FAQ provided by Metrocem Cement Limited

Portland cement is a finely ground mixture of calcium aluminates and silicates capable of setting and hardening by chemical reaction with water. It is a binding material used in engineering constructions and is obtained by the calcinations of calcareous and argillaceous materials. The calcined product which is called clinker is then finely pulverized by grinding in to a very fine powder and is finally mixed with gypsum to obtain cement.

Joseph Aspdin, an English mason who patented the product in 1824, named it Portland Cement, because it produced a concrete that resembled the color od the natural lime stone quarried on the Isle of Portland, a peninsula in the English Channel.

Clinker is produced in Kiln by mixing and burning at some regulated temperature (about 1450° C) measured proportion of which is lime stone & clay.

Fly ash (pulverized – fuel ash) is a material generated by combustion of pulverized coal in thermal power stations and precipitated in fabric bag filters or in electrostatic precipitators. The particle size of fly ash 0.05 to 0.2 mm. Fly ash is a vitrified product in which the oxides and Sio2 are the active hydraulic components & have the characteristic those are found in Clinker. Quality of Portland fly ash cement depends on the quality of fly ash that is mixed. The quality of the coal and the burning conditions in the boiler influences the quality of fly ash. Ash mixing procedure & ration is the major deciding factors in producing good quality Portland fly ash cement.

Blast furnace slag is a by product in the manufacturing of pig iron. Blast furnace slag shall consist of at least two-thirds by mass of Calcium Oxide (CaO), Magnesium Oxide (MgO) and Silicon dioxide (SiO2).The remainder contains Aluminium Oxide (Al2O3) together with small amounts of other compounds.

It is the time required to change the stage of cement paste from fluid to a rigid One.

Compressive strength is the usual primary requirement of good concrete in it’s hardened state and this is an overall measure for the quality of cement and concrete. In order to ascertain the compressive strength of cement and concrete, a predetermined ratio of cement, aggregate and water are thoroughly mixed and specimens are cast. These specimens are cured and stored in specified environmental conditions prior to testing it’s compressive strength at various ages like 3 days, 7 days, 28 days etc. Many of the desirable properties of concrete like durability, impermeability, abrasion resistance etc., are highly influenced by compressive strength. Water cement ratio, workability and maximum size of aggregate also affects the compressive strength of the concrete.

Soundness provides an index of potential delayed expansion caused by hydration of Calcium oxide, or Magnesium oxide, or both when present in Portland cement. This, in turn, provides information on the volume stability of concrete when Portland cement is used.

The two main raw-materials used in making Portland cement are Clinker and Gypsum.

Gypsum is added to the cement production process mainly for the purpose regulating its setting time. It prevents flash setting and makes the Concrete workable for hours. In addition ,it also influences other cement properties like grind ability, sensitivity to storage, volume stability and development of strength.

In blended cement production, some portion of the clinker component is replaced with a suitable pozzolan or a slag or limestone or fly ash etc. The use of these mineral will improve the durability and the pore structure of the hardened concrete by lowering its lime content and porosity as a consequence of the enhanced development of calcium silicate hydrates.

False set is used to describe the premature stiffening of the cement paste which some time occurs within 10-15 minutes of mixing with water which is caused by dehydration of Gypsum dehydrated due to high temperature during cement grinding process. However the plasticity of the paste is regained on remixing.

The term ‘drying shrinkage’ can be defined as the decrease in length of the cement test specimen, where the decrease is caused by any factor other than externally applied forces under stated environmental conditions. The terms includes the net effect of a variety of phenomena, including hydration of the cement, tending to bring about both increase and decrease in length during the period in which the cement test specimens are stored under stated environmental conditions.

The term ‘drying shrinkage’ can be defined as the decrease in length of the cement test specimen, where the decrease is caused by any factor other than externally applied forces under stated environmental conditions. The terms includes the net effect of a variety of phenomena, including hydration of the cement, tending to bring about both increase and decrease in length during the period in which the cement test specimens are stored under stated environmental conditions.

This indicates the volume of air (and other gases, if any) in a freshly mixed cement mortar, expressed as a percentage of total volume of the mortar. The test for air content is essential to assess the cement for its requirement with respect to air-entraining requirement.

Special cements are usually developed and produced to meet performance and durability requirement in particular.

• Improved strength development.
• Increased resistance to chemical attack.
• Improved compatibility with reactive – aggregates.
• Suitability for use at elevated temperature and pressures.
• Suitability for use in special application.
• Applicability in architectural purposes.

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. Concrete is a hard, strong construction material consisting of sand, conglomerate gravel, pebbles, broken stone, or slag in a mortar or cement matrix. Concrete gets stronger as it gets older. Portland cement is the generic term for the type of cement used in virtually all concrete. Cement compromises from 10 to 15 percent of the concrete mix, by volume. In the process called hydration, the cement and water harden and bind the aggregates into a rocklike mass. This hardening process continues for years that it brings the idea that concrete gets stronger and tougher it gets older.

Curing is a very important step in concrete construction. When concrete has been cured properly, it greatly increases its strength and durability. Through the result of hydration, concrete hardens. It is the chemical reaction between cement and water. Hydration occurs only if water is available and if the concrete’s temperature stays within a suitable range. During the curing period which is from five to seven days after the placement for conventional concrete, the concrete surface needs to be kept moist to allow the hydration process. The 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.

The compounds present in Portland cement react with water to form a cementations crystalline structure that adheres to the sand and aggregate. This helps in binding the mass together and increases its strength till it becomes very hard.

Hardening of cement is caused by some reactions described by the process of liberation of heat. This is referred to as heat of hydration. In mass concreting jobs like, dams where dissipation of heat is not possible, the temperature of concrete rises. On subsequent cooling, cracks develop. Hence it is very necessary to use cement with low heat of hydration in massive structures.

1. Low heat of hydration. Due to which thermal crack does not develop in the concrete.
2. PCC (Portland Composite Cement) = OPC (Ordinary Portland Cement) + Fly ash

1st Step Reaction:

2nd Step Reaction: