FAQ

What is cement?
  • A  finely ground grey powder
  • An inorganic compound of calcareous and argillaceous materials
  • Used in combination with sand, aggregates, water and admixtures to get a versatile and economical construction material called “concrete”
  • It is the main compound which binds all ingredients and gives strength to the concrete
What are the raw materials used to manufacture cement?

Calcareous (limestone obtained from mines) and argillaceous (clay or bauxite) materials and iron ore are the most commonly used raw materials in the manufacture of cement.

How is cement manufactured?

The cement manufacturing process has four stages:

  • Extraction of limestone from the mines
  • The blending of ground limestone, clay or bauxite and iron ore or laterite in the right proportions which is then clinkered in rotary kilns at a high temperature to form “clinker”
  • Grinding of clinker with gypsum to form cement
  • Finally, storing it in silos, testing and dispatch
What is clinker?

Clinker is made by heating limestone with small quantities of corrective materials to 1400°C- 1500°C in a kiln. It is ground with gypsum to a powder to manufacture cement.

What is the function of gypsum?

Gypsum functions as an agent which controls the setting time of cement.

What are the major compounds present in cement?
  • Tri-calcium Silicate(C3S)
  • Di-calcium Silicate(C2S)
  • Tri-calcium Aluminate(C3A)
  • Tetra-calcium Alumino-Ferrite(C4AF)
What are the types of cement generally available in the market?

Commonly available cements are ORDINARY PORTLAND CEMENT(OPC) 53 GRADE conforming to IS 12269, ORDINARY PORTLAND CEMENT (OPC) 43 GRADE conforming to IS 8112, PORTLAND POZZOLANA CEMENT(PPC) conforming to IS 1489(PART I) AND PORTLAND SLAG CEMENT(PSC ) conforming to IS 455.

What are different types of Cement & Its BIS Specification?
S.No.SPECIFICATION OF CEMENTBIS SPECIFICATION NO.
OPC
133 Grade OPCIS 269 : 1989
243 Grade OPCIS 8112 : 1989
353 Grade OPCIS 12269 : 1987
Blended Cement
1Portland Slag Cement(PSC)IS 455 : 1989
2Portland Pozonnana Cement ( PPC-FlyAsh Based)IS 1489 (Part 1) : 1991
3PPC (Calcined Clay Based)IS 1489 (Part 2) : 1991
4Masonry CementIS 3466 : 1988
Special Cement
1High Alumina Cement for Structural UseIS 6452 : 1989
2Super Sulphate CementIS 6909 : 1990
3Rapid Hardening Portland CementIS 8041 : 1990
4White Portland CementIS 8042 : 1989
5Hydrophobic Portland CementIS 8043 : 1991
6Sulphate Resisting Portland CementIS 12330 : 1988
7Low Heat Portland CementIS 12600 : 1989
8Oil Well CementIS 8229 : 1986
What do the labels 53 Grade, 43 Grade and 33 Grade cement indicate?

Grades 53, 43 and 33 indicate the compressive strength of cements in Newton for one mm2 area i.e. Minimum Strength of 53 NEWTON/SQ.MM(N/SQ.MM) OR 53 MPa at 28 DAYS and similar minimum strength of 43 N/SQMM for OPC 43 GRADE at 28 DAYS.

What is “setting of cement”?

Setting is the stiffening of the cement paste which broadly refers to it changing from its fluid state to a rigid one.

What is “hardening of cement”?

Hardening is the gain of strength in cement.

What are the reasons for slow setting and quick setting of cement mortar/concrete?

Slow setting is due to:

  • Salts and chemicals in water, sand and aggregates
  • Application of less cement mortar/concrete
  • Cold weather conditions
  • High percentage of impurities seen usually in adulterated cement
  • Higher water to cement ratio

 

Quick setting is due to:

  • Addition of low quantity gypsum
  • Hot weather conditions
  • High cement fineness
How should cement be stored?

Proper storage of cement permits easy access for inspection and identification. Cement should be stored in suitable weather-tight structures to protect it from dampness. It should not be piled higher than ten bags in a stack and has to be arranged in header and stretcher fashion as far as possible. While removing the bags for use, the”First in, first out” rule should be applied.

What is mortar?

Mortar is a mixture of sand, cement and water.

What is concrete?

Concrete is a mixture of cement, water, aggregates(fine as well as coarse) and chemical/mineral admixtures.

What should be the quality of aggregate for good concrete?

Aggregate for concrete should be inert, dense, hard, durable, structurally sound, capable of developing good bond with the cement paste, weather resistant and unaffected by water.

What are the sources of aggregate?

Natural sand, gravel, pebbles, rocks crushed to the required size and manufactured aggregates like blast furnace slag are sources of aggregate. IS 456-2000 stipulates that the aggregates should conform to IS 383-1970.

What is the function of water in concrete?

It is with water that the cement undergoes the process of hydration to form a gel with binding property that covers the aggregates uniformly when mixed. However, water should be added proportionally for the required consistency since excess water is deterrent to concrete.

Can any type of water be used for concreting?

It is important that water used in concrete should be free from impurities and generally, potable quality is considered best. Bore water needs to be checked before being used because it often contains impurities which affect setting times and reduce the strength of concrete. Sea water is not suitable for reinforced concrete as it causes corrosion. IS456-1978 has established the maximum possible limits of solids in water used here.

What are the factors that affect the workability of concrete?

The factors that affect the workability of concrete are water content, aggregate type and grading, aggregate to cement ratio, presence of admixtures and fineness of the cement.

What is meant by batching?

Measurement of various ingredients of concrete like sand, metal and cement for each batch of concrete as per design is called batching. Volume batching and weight batching are the two types currently used.

What are the factors that affect the strength of concrete?

The factors that affect the strength of concrete are water-cement ratio, shape and size of aggregate, aggregate-cement ratio, degree of compaction, age of concrete, cement content and air entrainment and moisture in aggregate.

What is segregation of concrete?

Segregation of concrete can occur in two forms. In the first, the coarser particles tend to separate out while concrete is conveyed along a slope (for e.g. sloped roof and staircase waist slab) or non-homogeneous coarse aggregates settle below leaving the slurry with fine aggregate above. The second form of segregation occurs in wet mixes by separation of grout from the mix. This happens due to excess vibration of the concrete and dropping it from a height.

What are the types of cracks and the reasons for their development?

A crack develops whenever stresses in the structural component exceed its strength. This may be due to external forces, foundation settlement, thermal changes, chemical action, etc.

Plastering Cracks: Plastering cracks appear because of evaporation of water, increased thickness, very fine sand, low quality bricks, non-uniform plastering, improper seasoning of walls, use of incorrect proportions in the mix, improper filling of joints, insufficient curing or excess free lime or Magnesium oxide.

 

RCC cracks: RCC cracks due to insufficient reinforcement and cover, corroded steel, high slump, improper vibration, volume changes, excessive coarse aggregates, expansion of concrete, lack of sufficient quantity of cement, electrical conduits not placed at sufficient depth in the concrete.

What do you mean by M20, M30, and M40 concrete?

M20, M30 and M40 Grades of concrete indicate compressive strength of concrete in Newton for one mm2 area at 28 days. M20 is 20 N per mm2 (minimum) and similarly M30 is 30 N per mm2(minimum) and so on.

What is water-cement ratio?

The quantity of water used for mixing of concrete per unit volume related to the quantity of cement is called water-cement ratio and is expressed as the ratio of the weight of water to the weight of cement used in a unit volume of concrete.

What is curing?

Maintaining a favorable environment for the continuation of chemical reaction that is responsible for hardening of cement is called curing.

What methods are commonly employed to ensure sufficient moisture for curing?

Ponding, continuous sprinkling, covering with wet cloth, polyethylene sheets, applying sealing coats commonly known as “Curing compounds” are some of the methods usually employed for curing.

When should curing begin?

Curing of all exposed surfaces should be started as soon as possible after the concrete has set. This will depend on the type of cement, admixture used and prevailing weather conditions and is best decided based on a field inspection. It should be remembered that curing at the earliest is very important. It is difficult and expensive to alter or repair concrete once it is placed and set. Improperly cured concrete has been the one of the causes for early deterioration and reduced service life of concrete structures. Such concrete is porous and has reduced durability and strength. Hence, constant and strict supervision of curing is necessary during construction. The strength gained by concrete at the time of stripping of formwork should be adequate and this depends on how well the concrete is cured.

What are the common mistakes which affect the quality of concrete?
  • Use of too much or too little water for mixing
  • Improper mixing of aggregate with cement
  • Improper grading of aggregates resulting in segregation or blending of concrete
  • Inadequate compaction of concrete
  • Using concrete that has already begun to set
  • Placing of concrete on a dry foundation without properly wetting it with water
  • Use of aggregate containing foreign materials such as earthy matter, clay or lime
  • Use of water containing solids, chemicals, etc.
  • Leaving the finished concrete surface exposed to sun and wind during the first ten days after placing without curing
How is plastic-shrinkage cracks prevented?

The measures that have been found effective in reducing plastic-shrinkage are:

  • Moisten the subgrade forms and the aggregates that are dry and absorptive
  • Erect temporary windbreakers to reduce wind velocity over the concrete surface
  • Erect temporary sunshades to reduce the temperature of the concrete surface
  • Keep the fresh concrete temperature low by cooling the aggregate and mixing water
  • Protect the concrete with temporary coverings such as polyethylene sheeting during any appreciable delay between placing and finishing
  • Reduce the time between placing and start of curing by eliminating delays during construction
  • To minimize evaporation protect the concrete immediately after finishing with wet burlap, fog spray, or a curing compound
  • Strict adherence to specifications regarding evaporation rates and cessation of concrete placement if relative humidity is low and temperature and wind speeds are high
  • Time may not permit utilization of such precautions in many cases, so careful testing and selection of materials which are least susceptible to plastic shrinkage cracking is important
What are the major advantages of blended cements (PPC/PSC) over OPC/Graded Cement?
  • Continuous increase in strength with time
  • Increased resistance against sulphate attack
  • Improved workability
  • Better cohesiveness of fresh concrete
  • Reduced heat of hydration and thermal cracking
  • Reduced corrosion of concrete
  • Better water tightness
  • Reduced Alkali – Silica reaction
  • Better resistance to mild acidic water and aggressive environment
  • Low drying shrinkage