Chapter 1 Cement

Chapter 1: Cement 1st Ed, Civil Engineering Materials

Prepared by: Ahmad Fahmy Kamarudin, January 2010

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Chapter 1CEMENT

1.1. Types of Cement1.2. Chemical Composition1.3. Testing of Cement1.4. Manufacturing of Cement1.5. Method of Cement Storing

Cement is a manufactured construction material and widely used forstructural constructions such as buildings, bridges, tunnels, dams, factories,pavement and etc. It is an instant glue and capable to bond mineralfragments into compact whole. There are variety type of cements can befound in the market. Each type is manufactured under certain conditiondepending on its special properties. However today, Portland cement is themost commonly used as mortar and concrete in structural construction.

Portland cement was patented by Joseph Aspdin in 1824 and was namedafter the limestone cliffs on the Isle of Portland in England.

1.1 Types of CementCement is manufactured with two basic raw ingredients called calcareousand an argillaceous material. The cement in making of concrete has theproperty of setting and hardening under water by virtue of chemicalreaction with it and this type of cement is called hydraulic cement.

Mortar:A mixture of cement, fine aggregates or sand and waterto form a paste

Concrete:A mixture of cement, sand, coarse aggregates and waterunder certain ratio

Calcareous:The calcareous material is a calcium oxide, such as limestone, chalk, or oystershells.

Argillaceous:Argillaceous is a combination of silica and alumina that can be found from clay,shale, and blast furnace slag.



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Different concrete applications require cements with different properties.Some applications require rapid strength gain to expedite the constructionand other applications require low heat hydration to control volume changeand associated shrinkage cracking. Hence, each type of cement ismanufactured by altering the ratios of four basic compounds namelyTricalcium Silicate, Dicalcium Silicate, Tricalcium Aluminate andTetracalcium Aluminoferitte to fit the applications.

Table 1.1: Main compound of Portland cementName of Compound Usual Abbreviation Reaction

Tricalcium SilicateDicalcium SilicateTricalcium AluminateTetracalcium Aluminoferitte

C3SC2SC3A

C4AF

QuickSlow

Very quickNot very important

Cements can be classified into two categories namely hydraulic cementand high alumina cement.

1.1.1 Hydraulic CementHydraulic cement is consists of silicates and aluminates of lime. This type ofcement can be classified as;

Setting TimeSetting refers to the stiffening of the cement paste or thechange from a plastic state to a solid state. The setting timerefers to changes of the cement paste from fluid to rigid.Setting is usually described in two levels namely, initialsetting and final setting.

Initial SettingInitial setting is defined as the beginning of the noticeablestiffening in the cement paste and it’s corresponding to therapid rise temperature. This normally takes about 45 – 175minutes.

Final Setting TimeThis refers to completion of setting which correspond to thepeak temperature in the cement paste. The stiffening ofcement paste increase as the volume of the gel increases andthe stage at which this is completed, the final hardeningprocess begins. It normally takes between 3 to 10 hours forthis to happen.

HardeningThis is referred to the gained of the strength of the cementpaste. Actually during the setting time, the cement gained verylittle strength.



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a. Natural CementNatural cements are powders obtained from certain natural rocks (clayeylime stone type) which are quarried, crushed and processes. Enoughheat is required to dry off carbonic acid gasses. Besides, it is brown incolour and sets slowly or quickly when mixed with water, depending onthe amount of clay in the limestone. The strength is low and not used forconcrete work.

b. Aluminous CementThe chief ingredients of aluminous cement are calcareous and aluminiousmaterials (limestone or chalk and bauxite). These are heated to atemperature of 1400oC and the whole mass is grinded to powder form.

c. Portland CementThe hardening of Portland cement is a chemical process during whichheat is evolved. Modified forms of Portland based on different ratio of fourmain compositions are made, to suit the varying demands of differentkinds of structural application.

Table 1.2: Classification of Portland cementsName Application

NormalGeneral concrete work when the special properties ofother types are not needed. Suitable for floors,reinforced concrete structures, pavements, etc.

Moderate SulfateResistance

Protection against moderate sulfate exposure, 0.1-0.2% weight water soluble sulfate in soil or 150-1500ppm sulfate in water (sea water). Can bespecified with moderate heat of hydration, making itsuitable for large piers, heavy abutments, andretaining walls. The moderate heat of hydration is alsobeneficial when placing concrete in warm weather.

High Early ofHydration

Used for fast-tract construction when forms need to beremoved as soon as possible or structure need to beput in service as soon of possible. In cold weather,reduces time required for controlled curing.

Low Heat of Hydration Used when mass of structure, such as large dams,requires careful control of heat of hydration.

High SulfateResistance

Protection from severe sulfate exposure, 0.2-2.0%weight water soluble sulfate in soils or 1500-10,800ppm sulfate in water



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i) Ordinary Portland Cement – OPC ( BS 12 : 1971)OPC has a medium rate of hardening and is suitable for mosttype of work. It is the one most commonly used for structuralpurposes when the special properties specified for other fourtypes of cement are not required.

ii) Rapid Hardening Portland Cement – RHPC (BS 12 : 1971)RHPC hardens rather more rapidly than OPC. It is similar inchemical composition to OPC but the proportions of the variouscompounds may be slightly different, and it is finely ground.Due to its finer grinding, it will increase the rate of hydration atearly ages, and this leads to the increased rate of earlyhardening as implied by the name.

This early strength is achieved by increasing C2S and C3Acontent of the cement and finer grinding. Since it has high heatevaluation, RHPC should not be used in large masses. With15% of C3A, it has lower sulfate resistance. The may be limitedto obtain moderate sulfate resistance or to 5% when highsulfate resistance is required.

Rapid-hardening Portland cement should not be regarded asquick-setting cement. The setting time specified in BS 12:1971for RHPC is similar as specified for OPC

What is cementhydration?

Hydration is chemical reaction between cement particles and water. Thefeatures of this reaction are the change in matter, the change in energy level,and the rate of reaction. Example:

Tricalcium silicate + Water à Calcium silicate hydrates (C-S-H) + Calciumhydroxide

C-S-H makes the hydrated cement paste strong and calcium hydroxide issusceptible to attack by sulfate and acidic water



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iii) White and Coloured Portland Cement (BS 12 : 1971)Generally used for decorative work. It is made by using Chinaclay in place of ordinary clay to exclude impurities, especiallyiron oxide and limestone.Coloured cements are made by mixing pigments with Portlandcement

iv) Low Heat Portland Cement – LHPC (BS 1370: 1974)LHPC hardens and evolves heat more slowly than OPC. It hasslightly different chemical composition. It is obtained byincreasing the proportion of C2S and reducing C3S and C3A. Itthus hydrates more slowly and evolves heat less rapidly thanOPC. The strength of LHPC is slow developed but the ultimatestrength is same. However, the initial setting time is greaterthan OPC.

v) Portland Blustfurnace Cement - PBC (BS 146: 1973)PBC is made by grinding a mixture of OPC clinker with selectedgranulated blast furnace slag. The proportion of slag is limitedby the British Standard to not more than 65% of the finishedcement.

The properties of blast furnace cement are very similar to thoseof OPC but it hydrates slower than those of Portland cement sothis cement evolves less heat and hardens more slowly thanOPC. The resistance to sulfate is often considered to beintermediate between that of sulfate-resisting Portland cement.

Setting Time

CementType

Initial SettingTime, minutes

(min)

Final SettingTime, minutes

(max)OPC 30 600

RHPC 30 600

LHPC 60 600

Trace Setting Timefor OPC, RHPC andLHPC



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vi) Sulfate-Resisting Portland Cement – SRPC (BS 4027: 1972)SRPC is specified where there is extensive exposure to sulfate.Typical applications include hydraulic structure exposed towater with high alkali content and structures subjected toseawater exposure. The surface resistance to SRPC isachieved by reducing the C3A content to a minimum since thatcompound is most susceptible to sulfate attack. It usually has ahigher content of C4AF. Concrete made with this cement ismore resistant to attack by sulfate compounds which may befound dissolved in ground water and which are present in seawater. SRPC tends to be darker in colour than OPC.

vii) High Strength Portland Cement – HSPCHSPC is produced from the same material as the case of OPC.The higher strength achieved by increasing C3S content andalso by finer grinding of clinker. The initial and final settingtimes are the same as that of OPC. At higher water cementratios, the HSPC has about 80% higher strength and at lowercement ratio 40% higher strength than OPC.

viii) Masonry Cement (BS 5224: 1976)For hand work such as rendering and bricklaying, mortarcomposed only of Portland cement and sand are not ideal.Such mortars harden too quickly, are too strong, and lack theplasticity and water retention desirable in a masonry mortar. Ithas been customary to overcome this difficulty by mixing limewith the cement mixtures. Masonry cement, under variousbrand names consists of Portland cement with a fine inertadmixture and plasticizing agent.

Table 1.3: Minimum compressive strength of concrete cube specifiedby BS 4550 for Portland cement

Compressive Strength (N/mm2)Type of Portland Cement 3 days 7 days 28 days

OPC 13 - 29RHPC 18 - 33

PBC 8 14 22

LHPC 5 - 19

SRPC 10 - 27



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1.1.2 High Alumina CementHigh alumina cement is quite different both in composition and propertiesfrom Portland cement. It is comparatively slow-setting but rapid hardening,thus, produces very high early strength. As a considerable amount of heat isgenerated during the setting and hardening process, it should not be used inrich mixes or large masses. It is essential that the concrete be keptcontinuously wet for at least 24 hours from the time it begins to harden.About 80%of the ultimate strength is developed at the age of 24 hours. Highalumina cement has an initial setting time about 4 hours and final setting timeabout 5 hours. The heat that generated during the hardening period has oneadvantage, as it enables the concrete to be placed at lower temperaturesthan OPC.

For the same water cement ratio, the alumina cement is more workable thanPortland cement. If high alumina cement concrete is used in place wheremoisture and a high temperature present simultaneously, there will be a lossstrength whether these conditions occur early of late in the life of theconcrete. High alumina cement concrete is more resistant than OPC to theaction of sulfates, therefore suitable under sea water applications. Thechemical oxide composition for high alumina cement is as shown in Table1.4.

The raw materials are limestone or chalk and bauxite which are crushed intolumps not exceeding 100mm. The materials are heated to the fusion point atabout 1600oC. The solidified material is fragmented and then ground to afineness of 2500-3200 cm2/g. The product of very dark grey powder ispassed through magnetic separators to remove metallic iron. The aluminacement is considerably more expensive.

Table 1.4: Chemical oxide composition for High Alumina cement

Alumina (Al2O3) 39%Ferric Oxide (Fe2O3) 10%

Lime (CaO) 38%

Ferrous Oxide (FeO) 4%

Silica (SiO2) 6%



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1.2 Chemical CompositionSeveral types of cements can be obtained by changing the percentages ofchemical composition. Table 1.5 shows the chemical composition limit ofPortland cement:

Table 1.5: Chemical composition limit of Portland cementName of Raw Material Chemical Composition Percentage Limit

LimeSilicaAluminaIron OxideMagnesiumAlkalis (Soda and or/ potash)Sulphur Trioxide

CaOSiO2

Al2O3

Fe2O3

MgONa2O,K2O

SO3

60 – 6717 – 25

3 – 80.5 – 60.1 – 4

0.2 – 1.31 - 3

The interaction of Portland cement raw materials are interacted in kiln byforming complex chemical compounds. Calcination in the kiln restructuresthe molecular composition by producing four main chemical compounds.

Table 1.6: Main compounds of Portland cement

Name of Compound Chemical FormulaUsual Range by

Weight (%)

Tricalcium SilicateDicalcium SilicateTricalcium AluminateTetracalcium Aluminoferitte

3CaO.SiO2

2CaO.SiO2

3CaO.Al2O3

4CaO.Al2O3.Fe2O3

45 – 6015 – 306 – 126 - 8

The minor compounds such as magnesium oxide, titanium oxide,manganese oxide, sodium oxide, and potassium oxide are represented a fewpercentages by weight of cement.

1.3 Testing of Cement

1.3.1 Setting

Vicat Set Time ApparatusSetting time can be determined with the Vicat apparatus. The Vicat testrequires sample of cement using the amount of water required for normalconsistency according to a specified procedure.



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Procedure:The 1 mm (0.04in) diameter needle is allowed to penetrate the paste for 30seconds and the amount of penetration is measured. The penetrationprocess is repeated every 15 minutes until a penetration of 25 mm (1in) orless is obtained. By interpolation, the time when a penetration of 25 mmoccurs is determined and recorded as the initial set time. The final set time iswhen the needle does not penetrate visibly into the paste.

Figure 1.1: Vicat set time apparatus

Gilmore Set Time ApparatusThe Gilmore requires a normal consistency cement paste sample. A pat witha flat top is molded and the initial Gilmore needle is applied lightly to itssurface. The application surface is repeated until the pat bears the force ofthe needle without appreciable indentation, and the elapsed time is recordedas the initial time. This process is then repeated with the final Gilmore needleand the final set time is recorded.

Figure 1.2: Gilmore set time apparatus



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SoundnessSoundness of the cement paste refers to its ability to retain its volume aftersetting. Expansion after setting, caused by delayed or slow hydration orreactions, could result if the cement is unsound. The autoclave expansiontest is used to check the soundness of the cement paste. In this test, cementpaste bars are subjected to heat and high pressure, and the amount ofexpansion is measured. ASTM C150 limits autoclave expansion to 0.8%.

Figure 1.3: Cement autoclave expansion

Compressive StrengthCompressive strength of mortar is measured by preparing 50mm (2in.) cubesand subjecting them to compression according to ASTM C109. The mortar isprepared with cement, water and standard sand (ASTM C778). Minimumcompressive strength values are specified by ASTM C150 for differentcement types at different ages. The compressive strength of mortar cubes isproportional to compressive strength of cylinders. However, the compressivestrength of the concrete cannot be predicted accurately from mortar cubestrength, since the concrete strength is affected by aggregate characteristics,the concrete mixing and the construction procedures.

1.4 Manufacturing of CementProduction of Portland cement deals with two basic raw ingredients namelycalcareous and argillaceous. These materials are crushed and stored in thesilos. The raw materials, in the desired proportions, are passed throughgrinding mill, using either wet or dry process. The ground material is storeduntil it can be sent to the kiln.



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Modern dry process cement plants use a heat recovery cycle to preheat theground material, or feed stock, with the exhaust gas from the kiln. Someplants use a flash furnace to further heat and feed stock. Both the preheaterand flash furnace improves the energy efficiency of cement productions. Inthe kiln, the raw materials are melted at temperatures 1400oC to 1650oC,changing the materials into cement clinker. The clinker is cooled and stored.The small amount of gypsum is added to regulate the setting time of thecement in the concrete.

The finished product may be stored and transported in either bulk or sacks.The cement can be stored for long periods of time, provided it is kept dry.

Figure 1.4: Portland cement manufacturing processes



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Tutorial 1

Q1: What is the best storing system for cement?. ExplainQ2: How to speed up the strength development of concrete?. Explain.Q3: What are the effects of non-potable water on concrete quality? Explain.

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Chapter 1 Cement