Cement is a binder, a substance that sets and hardens and can bind other materials together. Cements used in construction can be characterized as being either hydraulic or non-hydraulic, depending upon the ability of the cement to be used in the presence of water.
Non-hydraulic cement will not set in wet conditions or underwater, rather it sets as it dries and reacts with carbon dioxide in the air.
It can be attacked by some aggressive chemicals after setting.Hydraulic cement is made by replacing some of the cement in a mix with activated aluminium silicates, pozzolanas, such as fly ash.
The chemical reaction results in hydrates that are not very water-soluble and so are quite durable in water and safe from chemical attack.
This allows setting in wet condition or underwater and further protects the hardened material from chemical attack (e.g., Portland cement).
Use of Cement
Cement mortar for Masonry work, plaster and pointing etc.
Concrete for laying floors, roofs and constructing lintels,beams,weather-shed,stairs,pillars etc.
Construction for important engineering structures such asbridge,culverts,dams,tunnels,light house,clocks,etc.
Construction of water,wells, tennis courts,septic tanks, lamp posts, telephone cabins etc.
Making joint for joints,pipes,etc.
Manufacturing of precast pipes,garden seats, artistically designed wens, flower posts, etc.
Preparation of foundation, water tight floors, footpaths, etc.
Types of Cements
Many types of cements are available in markets with different compositions and for use in different environmental conditions and specialized applications. A list of some commonly used cement is described in this section:
Ordinary Portland cement
Ordinary Portland cement is the most common type of cement in general use around the world. This cement is made by heating limestone (calcium carbonate) with small quantities of other materials (such as clay) to 1450°C in a kiln, in a process known as calcination, whereby a molecule of carbon dioxide is liberated from the calcium carbonate to form calcium oxide, or quicklime, which is then blended with the other materials that have been included in the mix. The resulting hard substance, called ‘clinker’, is then ground with a small amount of gypsum into a powder to make ‘Ordinary Portland Cement'(often referred to as OPC). Portland cement is a basic ingredient of concrete, mortar and most non-specialty grout. The most common use for Portland cement is in the production of concrete. Concrete is a composite material consisting of aggregate (gravel and sand), cement, and water. As a construction material, concrete can be cast in almost any shape desired, and once hardened, can become a structural (load bearing) element. Portland cement may be grey or white.
This type of cement use in construction when there is no exposure to sulphates in the soil or ground water.
Lime saturation Factor is limited between i.e. 0.66 to 1.02.
Free lime-cause the Cement to be unsound.
Percentage of (AL2O3/Fe2O3) is not less than 0.66.
Insoluble residue not more than 1.5%.
Percentage of SO3 limited by 2.5% when C3A < 7% and not more than 3% when C3A >7%.
Loss of ignition -4%(max)
Percentage of Mg0-5% (max.)
Fineness -not less than 2250 cm2/g.
Rapid hardening Portland cement
It is firmer than Ordinary Portland Cement
It contains more C3S are less C2S than the ordinary Portland cement.
Its 3 days strength is same as 7 days strength of ordinary Portland cement.
Low heat Portland cement
Heat generated in ordinary Portland cement at the end of 3days 80 cal/gm. While in low heat cement it is about 50cal/gm of cement.
It has low percentage of C3A and relatively more C2S and less C3S than O.P. Cement.
Reduce and delay the heat of hydration. British standard ( B S. 1370 : 1974 ) limit the heat of hydration of this cement.
Sulphate resisting Portland cement
Maximum C3A content by 3.5% and minimum fineness by 2500 cm’/g.
Firmer than ordinary pot land cement.
Sulphate forms the sulpha-aluminates which have expensive properties and so causes disintegration of concrete.
Sulphate resisting Portland cement
For this cement, the silage as obtained from blast furnace is used
The clinkers of cement are ground with about 60 to 65 percent of slag.
Its strength in early days is less and hence it required longer curing period. It proves to be economical as slag, which is a Waste product, is used in its manufactures.
As per Indian standard, the proportions of Pozzolana may be 10 to 25 % by weight. e.2. Burnt clay, shale, Fly ash.
This Cement has higher resistance to chemical agencies and to sea water because of absence of lime.
It evolves less heat and initial strength is less but final strength is 28 days onward equal to ordinary Portland cement.
It possesses less resistance to the erosion and weathering action.
It imparts higher degree of water tightness and it is cheap.
White Portland cement
Grey colour of O.P. cement is due to presence of Iron Oxide. Hence in White Cement Fe,,O, is limited to 1 %. Sodium Alumina Ferrite (Crinoline) NavAlF6 is added to act as flux in the absence of Iron-Oxide.
It is quick drying, possesses high strength and has superior aesthetic values and it also cost lee than ordinary Cement because of specific requirements imposed upon the raw materials and the manufacturing process.
White Cement are used in Swimming pools, for painting garden furniture, moulding sculptures and statues etc.
The Cement of desired colour may be obtained by mixing mineral pigments with ordinary Cement.
The amount of colouring material may vary from 5 to 10 percent. If this percentage exceeds 10percent, the strength of cements is affected.
The iron Oxide in different proportions gives brown, red or yellow colour. The coloured Cement are widely used for finishing of floors, window sill slabs, stair treads etc.
This type of cement is produced by adding an expanding medium like sulphoaluminate and a stabilising agent to the ordinary cement.
The expanding cement is used for the construction of water retaining structures and for repairing the damaged concrete surfaces.
High alumina cement
This cement is produced by grilling clinkers formed by calcining bauxite and lime. It can stand high temper lures.
If evolves great heat during setting. It is therefore not affected by frost.
The various constituents combine in burning and form cement clinker. The compounds formedin the burning process have the properties of setting and hardening in the presence ofwater.They are known as Bogue compounds after the name of Bogue who identified them. These compounds are as follows: Alite (Tricalcium silicate or C3S), Belite (Dicalcium silicate or C2S), Celite (Tricalciumalluminate or C3A) andFelite (Tetracalciumalumino ferrite or C4AF).
It is supposed to be the best cementing material and is well burnt cement.It is about 25-50% (normally about 40 per cent) of cement. It renders the clinker easier to grind,increases resistance to freezing and thawing, hydrates rapidly generating high heat and developsan early hardness and strength. However, raising of C3S content beyond the specified limitsincreases the heat of hydration and solubility of cement in water. The hydrolysis of C3S is mainly responsible for 7 day strength and hardness. The rate of hydrolysis of C3S and the character of gel developed are the main causes of the hardness and early strength of cement paste. The heat of hydration is 500 J/g.
It constitutes about 25-40% (normally about 32 per cent) of cement. It hydrates andhardens slowly and takes long time to add to the strength (after a year or more). It impartsresistance to chemical attack. Rising of C2S content renders clinker harder to grind, reducesearly strength, decreases resistance to freezing and thawing at early ages and decreases heat ofhydration. The hydrolysis of C2S proceeds slowly. At early ages, less than a month, C2S has little influence on strength and hardness. While after one year, its contribution to the strength and hardness is proportionately almost equal to C3S. The heat of hydration is 260 J/g.
It is about 5-11% (normally about 10.5 per cent) of cement. It rapidlyreacts with water and is responsible for flash set of finely grounded clinker. The rapidity ofaction is regulated by the addition of 2-3% of gypsum at the time of grinding cement. Tricalciumaluminate is responsible for the initial set, high heat of hydration and has greater tendency tovolume changes causing cracking. Raising the C3A content reduces the setting time, weakens resistance to sulphate attack and lowers the ultimate strength, heat of hydration and contractionduring air hardening. The heat of hydration of 865 J/g.
It constitutes about 8–14% (normally about 9 per cent) of cement. It isresponsible for flash set but generates less heat. It has poorest cementing value. Raising theC4AF content reduces the strength slightly. The heat of hydration is 420 J/g.
Hydration of Cement
In the anhydrous state, four main types of minerals are normally present: alite, belite, celiteand felite. Also present are small amounts of clinker sulfate (sulfates of sodium, potassium and calcium) and gypsum, which was added when the clinker was ground up to produce the familiar grey powder.
When water is added, the reactions which occur are mostly exothermic, that is, the reactions generate heat. We can get an indication of the rate at which the minerals are reacting by monitoring the rate at which heat is evolved using a technique called conduction calorimetry.Almost immediately on adding water some of the clinker sulphates and gypsum dissolve producing an alkaline, sulfate-rich, solution.Soon after mixing, the (C3A) phase (the most reactive of the four main clinker minerals) reacts with the water to form an aluminate-rich gel (Stage I on the heat evolution curve above). The gel reacts with sulfate in solution to form small rod-like crystals of ettringite. (C3A) reaction is with water is strongly exothermic but does not last long, typically only a few minutes, and is followed by a period of a few hours of relatively low heat evolution. This is called the dormant, or induction period (Stage II).The first part of the dormant period, up to perhaps half-way through, corresponds to when concrete can be placed. As the dormant period progresses, the paste becomes too stiff to be workable.At the end of the dormant period, the alite and belite in the cement start to react, with the formation of calcium silicate hydrate and calcium hydroxide. This corresponds to the main period of hydration (Stage III), during which time concrete strengths increase. The individual grains react from the surface inwards, and the anhydrous particles become smaller. (C3A) hydration also continues, as fresh crystals become accessible to water.The period of maximum heat evolution occurs typically between about 10 and 20 hours after mixing and then gradually tails off. In a mix containing OPC only, most of the strength gain has occurred within about a month. Where OPC has been partly-replaced by other materials, such as fly ash, strength growth may occur more slowly and continue for several months or even a year.Ferrite reaction also starts quickly as water is added, but then slows down, probably because a layer of iron hydroxide gel forms, coating the ferrite and acting as a barrier, preventing further reaction.
Products of Hydration
During Hydration process several hydrated compounds are formed most important of which are, Calcium silicate hydrate, calcium hydroxide and calcium aluminium hydrates which is important for strength gain.
Calcium silicate hydrate
This is not only the most abundant reaction product, occupying about 50% of the paste volume, but it is also responsible for most of the engineering properties of cement paste. It is often abbreviated, using cement chemists’ notation, to “C-S-H,” the dashes indicating that no strict ratio of SiO2 to CaO is inferred. C-S-H forms a continuous layer that binds together the original cement particles into a cohesive whole which results in its strong bonding capacity. The Si/Ca ratio is somewhat variable but typically approximately 0.45-0.50 in hydrated Portland cement but up to perhaps about 0.6 if slag or fly ash or microsilica is present, depending on the proportions.
The other products of hydration of C3S and C2S are calcium hydroxide. In contrast to theC-S-H, the calcium hydroxide is a compound with distinctive hexagonal prism morphology. It constitutes 20 to 25 per cent of the volume of solids in the hydrated paste. The lack ofdurability of concrete is on account of the presence of calcium hydroxide. The calcium hydroxide also reacts with sulphates present in soils or water to form calcium sulphate which further reacts with C3A and cause deterioration of concrete. This is known as sulphate attack. To reduce the quantity of Ca (OH)2 in concrete and to overcome its bad effects by converting it into cementitious product is an advancement in concrete technology.The use of blendingmaterials such as fly ash, silica fume and such other pozzolanic materials are the steps toovercome bad effect of Ca(OH)2 in concrete. However, Ca(OH)2 is alkaline in nature due to which it resists corrosion in steel.
Calcium aluminium hydrates
These are formed due to hydration of C3A compounds. The hydrated aluminates do notcontribute anything to the strengthof concrete. On the other hand, theirpresence is harmful to the durabilityof concrete particularly where theconcrete is likely to be attacked bysulphates. As it hydrates very fast itmay contribute a little to the earlystrength.
Various tests on cement
Basically two types of tests are under taken for assessing the quality of cement. These are either field test or lab tests. The current section describes these tests in details.
There are four field tests may be carried out to as certain roughly the quality of cement.There are four types of field tests to access the colour, physical property, and strength of the cement as described below.
The colour of cement should be uniform.
It should be typical cement colour i.e. grey colour with a light greenish shade.
Cement should feel smooth when touched between fingers.
If hand is inserted in a bag or heap of cement,it should feel cool.
Presence of lumps
Cement should be free from lumps.
For a moisture content of about 5 to 8%,this increase of volume may be much as 20 to 40 %,depending upon the grading of sand.
A thick paste of cement with water is made on a piece of thick glass and it is kept under water for 24 hours.It should set and not crack.
Six laboratory tests are conducted mainly for assessing the quality of cement. These are: fineness, compressive strength, consistency, setting time, soundness and tensile strength.
This test is carried out to check proper grinding of cement.
The fineness of cement particles may be determined either by sieve test or permeability apparatus test.
In sieve test ,the cement weighing 100 gm is taken and it is continuously passed for 15 minutes through standard BIS sieve no. 9.The residue is then weighed and this weight should not be more than 10% of original weight.
In permeability apparatus test,specific area of cement particles is calculated.This test is better than sieve test.The specific surface acts as a measure of the frequency of particles of average size.
This test is carried out to determine the compressive strength of cement.
The mortar of cement and sand is prepared in ratio 1:3.
Water is added to mortar in water cement ratio 0.4.
The mortar is placed in moulds.The test specimens are in the form of cubes and the moulds are of metals.For 70.6 mm and 76 mm cubes ,the cement required is 185gm and 235 gm respectively.
Then the mortar is compacted in vibrating machine for 2 minutes and the moulds are placed in a damp cabin for 24 hours.
The specimens are removed from the moulds and they are submerged in clean water for curing.
The cubes are then tested in compression testing machine at the end of 3days and 7 days. Thus compressive strength was found out.
The purpose of this test is to determine the percentage of water required for preparing cement pastes for other tests.
Take 300 gm of cement and add 30 percent by weight or 90 gm of water to it.
Mix water and cement thoroughly.
Fill the mould of Vicat apparatus and the gauging time should be 3.75 to 4.25 minutes.
Vicat apparatus consists of aneedle is attached a movable rod with an indicator attached to it.
There are three attachments: square needle,plungerand needle with annular collar.
The plunger is attached to the movable rod.the plunger is gently lowered on the paste in the mould.
The settlement of plunger is noted.If the penetration is between 5 mm to 7 mm from the bottom of mould,the water added is correct.If not process is repeated with different percentages of water till the desired penetration is obtained.
This test is used to detect the deterioration of cement due to storage.The test is performed to find out initial setting time and final setting time.
Cement mixed with water and cement paste is filled in the Vicat mould.
Square needle is attached to moving rod of vicat apparatus.
The needle is quickly released and it is allowed to penetrate the cement paste.In the beginningthe needle penetrates completely.The procedure is repeated at regular intervals till the needle does not penetrate completely.(upto 5mm from bottom)
Initial setting time =<30min for ordinary Portland cement and 60 min for low heat cement.
The cement paste is prepared as above and it is filled in the Vicat mould.
The needle with annular collar is attached to the moving rod of the Vicat apparatus.
The needle is gently released. The time at which the needle makes an impression on test block and the collar fails to do so is noted.
Final setting time is the difference between the time at which water was added to cement and time as recorded in previous step,and it is =<10hours.
The purpose of this test is to detect the presence of uncombined lime in the cement.
The cement paste is prepared.
The mould is placed and it is filled by cement paste.
It is covered at top by another glass plate.A small weight is placed at top and the whole assembly is submerged in water for 24 hours.
The distance between the points of indicator is noted.The mould is again placed in water and heat is applied in such a way that boiling point of water is reached in about 30 minutes. The boiling of water is continued for one hour.
The mould is removed from water and it is allowed to cool down.
The distance between the points of indicator is again measured.The difference between the two readings indicates the expansion of cement and it should not exceed 10 mm.
This test was formerly used to have an indirect indication of compressive strength of cement.
The mortar of sand and cement is prepared.
The water is added to the mortar.
The mortar is placed in briquette moulds.The mould is filled with mortar and then a small heap of mortar is formed at its top.It is beaten down by a standard spatula till water appears on the surface.Same procedure is repeated for the other face of briquette.
The briquettes are kept in a damp for 24 hours and carefully removed from the moulds.
The briquettes are tested in a testing machine at the end of 3 and 7 days and average is found out.