• Indiana |
| Water Systems |
• Kansas |
| • New York City |
• Massachusetts |
| • Philadelphia |
• Michigan |
| • Boston |
• Minnesota |
| • Washington, DC |
• New York |
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• Texas |
| Military Fortifications |
• U.S. Capitol |
| 51 Third System forts, including: |
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| • Fort Sumter, S.C. |
Museums |
| • Fort Adams, Newport, R.I. |
• Smithsonian Institution National Portrait Gallery |
| • Fort Warren, Boston Harbor |
• Smithsonian Castle |
| • Fort Hamilton, New York Harbor |
• American Museum of Natural History |
| • Fort Trumbull, New London, Conn. |
• National Building Museum |
| • Fort Washington, Md. |
|
| • Fort Jefferson, Gulf of Mexico |
Other Government Buildings |
| • Fort Taylor, Key West, Fla. |
• Baltimore, Md., City Hall |
| • Fort Gaines, Mobile, Ala. |
• Richmond, Va., Old City Hall |
| • Fort Point, San Francisco, Calif. |
• Milwaukee, Wis, City Hall |
| • Fortress Alcatraz, San Francisco, Calif |
• U.S. Treasury Dept. |
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• Eisenhower Executive Office Building, Washington,
DC |
| Monuments |
• Buffalo, N.Y., County Building |
| • Washington Monument (lower 150 feet) |
• Fort Worth, Texas, Courthouse |
| • Statue of Liberty Pedestal |
• Dallas, Texas, Old Red Courthouse |
| • Confederate Monument, Savannah, Ga. |
• St. Louis, Mo., Old Post Office |
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• Milwaukee, Wis., Library |
| Bridges |
|
| • Brooklyn Bridge |
|
| • Roebling Suspension Bridge, Cincinnati, Ohio |
|
| • Smithfield Street Bridge, Pittsburgh, Pa. |
|
| • High Bridge, New York, N.Y. |
|
| • Stone Arch Bridge, Minneapolis, Minn. |
|
| • Eads Bridge, St. Louis, Mo. |
|
Source: Edison, M. P., “The American Natural Cement Revival,”
Standardization News, ASTM International, West Conshohocken,
PA, Vol. 34, No. 1, pages 34 to 39, January 2006.
 |
| Figure 1. Decline of natural cement production and rise of
portland cement production around the turn of the 20th century
(data from R. W. Lesley; J. B. Lober, and G. S. Bartlett,
History of the Portland Cement Industry, International
Trade Press, Chicago, 1924.) |
How is white cement different and why is it used in decorative concrete?
 There are only slight chemical and physical differences between
gray portland cement and white portland cement. These differences
are due to raw material differences and sometimes, though not always,
slight differences in manufacturing. The goal is to minimize the
oxides (particularly iron and manganese) that impart the grayish
color normally associated with portland cement.
 |
| Source: “What is White Cement,”
Concrete Technology Today, PL991, Portland Cement Association,
April 1999. |
Cements have difference strengths based on their physical and chemical
composition. White cement, which is typically ground a little finer
than gray cement, often has characteristics that make it behave
somewhat intermediate between general use cements like ASTM C 150
Type I or ASTM C 1157 Type GU and their high early strength counterparts,
C 150 Type III or C 1157 Type HE.
 White
portland cement is perfect for decorative concrete because it provides
a neutral tinting base. This allows for the brightest, richest colors
to be achieved in concrete, and requires the least amount of pigment
to accomplish the chosen color. Conversely, where muted colors are
wanted, white cement allows for the cleanest pastels. And, where
no color at all is desired, white cement allows for bright white
concrete to be made. It is important to note that sand acts like
a pigment, too, so sand color should be considered when formulating
the concrete. White sands, either manufactured or natural, are good
choices for decorative concrete.
White cement could be used for any of the same applications where
gray cement is used, but it rarely is. As a premium product, it
is best suited to specialty applications where appearance is a high
priority, like precast and stamped concrete. It’s also used
to make colored mortars and stucco finish coats.
ASTM C 150, Specification for Portland
Cement
ASTM C 1157, Performance
Specification for Hydraulic Cement
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