Calcareous (Limestone Obtained from mines), argillaceous materials (Clay, Bauxite) and Iron are the commonly used raw materials in the manufacture of cement.
Manufacture of cement comprises of four stages viz.
|Chemical Compounds||Range in *OPC%||Reaction Rate||Strength Attained||Liberation of Heat|
|Di-Calcium silicate (C2S)||25-40||Slow||Less initially & high latter||Low|
|Tri-calcium Aluminate (C3A)||8-11||Fast||Low||Very High|
Gypsum is added to control the 'setting of cement'
Setting is the stiffening of the cement paste. Broadly it refers to change from fluid to rigid state.
Hardening is gaining of strength in cement. This process of gaining of strength continuous up to one year(approx.) rapidly and gradually after one year and up to 5 years.
The rate of hydration of cement depends on the fineness of cement and for a rapid development of strength high fineness is necessary.
Cement may sometimes contain free lime, which may cause expansion. Hence soundness test is conducted to see that even if uncombined lime is present in cement whether it is within prescribed limits. If it is within the prescribed limits and if hardening is normal then it is said to be sound.
Grades 53, 43 and 33 indicate the compressive strength of cement in Newtons for one mm2 area i.e. 53 grade means a compressive strength of 53 N/mm2 attained after a curing period of 28 days. Similarly 43 and 33 grades may be defined.
The colour of cement depends on the raw materials used. If the raw materials contain more iron resulting cement will be darker/greener. Colour has no significance on any properties of cement. The strength of cement is dependent on the correct proportion of raw material mix and hence mineral phases in the clinker.
Slow setting of cement is due to salts and chemicals in water, sand and aggregate. Cold Weather, less cement content in the mortar/concrete. High % of impurities, adulterated cement, improper water cement ratio. Quick setting of Cement is due to: addition of low quantity of Gypsum, hot weather condition and high cement fineness.
Proper storage of cement shall permit easy access for inspection and identification.Cement shall be stored in suitable weather tight structures, to protect from dampness.It shall not be piled more than ten bags in a stack and shall be arranged in header and stretcher fashion as far as possible. While removing the bags for use, "FIRST IN FIRST OUT" rule shall be applied.
Mortar is a mixture of sand, cement and water.
Concrete is the most versatile moldable construction material ever known to mankind.It bears excellent fire and weather resistant properties. Most intricate shapes can be achieved with concrete. It is very good in compression though weak in tension.It is supplemented by steel to achieve tensile strength. Centuries old monuments bear testimony to its life.
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 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 cement undergoes the process of hydration to form a gel with binding property covering the aggregates uniformly on mixing. However water for mixing shall be limited to achieve a required consistency as excess water is deterrent to concrete.
It is important that water used in concrete should be free from impurities. Generally it should be of potable quality. Bore water would need checking before using because it often contains impurities which affect setting time and reduce the strength of concrete. Sea water is not suitable for reinforced concrete as it causes corrosion. IS456-1978 gives the maximum possible limits of solids in water.
An admixture is a chemical formulation generally added to concrete while mixing to induce certain desirable properties to it like better workability with water reducing admixture, delayed setting with retarders, improve the rate of hardening with accelerating admixtures, and air entertainment for better flow and lubrication.
Concrete is strong in compression and weak in tension. A plain concrete beam when it is subjected to bending fails at the bottom side due to tension. Hence to make concrete strong in tension, steel is embedded so that it becomes strong both in tension and compression. The concrete embedded with steel is called `Reinforced Cement Concrete'.
The factors that are affecting the workability of concrete are water content, aggregate type and grading, aggregate/cement ratio, presence of admixtures and fineness of cement.
Measurement/Dosing of various ingredients of concrete like sand,metal and cement for each batch of concrete as per design is called batching. Volume batching and weigh batching are the two types of batching in vogue.
The different stages of `mixing of concrete' are : To locate a correct spot on a firm ground close to the concrete site, positioning the mixture machine conveniently oriented to feed the hopper, emptying the drum of concrete on the platform prepared. To stack metal,sand and cement as per design mix. To work out the quantities per batch. To feed the drum with required quantity of water. To feed the drum with the contents of hopper and allow the mix in the rotating drum for 1 & 1/2 to 2 minutes till a homogeneous mix is obtained. To empty the drum of concrete on the platform and leading it to the concreting spot.
The factors affecting the strength of concrete are water cement ratio, shape and size of aggregate, Aggregate/cement ratio. Degree of compaction, Age of concrete and Air entertainment and moisture in aggregate.
Segregation of concrete can occur in two forms. In the first the coarser particles tend to separate out while concrete being conveyed along a slope(for e.g. sloped roof and staircase waist slab) or not being homogeneous coarse aggregate settle below leaving the slurry with fine aggregate. The second form of segregation occurs in wet mixes by separation of grout from the mix. This happens due to over vibration of concrete and dropping of concrete from a height.
A crack is developed 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 appear because of evaporation of water, increased thickness, very fine sand, low quality bricks, non-uniform plastering, improper seasoning of walls, use of different mix proportion , improper filling of joints, insufficient curing, more free lime/MgO.
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 concrete.
M20, M30, M40 Grades of concrete indicates compressive strength of concrete in Newtons for one mm2 area which is called the characteristic strength of concrete after 28 days. M20 mean 20 N/mm2 similarly M30 is 30 N/mm2.
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.
Maintenance of a favourable 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, sealing coat applied as a liquid commonly known as "Curing compounds" are some of the methods commonly employed for curing.