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Cement

Cement binds the sand and coarse aggregate together to fill voids in between sand and coarse aggregate particle to form a compact mass. Cement can be classified as



1

OPC : Ordinary Portland cement

Ordinary Portland cement is the binding agent in concrete. It is a hydraulic cement that, when combined with water, hardens into a solid mass. As a material, OPC has been used for well over 175 years .

In 1824, Joseph Aspdin, a Leeds mason took out a patent on a hydraulic cement that he coined "Portland" cement (Mindess and Young, 1981). He named the cement because it produced a concrete that resembled the color of the natural limestone quarried on the Isle of Portland, a peninsula in the English Channel.

2

PPC : Portland Pozzolana Cement

This type of cement is most common type available now in market.

This is made by blending 10-25% reactive pozzolana like flyash or calcined clay with OPC. Addition of pozzolana makes cement sensitive to curing and it requires longer curing than OPC.


3

SRPC : Sulphate Resistant Portland Cement

Sulphate-resisting Portland cement (SRPC) is a special purpose cement used where sulfates are present in concentrations that would damage Normal Portland cement concrete or mortar.

Sulfate resistance is achieved by adjustments to chemical composition to limit the level of tricalcium aluminate (C3A) in the SRPC


4

BFSC : Portland blast furnace slag cement

This type of cement constitutes about 10% of cement produced in India. The slag forms 25-60% of the cement.

Every ton of cast iron produce 0.3 tons of furnace slag which can be used in cement industry. During its setting the Ca(OH)2 liberated by OPC hydration acts as an activator for the slag.

Even though it is equated with OPC it behaves more like PPC and has lower heat of hydration and better sulphate resistance.

At present BFSC produced in India is only Grade-33 and there are proposals to make Grade-43 cements with 45-70% slag content. BFSC with more than 50% slag has good sulphates resistance.


5

Hydrophobic cement

In places of high rainfall and humidity normal cement tends to set when stored, due to mositure present in the atmosphere. By grinding the cement clinker with water repellent film forming substance like oleic acid, a water repellent film is formed around cement particle during manufacturing itself. This prevent setting of cement during storage.

During mixing with aggregates this film is broken and cement behave as ordinary cement.


6

RHPC : Rapid Hardening Portland cement

Rapid Hardening Portland cement (RHPC) is a special purpose cement used in concrete to ensure a higher rate of early age strength development than that typically achieved using Normal Portland cement (NPC).

The improved early age strength performance of RHPC is principally achieved through increased product fineness.


7

Blended cement

For economy a mixture of portland cement, blast furnace slag and flyash is allowed to be used in some countries. This is known as blended cement. In India this type of cement is not produced.



Classification of Cement



OPC: Ordinary Portland cement

Chemical composition of Portland Cement:

  • C3S: Tricalcium Silicate = 3CaO.SiO2 (50%)
  • C2S :Dicalcium Silicate = 2CaO.SiO2 ( (25%)
  • C3A :Tricalcium Aluminate = 3CaO.Al2O (10%)
  • C4AF :Tetracalcium Aluminoferrite = 4CaO. Al2O3.Fe2O3 (10%)
  • Gypsum (5%)

  • 1. C3S: Tricalcium Silicate


  • Hydrates and hardens rapidly and is largely responsible for initial set and early strength.
  • Early strength of cement is higher with increased percentages of C3S.

  • 2. C2S : Dicalcium Silicate

  • Hydrates and hardens slowly.
  • Contributes largely to strength increase at ages beyond one week.
  • Responsible for long term strength

  • 3. C3A :Tricalcium Aluminate

  • Liberates a large amount of heat during the first few days of hydration and hardening.
  • Also contributes slightly to early strength development.
  • Gypsum added to the cement slows down the hydration rate of C3A.
  • Cements with low percentages of C3A are especially resistant to soils and waters containing sulfates.

  • 4. C4AF :Tetracalcium Aluminoferrite .

  • Assist in the manufacture of Portland Cement by allowing lower clinkering temperature.
  • Also act as a filler
  • Contributes very little strength of concrete eventhough it hydrates very rapidly.
  • Also responsible for grey colour of Ordinary Portland Cement

  • Hydration of Cement

  • In the presence of water the cement compounds chemically combined with water (hydrate) to form new compounds that are the infrastructure of the hardened cement paste in concrete.
  • Both C3S and C2S hydrate to form calcium hydroxide and calcium silicate hydrate (CSH). Hydrated cement paste contains 15% to 25% Calcium hydroxide and about 50% calcium silicate hydrate by mass. The strength and other properties of hydrated cement are due primarily to calcium silicate hydrate.
  • C3A reacts with water and calcium hydroxide to form tetracalcium aluminate hydrate.
  • C4AF reacts with water and calcium hydroxide to form calcium aluminoferrite hydrate.

  • Relative Reactivity of Cement compounds


    Relative volume of major compounds in hydrated cement paste


    Cement -Manufacture


    The Cement Manufacturing Process


    Sulphate Resistant Portland Cement

    Properties

  • Applied at place where there is exposure to sulphate such as used in concrete below ground
  • Higher content of Tetracalcium Aluminoferrite & reducing the Tricalcium aluminate to aluminium
  • Has darker color than OPC
  • SRPC is made by fusing together a precisely controlled blend of very finely ground limestone, shale and iron oxide at high temperatures to form cement clinker.
  • A small quantity of gypsum is added to this clinker before grinding to produce the final fine powder - Sulfate-resisting Portland cement.
  • SRPC is manufactured in a modern dry process works at Castlemungret, Co.
  • Limerick and is supplied either in bulk or in bags.

  • Advantages

    Long Term Strength Development

  • A significant characteristic of SRPC is the ability to continue significant strength development beyond the normal 28 day period.
  • The benefits of this are obvious, considering that 28 days is a very short period in the life of most concrete structures.
  • High Strength Concrete
  • Studies have shown amazing benefits of the use of SRPC in high performance concrete.
  • The lower reactivity of the slag makes it easier to control slump in the first two hours that follow mixing,
  • as well as to reduce the amount of heat developed during hydration at higher cement contents.
  • Heat of Hydration .
  • Sulphate Resistant Portland Cement have a much lower heat of hydration than other cements, making them ideal for mass concrete work.
  • The lower heat of hydration results in smaller temperature gradients within mass concrete structures, thereby minimising thermal cracking.
  • Pure Water and Acid Attack.
  • The use of SRPC is recommended in applications where concrete is exposed to pure water and/or acidic conditions.
  • Examples are water pipes and reservoirs, soft drink factories and sewage works.
  • Sulphate Attack
  • SRPC provides higher resistance to sulphate attack for applications specifically requiring sulphate resistance.
  • Applications include sewage works and coastal environments.
  • Alkali Aggregate Reactivity
  • SRPC has been shown to reduce the risk of alkali aggregate reaction in concretes containing reactive aggregates.

  • Rapid Hardening Portland cement
    Properties

  • Similar chemical composition as OPC but different proportion
  • Causes to the increased rate of early hardening
  • Concrete made with RHPC develops in 7 days the same strengths
  • High early strength is achieved by increasing the CS & CA content
  • Properties

  • More finely grounded than OPC
  • Allows formwork to be struck earlier
  • Providing savings either the quantity of formwork required in time
  • Produce heat earlier than OPC, so it can be used in cold weather
  • Qualities

  • RHPC is produced using carefully selected raw materials.
  • Strict quality control throughout each stage of the manufacturing process ensures that a consistent final product is achieved.
  • Strength

  • Rapid Hardening Portland cement achieves higher earlier strengths than OPC because it is more finely ground.
  • Rapid Hardening Portland cement is chemically similar to OPC and as such concrete mixes should be designed with this in mind.
  • In particular, optimum ultimate strength of concrete using RHPC will depend, like OPC, on careful attention to the water/cement ratio, consistent with ensuring satisfactory placing and compaction.

  • Physical Tests Of Cement

  • Fineness
  • Consistency
  • Soundness
  • Setting Time
  • Compressive Strength
  • Heat of Hydration

  • TESTS FOR FINENESS (IS 4031 )

  • The first requirement is that 90% of cement should pass IS 90 mircons. Indian standard also specifies fineness test by blaine’s air permeability method as described in IS 4031-1968
  • 95% of cement particles are smaller than 45 micrometer, with the average particle around 15 micrometer.
  • Fineness of cement affects heat released and the rate of hydration.
  • The principle is based on relation between the rate of flow of air through a cement bed and surface area of particles comprising the cement bed of given porosity
  • It should be at least 225 m2/kg
  • More is the fineness of cement more will be the rate of hydration.
  • Thus the fineness accelerates strength development principally during the first seven days

  • Test of Normal or standard Consistency (IS 4031)

  • Consistency refers to the relative mobility of a freshly mixed cement paste or mortar or its ability to flow.
  • Normal or Standard consistency of cement is determined using the Vicat’s Apparatus. It is defined as that percentage of water added to form the paste which allows a penetration of 10 ? 1 mm of the Vicat plunger.
  • It is the consistency at which the vicat plunger G of 10mm diameter and 50mm length will penetrate 33-35 mm within 3 to 5 minutes of mixing .
  • The test procedure is to carry at least three trial experiment by mixing the cement with distilled water varying from about 24 to 27 % of weight of cement .

  • Test for Soundness (IS 4031)

  • Soundness is the ability of a hardened paste to retain its volume after setting.
  • A cement is said to be unsound (i.e. having lack of soundness) if it is subjected to delayed destructive expansion.
  • The Soundness test is an indication of excess of lime caused by inadequate burning of cement or excess of magnesia or sulphates .
  • Le chatelier’s Test

    This test shows unsoundness dur to lime only. Unaerated cement paste at normal consistency is first tested for expansion. If the test results does not satisfy requirement of 10mm expansion .

    Another test shall be made after aeration of cement by spreading of sample to a depth of 75mm at relative humidity of 50% to 80% for 7 days the expansion in this aerated cement test should not be more then 5 mm

    Autocalve test

    Autocalve Test is used for both lime and magnesia excess indication .
    All the cement having a magnesia content more then 3 % is to be tested for soundness by this test with unaerated cement .
    The test consist of heating bar made of cement paste with water of normal consistency and measuring its expansion


    Test for Setting Time

    The Setting Time is also determined by vicat’s needle on cement paste of normal consistency . For this test , we use 1mm square needle , this time to penetrate 33-35mm is taken as initial setting time.

    For final setting time we use special needle (5mm dia) and time at which this needle will not penetrate more then 0.5mm is taken as final set.


    Compressive Strength

    This is most important test. The compressive strength of cement is determine from cube of face 50cm2 in area made of cement mortar with one part cement and three part of standard sand (conforming to IS 650-1966) by weight and water corresponding to 25% normal consistency plus three percent of combined weight of cement and sand .


    Heat of Hydration (IS 4031-1968)

    Hydration of cement is chemical reaction and it produce heat . In very massive construction this effect can rise the temperature of concrete as much as 50°C . In such cases , we should use low heat cements or adopt cooling method.

    The test is carried out by calorimeter. Low heat cement should satisfy following criteria

  • In 7 days, heat generated should not be more than 65 calories/gm of cement.
  • In 28 days, heat generated should not be more than 75 calories/gm of cement.
  • Cements do not generate heat at constant rate as illustrated in Figure for a typical Portland cement


    Storage of Cement

  • Cement is moisture-sensitive material; if kept dry it will retain its quality indefinitely.
  • When exposed to moisture, cement will set more slowly and will have less strength compared to cement that kept dry.
  • At the time of use cement should be free-flowing and free of lumps.


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