VŠB - Technical University of Ostrava Faculty of...

1

VŠB - Technical University of Ostrava

Faculty of Metallurgy and Materials Engineering

INORGANIC BINDERS (study materials)

Michaela Topinková

Ostrava 2015

http://www.vsb.cz/en

http://www.fmmi.vsb.cz/en/

INORGANIC BINDERS

2

Title: Inorganic Binders

Author: Ing. Michaela Topinková

Edition: first, 2013

Number of pages: 60

Study materials for the field of study Heat Engineering and Ceramic Materials (study program in

Metallurgical Engineering) of the follow-up Master’s degree program at the Faculty of Metallurgy and

Materials Engineering.

Proofreading: not performed.

© Michaela Topinková

© VŠB – Technická univerzita Ostrava

INORGANIC BINDERS

3

HOW TO STUDY

Inorganic binders

For the subject Inorganic Binders of the 3rd semester of the study programme Heat

engineering and ceramic materials you received a package containing the integrated study

textbook for the combined study including the instructions for study.

Prerequisites

The course has no prerequisites.

The aim of the course and learning outcomes

An overview of the theoretical aspects and related manufacturing processes of preparing the

main types of construction binders. The procedures for testing the properties of binders and

matrix composites. Use of secondary raw materials.

After studying the course, students should be able to:

knowledge outputs:

Students will be able to characterize the technology of the various types of inorganic binders.

Students will be able to formulate the basic processes during hydration of various inorganic

binders

skill outputs:

Students will be able to use their knowledge to make decisions regarding the suitability of

various inorganic binders in practice.

Who is the subject intended for

The course is included in the Master study in the field of study Heat Engineering and ceramic

materials within the study programme Metallurgical Engineering, but it can be studied

students from any other fields of study if they meet the required prerequisites.

The educational support is divided into sections, chapters, which correspond to logical

division of the learning material, but their length differs. The estimated time to study chapters

may vary considerably, therefore large chapters are divided into numbered subsections

according to the structure described below.

When studying each chapter we recommend the following steps:

Carefully read the entire text of each chapter. At the end of each chapter there is a list of

concepts that students should know and be able to explain. Furthermore, at the end of each

chapter there are control questions that students should be able to answer. During the study, it

is appropriate to use the recommended literature to supplement or enhance the information

provided.

Way of communication with teachers:

During the semester, teaching will be organized in the form of several blocks lasting a few

hours each, within which the students are familiarized in detail with the course content and

the ways of communication with the teacher. The students will receive specific requirements

INORGANIC BINDERS

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concerning the organization of teaching and requirements for obtaining credit or passing an

exam at the first lesson at the beginning of the semester. The teacher addresses students‘

questions individually, through personal consultation or via email, or in the case of broader

interest, a consultation is arranged with a group of students.

Subject guarantor: doc. Ing. Jozef Vlček, Ph.D.

Teacher: Ing. Michaela Topinková

Contact information:

Ing. Michaela Topinková

Address: Studentská 11, Ostrava-Poruba, office č. N422

Telephone: 597 321 622

Email: [email protected]

INORGANIC BINDERS

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Contents

INORGANIC BINDERS .................................................................................... 1 Michaela Topinková ............................................................................................................................ 1

1. INTRODUCTION TO BINDERS ............................................................... 6 Summary of the concepts of the chapter (subchapter) ........................................................................ 8 Questions on the explained topic ......................................................................................................... 8

BUILDING BINDERS - CEMENS ................................................................... 9

2. PLASTER BINDERS ................................................................................... 9 Summary of the concepts of the chapter (subchapter) ...................................................................... 14 Questions on the explained topic ....................................................................................................... 14

3. CALCIUM BINDERS ................................................................................ 15 3.1. Classification and production of lime.................................................................................... 15 Summary of the concepts of the chapter (subchapter) ...................................................................... 21 Questions on the explained topic ....................................................................................................... 21 3.2. Lime properties, lime slaking ................................................................................................ 22 Summary of the concepts of the chapter (subchapter) ...................................................................... 25 Questions on the explained topic ....................................................................................................... 25

4. MAGNESIUM BINDERS .......................................................................... 26 Summary of the concepts of the chapter (subchapter) ...................................................................... 27 Questions on the explained topic ....................................................................................................... 27

5. CEMENT ..................................................................................................... 28 5.1. Portland cement ..................................................................................................................... 28 Summary of the concepts of the chapter (subchapter) ...................................................................... 33 Questions on the explained topic ....................................................................................................... 34 5.2. Chemical and physical processes in the formation of clinker, clinker minerals ................... 35 Summary of the concepts of the chapter (subchapter) ...................................................................... 40 Questions on the explained topic ....................................................................................................... 41 5.3. The technology of cement production (machine) .................................................................. 42 Summary of the concepts of the chapter (subchapter) ...................................................................... 46 Questions on the explained topic ....................................................................................................... 46 5.4. Cement hydration .................................................................................................................. 47 Summary of the concepts of the chapter (subchapter) ...................................................................... 51 Questions on the explained topic ....................................................................................................... 51 5.5. Properties of cement .............................................................................................................. 52 Summary of the concepts of the chapter (subchapter) ...................................................................... 54 Questions on the explained topic ....................................................................................................... 54 5.6. Other cements ........................................................................................................................ 55 Summary of the concepts of the chapter (subchapter) ...................................................................... 57 Questions on the explained topic ....................................................................................................... 57

6. AERATED CONCRETE ........................................................................... 58 Summary of the concepts of the chapter (subchapter) ...................................................................... 60 Questions on the explained topic ....................................................................................................... 60

7. REFERENCES ............................................................................................ 61

INTRODUCTION TO BINDERS

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1. INTRODUCTION TO BINDERS

Time to study: 90 minutes

Objective After studying this section, you will be able to

Define the concept of binder

Divide binders according to various criteria

Lecture

Binders are inorganic substances which are usually produced by heat treatment of natural raw

materials of suitable composition. Binders are agents (mixture of substances), which have the ability

of self-hardening, thus connecting granular systems in a rigid compact whole. Into the whole, binders

can also accommodate a filler = composite material. Mixing with the desired quantity of water results

in a well workable mass, which subsequently solidifies and hardens. Correct function is ensured by the

following properties of binders:

1. At the beginning of the action, binders (binders + fillers) must be fluid so that they can be

distributed on the surfaces and in the pores of the granular system.

2. The binder in the liquid state must wet the surface of the bound material to create adhesive

joints.

3. After a suitable period, the binder must lose its fluidity (plasticity), it must spontaneously

solidify. Solidification is happening by:

a) physical processes

b) chemical processes (reactions that accompany physical processes)

4. Adhesive joints must remain fixed even after drying of the binder.


7

The main cases of the binder distribution in a granular system are shown in Fig. 1.

Fig. 1 the binder distribution in a granular system: the case of a) and b) - small amounts of the binder,

c) - a large amount of the binder

In the case a) the binder is distributed in an appropriate manner, it will depend on the joint strength

and the strength of the binder (the whole will have good strength); in the case b) solidification will not

occur, connection of the system is only in a small area; in the case c) predominantly the strength of the

binder alone is applied – the system will not have high mechanical properties, high content of the

binder acts like pores.

The resulting strength of the system will depend on the size, shape and distribution of particles and

pores. Hardening of the system has two parts – solidification and hardening. Solidification is

characterized by the gradual depletion of deformability (losing fluidity). This stage gradually passes

into the second stage – hardening. Here, the system acquires mechanical properties. The transition

between solidification and hardening is called the bond (rapid binders). During solidification and

hardening, complex chemical reactions that we include in the process called hydration (reaction of the

components with water) frequently proceed. The chemical process itself, however, does not lead to

hardening, if not accompanied by a physical action, which develops a new mechanically stable

structure.

The substances that are binders are classified as follows:

1. Direct binders – they react directly with water, which results in solidification and hardening

(gypsum, lime, cements)

2. Latent hydraulic binders – systems (slag, fly ash), which do not solidify and harden with

water, but only in the presence of activators. The activator can be CaO or sodium silicate

(water glass).

From a practical point of view, we distinguish two main groups of organic binders:

1. Technical binders (e.g. phosphate binders, water glass)

2. Building binders – CEMENTS (e.g. cement, lime, gypsum) – they are the main functional part

of mortar

a) b)

=

>

vz

ni

k

C

S

H

ge

lů

c)


8

Cements are further subdivided into three groups:

A. Air

B. Hydraulic

C. Special

Air cements are characterized in that, after mixing with water, they solidify and hardens, but they are

stable only in air. When storing them in a humid environment or water, their strength decreases and

they often disintegrate. These include gypsum binders, magnesium binders and air lime.

Hydraulic cements are characterized in that products prepared from them harden even in water and

they are stable on storage in air and in water. These include hydraulic lime and cements.

Special cements are characterized by several other properties, such as e.g. high temperature resistance

(aluminous cement).

Summary of the concepts of the chapter (subchapter)

Binder

Solidification and hardening

Classification of binders

Cements

Questions on the explained topic

1. What is a binder?

2. Name the properties of binders.

3. How do we classify binders?

4. Explain the term of latent hydraulic binder.

5. How do we classify cements?

PLASTER BINDERS

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BUILDING BINDERS - CEMENTS

2. PLASTER BINDERS

(DIRECT BINDERS, AIR BINDERS)


Objective After reading this paragraph, you will be able to

describe the sequence of gypsum transformations,

explain the process of plaster solidification and hardening, enumerate the properties and uses of gypsum.

Lecture

Plaster results from partial or total dehydration of gypsum CaSO4. 2 H2O ( 2HSC ). For the production of

gypsum binders, both natural raw materials and waste from the chemical industry are used. Relatively

pure dihydrate deposits occur naturally; it is a soft mineral. In the Czech Republic, gypsum occurs

primarily in Opava region and in Spišská Nová Ves. Another source, recently gaining in importance, is

the dihydrate falling away in the chemical industry (e.g. in the production of H3PO4 – phosphogypsum

contains impurities of P2O5, citric acid – citric gypsum).

The sequence of gypsum transformations

Gradual dehydration to other products, namely such products that react back to gypsum upon contact

with water while solidifying and hardening.

The sequence of transformations at heating the calcium sulphate dihydrate is shown in Fig. 2, where

the approximate temperatures are also shown at which the corresponding transformation takes place at

practically usable speed. Sequences of transformations are given for conditions that are important for

technical production processes.

Abbreviations used:

DH – calcium sulphate dihydrate ( 2HSC )

HH – calcium sulphate hemihydrate ( 0,5HSC )

AH – calcium sulphate anhydrite ( SC )

PLASTER BINDERS

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Fig. 2 Sequence of transformations of gypsum

α hemihydrate and anhydrite III formed in a saturated water vapour or in aqueous suspension.

Large, well developed crystals are formed. These forms then moisturize more slowly and have a

higher ultimate strength. They are used for floors, mortar for bricklaying and plastering.

β hemihydrate and anhydrite III are formed by dry hydration. The shape of the crystals is not

regular, they are smaller in size. These forms react with water quickly. They are used for stucco and

modelling work, structural panels, gypsum moulds for shaping pottery.

In fact, the α and β forms represent two limit forms, which differ morphologically and between which

there is a continuous series of transitional forms.

When gypsum is further heated, insoluble anhydrite II is formed. It cannot be used as a direct binder

(reaction with water proceeds so slowly that solidification practically does not occur). The reaction of

solidification and hardening needs an activator (CaO).

At temperatures above 800 °C, a so called Estrich plaster is formed (AH I and CaO mixture). CaO is

a small amount, only about 2-4 %, it is used as a catalyst for hydration, thus allowing solidification.

This is a slow-setting plaster.

Solidification and hardening

Solidification and hardening is the essence of the hydration process. First it is necessary to consider

the ratio of solubility of individual phases of CaSO4 - H2O in water. Fig. 3 shows that the most soluble

form at temperatures of up to 100 °C is hemihydrate.

CSH2 CS III CS II CS I + CaO CSH0,5

DH HH AH III AH II AH I

α β α β α β +CaO

+H2O +H2O +H2O +H2O

100-180°C 170-230°C 400-700°C > 800°C

PLASTER BINDERS

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Fig. 3 Solubility of dihydrate (DH), hemihydrate (HH) and anhydrite (AH) in water (from different

sources, according to Hlaváč, 1981).

Dissolution takes place through the solution and begins by the initial product dissolving in water:

220,5 HSCOHHSC Exothermic process

Dihydrate has, particularly at lower temperatures, substantially lower solubility (Fig. 3). In an aqueous

suspension, hemihydrate forms a saturated solution, which is, however, supersaturated with respect to

dihydrate, which begins to separate from it. Elimination takes place by forming dihydrate nuclei on the

surface of yet undissolved hemihydrate crystals. From the nuclei, acicular crystals then grow towards

the solution which intertwine with each other to create a fixed end matted structure (Fig. 4). This is so

called hydration through a solution. The actual chemical process (reaction with water) would not lead

to strengthening of the system if not accompanied by other physical action that leads to rebuilding of

the structure.

HH (different forms)

A III

A II

DH

0 30 60 90 120

temperature (°C)

solu

bil

ity

(g

CaS

O4/1

00

gH

2O

)

0,8

0,4

0,6

0,2

1,0

PLASTER BINDERS

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Fig. 4 Elimination of DH crystals on the surface of HH in the initial stage of plaster solidification

(Hlaváč, 1981). The kinetics of the process of solidification and hardening of plaster

PLASTER BINDERS

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The main processes which take place during solidification and hardening of plaster are: dissolving of

hemihydrate, dihydrate nucleation and growth of the dihydrate crystals. These processes limit the rate

of hydration, and thus the hardening, which proceeds simultaneously with hydration (Fig. 5).

a) DH nucleation

b) DH crystal growth

c) HH dissolution

Fig. 5 Time-dependent degree of hydration in an aqueous suspension of hemihydrate plaster (Hlaváč

1981).

ad a) On the curve it is seen that relatively reluctantly heterogeneous nucleation occurs. This stage

can be affected in the following ways:

1. Increasing speed – in slow-setting plaster

- surface area (fine particles – better, faster responses)

- increase in the HH solubility (adding water and K2SO4)

2. Slowing down – for a better and longer processability time of quick-setting gypsum

- blocking HH by the addition of fish glue (grains are wrapped in a thin layer of HH)

ad b) After nuclei are formed, crystals already grow rapidly (the rise of the curve). When the HH free

surface is reduced so that it will not pass CaSO4 into the solution in sufficient quantities,

hydration will slow down and dissolution becomes the control action.

ad c) Dissolution (diffusion transport) – hydration slows down.

a) b) c)

time

deg

ree

of

tran

sfo

rmat

ion

HH

->D

H

PLASTER BINDERS

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Properties and applications of plaster

ρR 2.60 – 2.73 g.cm-3

ρP 2.9 g.cm-3

beginning of solidification 4 – 8 min (R) 2 – 5 h (P)

end of solidification 15 – 40 min (R) 6 – 8 h (P)

compressive strength 5 – 30 MPa

R – quick-setting plaster P – slow-setting plaster

Slow-setting plaster – floors, mortar for bricklaying and plastering, bedding.

Quick-setting plaster – ceramic moulds, modelling work, building elements = PLASTERBOARD.


Transformation of gypsum

Solubility of the phases of the system CaSO4 – H2O in water

Hydration of plaster

Properties of plaster


1. Explain the sequence of gypsum transformations.

2. What is the difference between α and HH and AH III?

3. Explain the concept of Estrick gypsum.

4. What is the solubility of the various phases of CaSO4 – H2O in water?

5. Define major actions during solidification and hardening of plaster.

6. Explain the difference between slow and fast setting plaster.

7. What is the most famous use of plaster?

CALCIUM BINDERS

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3. CALCIUM BINDERS


3.1. Classification and production of lime


Objective After studying this paragraph, you will be able to

clarify the concepts of air and hydraulic lime,

enumerate criteria of lime production, classify furnaces for lime production.

Lecture

What is binder (it reacts with water) is CaO. Lime = technical name for CaO of different degrees of

purity. As cement, it is made by decarbonisation of natural limestone. The main essence of limestone

(it is a rock) is a mineral calcite CaCO3, or dolomite CaMg (CO3)2. Limestones containing more than

10 % of dolomite are known as dolomite limestones. Natural limestone is mined in large quantities

and processed in various branches (besides the construction industry, it is metallurgical, chemical,

glass industry, etc.).

Limestone deposits in the Czech Republic: Hranice, Kotouč near Štramberk, Moravian Karst,

Čížkovice, Prachovice, the area between Prague and Beroun.

Generally, there are two main types of lime for construction purposes:

1. Air lime having a high CaO content and possibly a smaller MgO content (the sum

CaO+MgO> 85%). It reacts with water to form Ca (OH)2, and it is soluble in water.

2. Hydraulic lime, which arises from less pure limestone containing more than 10 % of so called

hydraulic components, i.e. SiO2, Al2O3 and Fe2O3. By firing the raw material we obtain, in

addition to CaO, compounds with CaO (silicates, aluminates and ferrites of calcium). The

result of hydration is then Ca(OH)2 and hydrate compounds of Si, Al, Fe, which are insoluble

in water.

As the name implies, the first type of lime solidifies and is stable only in air, whereas the second type

is also hardened under water while insoluble products are formed.

CALCIUM BINDERS

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Production of lime

Decomposition of limestone in the kiln at 1250 - 1350 °C:

CaCO3 → CaO + CO2

Production criteria

1. Chemical composition of the raw material.

2. Structure of limestone (if the material is friable or compact).

3. The size of the grains: coarse limestones (particles larger than 1 mm), fine-grained limestone

(particles smaller than 1 mm, max. 0.1 mm)

An important criterion of lime production is the limestone structure. Fine-grained limestones with

grain size of 0.1 mm are preferable, lime made from them gives mortars greater deformability. The

time necessary for the complete decomposition of limestone depends on the initial particle size.

Selecting the firing temperature according to the structure of limestone is shown in Fig. 6.

Fig. 6 Dependence of the time required for the complete decomposition of limestone on the

temperature and on the size of pieces (Hlaváč 1981).

If the feedstock is up to 5 cm, the firing time between 1200 °C and 1300 °C is 1 – 2 hours. For larger

grains, the burning time of the feedstock is substantially longer. The larger the size of the initial

surface (smaller particles), the faster the decomposition of limestone, and also the higher the firing

10cm

20cm

5 cm

1000

1200

1300

900

time (h)

tem

per

ature

(°C

)

CALCIUM BINDERS

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temperature, the shorter the decomposition time. Depending on the size of the feedstock, we carry out

a suitable firing.

Decomposition of limestone can also be accelerated by reducing the concentration of CO2 above the

carbonate surface, either by a rapid draining from the furnace or by partial reduction of carbon dioxide

to carbon monoxide.

Large pieces are easiest to fire to high quality lime because high temperature is needed for complete

decomposition of inner parts (so that time of firing is economically viable), therefore surface layers

may be over-burnt (sintering is applied on the surface of grains) connected with shrinking and closing

the pores and it is associated with under-fired internal parts. The solution is again small and uniformly

sized grain of the feedstock.

The differences in chemical composition and physical properties of limestones mean that the optimum

firing mode can be set for each type limestone only experimentally. This allows to obtain highly

reactive lime with low shrinkage and high porosity.

Furnaces for lime production

1. Circular furnace – they are no longer used nowadays. They had to have a quality lining

(fireclay) due to higher temperatures and a good exhaust of CO2.

The main drawback – difficult filling and emptying the chambers, difficult mechanization, work

in a hot environment.

The main advantage - they could be used for all kinds of limestone, a really high quality lime was

produced there.

Fig. 7 Circular furnace diagram (Herainová, 2003)

1. furnace body, 2. furnace, 3. stoking holes, 4. exhaust vents, 5. entrances to the furnace

1 2 4 5 3

CALCIUM BINDERS

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2. Vertical shaft furnace – counter flow

The most common type of furnace for lime production nowadays are vertical shaft furnaces. Lime is

burnt relatively quickly there, at higher temperatures than in circular furnaces. There is a large number

of different modifications of shaft furnaces. A significant improvement is the construction of the shaft

furnace with internal heating, which generates heat within the charge (limestone mixed with solid fuel

– coke is fed to the furnace). The air preheated at the bottom, cooling zone is fed to combustion, and

simultaneously the burnt lime is cooled there. Furnace lining is typically fireclay, more recently, in the

hot zone magnesite it used.

Fig. 8 Diagram of a shaft furnace

air CaO

gas burners

CaCO3 + solid fuel (coke)

exhaust gas

calcining zone - Here the coke is

burnt and charge

is heated

combustion air is preheated by

passage in the furnace and

simultaneously it cools the

resulting CaO

CALCIUM BINDERS

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Advantages:

- low thermal energy consumption

- the possibility of mechanization of the feedstock batching and limestone collection

- low investment costs

Disadvantages:

- the raw material must be adjusted to a uniform size of pieces to ensure a good passage of

gases through the filling of the furnace

3. Shaft furnace – type MAERZ - parallel flow

Both shafts are filled with the feedstock from above. Fuel and combustion air comes in parallel flow

also from above, through one shaft and the outgoing gas preheats the second chamber. Then, the

process reverses, combustion takes place in the second chamber and preheating in the first one.

Cooling air cools the formed CaO (it is continuously drawn off from the bottom), and subsequently

mixed with other gases in the transition channel. After Reversing the furnace operation, the shaft 2

heats and the entire process is done similarly. The furnace is equipped for automatic operation.

Fig. 9 is a diagram two-shaft parallel-flow furnace of the type Maerz (Hlaváč, 1981)

cooling air

exhaust gas

fuel

combustion

air

1 2

CALCIUM BINDERS

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4. A rotary furnace – counter flow

Fine forms of limestone are used here, the feedstock is preheated by exhaust gases.

Advantages:

- High performance (up to 1000 t per day).

- Possibility of mechanization and automation.

Fig. 10 Diagram of the rotary furnace for lime production (Škvára, 1995)

CaCO3

furnace

burner

cooler

(shaft, planetary, etc.)

lime

air

air

CALCIUM BINDERS

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Air lime

Hydraulic lime

Criteria for lime production

Circular furnace

Shaft furnace – counter flow

Shaft furnace - parallel flow

Rotary Furnace


1. What is the difference between air and hydraulic lime?

2. Define the concepts of over-burnt and under-fired.

3. Explain the production of lime.

4. Enumerate furnaces for lime production, their advantages and Disadvantages.

CALCIUM BINDERS

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3.2. Lime properties, lime slaking



define the properties of lime,

evaluate lime according to various criteria,

explain lime “slaking”,

define solidification and hardening of lime.

Lecture

Properties of CaO

- cubic system; ρCaO = 3.34 – 3.40 g.cm-3

- Tt = 2800 °C (the surface tends to sinter)

- In the case of MgO content – a compound is formed between CaO and MgO only at 2370 ° C.

Up to this temperature, they coexist as two oxides, they do not interact.

- Porosity of lime is characterized by bulk density, which is directly related to porosity.

𝑃 =𝜌 − 𝜌,

𝜌∙ 100 (%)

where ρ is the density (kg.m-3

)

ρ´- bulk density (kg.m-3

)

P - porosity (%).

When P> 50% is ρCaO = 1.45 to 1.65 g.cm-3, in practice, we cannot achieve → P = 37-46%, which

corresponds to ρCaO = 3.34 g.cm -3

.

CALCIUM BINDERS

23

Evaluation lime

1. CaO activity – indicates the highest temperature reached during slaking under the defined

conditions and the relevant time (Fig. 11). For common types of CaO, it is at least 70 °C

achieved for 1-12 minutes.

Fig. 11 Lime activity

2. Lime substance – the amount of the putty resulting from slaking of 1 kg CaO (water is added

while the reaction is taking place). It is expressed in litres/kg of lime.

3. Unslakeable share – it is performed by starting to slake 2 kg of lime, followed by mesh

analysis (capture on mesh sized 1.6 mm). The rest must not be larger than 9-12%.

Slaking lime

CaO + H2O → Ca(OH)2 strongly exothermic reaction

For 1 kg of CO about 0.32 litre of water is needed (in theory), in practice it is up to 3 times more water

as it partly evaporates during the reaction.

There are two types of slaking according to whether it takes place in excess of water or with a little

amount water:

1. Reaction “through the solution” – in excess of water (240 to 320 litres per 100 kg of lime).

When slaking lime in excess of water, a supersaturated solution Ca(OH)2 is formed, from

which then the hydroxide crystallizes. Crystals are formed on the solid phase (CaO). In a slow

rise in concentration, few nuclei growing to relatively large crystals are formed. If the

concentration increases rapidly until the supersaturation of the solution, many nuclei which

cannot grow to a large size are created.

T more active lime

The least active lime

C

4

A

F

CALCIUM BINDERS

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Slaking should take place at a suitable speed and it is adjusted by addition of water so that the

slaking temperature did not rise above 100 °C. The slaking process can be accelerated by the

addition of H3BO4, NaCl, CaCl2. Conversely, this process can be slowed down using gypsum,

Na2SO4, etc. The wet slaking method is especially suitable for lump lime.

2. “Dry” slaking – a small excess of water (60 to 70 ml per 100 kg of lime). Due to heat

developed during hydration, excess water partly evaporates and a powdery product containing

a small amount of adsorbed water is formed. Slaking can be carried out in mixers similar to

concrete ones, where lime pulp is sprinkled with water and then slating continues in tanks for

about 24 hours. It is suitable for fine lime and low consumption of lime. It is economically

most convenient to deliver the resulting hydrated lime to the construction sites, since it does

not carry excess water, and does not require special containers.

Ca(OH)2 formed as a result of slaking is stable to humidity, but reacts readily with CO2, even at a low

concentration. Relatively rapid carbonization occurs with CaO in the presence of moisture, while in a

dry environment CaO reacts with CO2 only at the temperature above 300 °C.

Ca(OH)2 + CO2 → CaCO3 + H2O RE-CARBONIZATION

The reaction proceeds slowly in depth, because CaCO3 shell is formed on the surface, which prevents

further the reaction. The advantage is that the system is hardened, strengthened, and it is chemically

resistant.

Setting and hardening of lime

The mechanism of setting and hardening of lime is not fully understood yet. The participation of

several processes is assumed. First, lime mortar solidifies by simply drawing off water through porous

masonry. Solidification then proceeds by the formation and drying of the hydroxide gel network.

Another process is slow recrystallization of Ca(OH)2, which is probably involved in the hardening

process. Concurrently, carbonization is even slower.

Solidification is caused by evaporation of water present and hence the mortar produced from air lime

does not solidify in water at all.

Use of lime

- binder for mortars (plaster, etc.),

- production of aerated concrete (fly ash + CaO + gypsum + H2O and Al),

- sand-lime bricks (silica sand + CaO + H2O).

CALCIUM BINDERS

25


Properties of lime

Activity of lime

Slaking lime

Setting and hardening of lime


1. How is defined lime porosity defined?

2. Explain the concept of activity of lime.

3. What does mean “slaking” lime mean?

4. Name kinds of slaking.

5. Explain the mechanism of setting and hardening of lime.

6. What is the most common use of lime?

MAGNESIUM BINDERS

26

4. MAGNESIUM BINDERS




define raw materials for Sorel cement production,

explain the advantages and disadvantages of Sorel cement.

Lecture

They are also called, according to its discoverer, Sorel cement. They are produced by mixing so called

caustic magnesite with MgCl2. Caustic magnesite is very reactive MgO resulting from firing at a

temperature of 700-800 °C from MgCO3 (magnesite), which is mixed with an aqueous solution

MgCl2.

MgO itself and its mixtures with aqueous solutions of other compounds (MgSO4, FeSO4, ZnCl2) have

the ability of solidification to a limited extent. However, the products do not attain the strength of the

mixture with MgCl2. Instead magnesite, dolomite can also be used to prepare mixtures; it must be pre-

fired to such a temperature that only decarbonisation of MgCO3 occurred and CaCO3 remained

undecomposed because the resulting CaO impairs volume stability, mechanical strength and resistance

of mortar against moisture.

The ratio of MgO:MgCl2 = 2:1-5:1, during hydration, the compound 5Mg(OH)2.MgCl2.7H2O

(hydroxide, chloride III) is formed. It is stable only at a lower concentration of MgCl2, at a higher

concentration it passes into hydroxide chloride II (which only solidifies, but it does not harden). We

can say that the main functional product and strength agent is hydroxide chloride III.

Disadvantages

- low resistance to humidity,

- low resistance to elevated temperature,

- corrosive effects on metallic materials.

MAGNESIUM BINDERS

27

Advantages

- It can hold a lot of filler (multiple quantities compared to cement)

a) organic – sawdust, granulated cork etc.

b) inorganic – quartz sand, asbestos, etc.

Use

- production of xylolite (mixed with sawdust → production of wooden floorboards),

- production of Heraklit (with wood wool → floors),

- on highly stressed floors of the factory halls (with quartz sand).

Properties

- solidification is completed within 6 hours,

- compressive strength of 60 to 100 MPa after 28 days.


Sorel cement

Caustic magnesite

Hydration of magnesium binders


1. What does caustic magnesite refer to?

2. What results from Sorel cement hydration?

3. What is the ratio of the main raw materials for the production of Sorel cement?

4. What advantages and disadvantages of using magnesium cement do you know?

5. Best-known use of magnesium cements.

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28

5. CEMENT

HYDRAULIC BINDERS

5.1. Portland cement


Target After studying this section, you will be able to

classify cements into groups,

define the concept of Portland cement,

know the composition of the raw material mixture for cement

production.

Lecture

Cements are currently the most widely used binder in construction. Cement is a hydraulic binder, i.e. a

finely ground inorganic substance; when mixed with water, it forms a slurry that sets and hardens as a

result of hydration reactions and processes. After hardening, it retains its strength and stability also in

water.

Hydraulic cement hardening is due to the hydration of calcium silicates and aluminates. Active

hydraulic cement components are compounds of CaO and SiO2, Al2O3, and Fe2O3 or other

compounds of similar type. The total content of active calcium oxide (CaO) and active silicon dioxide

(SiO2) must be at least 50 % of cement weight.

From the chemical point of view (the predominant active compound) cements can be divided into

three groups:

A. Siliceous (silicate) cement – the most prominent representative is Portland cement. Cements

from natural or artificial hydraulic material (e.g. from slag) are similar in composition, but the

production method and mechanism of solidification are different.

B. Aluminate (alumina) cements.

C. Other cements – e.g. ferrite, barium, etc. (a few percent).

In view of the composition, cements of the first two groups belong to the system CaO – SiO2 – Al2O3

– Fe2O3 with small amounts of minor components. Because the Fe2O3 content is relatively low, the

CEMENT

29

area of the cement composition can approximately be expressed in the three-compound system CaO –

SiO2 – Al2O3 (Fig. 12).

Fig. 12 Areas of the composition of Portland cement (PC), basic blast furnace slag (VS) and aluminate

cement (HC), (Hlaváč, 1981)

Portland cement

History

- The patent for the manufacture of Portland cement: 1824, John Aspdinov.

- Portland because concrete made from it resembles the limestone from the English island of

Portland.

- The first cement factory in the Czech Republic: 1865 in Bohosudov.

Portland cement is characterized by:

1. The production process consisting in firing a raw material mixture to sinter, thus so called Portland

clinker is formed, from which Portland cement is acquired by fine grinding with additives.

CaO Al2O3

SiO2 SiO2

C2S

C3S

C3A C12A7 CA CA2

HC

PC

VS

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30

2. Mineralogical (phase) composition, which resulted from high temperature reactions in the raw

material mixture; the product is a heterogeneous composition comprising mainly silicates, to a lesser

extent, calcium aluminates and ferrites and a glass phase.

Materials for the production of Portland cement

The main raw materials

1. Soiled limestone – containing calcite as the dominant mineral (from 75 to 80 wt. %), the rest

of the content are clay components, quartz, iron compounds. To such raw material it is no

longer necessary to add clay and other compounds containing SiO2, Al2O3, Fe2O3.

2. Clays – they are used only when if we have high-percentage limestone (high CaCO3 content).

Deposits are preferably close to a cement plant (easier and cheaper transport).

Additional materials

They are added if the content of any of the components in the basic raw materials is not sufficient.

1. Burnt pyrites – correction of Fe2O3

2. Silica sand – correction of SiO2

3. Bauxite – Al2O3

Auxiliary materials

1. Setting regulator – gypsum 2HSC

2. Latent hydraulic substance – finely ground GVS

From the raw materials, so called cement clinker is prepared by firing. The resultant clinker contains

the main clinker minerals:

C3S 3CaO.SiO2 Tricalcium silicate

C2S 2CaO.SiO2 Dicalcium silicate

C3A 3CaO.Al2O3 Tricalcium aluminate

C4AF 4CaO.Al2O3.Fe2O3 Tetracalcium alumoferrite

The average chemical composition of the clinker is: 65 % C, 21 % S, 6 % A, 3 % F.

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31

The composition of the raw material mixture

The aim is that all CaO from the raw materials reacted during the thermal treatment to compounds

capable of hydraulic solidification. There is only a limited number of such compounds, and the entire

production process, from preparing the mixture through firing to cooling, must be done so that these

desired compounds (the main clinker minerals) are formed.

Three modules were proposed and used on a long-term basis to achieve appropriate composition of

the cement raw material.

A) Hydraulic module MH – the ratio between the C content and the sum of S, A, F

𝑀𝐻 =C

S+A+F ≈1,7-2,4

The lower limit of MH – more C2S than C3S in clinker (lower strength, but less energy consumption for

the production).

Upper limit MH – more C3S, means longer firing of the raw materials, higher temperatures, but better

and higher quality cement (higher strength).

B) Silicate module MS – the ratio between S and the sum of A and F.

𝑀𝑆 =S

A+F ≈1,7-2,7

The lower limit of MS – less minerals containing S, cements are less expensive with a lower chemical

resistance, they are easier to grind.

The upper limit of MS – chemically resistant product with an increased proportion of S (more C3S in

clinker), firing is more expensive.

C) Aluminate module MA – the ratio between A and F.

𝑀𝐴 = A

F ≈1,5-2,5

A higher value of MA means a higher initial strength, but lower chemical resistance.

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32

Besides the aforementioned modules, there is an attempt to express the relationship between CaO and

hydraulic components. This value is called the degree of saturation SS (weight representation of the

components in minerals). In other words, MH specifies SS. The calculation derivation follows:

C3S= 3∙C

S=2,80

C2S= 2∙C

S=1,86

C3A= 3∙C

A=1,65

C4AF={C3A+CF →1,65+0,35C2A+ C2F →1,1+0,7

These ratios indicate the amount of C necessary for the formation of clinker minerals C3S, C3A and

C4AF (except C2S):

C=2,8 ∙S+1,65 ∙A+0,35 ∙F Bogue

Based on experience, the relationship was adapted for C3S, C2A and C2F:

C=2,8 ∙S+1,1 ∙A+0,7 ∙F Kűhl

Today, the following compromise is used in the cement work most commonly:

C=2,8 ∙S+1,18 ∙A+0,65 ∙F Lea+Parker

These equations represent the amount of C necessary for complete reaction to the respective clinker

minerals.

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33

The degree of saturation is then determined as:

𝑆𝑆 = CSKUTCTEOR

∙ 100 (%)

𝑆𝑆𝑩 = CSKUT

2,8 ∙S+1,65 ∙A+0,35 ∙F ∙100 (%)

𝑆𝑆𝑳𝑷 = CSKUT

2,8 ∙S+1,18 ∙A+0,65 ∙F ∙100 (%)

Numerical difference between thus expressed degrees of saturation is not great. It then follows from

the reaction that at the maximum value of SS = 100, all CaO contained in the raw material mixture

reacted to clinker (or other) compounds. But this would mean in practice that the reaction was

finishing too long. Conventional cements have SS = 85-95 %. This means that we always give less

CSKUT than CTEOR. For good clinker, the content of free CaO in clinker must not be greater than 2 %

(usually it is 0.5 %).

The main significance of those relationships is that they allow calculation of the raw material mixture

so as to obtain the maximum number of the desired hydraulic products and to make it possible for all

of CaO to react.

Thus the necessary criteria for setting the raw material mixture are: Degree of saturation SS, Silicate

module MS, Aluminate module MA.


Portland cement

Raw materials for cement production

Cement clinker

Hydraulic module

Silicate module

Aluminate module

The degree of saturation

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34


1. General classification of cements.

2. Draw composition of Portland cement in the ternary diagram of the C-S-A.

3. Enumerate the main clinker minerals.

4. What are the raw materials for cement production?

5. Using what do we define the composition of the raw material mixture?

6. Explain the concept of the degree of saturation.

7. What criteria are necessary for the appropriate adjustment of the raw material mixture?

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35

5.2. Chemical and physical processes in the formation of clinker, clinker

minerals



describe the reactions of the main raw materials for the production of clinker,

define the clinker minerals,

define potential phase composition of clinker.

Lecture

Decomposition of the main raw materials

Limestone

CaCO3800°C→ CaO+CO2

Clay

AS2H2 (kaolinite)550-600°C→ AS2

950°C→ A2S3 (spinel)+ Samorphous

>1100°C→ A3S2 (mullite)+ SKR(cristobalite)

Sand

β-quartz 573°C→ α-quartz

>1200°C→ cristobalite

Reaction of the raw materials among themselves

The mixture of raw materials prepared according to the principles described in the previous chapters,

is heated to temperatures around 1450 °C. With gradually increasing temperature, especially water is

CEMENT

36

removed from the mixture. Drying is completed at 200 °C. The processes that are important for the

production of clinker begin at considerably higher temperatures. They are:

A) decomposition of solids (dehydration of clay minerals, decomposition of CaCO3)

B) the mutual reaction of the components in the solid state, later the participation of the melt,

C) melting of eutectics, dissolution of solids in the melt.

Decomposition of solids occurs first. Dehydroxylation of clay minerals takes place between 550-600

°C. These reactions produce reactive products which then react approximately above the temperature

of 700 °C with CaO (or with CaCO3), for example:

CaCO3 + AS2 → CA + C2S + CO2

Thus, the first reaction products are formed in a solid state (CA, C2S). CA arises before C2S,

simultaneously with the formation of the CA, C2F and C4AF begin to form.

At about 800 °C, calcination (calcite decomposition) to CaO and CO2 occurs. From about 900-950 °C,

calcium aluminates are formed, mainly C3A:

C + CA → C3A

Above the temperature of 1250 °C, clinker liquid phase begins to emerge, and from this temperature

C3S begins to form:

C2S+C→ C3S

Simultaneously with this reaction, the formation of C3A, C4AF, C2F and other compounds proceeds.

Reactions resulting in the formation of clinker minerals do not gain sufficient speed below the

temperature range 1350 - 1450 °C. This temperature range is most important for the production of

clinker, mainly because the most desirable clinker mineral C3S is formed there, which carries the

typical characteristics of Portland clinker. Below the temperature of 1250 °C, this compound is

unstable, it decomposes back to C2S and C. The decomposition can be prevented by rapid cooling of

clinker.

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37

Fig. 13 Formation of basic clinker minerals depending on the time and firing temperature. Legend: 1 –

clay decomposition, 2 – limestone decomposition, 3 – formation of minerals, 4 – formation of clinker,

5 – cooling, 6 – heating.

Reaching the maximum temperature of 1420 - 1450 ° C is followed by rapid cooling of the clinker to

preserve phase composition, which was formed at a high temperature and was close to equilibrium.

Clinker falls into the coolers and it is intensely purged by cold air. The main reasons:

1. To keep as much C3S as possible – during slow cooling it would decompose to C2S and C.

2. Slow cooling would cause a modification conversion of β-C2S to -C2S; it does not have the

typical characteristics, clinker would be spoilt. Due to a large volume change (10 %), this

transformation leads up to the spontaneous clinker decomposition. C2S exists as α, α´ (high-

temperature forms), β (metastable form), (low- temperature form) (Fig. 14).

Fig. 14 Polymorphic forms of C2S.

T

p

α

α’ β

γ

C2S

1

2 3 4

5

ΔHo

ΔHr

T 550 700 800 1250 1450

6

6

6

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38

3. At high temperatures, the furnace contains 15 % of the melt. During rapid cooling, the melt is

super cooled predominantly on glass and MgO remains dissolved in it, which would otherwise

be crystallized (MgO has an adverse effect of expansion at sufficiently slow hydration).

Clinker minerals

The phase (mineralogical) clinker composition, i.e. the presence and relative proportion of so called

clinker minerals has a major effect on the final properties of clinker and cement. An important tool

when considering the phase composition of clinker are phase diagrams that allow graphic subtraction

or calculations of shares of each phase. In Portland clinker, more than 25 mineral phases has been

described so far, four major compounds are decisive for the properties of cement:

C3S 3CaO.SiO2 Tricalcium silicate

C2S 2CaO.SiO2 Dicalcium silicate

C3A 3CaO.Al2O3 Tricalcium aluminate

C4AF 4CaO.Al2O3.Fe2O3 Tetracalcium alumoferrite

Pure minerals must be distinguished from technical ones, which are usually altered in clinker by the

presence of other components or impurities in a solid solution. This changes their basic properties

(melting points, temperatures of modifying transformations, etc.). These technical phases are referred

to as follows:

1. ALITE – solid solution C3S (it contains foreign ions but retains the structure of C3S). The alite

content affects the speed of hardening and strength of cement. Pure C3S is stable only in the

range 1250 °C to 2070 °C, below the temperature of 1250 °C it decomposes to C2S and C.

Although C3S (or alite) is unstable below the temperatures of 1250 °C, it may be preserved as

metastable phase by rapid cooling, since the decomposition reaction does not take place in the

limited time.

2. BELITE – solid solution C2S (especially β-C2S ) – shows slower but longer lasting increase in

strength than alite.

3. CELITE – clinker containing C3A, C4AF and glass

a) Light inter-matter – under the microscope it is bright, but in reality it is dark (it

contains ferrites – C4AF),

b) Dark inter-matter –under the microscope it is dark, but in reality it is light (it

contains aluminates, C3A).

Furthermore, the following compounds occur in clinker: 2 % of free CaO at maximum, 2 % of free

MgO at maximum, glassy phase from 5 to 15% (its content is dependent on the conditions and the rate

of cooling of clinker).

Clinker composition:

Alite 65% or more

Belite 10-25%

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39

Celite 8-20% (light inter-matter is represented more than dark)

Cement strength is primarily affected by the content of C3S and C2S. C3A and C4AF contribute to

strength only at the beginning (Fig. 15)

Fig. 15 The development of strengths of major clinker minerals.

Assessment of clinker

What is decisive for the properties of clinker is not the chemical composition, but the phase

composition (mineral content). Basic rules of phase equilibria allow the determination of the phase

composition for the system by calculation. For certain chemical composition of clinker there is only

one possible proportion of the contents of four of the equilibrium crystalline phases according to the

phase rule. This proportion is called the potential phase composition of clinker.

The method of calculation was suggested by Bogue. For the calculation of the potential composition as

the equilibrium solid phase, C3S and C2S, C3A and C4AF are considered (we consider only 4 phases,

not the glassy state – we calculate it subsequently). Quantitative mineralogical composition is

accurately determined by calculating from the chemical composition. The balance is based on the

known stoichiometric composition.

C3S

β - C2S

C3A

C4AF

360 180 90 28

20

40

60

80

time (days)

com

pre

ssiv

e st

ren

gth

MP

a

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40

We consider, for example, the following composition:

oxide C3S C2S C3A C4AF

x y z v

CaO 73.7 65.1 62.3 46.2

SiO2 26.3 34.9 - -

Al2O3 - - 37.7 21.0

Fe2O3 - - - 32.8

The weight percentages of the individual components in clinker (x, y, z, v) must match the contents of

these components in each stage.

Then:

CaO = x ∙ 0,737 + y ∙ 0,651 + z ∙ 0,623 + v ∙ 0,462

SiO2 = x ∙ 0,263 + y ∙ 0,349

Al2O3 = z ∙ 0,377 + v ∙ 0,21

Fe2O3 = v ∙ 0,328

We solve the 4 equations with four unknowns, in the solution we obtain the representation of the

minerals in clinker in the particular chemical composition.

% C3S = 4,07 ∙ C - 7,6 ∙ S - 6,72 ∙ A - 1,43 ∙ F

% C2S = 8,60 ∙ S + 5,07 ∙ A + 1,08 ∙ F - 3,07 ∙ C

% C3A = 2,65 ∙ A - 1,69 ∙ F

% C4AF = 3,04 ∙ F

It is the most accurate method for determining the composition of clinker – the potential clinker

composition is obtainable at thermodynamic equilibrium. The advantage of this calculation is that,

according to the calculated content of C3S and C2S, we can deduce the ultimate strength of the cement

and that we need to know the content of C3A and C4AF for dosing gypsum while grinding clinker for

the cement production.


Decomposition of the main raw materials

Mutual reaction of the components in the solid state

Clinker minerals

Alite, belite, celite

Assessment of clinker

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41


1. Explain the decomposition reactions of the main raw materials for the production of clinker.

2. Processes important for the formation of clinker.

3. Describe the reactions between the raw materials in the production of clinker.

4. The formation of clinker minerals, depending on the temperature and time of firing.

5. Why rapid cooling follows immediately after firing clinker?

6. Define clinker minerals.

7. Define technical minerals.

8. Explain the development of strength of the principal clinker minerals.

9. What is the most common method for determining the composition of clinker?

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42

5.3. The technology of cement production (machine)



explain the production of clinker in cement furnaces,

explain the production of cement,

enumerate cement producers in the Czech Republic.

Lecture

The three main stages of the cement production:

1. Preparing the raw material mixture – mining, crushing, grinding, homogenization.

2. The production of clinker – heat treatment (firing) of the raw material mixture into clinker.

3. The production of cement - grinding clinker with additives.

According to the process of mixing, grinding and homogenization of the raw material mixture and its

forms during firing into clinker, there are two ways of producing cement – wet and dry.

Wet method of the clinker production

Limestone is crushed when dry and then ground when wet. The water content in the resulting raw pulp

thus formed is 33 to 40 %. The following step is injecting into a long rotary furnace (150-200 m),

which has zones:

- drying: up to 200 °C,

- preheating: 200 to 800 °C,

- calcining (decarbonizing) 800 - 1200 °C, decomposition of CaCO3 occurs there, and the

formation of the first clinker minerals,

- exothermic: 1300 °C, the formation of clinker minerals continues,

- cooling zone: 1100 - 1000 °C, discharging holes, clinker falls into the cooler (drum, planetary,

today grate ones)

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43

The furnace rotates about the axis with a frequency of about 1 to 2 revolutions per minute. Lining is of

fireclay up to 1000 °C, for higher temperatures of magnesite or magnesium-chromite. Most highly

stressed points are exit holes and space around the burners (lining of quality magnesium-chromite or

refractory concrete. Magnesium-chromium contains dangerous trivalent chromium, which passed

partly into clinker and spoilt it. Therefore, its use was prohibited and it was replaced with magnesium-

spinel).

Advantages

- clinker is homogeneous,

- the feedstock does not need to be dries,

- the raw material is easily ground,

- the environment dustiness is eliminated.

Disadvantages

- high water consumption,

- high consumption of heat for the subsequent drying of the pulp in the furnace.

In the Czech Republic, this method is not used.

Dry method of the clinker production

The raw material must be dried before grinding. A powder mixture is usually preheated in suspension

flue gases. Then a calciner follows (the decomposition of CaCO3 occurs almost entirely here). The raw

material enters the furnace at about 900 °C. It is a short rotary furnace (60 to 130 m) (Fig. 16).

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44

Fig. 16 A simplified diagram of the rotary furnace with a precalciner (Šauman, 1993)

Figure legend

1 – I, II, III, IV – heat exchangers; V – precalciner

2 - input of raw materials

3 – burner

4 - guiding tertiary air

5 - removal of solids

6 - burner

7 - clinker cooler

8 – clinker discharge

9 - rotary furnace

10 - fan

IV

III

II

I

V

1

2

3

4

5

6

7 8

9 10

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45

Advantages

- high efficiency production,

- high thermal efficiency of firing due to preheating of the raw material,

- lower heat consumption for preheating and firing the raw material mixture.

Disadvantages

- dustiness

- clinker is not as homogeneous as when wet.

Cement production

Cooled clinker is ground in the grinding mills. Only after grinding it gains the ability of sufficiently

fast reaction with water and solidification. Fineness of grinding is a critical production operation due

to the use of cement. Finely ground cements hydrate faster (they have a larger specific surface area),

having greater initial and final strength. The minimum fineness of grinding is 225 m2.kg

-1.

Clinker is ground with the addition of 2-6 % of gypsum ( 2HSC ), now in the form of energy gypsum or

chemical gypsum. It serves as a setting regulator. Only clinker + gypsum = cement. Sometimes latent

hydraulic substances LHL (GVS, volcanic ash – pozzolan, power station fly ash) are added – they

regulate the mixing properties of cement

Common cements:

CEM I Portland cement

CEM II Mixed Portland cement

CEM III blast furnace cement

CEM IV Pozzolanic cement

CEM V mixed cement

Furthermore, following the Roman numeral, the strength class is indicated: 32.5; 42.5; 52.5 (the value

means compressive strength in MPa after 28 days of hydration. Other marking of cement:

R fast setting cement (high early strength)

N normal solidification rate

L low initial strength

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46

Cement production in the Czech Republic

1. Českomoravský cement, a.s.

- plant Králův Dvůr

- plant Radotín

- plant Mokrá u Brna

2. Holcim (Czech Republic), a.s. – Prachovice

3. Cement Hranice, a.s.

4. Lafarge Cement, a.s. - Čížkovice


Wet method of the clinker production

Dry method of the clinker production

Cement production

Producers of cement in the Czech Republic


1. Cement production phases

2. Explain the wet method of the cement clinker production.

3. Explain the dry method of the cement clinker production.

4. How to make cement?

5. What are the common cements?

6. Name the major cement producers in the Czech Republic.

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47

5.4. Cement hydration



explain hydration of cement through solution

explain the topochemical mechanism of hydration,

define the process of hydration of the clinker minerals.

Lecture

Cement clinker + 2HSC = cement

The cement hydration itself is a complex set of reactions between the clinker minerals, gypsum and

water. Gradually, creating real structure, characterized by decreasing the pore volume, is formed.

When mixed with water, cement solidifies and gradually acquires strength. Due to the reaction with

water, initially anhydrous cement mineral phases are converted to moisturizing products (new

compounds containing chemically bound water). These moisturizing products are insoluble in water

and are stable in the aquatic environment. The process of hydration (hardening of the system) has two

aspects:

a) chemical – four principal clinker minerals react with water to form hydro-silicates and

calcium hydro-aluminates,

b) physical – i.e., changes in structure. They take place simultaneously with chemical reactions

in physical processes in which hydrates grow into each other, and the system is strengthened.

First, C3A reacts with water, then gradually C3S, C4AF and C2S.

There are two theories of hydration of cement clinker.

1. Le Chatelier - the hydration process through the solution. Grains of the initial particles after

addition of water begin to dissolve into the solution, hydrate compounds begin to appear in the

solution, and these are less soluble in water, therefore they crystallize from the solution. Thus,

the solution is diluted again and the initial phase can dissolve again. Simultaneous hardening

process is possible, because the discharged crystals have acicular or plate character and they

CEMENT

48

form a felt-like network which is connected by adhesive forces. The product is represented by

crystalline phases. They are large crystalline formations that grow very little.

2. Michaelis – a topochemical mechanism. Input grains, once they get water, form a thick gel

envelope soaked with water on the surface. Grain kernels begin to suck this water from the

surface for further hydration – it also varies – the envelope on a particle dries up and forms a

crystalline needles. The needles of neighbouring particles begin to grow into each other, thus

strengthening the system. Fine crystals, much interconnected are thus formed. Strength of this

system is much higher.

Both mechanisms are involved in the overall hydration of clinker, which results in big crystals and

fine gel crystals.

The actual process of cement hydration can be divided into four stages:

A. Pre-induction period – it takes place in the first minutes. Rapid initial dissolution of alkali

sulphates and aluminates occurs. Initial hydration of C3S. Formation of the AFt phase

(ettringite).

B. The induction period – it takes place in the first hours of hydration. Loss of silicates and

Ca(OH)2 and C-S-H nucleus formation occurs. The emergence of AFt and AFm

(monosulphate).

C. Accelerating step – about 3-12 hours. Rapid chemical reaction C3S takes place to form

Ca(OH)2, and C-S-H.

D. The final step – formation of the C-S-H and Ca(OH)2 phases controlled by diffusion.

Conversion of ettringite to monosulphate (for cements containing low amounts of C3A and a

significant proportion of gypsum monosulphate, it usually does not take place).

Hydration through solution

It takes place with C3A and C4AF.

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49

it can, but does not have to C3A

C4AF

+H C3A . 3CSH32

3H6

ettringite gel +CSH2 C3A . 3CSH14~16

3H6

(AFm= monosulphate)

)

ettringit AFt (aluminoferrite trisulphate)

by

=> the system solidifies, but it does not hold together – false solidification – we have to prevent this

process =>

Gypsum forms so called ettringite gel on the surface of the grains thereby slowing the reaction with

water (Fig. 17). It is believed that the formation of very fine-grained ettringite on the particles of

clinker causes a delay in solidification, because due to this so called protective layer, fine particles to a

large extent prevent further reactions; besides, these formations of very small size cannot form a solid

structure. Only due to their conversion into needle-shaped or rod-shaped crystals of ettringite a solid

structure is formed.

The optimum amount of gypsum added has not been clearly established. Excess gypsum causes the

formation of ettringite, which takes place even after completion of solidification, so that it can cause

uncontrollable expansion with consequent disruption of the cement microstructure. Conversely, a low

addition of gypsum results in premature formation of the AFm phase, reducing the nucleation of the

Ca(OH)2 and C-S-H phases, which leads to a deceleration of the increase in strength in the initial

period.

Fig. 17 Formation of ettringite gel on the surface of the grains.

C3A

gypsum

CEMENT

50

Topochemical mechanism of hydration of C3S and C2S

C3S is the main component of cement as its proportion in clinker is generally higher than 55-60%. A

significant part hydrates within 30 days, another portion within 1 year. The C3S reaction with water

can be expressed as follows: C3S + H → C-S-H + CH. The reaction cannot be precisely quantified

stoichiometrically because C-S-H gel formed together with portlandite is characterized by varying

composition.

Fig. 18 The stages of C3S hydration (Šauman, 1993). In contact with water (1) an electric bilayer

consisting of movable Ca2+

ions and an immovable silicate layer (2) is subsequently formed. The

system is further developing, and it is slowed down primarily by the fact that the ions must penetrate

the electric bilayer. Gradually, nuclei are formed in the solution (Ca2+

, OH- and silicate ions) from

clusters of atoms; they must achieve the critical size necessary for their growth. Subsequently, either

Ca(OH)2 or C-S-H gel (3) is formed.

The reaction of C3S and C2S with water can then be written as follows:

Layer rich in Si –

immovable silicate

layer movable ions Ca2+

1) 2) 3)

C3S

H2O

C3S C3S

C-S-H

Ca2+

Ca2+

Ca2+

Ca2

+

Ca2+

OH-

OH-

OH-

OH-

Ca(OH)2

Plate crystals

Ca(OH)2

Ca2+

Ca2+

Ca2+

Ca2+

OH-

OH-

OH-

OH-

Ca2

+

OH-

Ca2+

OH-

OH-

Ca2+

C3S

C2S

+H=> formation of CSH

gels

+CH

C-S-H I

C-S-H II

+CH

Long time (even years)

CxSyHz + CH

=> formation of definitive minerals

=> most frequently tobermorit

C5S6H5

CEMENT

51

After completion of the hydration processes, the cement paste represents a very complicated system

that consists of an amorphous, poorly crystalline and crystalline phases, which to a lesser or greater

extent contribute to the achieved physical and chemical properties.

The final products of hydration using clinker and gypsum are:

ad 1. AFt (ettringite), AFm (monosulphate), partly C4(A,F)H13

ad 2. C-S-H I, CxSyHz, Ca(OH)2

Ca(OH)2 contributes significantly to the final mechanical strength, but of all final products of

hydration, it is most soluble in water..


The hydration process through the solution (Le Chatelier)

Topochemical hydration mechanism (Michaelis)

Cement hydration

The final products of the cement hydration


1. Explain the process of hydration through the solution.

2. Explain the topochemical hydration mechanism.

3. Describe the actual process of cement hydration.

4. Explain hydration of C3A and C4AF.

5. Why is it necessary to add gypsum to clinker?

6. Explain the hydration of C3S and C2S.

7. Define the end products of cement hydration.

CEMENT

52

5.5. Properties of cement



enumerate macro-properties of the product,

explain the essential properties of cement.

Lecture

Macro-properties of the product

There may be three types of pores in hardened cement stone.

1. Gel pores – the smallest ones, produced in gelatinous bodies, usually in the size of 2-4 nm;

external water cannot reach them – they are not dangerous, it is the most resistant part of the

hardened system.

2. Capillary pores – size in microns (1-10 μm), they were formed after the addition of large

amounts of water – excess water which was not consumed for the formation of gels,

evaporated, created “paths” permeable for external water. Those pores cause corrosion

phenomena (causing destruction of concrete by additional hydration).

a) CaO content – additional response to Ca(OH)2 – large volume change – concrete rupture

(content only up to 2 %).

b) MgO content – additional reaction to Mg(OH)2 – again concrete rupture – content of up to

5 %.

c) Much C3A little 2HSC – reaction with water slowly finishes – ettringite baAl.

CEMENT

53

3. Large pores – size mm, they are the largest and arise due to:

a) poorly mixing concrete (closed air bubbles)

b) poor grain structure,

c) artificially – aerated concrete (lightweight structures).

Pores 2 and 3 relate to water ratio. Optimum water ratio is 𝑤 𝑐⁄ = 0,25 ~ 0,55 (the theoretical amount

of water for the formation of the clinker minerals). Little water → hydration reactions takes place only

partially, a lot of water → a porous product permeable to water and with reduced strength.

Essential characteristics of cement – cement tests

1. Cement must be ground to certain fineness. With increasing fineness of cement, concrete

strength is growing. It is assessed using sieve analysis and measurement of the specific surface

area (Blaine). Portland cement must have a value greater than 225 m2.kg

-1.

2. Time and the course of solidification (Vickat)

a) Start of solidification – Vickat needle 1 mm above the bottom. An important property

associated with sufficient time from making concrete mix to its thorough mixing,

transportation to the destination and the actual processing.

b) End of solidification – needle less than 1 mm below the surface of the slurry.

The time of preparing the slurry until the end of solidification – setting time. Portland cement

has a setting time of about 45 minutes and the maximum setting time of 12 hours.

3. The density of cement – determination by a Vickat devices – the densimetric roller. Normal

density of cement slurry corresponds to such a consistency at which the densimetric roller

stops 5-7 mm from the ground. The amount of water with which normal density is achieved is

about 25 to 29 % per weight of Portland cement. The stated amount corresponds to the water

needed for complete hydration of the clinker minerals. The excess water increases the

porosity, thus reducing strength.

The density is dependent on the mineralogical composition of clinker. For Portland cement it

is about 3100 – 3200 kg.m-3

.

4. Cement strength – flexural and compressive. Mortar is prepared (3 parts of sand + 1 part of

cement + 0.5 part of water) → moulds 4x4x16 cm, wet storage, after 24 hours, demoulded and

immersed in water. Tests according to the standard after 2, 7 and 28 days. Strength of Portland

cement after 28 days of hydration is 32.5 to 47.5 MPa.

Cement strength are influenced mainly by mineralogical composition and fineness (particle

size). Early strength is significantly affected by alite. With its increasing amount, strength is

increasing.

CEMENT

54

5. Volume stability (Le Chatelier sleeve). If the cement volume is not constant, it can cause

significant reductions in concrete strength, in the extreme case, its destruction. (This is

primarily a content of free CaO and MgO.)

6. The heat of hydration – the amount of heat released in the hydration process. It is measured

calorimetrically. In normal cement at hydration after 28 days → 420 kJ.kg-1

.


Gel, capillary and large pores

Tests of cement


8. What kinds pores can there be in hardened cement stone?

9. Why is the optimum water-cement ratio important?

10. Name the essential characteristics of cement.

11. Describe tests performed using the Vickat device.

CEMENT

55

5.6. Other cements



define the concept of aluminous cement,

enumerate other types of cement.

Lecture

Aluminous cement

The main ingredient of aluminous cement is CaO.Al2O3 (CA). In the area of the primary

crystallization, the composition should be in the phase diagram C-A-S (Fig. 19)

Fig. 19 Occurrence of aluminous cement in the ternary diagram C-A-S.

C A

S

C3A C12A7 CA CA2

CEMENT

56

Raw materials for the production

Limestone and natural bauxite (ratio 1: 1). Chemical composition varies widely, depending on oxide

content (3-10 % of SiO2, 35-45 % of CaO, 35-50 % of Al2O3, 10 % of Fe2O3). The raw materials

should contain little SiO2 in order to suppress excessive amounts of C2AS (gehlenite) and C2S.

The main ingredients of aluminous cement are Al2O3 and CaO. According to the CaO content, we

divide them into high-calcareous (above 40 % of CaO) and low-calcareous (below 40 % of CaO).

Production

Two ways:

1. Melting of the crushed material in a flame or electric furnace at 1600 °C.

2. Sintering the granules of the ground material in a rotary furnace at 1250 – 1350 °C.

In contrast to Portland cement, the product is supposed to be cooled slowly, so as to create the desired

mineralogical composition.

In addition to the main clinker minerals CA + CA2, C4AF, C2AS (gehlenite) and melt are formed. The

resulting clinker is ground and we directly acquire aluminous cement. The mineral CA solidifies very

slowly and hardens quickly, the mineral CA2 is decisive for the final strength of cement.

Hydration

During aluminous cement hydration, principal clinker minerals react with water to form CAH10,

C2AH8, Al(OH)3 (topochemical mechanism – very fast acquisition of strength). The hydration reaction

proceeds rapidly. Metastable CAH10 is the bearer of early high strength.

Properties

1. Very fast acquisition of strength (within 24 hours, strength as Portland cement after 28 days).

2. High final strength (60-100 MPa).

3. Higher heat of hydration (possibility to concrete in a slight frost).

4. Cement resistant to sulphate weathering

Note: Sulphate water – sea water, mineral water. These waters destroys Portland cement – additional

reactions may take place in it resulting in ettringite formation associated with a change in volume and

concrete “rupture” according to the reaction: C3A + 2HSC = C3A . 3 32HSC

5. Lower porosity than Portland cement.

6. Resistance to elevated and high temperatures, which increases with a decrease in SiO2 content

and an increase in Al2O3 (it is used in refractory concrete).

CEMENT

57

Use

For the production of refractory concrete for monolithic furnace linings.

Note: In the construction industry, use of aluminous cement is prohibited in the Czech Republic. Due

to conversion of metastable phases of CAH10 and C2AH8 at temperatures above 20 °C there is an

increase of porosity and reduction of strength = destruction of the structure.

Other cements

A. Mixed cement – cement with LHL (rapidly chilled blast furnace slag), gypsum is used as an

activator → formation of the main binder phase of ettringite.

B. Expandable cements – any cement with water is connected with volumetric contraction. For

these cements, we try to increase the expansion volume. The chemical composition is set so as

to form as much ettringite and portlandite as possible. The use of pure MgO (about 5 %)

invokes appropriate concrete expansion. The expansion is due to osmotic pressure. It is used

for small applications with a particular focus.

C. White cement – manufactured from white high percentage limestone and kaolin containing

iron compounds up to 1 %.


Aluminous cement

Mixed cement

Expansive cement

White cement


1. Draw the composition of aluminous cement in the ternary system C-A-S.

2. Define the raw materials for the production of aluminous cement.

3. Explain the principle of the production of aluminous cement.

4. Name the main characteristics and the use of aluminous cement.

5. What other types of cement do you know?

AERATED CONCRETE

58

6. AERATED CONCRETE



explain the processes in the production of aerated concrete,

define the concept of autoclaving.

Lecture

The kind of lightweight concrete with good heat and sound insulating properties. It is directly

lightweight concrete, which means that the lightening was achieved during the production by creating

pores directly to the mass of concrete.

Raw materials

1. Binders – cement based on Portland clinker, lime (pure, unslaked), a mixture of lime +

cement.

2. Fillers – silica materials (sand, power station fly ash), SiO2 content > 90 %.

3. Water

4. Gypsum – setting regulator (today rather energy gypsum).

5. Aerogenous substances – finely ground metal aluminium mixed with water with the addition

of surfactants that degrease powder.

6. NaOH – to increase the environment alkalinity.

7. Crystallization nuclei – to increase the strength and reduce the time of autoclaving (production

waste).

AERATED CONCRETE

59

Processes during the production

1. Cement hydration (formation of C-S-H phases and Ca(OH)2, which reacts with finely divided

SiO2 at normal temperature to produce additional C-S-H phases).

2. Aluminium reacts with alkali according to the equation: H2O + Al + NaOH → H2↑ + NaAlO2

Production technology

The raw materials are mixed, aerated concrete slurry is finally added (crystallization nuclei) and

aluminium. It is necessary to harmonize the development of hydrogen and setting of the material so

that the result is a bubbled product full of pores. The material is poured into a “pan” (mould), where it

rises (50-60 cm). The top is then cut off (levelled) – the result is aerated concrete sludge with

crystallization nuclei, we bring them into the fresh mixture and thereby accelerate the process of

formation of the minerals. Subsequently, the mixture is cut into appropriate size of shape bricks and

then fed into the autoclave where the temperature is 170-190 °C and the pressure is 1.2 MPa. This

accelerates the hydration of cement and the reaction of formation of calcium silicate hydrates (C-S-H).

Autoclaving time has influence too – recrystallization occurs, all the hydrothermal reactions have to

take place (Fig. 20). The result is four stages of autoclaving:

1. Evacuation (0.5 hour)

2. Rise (1.5 - 3 hours)

3. Endurance (6-10 hours)

4. Fall (1.5 - 3 hours)

Fig. 20 Autoclaving stages during the production of aerated concrete.

0,5 1,5-3 6-10 1,5-3

1. 2. 3. 4.

1,2

saturated water

vapor pressure

[MPa]

t[h]

60

The actual reaction between Ca(OH)2 and SiO2 occurs during the third stage of isothermal heating

Properties

1. What is the lowest density (400 to 700 kgm-3

) at high mechanical strengths.

2. Low water absorption.

3. Resistance to frost.

4. Compressive strength (depending on porosity).

5. Thermal conductivity (in the case of porous materials decreases with decreasing density, i.e.

with increasing porosity).

Use

Bricks, blocks, lintels, ceiling liners, insulation panels.


Raw materials for manufacture of aerated concrete

Autoclaving


1. Define the raw material for the production of aerated concrete.

2. Explain the main processes in the production of aerated concrete.

3. Describe the stages of autoclaving.

4. What are the features and use of porous concrete?

REFERENCES

61

7. REFERENCES

References that can be used to further studies

[1] HLAVÁČ, J. Základy technologie silikátů. SNTL, Praha, 1981, s. 516.

[2] ODLER, I. Special Inorganic Cements. Routledgemot E F & N Spon, 2002, pp. 416,

ISBN-0-419-22790-3.

[3] BRANDŠTETR, J., aj. Geopolymery, geopolymerní cementy a betony. Silika 15, 2005,

č. 7/8, s. 208-211.

[4] ŠKVÁRA, F. Technologie anorganických pojiv I. VŠCHT, Praha, 1995, s. 150. ISBN

80-7080-224-3.

[5] ŠKVÁRA, F. Technologie anorganických pojiv II. VŠCHT, Praha, 1995, s. 184. ISBN

80-7080-225-1.

[6] ŠAUMAN, Z. Maltoviny I. PC-DIR spol. s.r.o., Brno, 1993, s. 198. ISBN 80-214-0509-

0.

[7] SCHULZE, W a kolektiv. Necementové malty a betony. SNTL, Praha, 1990, s. 271.

[8] TICHÝ, O. Tepelná technika pro keramiky. ČSVTS – Silikátová společnost České

republiky, Praha, 2004, s. 211. ISBN 80-02-01570-3.

[9] ADÁMEK, J., NOVOTNÝ, B., KOUKAL. J. Stavební materiály. Akademické

nakladatelství CERM, s.r.o., Brno, 1996, s. 205. ISBN 80-214-0631-3.

[10] FIGUŠ, V. Maltoviny. Technická litreatura, n.p., Bratislava, 1962, s. 307.

[11] BRANDŠTETR, J. Struskoalkalické betony. Stavivo, 1984, č. 3, s. 110-114.

[12] JIRÁSEK, J., VAVRO, M. Nerostné suroviny a jejich využití [online]. Dostupné na

www: http://geologie.vsb.cz/loziska/suroviny/index.html.

[13] ROVNANÍKOVÁ, P. Materiály historických omítek [online]. Dostupné na www:

http://www.studioaxis.cz/images/pamatky/rovnanikovapavla.doc.

[14] BÁRTA, R. Chemie a technologie cementu. Praha, 1961. Nakladatelství ČSAV. 1108 s.

[15] SHI, C., KRIVENKO, P.V., ROY, D. Alkali-Activated Cements and Concretes.

Taylor&Francis. ISBN 10: 0-415-70004-3.

[16] KURDOWSKI, W. Chemia cementu. Warszava, 1991. Wydawnictwo naukove PWN.

479 s.

[17] SCHULZE, W. aj. Necementové malty a betony. Praha, STNL, 1990. 271 s. ISBN 80-

03-00188-9.

[18] BARBOSA, V.F.F., MacKENZIE, K., THAUMATURO, C. Synthesis and characterizat

ion of materials based on inorganic polymers of alumina and silica: sodium polysialate

polymer. International Journal of Inorganic Materials, 2000. Vol 2, pp. 309-317.

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