CHAPTER 2

CEMENT

Introduction

In the general sense, cement (Latin caementum) is any material with adhesive properties.

The term cement is also commonly used to refer more specifically to powdered materials which develop strong adhesive qualities when combined with water.

Cement has been around for at least 12 million years. When the earth itself was undergoing intense geologic changes

natural, cement that humans first put to use Eventually, they discovered how to make cement from other

materials. The Assyrians and Babylonians used clay for this purpose. Then the Egyptians discovered lime and gypsum mortar which they

used to build the pyramids. The Greeks made further improvements, and finally the Romans

perfected cement that produced structures of remarkable durability.

A mixture of cement and sand when mixed with water to form paste is known as cement mortar, whereas a mixture of cement, sand, gravel or crushed stone and water is called cement concrete.

The function of cement is: To bind the sand and coarse aggregate together. To fill the void in between sand and course aggregates

particles to form a compact mass.

Types of cement

Cement can be classified into two categories: Hydraulic cement High alumina cement

Hydraulic Cement Is the cement that would set and hardened under water by

virtue of a chemical reaction between the constituent compounds of cement and water.

Hydraulic cement consists mainly of silicates and aluminates of lime (C2S, C3S, C3A , and C4AF)

They may be classified as:1. Natural cement2. Portland cement3. Aluminous cement

1. NATURAL CEMENT

Natural cements are powders obtained from certain natural rocks (clayey lime stone type) which are quarried, crushed and processed.

Hydraulic cements results if enough heat is applied to dry off carbonic acid gases.

Brown in colour and sets slowly or quickly when mixed with water, depending on the amount of clay in the limestone.

Has low strength and now not used for concrete work because of the strength and other physical property are varies greatly

2. PORTLAND CEMENT (PC)

Portland cement are made by burning together calcareous (limestone) and argillaceous materials (clay) or other chemically similar suitable materials in a large rotary kiln at 1600⁰C to form clinker.

This clinker is ground to a fine powder with a small proportion of gypsum (calcium sulphate) which regulates the rate of setting when the cement is mixed with water.

Commonly used in construction work.

3. ALUMINOUS CEMENT

In aluminous cements the chief ingredient are calcareous (clay, shale) and aluminous materials (limestone or chalk and bauxite)

These are heated to a temperature of 1600⁰C and then the whole mass is grinded to powder form.

Hydraulic Cement

High alumina cement

High alumina cement is quite different both in composition and properties from Portland Cement (PC).

Has slow-setting but rapid-hardening, thus produces very high early strength.

About 80% of the ultimate strength is developed at the age of 24 hours.

More workable than PC. More resistant than PC – to the action of sulphates,

therefore suitable for under seawater application. More expensive than PC

PORTLAND CEMENT

There is a variety of cements in the market and each type is used under certain conditions due to its special properties.

Portland cement is commonly used cement.

The name Portland cement was given due to yellowish-gray color and quality of set cement which resembled stone quarried on the Isle of Portland, England.

Over the years, several types of Portland cement have been developed: ordinary, rapid hardening, sulphate resisting and white.

As their name simply, they are produced to provide special properties and effects which are value in appropriate applications.

They are containing the same active compound - only the proportion of each is different.

The main compounds in Portland cements are given in Table 1.

Name of Compound

Chemical Composition

Usual Abbreviation

Reaction

Tricalsium

Silicate

3CaO.SiO2 C3S Quick Reaction

Dicalsium

Silicate

2CaO.SiO2 C2S Slow Reaction

Tricalsium Aluminate

3CaO.A2O3 C3A Very Quick Reaction

TerraCalsium Aluminoferrite

4CaO.Al2O3.Fe2O3

C4AF Not very important

Raw Material(limestone/chalk + bauxite)

Crushed into lumps not exceeding 100 mm

Heated at 1600⁰C to form clinker

Clinker is ground to form cement powder

Types of portland cement

Portland cement are divided into 8 types:1. Ordinary Portland Cement (OPC)2. Rapid Hardening Portland Cement (RHPC)3. White & Colored Portland Cement4. Low Heat Portland Cement (LHPC)5. Portland – Blastfurnace Cement (PBFC)6. Sulphate – Resistance Portland Cement (SRPC)7. High Strength Portland Cement (HSPC)8. Masonry Cement

1. Ordinary Portland Cement (OPC)

Ordinary Portland Cement (OPC) is the most common cement used in general concrete construction when there is no exposure to sulphates in the soil or groundwater.

Has a medium rate of hardening. Suitable for most type of work.

2. Rapid-Hardening Portland Cement (RHPC)

More finely ground – increase the rate of hydration at early ages. Increase the rate of hardening. Very useful when concrete of high early strength is required or

when it is necessary to strike the form work quickly. Also useful when concreting in the cold weather. High early strength is achieved by increasing the C3S and C3A

content of the cement & finer grinding. The strength of 7 days with RHPC= 28 days with concrete made

with OPC. High heat evolution, RHPC is not suitable for large masses.

3. White & Coloured Portland Cement

Generally used for decorative work. White cement- made by using china clay. Coloured cement- by mixing a pigment with Portland Cement.

4. Low Heat Portland Cement (LHPC)

o Hardens & evolves heat more slowly than OPC.o Obtained by increasing the proportion of C2S & reducing C3S and

C3A.o Hydration more slowly. Resulting slow development of strength

but the ultimate strength is the same.o Initial setting is greater than OPC.o Developed for mass-concrete application. If OPC cement is used in

large masses that cannot lose heat by radiation, it liberates enough heat during hydration to raise the temp. (as much 50◦F or 60◦F) à develop shrinkage cracks

5. Portland- Blastfurnace Cement (PBFC)

Made by grinding a mixture of OPC clinker with selected granulated blastfurnace slag. (proportion of slag is limited by the BS to <65% of the finished cement

Properties are very similar to OPC but hydrates slower than OPC- evolve less heat & hardens more slowly than OPC.

Resistance to sulphate is often considered to be intermediate between that of SRPC.

Benefits of using Portland-Blastfurnace Cement: o Low heat of hydration

o Portland- Blastfurnace Cement, with lower heat of hydration compared to OPC is suitable for mass concreting works.

o BS 8110 Part 1 Clause 6.2 (b) recommended " the use of material with a lower release of heat of hydration to be considered from mass concreting works" and details of early thermal cracking are given in BS 8110 Part II Clause 3.8.4.

o Resistance to sulphate attack o Naturally occurring sulphates of sodium, potassium,

calcium or magnesium which can attack concrete are sometimes found in sea or dissolved in ground water adjacent to concrete structures.

o When evaporation takes place from an exposed surface, the sulphates may accumulate at that surface, thus increasing their concentration and potential for causing deterioration.

o BS 8110 stipulated that OPC is not to be used for Class 3 sulphate concentration.

o However, PBFC and SRPC is acceptable. o As most of the sea water and coastal environment falls

within the moderate sulphate condition, it appears that OPC is NOT the best option.

o Resistance to chloride attack

o Due to tropical climate, chloride attack is a severe problem for concrete construction in the marine environment.

o Chloride attack as measured by chloride diffusion into concrete is reduced by using Portland-Blastfurnace Cement.

o Resistance to alkali silica reaction(ASR) o Alkali Silica Reaction which causes concrete to crack and

disintegrate is caused by deleterious aggregates which react with the Sodium Equivalent in cement.

o The department of Civil Engineering in University Malaya has carried out an independent and extensive research on potential ASR of local aggregates and it is noted that 50% of all the location studied consist of deleterious aggregates that is very favorable for ASR to occur.

o Consequently, the construction industry should be made aware of this potential problem and ensure that the necessary preventive measures are being adhered to.

o In order to minimize the risk of ASR, a number of consultants have started to specify the control of ASR in the concrete specifications. Portland- Blastfurnace Cement is effective in reducing the risk of alkali silica attack caused by reactive aggregates.

6. Sulphate–Resistance Portland Cement (SRPC)

Is achieved by reducing C3A to a minimum since that compound is most susceptible to sulphate attack.

Higher content of C4AF. Sulphate compounds can be found in ground water and sea water. SRPC tends to be darker in colour than OPC.

Uses of SRPC

Where concrete structures are extensively exposed to sea water or ground water and soil containing soluble sulphates to ensure long-term protection from sulphates attack.

SRPC highly recommended for hydraulic structures exposed to water with high alkali content & structures subjected to seawater exposure:

Port facilities, e.g. jetties and other marine concrete structures.

Coastal protective structures, e.g. sea-walls, breakwater

Underground structure canals and culverts. Structures in swampy areas, e.g. footings, ground

beams, piles, etc.

Advantages of SRPC

High resistance to attack by sea water and soils containing soluble sulphates.

Lower susceptibility to dissolution and leaching. Lower heat of hydration and hence less risk of thermal

shrinkage. Greater resistance to cracking. Improve durability in aggressive soil conditions.

7. High Strength Portland Cement (HSPC)

Same materials as OPC. Higher strength – achieved by increasing C3A content & finer

grinding of the clinker. Initial and final setting times = OPC. At higher water cement ratios, the high strength concrete has about

80% higher strength and at lower water cement ratios 40% higher than OPC

8. Masonry Cement

Use mainly for plastering of brick wall. The cement mortar in plastic condition for quite sometime before

hardened. Consist of OPC + fine inert admixture + plasticizing agent. It is a homogeneous blend of controlled amounts of Portland

Cement, plasticizing material and air entraining agent, inter-ground to a high fineness to give consistent quality.

Application

Masonry Cement is an extremely versatile material. It is highly recommended for the following:

o -Bedding and pointing brickwork and blockwork. o -Interior and exterior plasteringo -Wall finishes due to its low susceptibility to efflorescence.

Its economics :-

Unlike conventional mortar which is a mixture of 4 ingredients, i.e. Portland Cement, lime, sand and water, masonry mortar requires only 3 ingredients, i.e. masonry cement, sand and water.

This means that masonry cement is: o Savings in labor cost. o Less wastage. o Minimum storage requirement.

Advantages

Masonry cement– the easy, efficient, economical and environment friendly way of mixing mortar. Its excellent water retaining property prevents premature loss of water, therefore ensuring:

Strong bonding Low drying shrinkage Better weather resistance Good workability Easier handling Smoother finish

Manufacture of portland cement

o The process of manufacture of cement consists essentially of :-

Terms and definition

WASH MILL Circular pit with revolving radial arms carrying rakes which

breaks up the lumps of solid raw materials BALL MILL

A mill where coarse materials are grounded to fine particles SLURRY

A liquid of creamy consistence with a water content of between 35% & 50% and only a fraction of the material about 2% (larger that 90µm BS sieve size)

Kept in a storage tanks such as:- Soda ash Na2 CO3 (Sodium Carbonate) à reduce

viscosity Sodium silicate Na2 SiO3 à reduce moisture

ROTARY KILN A large steel cylinder lined with fired bricks (refractory

lined). 2-5 m Ø, about 100-150 m long, thickness of steel sheeting

= 20m The cylinder is slightly inclined to the horizontal & rotates

about it’s axis at a speed of 1-2 times in 2 minutes.

CLINKER Is the product of slurry inside a rotary kiln where slurry

undergoes chemical changes due to high temp. and lime, silica& alumina recombine mass then fuses into ball of 3-25mm Ø

Grinding the raw materials (treatment of raw materials)

Mixing them intimately in certain proportions

Burning in a large-rotary kiln at a temperature of approximately 1300◦C to 1600◦C when the materials are

partially fused into balls known as clinker.

The clincker is cooled and ground to a fine powder with some gypsum added.

Manufacturing process (wet) Grinding and Mixing

• Chalk & clay are broken up & dispersed in water in a wash mill.

• The 2 mixtures are pumped, mixed in predetermined proportions & passed through a series of screens.

• The coarse materials are passed through a ball mill for secondary crushing & re-screened.

• The resulting cement slurry is then pumped into large storage tanks (sedimentations of the suspended solid is prevented by mechanical stirrers @ bubbling by compressed air.

• If limestone is used, it has to be blasted with explosives, quarried & crushed in primary and secondary crushers (may be stored in silos).

• The crushed limestone is then mixed with clay slurry in the required proportions.

• Limestone+clay all fed into a ball grinding mill (which is wet because of the presence of clay slurry)

• The crushing of limestone are grounded to the fineness of flour à resultant cements lurry is pumped into storage tanks. (from here onwards the process is the same regardless of the original nature of raw materials.

Burning in Rotary Kiln • Slurry is then passed into a rotary kiln. • The kiln rotates about its axis at a speed of 1-2 times in 2

minutes. • The rotation is controlled by inclination of the kiln & by

speed of rotations. • The slurry is fed at the upper thus end of the rotary kiln with

pulverized coal is blown by an air blast at the lower end of the kiln raising the temperature inside the kiln (1400-1500◦C).

• The coal (should not have too high ash content) deserves a special mention because up to 350 kg of coal is used to make a ton of cement.

• Oil or natural gas can also be used instead of coal. Cooling and Grinding

• The clinker is then dropped into a cooler situated below the kiln.

• The cool clinker is characteristically black. Glistering and hard.

• It is grounded with gypsum (2-5 %) à to prevent flash setting

• The grinding is done in a ball mill consisting of several compartments with progressively smaller steel balls to a size of 44 microns.

• The cement discharged by the mill is passed through a separator, and the fine particles being removed to a storage silo by air current while the coarser particles are passed through the mill once again.

Manufacturing Process (DRY)

In dry & semi dry process, the correct proportion of raw materials are crushed & fed into grinding mill (they are dried & reduced in size to a fine powder à raw meal).

Raw meal is pumped to a blending silo (final adjustment in requisite proportions of materials required are made).

The raw material is blended by means of compressed air to obtain a uniform & intimate mixture

Factor affecting the properties of cement

They include:

1. Chemical composition2. Fineness

a) Hydration of cementb) Setting timec) Soundnessd) Loss on ignition

1. Chemical composition

Raw materials used in the manufacture of PC consist mainly of lime, silica, alumina & iron oxide.

The four main compound change in % of them will produce different types of cement

2. Fineness

Is a measure of the sizes of particles of cement & expressed in term of specific surface of cement.

The more the fine the cement is ground, the greater will be its specific surface.

More fineness of cement, more rate of hydration & rapid development of strength.

The fineness is most important factor which determine the properties of cement :-

Finer grinding increases the speed with which the various constituents reacts with water but does not alter their coherent properties.

Fineness of grinding is of some importance in relation on the workability of concrete mixes.

Greater fineness increases the cohesiveness of a concrete mix.

Fineness increases the chance of shrinkage cracking (resulted from both cooling & drying of concrete)

In some special type of cement the strength increases slowly than normal though they are finely grounded.

Fineness of cement can be carried out via :- Sieve analysis through a 90 micron sieve Special surface area: Air permeability method Wagners turbidimeter method

Table 2.0 Fineness of Different Portland Cement

Cement Type Specific Surface Area

Ordinary Portland Cement 225m2/kg

Rapid Hardening Portland Cement

325 m2/kg

Low Heat Portland Cement 320 m2/kg

a) Hydration of Cement

Heat is liberated as cement set & hardened by reacting with water.

It is the rate in which heat is released governed the strength of concrete.

The rate of heat >>> depends on the composition of cement. Fineness >>> affect the rate of hydration & heat but total

amount of heat liberated in unaffected by fineness Table 3.0 Minimum Compressive Strength of Concrete Cubes

at 3 different ages (specified by BS 4550 for Portland Cement)

Type of Portland Cement

Compressive Strength(N/mm2)3 days 7

days28 days

i) OPC

13 - 29

ii) RHPC

18 - 33

iii) PBC

8 14

22

iii) LHPC

5 - 19

iv) SRPC

10 - 27

b) Setting Time

Is the time from the addition of water to the initial & final setting stage.

It also refers to the changes of the cement paste from fluid to rigid. It involves chemical reaction between cement & water, hydration

resolves in the formation of gel around each cement particles & temperature change.

Table 4.0 Setting Time for Different Portland Cement

Type of Portland Cement

Setting Time

Initial setting time, minutes (min)

Final setting time, minutes (max)

i) OPC 30 600

ii) RHPC 30 600

iii) LHPC 60 600

The means of controlling the rate at which cement stiffened is by inter grinding a measured quantities of gypsum.

Initial Setting Time

- the time of addition of water to cement to the beginning of the noticeable stiffening in the cement paste.

- correspond to the rapid rise in temp.- normally about 45-175 minutes.

Final Setting Time

- time of beginning of addition of water to cement to completion of setting i.e.cement paste becomes stiff.

- Correspond to the peak temp. in the cement paste - stiffening increases as the gel increases and the stage at

which this is complete, the final hardening process begins

- normally 3-10 hrs.

Hardening

- refer to the gain of the strength of the cement paste. (during setting time, cement gained very little strength)

- Setting time can be determined with the Vicat Apparatus (detailed are explained in BS 4550).

c) Soundness

o unsound cement --> excessive change in volume, particularly expansion of cement paste after setting.

o unsoundness due to presence of free lime & magnesia in cement.

o Free lime hydrates very slowly because it is covered by thin film of cement (prevents direct contact between lime and water).

o After setting of cement, the moisture penetrates into the free lime resulting in its hydration.

o Since slaked lime occupies a larger volume, the expansion takes place resulting in severe cracking.

o The unsoundness may be reduced by :-o Limiting the MgO content to less than 0.5%o Fine grindingo Allowing the cement to aerate for several dayso Thorough mixing

d) Loss on Ignition

Test done to determine the loss of weight when cement sample is heat to 900ºC - 1000ºC.

Loss of weight due to evaporation of moisture & CO2 which are present in combination with free lime or magnesia.

The presence of moisture causes prehydration of cement. Loss of weight à is a measure of the freshness of cement. Inert substances à hydroxide & carbonates of lime &

magnesium have no cementing property < loss of ignition, < quantity of inert substances, better the

cement The loss on ignition is determined by heating 1g of cement

sample in a platinum crucible at a temp of 900ºC - 1000ºC for min 15 minutes.

Normally the loss will be around 2%. Max allowable = 4%

Definition and Terms Chemical Composition

Differences in chemical composition, particularly with supplementary cementitious materials that could be less uniform than Portland cement, could affect early and ultimate strengths, heat released, setting time, and resistance to deleterious materials.

Fineness

The fineness of the cement or supplementary cementitious materials affects heat release and rate of hydration. Finer materials react faster, with a corresponding increase in early strength development, primarily during the first 7 days. Fineness also influences workability, since the finer the material, the greater the surface area and frictional resistance of the plastic concrete.

Soundness

Refers to the ability of the cement paste to retain its volume after setting, and is related to the presence of excessive amounts of free lime or magnesia in the cement or supplementary cementations material.

Setting Time

The setting time for the cement paste is an indication of the rate at which hydration reactions are occurring and strength is developing and can be used as an indicator as to whether or not the paste is undergoing normal hydration reactions.

False Set – false set or early stiffening of the cement paste is indicated by a significant loss of plasticity without the evolution of heat shortly after the concrete is mixed.

Compressive Strength – compressive strength is influenced by cement composition and fineness. Compressive strengths for different cements or cement blends are established by compressive strength testing of mortar cubes prepared using a standard graded sand.