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.
The process has four stages:
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.
Gypsum functions as an agent which controls the setting time of cement.
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.
|S.No.||SPECIFICATION OF CEMENT||BIS SPECIFICATION NO.|
|1||33 Grade OPC||IS 269 : 1989|
|2||43 Grade OPC||IS 8112 : 1989|
|3||53 Grade OPC||IS 12269 : 1987|
|1||Portland Slag Cement(PSC)||IS 455 : 1989|
|2||Portland Pozonnana Cement ( PPC-FlyAsh Based)||IS 1489 (Part 1) : 1991|
|3||PPC (Calcined Clay Based)||IS 1489 (Part 2) : 1991|
|4||Masonry Cement||IS 3466 : 1988|
|1||High Alumina Cement for Structural Use||IS 6452 : 1989|
|2||Super Sulphate Cement||IS 6909 : 1990|
|3||Rapid Hardening Portland Cement||IS 8041 : 1990|
|4||White Portland Cement||IS 8042 : 1989|
|5||Hydrophobic Portland Cement||IS 8043 : 1991|
|6||Sulphate Resisting Portland Cement||IS 12330 : 1988|
|7||Low Heat Portland Cement||IS 12600 : 1989|
|8||Oil Well Cement||IS 8229 : 1986|
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.
Setting is the stiffening of the cement paste which broadly refers to it changing from its fluid state to a rigid one.
Hardening is the gain of strength in cement.
Slow setting is due to:
Quick setting is due to:
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.
Mortar is a mixture of sand, cement and water.
Concrete is a mixture of cement, water, aggregates(fine as well as coarse) and chemical/mineral admixtures.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Maintaining a favorable environment for the continuation of chemical reaction that is responsible for hardening of cement is called 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.
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.
The measures that have been found effective in reducing plastic-shrinkage are: