chapter 1 cement civil engineering material

Chapter 1: Cement 1st Ed, Civil Engineering Materials

Chapter 1CEMENT

Types of Cement1.1. Chemical Composition1.2. Testing of Cement1.3. Manufacturing of Cement1.4. Method of Cement Storing

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

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

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

Prepared by: Ahmad Fahmy Kamarudin, January 2010

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

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

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

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


Different concrete applications require cements with different properties. Some applications require rapid strength gain to expedite the construction and other applications require low heat hydration to control volume change and associated shrinkage cracking. Hence, each type of cement is manufactured by altering the ratios of four basic compounds namely Tricalcium Silicate, Dicalcium Silicate, Tricalcium Aluminate and Tetracalcium 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 cement and high alumina cement.

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


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

Initial SettingInitial setting is defined as the beginning of the noticeable stiffening in the cement paste and its corresponding to the rapid rise temperature. This normally takes about 45 175 minutes.

Final Setting TimeThis refers to completion of setting which correspond to the peak temperature in the cement paste. The stiffening of cement paste increase as the volume of the gel increases and the stage at which this is completed, the final hardening process begins. It normally takes between 3 to 10 hours for this to happen.

HardeningThis is referred to the gained of the strength of the cement paste. Actually during the setting time, the cement gained very little strength.


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

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

c. Portland CementThe hardening of Portland cement is a chemical process during which heat is evolved. Modified forms of Portland based on different ratio of four main compositions are made, to suit the varying demands of different kinds of structural application.

Table 1.2: Classification of Portland cementsName Application

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

Moderate Sulfate Resistance

Protection against moderate sulfate exposure, 0.1-0.2% weight water soluble sulfate in soil or 150-1500ppm sulfate in water (sea water). Can be specified with moderate heat of hydration, making it suitable for large piers, heavy abutments, and retaining walls. The moderate heat of hydration is also beneficial when placing concrete in warm weather.

High Early of Hydration

Used for fast-tract construction when forms need to be removed as soon as possible or structure need to be put 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 Sulfate Resistance

Protection from severe sulfate exposure, 0.2-2.0% weight water soluble sulfate in soils or 1500-10,800 ppm 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 most type of work. It is the one most commonly used for structural purposes when the special properties specified for other four types of cement are not required.

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

This early strength is achieved by increasing C2S and C3A content of the cement and finer grinding. Since it has high heat evaluation, RHPC should not be used in large masses. With 15% of C3A, it has lower sulfate resistance. The may be limited to obtain moderate sulfate resistance or to 5% when high sulfate resistance is required.

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


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What is cement hydration?

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

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

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


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

iv) Low Heat Portland Cement LHPC (BS 1370: 1974)LHPC hardens and evolves heat more slowly than OPC. It has slightly different chemical composition. It is obtained by increasing the proportion of C2S and reducing C3S and C3A. It thus hydrates more slowly and evolves heat less rapidly than OPC. The strength of LHPC is slow developed but the ultimate strength is same. However, the initial setting time is greater than OPC.

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

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


Cement Type

Setting TimeInitial Setting Time, minutes

(min)

Final Setting Time, minutes (max)

OPC 30 600RHPC 30 600LHPC 60 600

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Trace Setting Time for OPC, RHPC and LHPC


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 to water with high alkali content and structures subjected to seawater exposure. The surface resistance to SRPC is achieved by reducing the C3A content to a minimum since that compound is most susceptible to sulfate attack. It usually has a higher content of C4AF. Concrete made with this cement is more resistant to attack by sulfate compounds which may be found dissolved in ground water and which are present in sea water. SRPC tends to be darker in colour than OPC.

vii) High Strength Portland Cement HSPC HSPC is produced from the same material as the case of OPC. The higher strength achieved by increasing C3S content and also by finer grinding of clinker. The initial and final setting times are the same as that of OPC. At higher water cement ratios, the HSPC has about 80% higher strength and at lower cement ratio 40% higher strength than OPC.

viii) Masonry Cement (BS 5224: 1976)For hand work such as rendering and bricklaying, mortar composed only of Portland cement and sand are not ideal. Such mortars harden too quickly, are too strong, and lack the plasticity and water retention desirable in a masonry mortar. It has been customary to overcome this difficulty by mixing lime with the cement mixtures. Masonry cement, under various brand names consists of Portland cement with a fine inert admixture and plasticizing agent.

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

1.1.2 High Alumina Cement


Type of Portland CementCompressive Strength (N/mm2)

3 days 7 days 28 daysOPC 13 - 29

RHPC 18 - 33PBC 8 14 22

LHPC 5 - 19

SRPC 10 - 27

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High alumina cement is quite different both in composition and properties from Portland cement. It is comparatively slow-setting but rapid hardening, thus, produces very high early strength. As a considerable amount of heat is generated during the setting and hardening process, it should not be used in rich mixes or large masses. It is essential that the concrete be kept continuously 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. High alumina cement has an initial setting time about 4 hours and final setting time about 5 hours. The heat that generated during the hardening period has one advantage, as it enables the concrete to be placed at lower temperatures than OPC.

For the same water cement ratio, the alumina cement is more workable than Portland cement. If high alumina cement concrete is used in place where moisture and a high temperature present simultaneously, there will be a loss strength whether these conditions occur early of late in the life of the concrete. High alumina cement concrete is more resistant than OPC to the action of sulfates, therefore suitable under sea water applications. The chemical oxide composition for high alumina cement is as shown in Table 1.4.

The raw materials are limestone or chalk and bauxite which are crushed into lumps not exceeding 100mm. The materials are heated to the fusion point at about 1600oC. The solidified material is fragmented and then ground to a fineness of 2500-3200 cm2/g. The product of very dark grey powder is passed through magnetic separators to remove metallic iron. The alumina cement is considerably more expensive.

Table 1.4: Chemical oxide composition for High Alumina cement

1.2 Chemical Composition


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

Lime (CaO) 38%

Ferrous Oxide (FeO) 4%

Silica (SiO2) 6%

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Several types of cements can be obtained by changing the percentages of chemical composition. Table 1.5 shows the chemical composition limit of Portland 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

CaOSiO2Al2O3Fe2O3MgO

Na2O,K2OSO3

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 by forming complex chemical compounds. Calcination in the kiln restructures the molecular composition by producing four main chemical compounds.

Table 1.6: Main compounds of Portland cement

Name of Compound Chemical Formula Usual Range by Weight (%)

Tricalcium SilicateDicalcium SilicateTricalcium AluminateTetracalcium Aluminoferitte

3CaO.SiO22CaO.SiO23CaO.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 few percentages 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 test requires sample of cement using the amount of water required for normal consistency according to a specified procedure.

Procedure:


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The 1 mm (0.04in) diameter needle is allowed to penetrate the paste for 30 seconds and the amount of penetration is measured. The penetration process is repeated every 15 minutes until a penetration of 25 mm (1in) or less is obtained. By interpolation, the time when a penetration of 25 mm occurs is determined and recorded as the initial set time. The final set time is when 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 with a flat top is molded and the initial Gilmore needle is applied lightly to its surface. The application surface is repeated until the pat bears the force of the needle without appreciable indentation, and the elapsed time is recorded as the initial time. This process is then repeated with the final Gilmore needle and the final set time is recorded.

Figure 1.2: Gilmore set time apparatus

Soundness


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Soundness of the cement paste refers to its ability to retain its volume after setting. Expansion after setting, caused by delayed or slow hydration or reactions, could result if the cement is unsound. The autoclave expansion test is used to check the soundness of the cement paste. In this test, cement paste bars are subjected to heat and high pressure, and the amound of expandsion is measure. 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.) cubes and subjecting them to compression according to ASTM C109. The mortar is prepared with cement, water and standard sand (ASTM C778). Minimum compressive strength values are specified by ASTM C150 for different cement types at different ages. The compressive strength of mortar cubes is proportional to compressive strength of cylinders. However, the compressive strength of the concrete cannot be predicted accurately from mortar cube strength, 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 namely calcareous and argillaceous. These materials are crushed and stored in the silos. The raw materials, in the desired proportions, are passed through grinding mill, using either wet or dry process. The ground material is stored until it can be sent to the kiln.


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Modern dry process cement plants use a heat recovery cycle to preheat the ground material, or feed stock, with the exhaust gas from the kiln. Some plants use a flash furnace to further heat and feed stock. Both the preheater and flash furnace improves the energy efficiency of cement productions. In the 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 the cement 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.


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Figure 1.4: Portland cement manufacturing processes


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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Types of Cement

chapter 1 cement civil engineering material

Documents

cement paste

stiffening of cement

mixture of cement

high alumina cement

method of cement storingcement

setting timesetting

completion of setting

property of setting