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, di-calcium silicate, tri-calcium aluminate and tetra-calcium 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.

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:

Why do concrete surfaces flake and spall?
  • 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 hydraulic cement, which means that it sets and hardens due to a chemical reaction with water. Consequently, it will harden under 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 shot blasting, grinding, scabbing, planning 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 bloodstains, 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.

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 OPC Cement?

OPC means Ordinary Portland Cement, which contain mixture of Portland clinker and Gypsum ground to a very fine powder.

What is PPC?

PPC means Portland Pozzolana Cement. It is manufactured by blending a mixture of Ordinary Portland cement and Pozzolana materials such as Fly Ash, in proportions not less than 15% and not more than 35% by weight of cement. The fineness of PPC will be greater than OPC.

What is the difference between OPC and PPC Cement?

Addition of Pozzolanic material such as fly ash in the cement manufacture by 15% to 35% is the only difference between OPC and PPC.

What is the setting time for OPC and PPC?

Same for both OPC and PPC. i.e. initial setting time 30 minutes and final setting time 600 Minutes. (10 hours)

What is the difference between 43 and 53 Grade?
A. 3 Days Strength 23 Mpa (min) 27 Mpa (min)
B. 7 Days Strength 33 Mpa (min) 37 Mpa (min)
C. 28 Days Strength 43 Mpa (min) 53 Mpa (min)

The differences are due to change in the potential phase composition of the clinker. The heat of hydration of 43 Grade is less than 53 Grade.

Why there is no grade for PPC?

Since gradation of cement is mainly due to the difference in potential phase composition of clinker such as C2S, C3S, etc., the addition of Pozzolana to cement manufactured will not hold good for any cement gradation for PPC. But PPC should meet out OPC 33 Grade strength on curing for 28 days as per BIS.

Why do you use Admixtures in concrete?

Admixtures enhance the properties of concrete or mortar in the plastic state and improve durability in the hardened state. Admixtures increase the efficiency of cementitious materials and/or improve the economy of the concrete mix.

What are Superplasticizers / High Range Water-Reducers

Superplasticizers are liquid chemical admixtures used in concrete because of its ability to perform as a highly effective wetting agent. Superplasticizers increase water reduction to achieve low water/cement ratios, increase strength gain, increases slump for ease of placement and creates a denser and more durable concrete.

What are Accelerators?

Accelerators are used to reduce the set time of concrete and increase early strength gain. Although the mechanism is not fully understood, it is believed that accelerating admixture increases the reactivity and hydration of C3S and C2S. The reaction is catalytic in nature, forming CSH (calcium silica hydrate) gel and increasing rate of reaction. This results with increased reaction of Hydrates, especially at early ages.

What are Retarders?

Retarders are admixtures that cause a decrease in the rate of cement hydration and lengthen the time of setting. Retarders remove the tendency of some cement to exhibit false set and counter the accelerating effect of high temperatures. They improve the workability and the finishability in hot weather and increase the transporting distance of the ready-mixed concrete. Retarders help to reduce the possibility of early dry-shrink cracking and reduce the maximum temperature rise in mass concreting, by extending the heat-dissipation period

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.

Why test concrete?

Due to their non-polarity, oils do not easily adhere to other substances. This makes oils useful as lubricants for various engineering purposes. Mineral oils are more suitable than biological oils, which degrade rapidly in most environmental conditions.

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. You can reduce the water/cement ratio by adding water-reducing admixtures or superplasticizers

How much does concrete weigh?

Fresh concrete weighs about 3900 pounds per cubic yard using normal weight aggregates. It weighs about 3500 to 3700 pounds per cubic yard when cured.

What are the most common tests for fresh concrete?
  • Slump i2s 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. 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 , 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.
What does 28 days 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.

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.

What is alkali-silica reactivity (ASR)?H

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.

What is fly ash?

Flyash is a byproduct of coal burning power plants. When used properly in concrete, flyash is an affordable mineral admixture that improves the quality of the mix. Concrete with flyash will typically have a higher ultimate strength, although early strength may be lower than with straight cement mixes. Sometimes users will complain about "too much flyash in the mix." That is rarely the problem. More often the problem is not enough cement in the mix.

Is concrete environmentally friendly?

Yes. Concrete is chemically inert and has no components made from petroleum products or other chemicals that might have an effect on our water supply due to rainwater runoff.

What is ready mix?

Ready mix, also known as ready mixed concrete, is concrete that is delivered in trucks that agitate and/or mix the concrete on the way to the job or at the job site. The concrete is delivered in a plastic or unhardened state.