Cement (sometime referred to as Portland cement or hydraulic cement) is one component of concrete. Cement is to concrete what flour is to a loaf of bread. Concrete is basically a mixture of 2 components: aggregates and paste. The paste, comprised of cement and water, binds the aggregates (sand and stone or gravel) into a rocklike mass as the paste hardens because of the chemical reaction of the cement and water. This reaction is called hydration.

Concrete "shrinks" slightly as it hardens. A normal shrinkage rate is approximately 1/8" per 100 linear feet. This shrinkage is caused by loss of excess water from the mix. Obviously, the "wetter" the mix, the higher the shrinkage rate. Control joints should be placed in the concrete at intervals equal to 2.5 times (in feet) the thickness of the slab. For example, a slab 4" thick should have control joints every 10 feet.

The surface of freshly placed concrete should be kept moist for at least 7 days. Contrary to popular belief, concrete should not be allowed to "dry out." If the concrete is allowed to "dry out," the ultimate strength gain will be considerably less than its designed strength. Proper curing will also minimize the potential of cracking. The simplest method of moist curing is wetting the surface of the concrete, then covering it with polyethylene.

These defects are generally a result of improper finishing of the concrete. As discussed earlier, the prime factor affecting concrete strength is water/cement ratio. If excess water is added to the surface of the concrete during placement and finishing, the water/cement ratio on the surface may be drastically increased. This condition greatly reduces the strength of the concrete on the surface. Unfortunately, this is where the wear takes place.

The term slump simply refers to the consistency of the concrete in a plastic state (prior to hardening). Slump is a measure of how wet or stiff the concrete is. Obviously, the more water used in producing the concrete, the wetter (or higher) the slump will be. Again, excessive water causes a dramatic loss of strength. Applications exist where higher slump (wetter) concrete is necessary because of difficulty in placement or specification requirements. These higher slumps can be attained through the use of water reducing admixtures (chemical additives).

Concrete's most rapid period of strength gain occurs in the first 7 days. The accepted time standard for measuring concrete strength is 28 days. However, concrete will continue to gain strength for long periods of time (months, even years!) As long as moisture is present to continue the chemical process of hydration.

Fly ash is a by-product from coal fired electric power generating plants. The inorganic or mineral constituents of the coal, such as clay, quartz and shale, fuse and chemically recombine during burning to produce various crystalline and glassy phases of fly ash. The fly ash is entrained in the flue gas and cools into spherical, usually hollow shaped particles. These particles are collected in electrostatic precipitators or bag houses and the gradation, or fineness, of the fly ash can be controlled by how and where the particles are collected. Fly ash reacts with the free lime generated by cement hydration to form cement-like compounds, which increase the strength and reduce the permeability of concrete.

The Setting time of Cement is the time required by concrete to harden and set itself according to the fromwork in which it has been placed. The amount of time taken by concrete to set is 7 hours to 10 hours depending on the conditions under which the concrete is placed in the formwork. But this does not mean that the formwork may be removed after 7 hours the initial setting of concrete does not impart enough strength for it to withstand its own self load so the forwork should be removed only after 7 to 10 days of proper curing.

No, the color of cement does not effect the strength of cement in anyway. It just gives a proper finish to the concrete. There is a general misconception that the cement that is darker in color has greater strength, this is not true and the cement that is lighter in color not only has the same strength but also has a more pleasing finish than the cement that is darker in color.

This is not true. Infact the advantages of addition of fly ash are many, a few of which are listed below.

• It reduces the permeability of concrete thus helping in making the     concrete structures water proof.
• The addition of fly ash also helps in better bonding with the reinforcement.
• It also increases the density of concrete by reducing the air voids.
• It gives a pleasing appearance to the finished concrete.

The water cement ratio is very important with regards to the strength of concrete. The strength of concrete is inversely proportional to the water cement ratio i.e. with the increase in water content the compressive strength of concrete decreases and vice versa.