Cement - Friedrich W. Locher - ebook

Cement ebook

Friedrich W. Locher

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The successful launch of the German standard work on Cement by Prof. Locher in 2000 is now being followed by the publication of the widely requested English language version "Cement" which takes special country-specific features and standards into account. The book is aimed at chemists, physicists, engineers and technologists in the Cement industry, in machine construction, the construction industry, materials testing and environmental protection. This clear and practical book will provide them with the understanding of the chemistry of Cement needed for their daily work. It will also make an ideal textbook for the study of building materials science at colleges and universities.

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LocherCement – Principles of production and use

Cementprinciples of production and use

Prof. Dr. rer. nat. Friedrich W. Locher

VLB-Meldung

Locher, Friedrich Wilhelm:CementPrinciples of production and useDüsseldorf: Verlag Bau+Technik GmbH, 2006

ISBN 3-7640-0420-7eISBN 978-3-7640-0536-8

© by Verlag Bau+Technik GmbHGesamtproduktion: Verlag Bau+Technik GmbH,Postfach 12 01 10, 40601 Düsseldorfwww.verlagbt.de

Foreword

The production and use of cement are complex processes in which important parts are played by the cost-effectiveness of the operations and the measures to protect the environment. An understanding of the material processes and interrelationships involved is necessary to grasp and solve the problems that arise. This involves cement chemistry, the scope of which has widened greatly since cement was first used for building and for a long time has also included the methods and discoveries of mineralogical and crystallographic as well as chemical and physical research. This book is intended to provide an overview of the current understanding of the essential facts of cement chemistry. It covers not only the composition and properties of the cements and the reactions that occur during their production and use but also the material problems of environmental protection.

The book is aimed at all chemists, physicists, engineers and technicians working in the cement industry, machine construction, the building industry, materials testing and environmental protection to provide them with the knowledge of the chemistry of cement necessary for their work. It is also intended for use as a textbook for the study of materials science in colleges and universities. I have therefore tried to show the interrelationships in a way that is as readily understandable as possible, to explain the technical expressions and to describe the principles of measurement on which the methods of technical and scientific investigation are based.

Since the beginning of the production of Portland cement in the middle of the 19th century the research in works laboratories and research establishments of the cement industry has been directed towards the problems occurring during the production and use of cement. The initial focus was on the optimum composition of the raw material mix with respect to burning the cement clinker and on the soundness of the masonry mortar produced from the cement. After that there was the much debated problem of the use of latent hydraulic and pozzolanic substances as cement constituents, and especially their influence on the durability of structures built with such cements. New problems arose with the increased introduction of energy-saving burning procedures and with the change of fuel from oil to coal as a result of the increasing cost of crude oil. The rapid spread of ready-mixed concrete, which caused a fundamental change in the requirements for the workability properties of the cement, also required intensive research work, especially in relation to the optimum retardation of the setting. For a long time the cement industry has become increasingly involved with environmental protection and with environmentally compatible utilization of high-caloric wastes in the firing systems of cement kilns. Here again, the emphasis has been set increasingly on chemical questions, such as the environmentally relevant constituents of gases and aerosols.

This English edition takes account of the changes in the standards for cement and concrete and in environmental protection law that have been introduced in Europe since the appearance of the German edition in 2000.

The book is based on the lectures that I gave at Clausthal Technical University from 1959 to 1999. My special thanks for their all-round interest and encouragement go to the members of the German Cement Works Association, its board and its management team. It was a great help when preparing the manuscript that I had access to the extensive international technical literature in the library of the Research Institute of the Cement Industry in Düsseldorf. I remember with thanks the willing support provided by my colleagues at the Düsseldorf Institute, at the Institute for Non-Metallic Materials of Clausthal Technical University, at the Institute for Rock Metallurgy at Aachen College, at the College Institutes in Weimar and Berlin and at other college institutions.

I would also like to thank Mr Robin B.C. Baker of Cranbrook, Great Britain, for his professional and dependable translation into English, as well as the staff of Verlag Bau+Technik GmbH for their cooperation and many suggestions as well as for the publication of this book.

I dedicate my book to my dear wife, Eva Locher, who has followed its development with great interest and active concern, and to our sons, Dietrich, Christian and Georg, to whom I am indebted for much advice on new developments in environmental protection and for help in dealing with the computer.

 

 

Ratingen, August 2005

Friedrich W. Locher

Table of contents

1

Classification of cements

1.1

Definition

1.2

European and German standard cements

1.2.1

General

1.2.2

Constituents of the European and German standard cements

 

1. Portland cement clinker (K)

 

2. Granulated blastfurnace slag (S)

 

3. Pozzolanic material (P and Q)

 

4. Fly ash (V and W)

 

5. Burnt shale (T)

 

6. Limestone (L, LL)

 

7. Silica fume (D)

 

8. Minor additional constituents

 

9. Calcium sulfate

 

10. Additives

1.2.3

Types of cement in DIN EN 197-1:2000 (Feb. 2001)

1.2.4

Application of DIN EN 197-1:2000 (Feb. 2001) in Germany

1.2.5

Requirements of the cements conforming to DIN EN 197-1:2000 (Feb. 2001)

1.3

Cements covered by the ASTM standards

2

History of cement

2.1

The material basis of hydraulic binders

2.2

Burning the cement clinker

2.3

Comminution of raw materials and cement

2.4

Environmental protection

2.5

Glassy blastfurnace slag

2.6

Cements with special properties

2.7

Cement standards

3

Cement clinker

3.1

Composition of cement clinker

3.1.1

General survey

3.1.2

Tricalcium silicate

3.1.3

Dicalcium silicate

3.1.4

Tricalcium aluminate

3.1.5

12/7 calcium aluminate

3.1.6

Monocalcium aluminate

3.1.7

Calcium dialuminate

3.1.8

Calcium aluminoferrite

3.1.9

Clinker compounds containing alkalis

3.1.10

Free CaO and free MgO (periclase)

3.1.11

Glass

3.1.12

Alinite

3.1.13

Calcium aluminosulfate 3CaO·3Al2O3·CaSO

3.1.14

Spurrite

3.2

Production of cement clinker

3.2.1

Introduction

3.2.2

Nature of the raw materials

3.2.3

Extraction and processing of the raw materials

3.2.4

Fuels

3.2.5

Process technology of burning and cooling cement clinker

3.2.6

Reactions during the burning and cooling of cement clinker

3.2.7

Factors affecting the reactions during clinker burning

 

1. Agglomeration of the kiln feed

 

2. Sintering behaviour of the kiln feed

 

3. Influence of additives on clinker formation and cement properties

3.2.8

Energy requirement of the burning process

 

1. Theoretical energy requirement for clinker formation

 

2. Energy requirement for evaporating the water

 

3. Enthalpy content of the kiln exhaust gases

 

4. Enthalpy content of the cooler exhaust air

 

5. Wall losses from preheater, kiln and cooler

 

6. Enthalpy content of the clinker on leaving the cooler

3.2.9

Factors affecting the fuel energy requirement

3.2.10

Influence of clinker cooling on the quality of the cement clinker and of the cement

3.2.11

Influence of the kiln atmosphere on the quality of the cement clinker and of the cement

3.2.12

Coating formation in cement kiln systems

3.3

Assessing the cement clinker

3.3.1

Microscopic assessment

3.3.2

Determination of phase composition by X-ray diffraction analysis

3.3.3

Calculating the phase composition

3.3.4

Lime saturation factor

3.3.5

Silica ratio

3.3.6

Alumina ratio

3.3.7

Free CaO, litre weight

4

Other main constituents

4.1

General survey

4.2

Granulated blastfurnace slag

4.3

Burnt shale (oil shale)

4.4

Natural pozzolanas

4.5

Synthetic pozzolanas

4.5.1

Fly ash

4.5.2

Silica fume

4.5.3

Rice husk ash

4.5.4

Calcined clay

4.6

Limestone

5

Grinding the cement

5.1

Grinding processes

5.2

Fineness, particle size distribution

5.3

Grindability

5.4

Grinding aids

6

Environmental protection during the manufacture of cement

6.1

General review

6.2

Dust

6.2.1

Dust emission

 

1. Nature and quantity of the dust

 

2. Industrial equipment for reducing dust emission

 

3. Measuring the dust emission

 

4. Limiting and monitoring the dust emission

6.2.2

Dust dispersion, dust precipitation

6.2.3

Effect of dust

6.3

Vaporizable constituents, recirculating systems, balances, emission and immission

6.3.1

Basic principles

6.3.2

Recirculating dust system

6.3.3

Alkalis

6.3.4

Sulfur

6.3.5

Fluoride

6.3.6

Chloride, bromide, iodide

6.3.7

Environmentally relevant trace elements

 

1. General survey

 

2. Nickel, chromium, arsenic, antimony

 

3. Zinc, lead

 

4. Cadmium

 

5. Thallium

 

6. Mercury

 

7. Volatility of trace elements

6.3.8

Emission of vaporizable constituents

 

1. Emission limitation

 

2. Measuring the emissions

 

3. Contribution by the fuels to the emission of trace elements

 

4. Emission prediction, reduction of emissions

6.3.9

Immission of vaporizable constituents, effect on the environment

 

1. Immission and immission limitation

 

2. Effect of thallium on plants

6.4

Gases

6.4.1

General review

6.4.2

Carbon dioxide

6.4.3

Carbon monoxide

6.4.4

Organic compounds

6.4.5

Sulfur dioxide

6.4.6

Nitrogen oxides

 

1. NO formation

 

2. NO2 formation, NO-NO2 recirculation, NOx

 

3. Factors affecting the NOx emission from cement kilns

 

4. Selective non-catalytic NO reduction (SNCR technology)

 

5. Limiting the NOx emissions

6.4.7

Removal of gaseous constituents

7

Cement hardening

7.1

Introduction

7.2

Hydration products

7.2.1

General survey

7.2.2

Calcium hydroxide, magnesium hydroxide

7.2.3

Calcium silicate hydrates

 

1. Solution equilibrium

 

2. Morphology, structure

 

3. Hydration of C3S and b-C2S

 

4. Structural elements of C–S–H

 

5. Calcium silicate hydrates at higher temperatures

7.2.4

Calcium aluminate hydrates

 

1. Solution equilibria, stable and metastable calcium aluminate hydrates

 

2. C4AH19, crystal structure and de-watering characteristics

7.2.5

Calcium ferrite hydrates

7.2.6

Sulfatic hydrates and related compounds

 

1. AFt compounds

 

2. AFm compounds

 

3. Syngenite

 

4. AFt and AFm compounds in hardened cement

7.2.7

Hydrogarnet

7.2.8

Gehlenite hydrate

7.3

Hydration reactions

7.3.1

Water requirement

7.3.2

Bleeding

7.3.3

Composition of the aqueous solution

7.3.4

Course of the hydration

 

1. Portland cement

 

2. Portland oil shale cement

 

3. Cements containing granulated blastfurnace slag

 

4. Cements containing pozzolana or fly ash

 

5. Cement with added silica fume

7.3.5

Setting

 

1. Setting reactions and progress

 

2. Factors affecting the reactivity of tricalcium aluminate Drel C3A

 

3. Factors affecting the quantity of available sulfate

7.3.6

Hardening

 

1. Cause and behaviour pattern of hardening

 

2. Influence of clinker composition

 

3. Influence of cement composition

 

4. Influence of fineness and particle size distribution

 

5. Influence of the water/cement ratio

 

6. Influence of additions

 

7. Influence of temperature, heat-treatment, delayed ettringite formation

 

8. High-pressure steam curing

7.3.7

Heat of hydration

 

1. General

 

2. Heat of solution calorimeter

 

3. Adiabatic calorimeter

 

4. Semi-adiabatic process

 

5. Heat flow calorimeter

 

6. Heats of hydration of the cements and their constituents

8

Constitution and properties of hardened cement paste

8.1

Water bonding

8.2

Specific surface area and particle size of the hydration products

8.3

Microstructure

8.3.1

Models

8.3.2

Void filling

8.3.3

“Outer” and “inner” hydration products

8.3.4

Contact zone between hardened cement paste and aggregate

8.4

Porosity

8.4.1

General survey

8.4.2

Methods of measurement

 

1. Pycnometer method

 

2. Saturation with a liquid

 

3. Capillary condensation

 

4. Mercury intrusion porosimetry

 

5. Microscopic measuring methods

 

6. Other methods

8.4.3

Results, conclusions

8.5

Strength

8.5.1

General

8.5.2

Influence of porosity

8.5.3

Specific strength of the hardened cement paste

8.5.4

Hardening, influence of the water/cement ratio and the degree of hydration

8.5.5

DSP and MDF materials

8.6

Deformation

8.6.1

General survey

8.6.2

Modulus of elasticity

8.6.3

Shrinkage and swelling

8.6.4

Creep

8.6.5

Thermal deformation

8.7

Permeability

8.7.1

General survey

8.7.2

Permeation

8.7.3

Diffusion

8.7.4

Capillary suction

8.7.5

Factors affecting the impermeability of hardened cement paste, mortar and concrete

8.7.6

Water-impermeable concrete

8.8

Effect on metals, corrosion protection

8.8.1

General review

8.8.2

Electrochemical reactions, standard potential

8.8.3

Corrosion reactions of iron

8.8.4

Carbonation of hardened cement paste, mortar and concrete

8.8.5

Action of chloride

 

1. Corrosion mechanism

 

2. Chloride combination, threshold value

 

3. Penetration of chloride into concrete

 

4. Factors affecting chloride-induced corrosion of steel reinforcement

8.8.6

Stress corrosion

8.8.7

Corrosion protection of the steel reinforcement

8.8.8

Corrosion and corrosion protection of non-ferrous metals

8.9

Resistance to chemical attack

8.9.1

General review

8.9.2

Action of substances which attack concrete

 

1. Dissolving attack

 

2. Expansive attack

 

3. Attack by seawater

 

4. Attack by soils

 

5. Attack by gases

8.9.3

Assessing the chemical attack

8.9.4

Structural preventive measures

8.9.5

Discoloration, efflorescence

8.10

Alkali-aggregate reaction

8.10.1

General survey

8.10.2

Alkali-sensitive silica and silicates in the aggregate

8.10.3

Mechanism of the alkali-silica reaction

 

1. Chemical processes

 

2. Effect of the alkali-silica reaction in concrete

 

3. Influence of the alkali-sensitive aggregate; “pessimum”

 

4. Alkali content of the cement; cement type

 

5. Composition of the concrete

8.10.4

Preventive measures against the concrete-damaging alkali-silica reaction

 

1. General review

 

2. Testing the alkali-sensitivity of aggregates containing silica

 

3. Ambient conditions

 

4. Concrete technology measures

8.10.5

Alkali-carbonate reaction

 

1. Alkali-sensitive carbonate rocks

 

2. Chemical reactions and expansion mechanism

 

3. Testing the alkali sensitivity of carbonate rocks

 

4. Concrete technology measures

8.11

Freeze-thaw resistance

8.11.1

Mechanism of freeze-thaw attack

 

1. Hydrodynamic pressure (hydraulic pressure)

 

2. Diffusion

 

3. Growth pressure of the ice crystals

 

4. Thermal expansion of the ice crystals

8.11.2

Course of the freeze-thaw attack

8.11.3

Factors affecting the freeze-thaw attack

 

1. Degree of filling of the pores, degree of saturation

 

2. De-icing agents

 

3. Aggregate

 

4. Air voids

 

5. Composition of the concrete

 

6. Carbonation

8.11.4

Testing the resistance to freeze-thaw and to freeze-thaw with de-icing agent

9

Standard cements with special properties, special cements

9.1

General review

9.2

Cement with high sulfate resistance

9.2.1

Characterization

9.2.2

Accelerated test methods

 

1. General review

 

2. Le Chatelier-Anstett test

 

3. ASTM C 452 - potential expansion of Portland cement mortar during sulfate attack

 

4. Sulfate expansion of low-cement mortar

 

5. Koch-Steinegger small prism method

 

6. The Wittekindt flat prism method

 

7. Comparison of the small prism and flat prism methods - investigations by the German Cement Works Association from 1957 to 1964

 

8. Other accelerated methods

9.2.3

Influence of cement composition and additions on sulfate resistance

 

1. Portland cement and blastfurnace cement with high sulfate resistance

 

2. Concrete additions

9.3

Cement with low heat of hydration

9.4

Cement with low effective alkali content

9.5

Regulated set cement (Quick-hardening cement)

9.6

Expansive cement

9.7

Oil well cement

9.8

Hydrophobic cement

9.9

Ultrafine binders

9.10

Cement for sprayed concrete

9.11

Masonry cement

9.12

Supersulfated cement

9.13

High-alumina cement

9.13.1

Definition and description

9.13.2

Manufacture

9.13.3

Composition

 

1. Chemical composition

 

2. Phase composition of standard high-alumina cement

 

3. Determining the phase composition of standard high-alumina cements

9.13.4

Hydration

9.13.5

Microstructure and properties of hardened high-alumina cement

9.13.6

Transformation of the hydration products and their effect on the properties of hardened high-alumina cement

9.13.7

Mixtures with other cements and cement constituents

9.13.8

Refractory concrete made with high-alumina cement

10

Environmental compatibility of cement and concrete

10.1

Cement dust

10.2

Alkaline action

10.3

Action of chromate

10.4

Fixation of environmentally relevant substances with cement

10.5

Radioactivity and concrete

10.5.1

Radioactive radiation

10.5.2

Half-life value of radioactive elements

10.5.3

Units of measurement for radioactivity

10.5.4

Radioactive exposure of human beings

10.5.5

Radioactivity of building materials

10.5.6

Radon

11

Literature

12

Index

13

Chemical Formulae

1 Classification of cements

1.1 Definition

Cement is a hydraulic binder, i.e. an inorganic, non-metallic, finely ground substance which, after mixing with water, sets and hardens independently as a result of chemical reactions with the mixing water and, after hardening, it retains its strength and stability even under water. The most important area of application is therefore the production of mortar and concrete, i.e. the bonding of natural or artificial aggregates to form a strong building material which is durable in the face of normal environmental effects. The difference between mortar and concrete is governed by the particle size of the aggregate, which in mortar has a maximum value of about 4 mm and in concrete can be as large as 32 mm but in special cases may be smaller or larger.

Hydraulic hardening is caused primarily by the formation of calcium silicate hydrates. Cements therefore consist of those substances, or mixtures of substances which, through reaction with the mixing water, form calcium silicate hydrates sufficiently rapidly in a quantity sufficient to provide strength and durability. However, other compounds, e.g. calcium aluminates, may also participate in the hardening process.

In contrast to these silicate cements the high-alumina cements consist predominantly of calcium aluminates. Their hardening is based on the formation of calcium aluminate hydrates.

1.2 European and German standard cements

1.2.1 General

In practically all countries there are standards for cement as a basic material for the production of mortar and concrete. Differences in economic and industrial development, in raw material deposits and in climatic conditions have led to the development of different construction materials and methods of construction in the different countries, and hence also to different types of cement. There are therefore also substantial differences in the national cement standards which, among other things, also affect the specifications for the durability of concretes produced from the cements.

In Europe the work on compiling the technical basis for a European cement standard has been in progress since 1975. The emphasis was initially on consistent test methods, which are listed in EN 196 [D 42]. A Europe-wide consistent designation of the types of cement, their compositions and cement strength classes was defined in the standard EN 197-1:2000 [E 26]. This was based on all the cements with calcium silicate hardening which are produced in the countries of central and western Europe for general use [A 19]. Cements with additional special properties (special cements) and cements with different hardening mechanisms are to be dealt with in further parts of this standard [E 26, S 193]. Of the 27 cements in EN 197-1 only 12 cements were included initially in the German cement standard DIN 1164 (October 1994) [D 51]. These 12 cements were included because they had already proved successful with regard to the durability of concretes produced from them. Since 1st April 2001 the European cement standard designated DIN EN 197-1:2000 (Feb. 2001) has had the status of a German standard. It therefore replaces DIN 1164-1 (Oct. 1994). The use of the cements specified in the new standard DIN EN 197-1 is regulated by DIN EN 206-1 and DIN 1045-2 [D 44, D 49, w 2] (Section 1.2.4).

1.2.2 Constituents of the European and German standard cements

Cement constituents defined in DIN EN 197-1:2000 (Feb. 2001) [D 43] are:

1.  Portland cement clinker (K)

2.  Granulated blastfurnace slag (S)

3.  Pozzolanic material (P and Q)

4.  Fly ash (V and W)

5.  Burnt shale (T)

6.  Limestone (L, LL)

7.  Silica fume (D)

8.  Minor additional constituents

9.  Calcium sulfate

10.  Additives

The constituents of cement are sub-divided into main and minor additional constituents [E 26, D 51]. Main constituents are the substances listed under 1 to 7, provided their content in the cement exceeds 5 % by mass. Minor additional constituents can be all the substances listed under 1 to 8, provided they have a maximum content of 5 % by mass in the cement, as well as inorganic mineral substances from clinker production. The data concerning the cement composition, and also concerning the proportions of calcium sulfate and additives, always relate to the total of all main and minor additional constituents in the cement without taking the calcium sulfate and additives into account.

1. Portland cement clinker (K)

Portland cement clinker is also known as cement clinker or just clinker. At least twothirds of it consists of the two calcium silicates, namely tri- and di-calcium silicate, which are richest in CaO and can react with the mixing water and harden reasonably rapidly. It is therefore a hydraulic substance.

2. Granulated blastfurnace slag (S)

Granulated blastfurnace slag is a granulated, rapidly cooled, and therefore predominantly glassy, basic blastfurnace slag. It is a latent hydraulic substance because it reacts only slowly with water, but when mixed with activators, such as cement clinker, it reacts and hardens relatively rapidly with the formation of calcium silicate hydrates. It must consist of at least two-thirds by mass of glassy slag and at least two-thirds of CaO, MgO and SiO2.

3. Pozzolanic material (P and Q)

Pozzolanic materials are natural or industrial substances which, because of their content of reactive silicon dioxide, SiO2, react when finely ground in the presence of water at normal ambient temperature with dissolved calcium hydroxide, form calcium silicate hydrates, and as a result can harden hydraulically. Reactive silicon dioxide, which is present either as free SiO2 or combined in aluminosilicates, is therefore essential for the pozzolanic hardening. Calcium aluminate hydrates, which can also contribute to the strength formation, are therefore also formed. The proportion of reactive CaO is unimportant. The content of reactive SiO2 content must be at least 25 % by mass.

Although fly ash and silica fume have pozzolanic properties they are dealt with separately in Sections 4 and 7.

Natural pozzolanas (P) are usually materials of volcanic origin or sedimentary rock of suitable chemical and mineralogical composition. This also includes trass as defined in DIN 51043 [D 64].

Industrial pozzolanas (Q) can be thermally treated and activated clays and shales, and aircooled slags from the extraction of lead, copper or zinc, provided they contain sufficient concentrations of reactive SiO2.

4. Fly ash (V and W)

Fly ash is obtained by electrostatic or mechanical precipitation of dust particles from the exhaust gases from furnaces. It may only be used for cement production if it comes from a furnace fired with pulverized coal. The fly ash is either an aluminosilicate or a calcium silicate depending on how the silicon dioxide is chemically combined. Because of the content of reactive silicon dioxide both types have pozzolanic properties, and calcium silicate fly ash also has hydraulic properties. In order to limit the content of incompletely burnt substances the loss on ignition must not exceed 5.0 % by mass.

Siliceous fly ash (V) is a fine powder, consisting predominantly of spherical and glassy particles, which has pozzolanic properties. It must contain less than 5 % by mass of reactive CaO and at least 25 % by mass of reactive SiO2.

Calcareous fly ash (W) is a fine powder with hydraulic and/or pozzolanic properties. The content of reactive CaO must not be less than 5 % by mass. Calcareous fly ash, containing between 5 and 15 % by mass of reactive CaO, must contain more than 25 % by mass of reactive SiO2.

5. Burnt shale (T)

Burnt oil shale has particular importance as a constituent of hydraulic binders. It is produced in a special furnace at temperatures of approximately 800 °C. Because of the content of calcium carbonate and sulfur in the natural starting material the burnt oil shale contains clinker phases, mainly dicalcium silicate and monocalcium aluminate, as well as small quantities of free CaO and calcium sulfate and larger proportions of pozzolanically reacting substances. In a finely ground state such burnt shales therefore exhibit not only hydraulic properties, such as those of Portland cement, but also pozzolanic properties. During strength testing in mortar in accordance with the cement standard DIN EN 196 [D 42], but after moist storage instead of water storage [D 51], finely ground burnt oil shale must reach a compressive strength of 25.0 N/mm2 at 28 days. It must also be sound when mixed with 70 % by mass of Portland cement [D 51, D 42].

6. Limestone (L and LL)

Limestone must meet the following requirements:

a) The limestone must contain at least 75 % by mass of CaCO3, calculated from the CaO content

b) The clay content, determined by the methylene blue adsorption [D 46] on the pulverized limestone, must not exceed 1.20 g/100 g

c) The total content of carbon TOC as a measure of the content of organic constituents [C 8, D 47] must not exceed the following values:

Limestone LL

0.20 % by mass

Limestone L

0.50 % by mass

7. Silica fume (D)

Silica fume consists of very fine spherical particles with a content of amorphous silicon dioxide SiO2 of at least 85 % by mass. Silica fume must meet the following requirements:

a) The loss on ignition must not exceed 4.0 % by mass.

b) The specific surface area (BET) [B 122, I 11] must be at least 15 m2/g.

8. Minor additional constituents

Minor additional constituents are natural or synthetic inorganic mineral substances which, after appropriate preparation, improve the physical properties of the cement, e.g. its workability or water retention, through their particle size distribution. They can be inert or have slightly hydraulic, latent hydraulic or pozzolanic properties. However, no requirements are set for them in this respect. They must be correctly prepared, i.e. selected, homogenized, dried and comminuted, to suit their state of production or delivery. They must not increase the water demand of the cement appreciably, impair the resistance of the concrete or mortar, or reduce the corrosion protection of the reinforcement.

9. Calcium sulfate

Calcium sulfate, in the form of gypsum CaSO4·2H2O or β-anhydrite (β-CaSO4), or as a mixture of these compounds, is added in small quantities to the cement during its manufacture to control the setting. β-anhydrite is the naturally occurring modification of water-free CaSO4, and is also known as anhydrite II. α-anhydrite (anhydrite I) is the hightemperature modification of CaSO4, and is stable only at temperatures above 1180 °C. If part of the water content of gypsum is removed hemihydrate CaSO4·1/2H2O is formed, while complete dehydration produces “soluble”γ-anhydrite, γ-CaSO4, also known as anhydrite III. The hemihydrate occurs in two forms, known as α- and β-hemihydrate. They both have the same crystal lattice and differ only in the way they are formed, and are therefore not polymorphic modifications. The more coarsely crystalline α-hemihydrate with lower water demand is formed when gypsum is dehydrated in an autoclave, and β-hemihydrate with a substantially greater specific surface area and higher water demand is formed by “dry” dewatering in rotary kilns or boilers at temperatures from 120 °C to 180 °C [B 66, W 55, H 80, H 12, g 1].

Gypsum and β-anhydrite occur naturally, but the calcium sulfates which are generated in various industrial processes can also be used as setting regulators. This applies in particular to chemical gypsum, which is generated during the extraction of phosphoric acid from calcium phosphates (phosphogypsum) or during the extraction of hydrofluoric acid from fluorspar (fluogypsum), as well as to FGD gypsum, i.e. gypsum from flue gas desulfurization plants, mainly in power stations.

10. Additives

For the purpose of the European and German standards cement additives are constituents which are used to improve the manufacture or properties of cement, e.g. grinding aids. The total quantity of these additives should not exceed 1 % by mass. If this value is exceeded the precise quantity must be stated on the packaging and/or on the delivery document. These additives must not promote corrosion of reinforcement or adversely affect the properties of the cement or of the concrete or mortar made from the cement.

1.2.3 Types of cement in DIN EN 197-1:2000 (Feb. 2001)

DIN EN 197-1:2000 [D 43] just contains cements for general use, and not cements with special properties. It differentiates between the following five main categories:

CEM I

Portland cement

CEM II

Portland-composite cement

CEM III

Blastfurnace cement

CEM IV

Pozzolanic cement

CEM V

Composite cement

The subdivision of these five main categories into a total of 27 types of cement together with their designations are shown in Table 1.1.

CEM I is Portland cement containing at least 95 % by mass of Portland cement clinker. The main category CEM II covers cements which, in addition to clinker, contain one or more main constituents in a proportion of between 6 and 35 % by mass (silica fume up to a maximum of 10 % by mass). This proportion is subdivided again at 20 % by mass. The cement with the lower proportion is designated as A, and the cement with the higher proportion as B. CEM III is the designation for three types of blastfurnace cement A, B and C containing between 36 % and 95 % by mass of granulated blastfurnace slag with subdivisions at 65 % and 80 % by mass of granulated blastfurnace slag. CEM IV denotes two types (A and B) of pozzolanic cement containing between 11 and 55 % by mass of pozzolana, with a subdivision at 35 % by mass of pozzolana. These cements must pass the pozzolana test (Section 4.4). CEM V comprises composite cements which, in addition to cement clinker (K), contain 36 % to 80 % by mass of granulated blastfurnace slag (S) and/or pozzolana of natural (P) and/or industrial (Q) origin and/or siliceous fly ash (V), and are subdivided into A and B at 60 % by mass.

1.2.4 Application of DIN EN 197-1:2000 (Feb. 2001) in Germany

98 % of cement deliveries in Germany are currently (mid 2001) accounted for by just 6 of the 27 types of cement covered by DIN EN 197-1. In addition to Portland and blastfurnace cements these are the two Portland slag cements and Portland limestone cement. The market share taken by these Portland composite cements has increased significantly in recent years at the expense of Portland cement. The other cements are mainly used regionally or for specific purposes [S 264, V 89, V 88].

As binders for engineering concrete construction the types of cement previously covered by the German cement standard DIN 1164, shaded in Table 1.1, have long fulfilled all the requirements for strength development and durability. However, there has not yet sufficient practical experience with the majority of the 15 cements newly included in the standard, so their areas of application have to be restricted. This is decided by the ambient conditions, characterized by the exposure classes laid down in the standards DIN EN 206-1 and DIN 1045-2 [D 44, D 49, w 2], to which the particular component is to be assigned [V 87, V 90]. For example, there are restrictions on the use of pozzolanic cements CEM IV, composite cements CEM V, blastfurnace cements CEM III/C and some Portland composite cements CEM II-M in components which are exposed to the action of frost and/or chloride. Standard cements meet the requirements of DIN EN 197-1:2000 (Feb. 2001) or DIN 1164 (Nov. 2000). They are monitored regularly both through factory production control and by an outside testing laboratory. Conformity of the cement with the requirements of DIN EN 197-1 is indicated by the EG conformity symbol (CE symbol) which is printed on the delivery documents and the bags together with the identifying symbol of the certifying laboratory and information about the cement [F 44]. Cements with special properties as defined in DIN 1164 (Nov. 2000) will continued to be identified by the German conformity symbol (Ü symbol).

 

Table 1.1: Cement types and composition of 27 cements as defined in DIN EN 197-1 (Febr. 2001) [D 43]. Proportion in % by mass The cements of previously applied DIN 1164 (Oct. 1994) [D 51] are characterized by shading

FortsetzungTable 1.1

1.2.5 Requirements of the cements conforming to DIN EN 197-1:2000 (Feb. 2001)

1. Strength

Table 1.2: Strength classes of cements as defined in DIN EN 197-1:2000 (Febr. 2001) and colour codes [D 51]

 

Table 1.3: Chemical requirements for the cements as defined in DIN EN 197-1:2000 (Febr. 2001) [D 43]

2. Physical and chemical requirements

According to DIN EN 197-1:2000 (Feb. 2001) [D 43] and according to the German standard DIN 1164-1 [D 52] the setting as tested in accordance with DIN EN 196 3 [D 42] must not start before 75 min for cements of the 32.5 strength classes, not before 60 min for cements of the 42.5 strength classes, and not before 45 min for cements of the 52.5 strength classes. There is no limit for the final setting time.

The measure for soundness is the expansion during the Le Chatelier test as defined in DIN EN 196-3 [D 42]; it must not exceed 10 mm.

The chemical requirements which must be fulfilled by the cements complying with DIN EN 197-1:2000 are listed in Table 1.3. The values relate to the sample in the condition as supplied.

3. Cements with special properties

European standards are not yet available for cements with special properties. The existing regulations have been retained in the new version of DIN EN 197-1 and summarized in DIN 1164 (Nov. 2000) [D 52]. The following cements with special properties are accordingly standardized in Germany:

– NW cements with low heat of hydration

– HS cements with high sulfate resistance

– NA cements with low active alkali content

The types of cement and the requirements are shown in Table 1.4[D 52].

 

Table 1.4: Requirements for cements with special properties as defined in DIN 1164 (Nov. 2000) [D 52]

1.3 Cements covered by the ASTM standards

The ASTM standards of the United States of America are also of general importance. They contain regulations for the following cements:

– Portland cement as specified in ASTM C 150 [A 46]

– blended hydraulic cement of defined composition as specified in ASTM C 595M [A 60],

– blended hydraulic cement with defined performance features as specified in ASTM C 1157M [A 66],

– expansive cement as specified in ASTM C 845 [A 64].

According to the terminology standardized in ASTM C 219 [A 49] with respect to cement, blended hydraulic cement is a cement which is made from two or more inorganic substances, of which at least one is not Portland cement or cement clinker, and is produced by intergrinding or by separate grinding and mixing. Instead of “blended hydraulic cement” the correct designation is therefore “cements made from several main constituents”. In a note in ASTM C 595M [A 60] it is pointed out that appropriate equipment and controls are necessary to ensure the homogeneity and uniformity of these cements.

The Portland cements specified in ASTM C 150 and the cements made from several main constituents specified in ASTM C 595 are listed in Table 1.5. With the exception of the cement types P LH and P SR, corresponding to ASTM types IV and V, all cements can also contain air-entraining additives, indicated by A (air entrainment). The expansive cement specified in ASTM C 845 and other cements produced in the USA are described in Section 9.

Table 1.5: Portland cements and cements consisting of several main constituents as defined in ASTM C 150 and C 595

With the exception of Portland cements IV and V all cements can contain air-entraining additives, designated by A

The designations have the following meanings

RH

rapid hardening

MS

moderate sulfate resisting

SR

high sulfate resisting

MH

moderate heat of hydration

LH

low heat of hydration

2 History of cement

2.1 The material basis of hydraulic binders

The name “cement” goes back to the Romans who used the term “opus caementitium” to describe masonry which resembled concrete and was made from crushed rock with burnt lime as the binder. The volcanic ash and pulverized brick additives which were added to the burnt lime to obtain a hydraulic binder were later referred to as cementum, cimentum, cäment and cement [Q 1, h 1, d 2, l 2].

The significance of the clay content for the hydraulic properties of the hydraulic lime produced from a natural mixture of limestone and clay was discovered by the Englishman John Smeaton (1724-1792) when he was preparing to build the Eddystone lighthouse near Plymouth and was looking for a binder for water-resistant mortar. In 1796 his compatriot James Parker used the name “Roman cement” for the Roman lime which he burnt from the marl nodules in London septarian clay. The Frenchman Louis-Joseph Vicat (1786-1861) and the German Johann Friedrich John (1782-1847) discovered independently of one another that mixtures of limestone and 25 to 30 % by mass of clay were the most suitable for producing hydraulic lime. The binder which Joseph Aspdin (1778-1855) produced by burning an artificial mixture of limestone and clay, and for which he obtained a patent in 1824 under the name of “Portland cement”, also at first corresponded in composition and properties to a Roman lime, as it had not yet been burnt to the sintering point. The artificial rock produced from it resembled Portland stone, an oölitic limestone which is quarried on the Portland peninsula in the county of Dorset on the Channel coast. When William Aspdin, the son of Joseph Aspdin, started to produce Portland cement in 1843 in a newly established works at Rotherhithe near London it became apparent, especially during the construction of the Houses of Parliament London, that this was far superior to “Roman cement”. This was mainly because a considerable proportion of the mix had been sintered during the burning process. The significance of this sintering had apparently been first recognized in 1844 by Isaac Charles Johnson (1811-1911) [Q 1, d 2, h 1]. The first German Portland cement on the English pattern was produced in Buxtehude in 1850. However, the basis for the manufacture of Portland cement in Germany was provided by Hermann Bleibtreu (1824-1881), who also built two cement works in Züllchow near Stettin (1855) and in Oberkassel near Bonn (1858).

In France the manufacture of Portland cement started around 1850 when a slow-setting binder was obtained from the sintered residues produced during the slaking of burnt lime by grinding them between mill stones.

Sintered cement clinker was first manufactured in the USA around 1870 by David Saylor who comminuted the raw material to homogenize it, and moulded the meal into bricks for burning.

Wilhelm Michaëlis (1840 - 1911) had a crucial influence on the continued development of cement. In his book “Die hydraulischen Mörtel” (The hydraulic mortar) published in 1868 he was the first to give accurate data on the most favourable composition for the raw material mix. Information about the lime limit, i.e. the highest possible CaO content in the raw material mix which can be combined with SiO2, Al2O3 and Fe2O3 during the burning, and about the processes occurring during the burning and cooling of cement clinker was first provided by the investigations of the brothers S.B. and W.B. Newberry (1897) [N 18] as well as by E. Wetzel (1911/1914) [W 37], E. Spohn (1932) [S 173], F.M. Lea and T.W. Parker (1935) [L 26] and H. Kühl (1936) [K 104].

2.2 Burning the cement clinker

Shaft kilns, which operated intermittently, were the only means available at first for the burning process. The first step towards continuous operation was the introduction of the Hoffmann annular kiln. The term “cement clinker” also came from this period as the kiln feed for burning in annular kilns was formed into bricks which were then burnt in a similar way to masonry bricks. The rotary cement kiln goes back to patents by the Englishman Frederick Ransome in 1885/86. Burning trials with rotary kilns in Germany began in 1897, and industrial clinker production started two years later [S 137, S 219]. The first grate preheater kiln (1929) and the first cyclone preheater kiln (1950) also came on stream in Germany.

2.3 Comminution of raw materials and cement

Jaw crushers were used for primary comminution of the hard raw materials and the clinker; grinding rolls were used for the rough grinding and millstone arrangements, which consisted of two mill stones with diameters of 0.8 m to 1.5 m on top of one another, for the fine grinding [S 137]. The mill feed was introduced through a central hole in the upper stone and comminuted in the gap between the stationary upper stone and the lower stone, which was driven by a central shaft.

The continued development was aimed primarily at increasing the fineness and raising the throughput. The cement had to be screened in order to achieve the fineness which at that time conformed to the standard of a maximum of 20 % residue on 0.2 mm. The grinding units were therefore combined with screening equipment with the disadvantages of high wear and low throughput. Introduction of the mechanical air separator in 1889 was therefore an important improvement.

Further grinding units were, among others, the edge mill for wet processing of the raw material, various types of roller mill and the Griffin mill, a pendulum ring-roller mill adopted from the USA for grinding cement. The head of the pendulum was formed as a grinding roller and circulated in a steel grinding ring. The comminution action was generated by the centrifugal force of the rotating pendulum [h 1].

Greater fineness with reasonable throughput was achieved principally through the tube mill which was introduced into the German cement industry in 1892. The first tube mills were single chamber mills with diameters of 1.2 m and lengths of 5 m to 6 m. They produced about 3 t cement per hour with a fineness of 15 % residue on 0.09 mm and a specific power consumption of 20 kWh/t. By 1920 the various types of tube mill had largely displaced the other mill designs for grinding both raw material and cement [S 137].

2.4 Environmental protection

In 1887 there was a report on the first dedusting systems in the German cement industry. Their purpose was to prevent dust nuisance and loss of material during the grinding and packing of cement. They consisted of dust catchment hoods, a suction system and a dust chamber in which woven fabrics or strands of hemp fibre were suspended [K 1, S 73]. Bag filter systems with cleaning by air or by rapping were introduced at about the same time and then rapidly gained general acceptance. In the first “Technische Anleitung” (Statutory Regulation) in § 16 of the industrial code of the German Reich of 1895 it was stipulated that the air extracted from comminution machines had to be dedusted, and could only be discharged when it was free from dust. After the first world war these instructions were also applied correspondingly to kiln systems [S 137].

The pioneering work on electrostatic dust collection, which started in 1906 in the USA, was carried out by F.G. Cottrell [D 84]. The first system for electrostatic dedusting of cement kiln exhaust gases came on stream in 1913 at the Riverside Portland Cement Co. in California [H 71]. Because of the success a further 14 cement works in the USA were then equipped with electrostatic precipitators. This was helped by the fact that the kiln dust could be used as a potassium fertilizer.

2.5 Glassy blastfurnace slag

The latent hydraulic properties of granulated, i.e. rapidly cooled and therefore largely glassy, blastfurnace slag were discovered in 1862 by Emil Langen, who showed that high strengths can be achieved by a mixture of granulated blastfurnace slag and burnt lime. In 1882 Godhard Prüssig (1828 - 1903) was the first to add granulated blastfurnace slag to Portland cement. In Germany the cement with a fairly low slag content was designated “iron Portland cement” in 1901; cement with a higher slag content has, since 1907, been called “blastfurnace cement”. The term “Hüttensand” (metallurgical sand) normally used in Germany for granulated blastfurnace slag goes back to Hermann Passow (1902) [P 17]. In 1908 H. Kühl discovered the sulfate activation of granulated blastfurnace slag on which the manufacture of supersulfated cement is based [k 6].

2.6 Cements with special properties

The first cement with high early strength, initially called high strength cement, was manufactured in 1912/13 in the Lorüns cement works (Vorarlberg) in Austria. This was a very finely ground Portland cement made from clinker which had been burnt at a fairly high temperature from a particularly carefully prepared raw material mix.

The first Portland cement with high resistance to sulfates was the “Erzzement” (iron-ore cement) for which Krupp-Grusonwerk in Magdeburg obtained a patent in 1901 and which was manufactured in the Hemmoor cement works. It had a low aluminium oxide content and a high iron oxide content, its alumina-iron ratio of 0.3 was therefore very low. The present highly sulfate-resisting Portland cements are similar to the so-called Ferrari cement, a Portland cement with an alumina-iron ratio 0.64 which was first manufactured in Italy on the basis of a patent awarded to F. Ferrari in 1919/20. The importance of a high blastfurnace slag content for sulfate resistance in blastfurnace cements has been known since the 20s.

White Portland cement was manufactured in small quantities at the Heidelberg Portland cement works as early as the 80s of the nineteenth century, and then also in various other cement works.

The development of oil well cements started around 1930 when greater well hole depths became necessary for extracting crude oil. This meant that the well holes needed to be lined with cements which stiffened and set only after a fairly long time, even at high temperatures and pressures.

The first publication relating to expansive cement appeared in 1920 by A. Guttmann [G 92] who obtained a patent for it in the same year [G 93]. Independently of this H. Lossier [L 99] became involved later with the development of expansive cements. The expansive cements currently manufactured in various countries are based on the work by V.V. Mikhailov in the USSR (1955) and by A. Klein in the USA (1958/61) [K 52].

Quick-setting cement was developed by the Portland Cement Association in the USA and a patent application was filed in 1968 [P 46]. Jet cement, for which a patent application was filed in 1970 [O 35], is manufactured in Japan on a similar basis.

The first high-alumina cement was manufactured in France during the first world war in 1914/18 on the basis of a patent, which went back to 1908, belonging to the French chemist J. Bied who found that crystallized melts composed of monocalcium aluminate can harden hydraulically and achieve high strengths.

2.7 Cement standards

The first cement standard was introduced in Germany in 1878 under the title “Normen für die einheitliche Lieferung und Prüfung von Portland-Cement” (Standards for uniform supply and testing of Portland cement). It was drawn up by the “Verein Deutscher Cement-Fabrikanten” (Association of German Cement Manufacturers), which had been founded in 1877 and was the predecessor of the present VDZ (German Cement Works Association). The standard for Portland cement was fundamentally revised in 1887 and 1909. The standard for iron Portland cement was introduced in 1909, and the standard for blastfurnace cement in 1917. The “Deutscher Normenausschuss DIN” (German Standards Committee), which has been in existence since 1917, first issued the DIN 1164 cement standard which combined the standards for the three types of cement in 1932 [W 65]. Cements were authorized for government buildings in France in 1885. The cement standards in Great Britain and the USA were written in 1904.

3 Cement clinker

3.1 Composition of cement clinker

3.1.1 General survey

Cement clinker consists essentially of tricalcium silicate, dicalcium silicate, tricalcium aluminate and calcium aluminoferrite. It is produced from a raw material mix which contains mainly calcium oxide CaO, silicon dioxide (silica) SiO2, aluminium oxide (alumina) Al2O3 and iron oxide Fe2O3 in certain proportions. When this mix is heated until it “sinters” new compounds are formed, the so-called clinker compounds or clinker phases. Their proportions, their designations and their chemical formulae are listed in Table 3.1. The designations alite and belite come from A. E. Törnebohm [T 54] who in 1897 named the phases which could be recognized as the main constituents under the microscope after the first letters of the alphabet as he did not yet know their compositions. The designations alite and belite are still used today in order to differentiate the silicates in the clinker, which always contain small quantities of aluminium, iron and magnesium oxides and alkalis, from the pure silicates. The designations celite and felite are no longer in current use; calcium aluminoferrite was designated celite and a certain form of the dicalcium silicate was designated felite.

 

Table 3.1: Potential phase composition of German cement clinker, calculated from chemical composition [V 47], and according to data of the Research Institute of the German Cement Industry, Düsseldorf)

Shortened forms are normally used in cement chemistry to simplify the chemical formulae, in which

C

CaO

M

MgO

S

SiO2

A

Al2O3

F

Fe2O3

N

Na2O

K

K2O

H

H2O

s

SO3

The clinker compounds and their properties are described in the following sections. Also covered are those compounds which occur as transition phases in certain temperature ranges during the burning of cement clinker, which can be formed in coatings in the kiln or preheater, or which are constituents of special cements or high-alumina cement.

Figure 3.1: CaO-SiO2 binary system [R 5, W 30]

 

Table 3.2: Modifications of tricalcium silicate, temperature ranges, stabilization by incorporation foreign ions in the crystal lattice [B 68, H 7, M 12]

Modification

Temperature range in °C

Incorporated ions as oxides in % by mass

 

 

Fe2 O3

Al2 O3

MgO

ZnO

T1

< 620

0…0.9

0…0.45

0…0.55

0…0.8

T2

620… 920

0.9…1.1

0.45…1.0

0.55…1.45

0.8…1.8

T3

920… 980

-

-

-

-

M1

980… 990

-

-

1.45…2.0

1.8…2.2

M2

990…1060

-

-

-

2.2…4.5

M3

1060…1070

-

-

> 1.2

-

R

> 1070

-

-

-

4.5…5.0

Type of Incorporation

 

2 Fe for 1 Ca + 1 Si

Al for Ca, Si and lattice vacancies

1 Mg for 1 Ca

1 Zn for 1 Ca

3.1.2 Tricalcium silicate

Tricalcium silicate is the compound in the binary CaO-SiO2 system which is richest in CaO (Fig. 3.1). It melts incongruently at 2150 °C with formation of solid CaO and liquid [G 89]. Below about 1250 °C it is unstable and decomposes into CaO and dicalcium silicate. However, this decomposition occurs only during very slow cooling or very long tempering in the temperature range below 1264±3 °C [L 114]. It is promoted by the presence of the decomposition products CaO and C2S, moisture or sulfate melts [M 106]. Foreign ions in the crystal lattice of the tricalcium silicate lower the decomposition temperature. Certain foreign ions can also delay the decomposition, and others can accelerate it.

The ions of divalent iron, Fe2+, are particularly important for industrial clinker production as they greatly accelerate the decomposition, which then takes place at about 1180 °C [W 78]. Cement clinker containing iron oxides must therefore always be burnt and cooled under oxidizing conditions as the iron is then present in trivalent form and is combined as calcium aluminoferrite.

In the metastable region below 1264 °C there are known seven modifications of the tricalcium silicate which can be differentiated by X-ray diffraction and/or with the aid of their optical crystal properties [B 68, M 11]. Their temperature ranges are listed in Table 3.2. In this table T represents triclinic, M monoclinic and R rhombohedral symmetry of the crystal lattice. The modifications which are metastable at the higher temperatures can be stabilized by solid solution of foreign irons in their crystal lattices, so that on cooling they are no longer converted into the modifications which are metastable at lower temperatures. One effective stabilizer, for example, is zinc [S 230] which, depending on its concentration, stabilizes every modification with the exception of T3 and M3. The limit of the solubility for zinc is reached at about 5.0 % by mass of ZnO [B 68, H 7]. The incorporation of titanium up to 1 % by mass promotes the formation of C3S, but higher levels inhibit it [K 20]. The main modifications occurring in industrial clinker are M1 and M3, which are stabilized by MgO. Clinker with less than about 0.8 % by mass of MgO only contains the M1 modification, and M3 is present in clinker containing at least 1.2 % by mass of MgO [M 12]. However, the MgO concentration required for stabilizing the M3 modification increases with rising sulfate content of the clinker. The crystal lattice of tricalcium silicate can take up a maximum of 2.0 % by mass of MgO. The limits of the absorptive capacity for the ions of aluminium and of trivalent iron are 1.0 % by mass of Al2O3 and 1.1 % by mass of Fe2O3. The lower absorption of Al2O3 and Fe2O3 is attributable to the fact that the ionic radius of Mg2+ differs less from that of Ca2+ than do those of Al3+ and Fe3+, and therefore the magnesium fits somewhat better into the tricalcium silicate crystal lattice than do aluminium and iron.

Tricalcium silicate is the compound to which cement owes its important properties. When finely ground and mixed with water to form a paste it hardens rapidly and reaches very high strengths. It is formed at high temperature by the chemical reaction of calcium oxide and silicon dioxide in the solid state, and can therefore, for example, be produced by heating a mixture of limestone and quartz sand. However, this requires the starting materials to be extremely finely ground, and the mixture of appropriate composition to be burnt for a sufficiently long time at temperatures of at least about 1500 °C. In the industrial clinker burning process this reaction takes place significantly faster at a temperature between about 1350 °C and 1500 °C in the presence of the clinker melt, which consists predominantly of calcium oxide, aluminium oxide and iron oxide.

The alite, i.e. the modifications of tricalcium silicate stabilized by various foreign ions, is generally more hydraulically reactive, and therefore hardens substantially faster, than pure tricalcium silicate [O 34]. The crystal lattices of the individual C3S modifications differ only slightly, which can be recognized from the very low transformation enthalpies of about 0.2 J/g to 4 J/g [R 30], so the differences in hydraulic reactivity may well be due mainly to the lattice dislocations caused by the incorporation of foreign ions.

3.1.3 Dicalcium silicate

Dicalcium silicate occurs when the clinker is not fully saturated with calcium oxide. It melts congruently at 2130 °C [G 89] (Fig. 3.1), is a very stable compound, and is formed initially in lime-rich mixtures as the product of a solid-state reaction.

Dicalcium silicate occurs in five modifications known as α-, α′H-, α′L-, β- and γ-C2S. The subscripts H and L indicate the high and low temperature modifications of α′-C2S. A general idea of the transformation behaviour, which has been investigated repeatedly (sometimes with differing results and conclusions), is provided by Fig. 3.2 [S 143, S 144, S 18, N 22, L 35, C 29]. From this it can be seen that the transformations β → γ during cooling and γ → α′L during heating are not reversible (irreversible, monotropic). During the β → γ conversion there is a substantial change in the crystal lattice, recognizable by the fact that, among other things, the density of the γ-modification of 2.94 g/cm3 is about 10 % lower than the density of the β-modification of 3.20 g/cm3. As a result of this change in modification an originally compact burning product cracks and rapidly disintegrates into dust (“dusting”) as soon as the temperature during cooling falls below about 500 °C. However, the transformation of the β- into the γ-modification is largely suppressed if, during the burning of samples consisting largely or completely of dicalcium silicate, the temperature does not exceed 1160±10 °C at which the α′L-modification is transformed during heating into the α′H-modification.

Figure 3.2: Changes between dicalcium silicate modifications [N 22, L 35]

Regarding the polymorphic transition β → γ a relationship to the size of the α′L-C2S crystals was observed [G 89, Y 7, L 35). Small crystals formed during burning below 1160 °C in the stability range of the α′L-C2S have less tendency to transform into the γ-modification than larger crystals which are formed during burning above 1160 °C in the stability range of the α′H-C2S. The reason for the different tendency to transformation is thought to be the hindrance to the formation of γ-C2S crystal nuclei able to grow. The coarsely crystalline α′H-C2S, which is produced when the C2S sample is heated to temperatures above 1160 °C, is attributed a crystal lattice with a higher degree of disorder than the finely crystalline α′L-C2S which is formed during burning below 1160 °C. As there are fundamental differences between the crystal lattices of β- and γ-C2S the formation of a γ-C2S crystal nucleus requires a correspondingly large amount of nucleation energy. This can apparently be supplied by the crystal lattice of the coarser α´L-C2S which, owing to its higher degree of disorder, contains more energy than the crystal lattice of the fine-grained α′LC2S [N 23, S 1