The Cements Used In Concrete Production Biology Essay

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Cement is a binder which mainly consists of compounds of calcium, silicon, aluminum, iron and small amounts of other materials. The cements used in concrete production are called hydraulic cements which set and harden after being combined with water.

In the earliest of the 19th century, Joseph Aspdin, a bricklayer, first made a hydraulic cement called Portland cement whose name was given since the hardened cement resembles the color and quality of Portland stone [ErdoÄŸan T.S., ErdoÄŸan Y.T. (2007), BaÄŸlayıcı Malzemelerin ve Betonun Onbin Yıllık Tarihi, ODTÜ Yayıncılık.].

Portland cement is produced by intimately mixing together calcareous and argillaceous, or other silica-, alumina-, and iron oxide bearing materials, burning them in a kiln at a temperature about 1450°C, and grinding the resulting clinker with a small amount of gypsum (3-5%) [Neville A. M., Properties of Concrete, Fourth Edition, Pearson Education Limited, England, 2003].

2.1.1. Manufacturing of Portland Cement Clinker

In order to produce Portland cement clinker, there are four basic elements which should be in the raw materials in an appropriate proportion. These raw materials are calcium oxide (CaO), silica (SiO2), aluminum oxide (Al2O3) and iron oxide (Fe2O3).

The sources of calcium oxide, which approximately constitutes the 65% of the Portland cement clinker, are limestone, marl, chalk and sea shells. The sources of silica are sand, clay and gravel. The sources of aluminum oxide are shale, bauxite, clay and sand. The sources of iron oxide are hematite, pyrite and clay. To produce high-quality cement, there is a certain limit for the raw materials and these typical raw materials proportions are given in Table 2.1.

Table 2.1 Typical raw materials proportions after inginition [O. Labahn, Cement Engineers' Handbook , Forth Edition, Bauverlag GmBH-Wiesbaden and Berlin, 1983.]

Oxide

Limiting Value (wt %)

Content (wt%)

CaO

60-69

65

SiO2

18-24

21

Al2O3

4-8

6

Fe2O3

1-8

3

MgO

<5,0

2

K2O, Na2O

<2,0

1

SO3

<3,0

1

As given in Figure 2.1, the crushed, ground and screened homogenous raw mix is fed into a rotary kiln at a temperature of 1250 - 1450°C. The reactions take place, during the Portland cement clinker production, are as follows:

In the evaporation zone, as the name implies both the free water and the adsorbed water of the raw materials is evaporated between 100 and 400 oC. In the clay dehydration zone, between 350 and 650 oC, clay minerals decompose. At 550 oC, calcium carbonate starts to decompose. In the decarbonation zone, calcium carbonate rapidly decomposes at 900 oC. In the burning zone, with the decomposition of calcium carbonates formation of clinker minerals start. At 1200 oC, formation of dicalcium silicate (C2S), tricalcium aluminate (C3A) and tetracalcium aluminoferrite (C4AF) start. After the formation of these clinker minerals, tricalcium silicate (C3S) formation is achieved between 1250 and 1450 oC. Then, the resultant clinker at a temperature 1450 oC is rapidly cooled in order to avoid the sintering reactions to reverse. The cooled clinker is ground with about 3-5% gypsum to a specified degree of fineness. The reason behind the gypsum addition is to adjust the setting time of the cement. Finally, the resulting product is commercial Portland cement so widely used throughout the world [O. Labahn, Cement Engineers' Handbook , Forth Edition, Bauverlag GmBH-Wiesbaden and Berlin, 1983 / Ramachandran V.S., Concrete Admixtures Handbook, Second Edition, Noyes Publications, United States of America, 1995].

Admixtures

Cement Silo

Fuel Preparation

Quarry

Raw Grinding

Homo Silo

Gypsum + Additive

Crusher

Raw material

Cooling

Finish Grinding

Preheater

Kiln

Filter

Stack

Clinker

Figure 2.1 Flow sheet of Cement Production

2.1.2. Chemical Composition of Portland cement

In cement, there are essentially four clinker phases which are listed in Table 2.2. Determination of these phases of cement is a very complex procedure. However, approximations for the clinker phases' composition for Portland cement can be achieved with its oxide analysis (see Table 2.1.) by using Bogue's formula.

Table 2.1 Cement Clinker Phases. [Cements, Graeme Moir]

Designation of the phase

Composition of the pure phase

Abbreviated Notation

Typical range

(mass %)

Alite or Tricalcium silicate

3CaO. SiO2

C3S

45-65

Belite or Dicalcium silicate

2CaO. SiO2

C2S

10-30

Aluminate or Tricalcium Aluminate

3CaO.Al2O3

C3A

5-12

Ferrite or Calcium Aluminoferrite

4CaO. Al2O3.Fe2O3

C4AF

6-12

C3S = 4.071 (total CaO - free lime) - 7.600SIO2 - 6.718A1203 - 1.430Fe203 (2.1)

C2S = 2.867SIO2- 0.7544C3S (2.2)

C3 A = 2.65A1203- 1.692Fe203 (2.3)

C4AF = 3.043Fe203 (2.4)

Hydration of Phases in Portland cement

The series of reactions take place during the hydration of cement with water which results setting and hardening of concrete. As stated in [Handbook of thermal analysis of construction materials Vangipuram Seshachar Ramachandran], the rate of hydration of cement depends on the crystal size, imperfections, particle size, particle size distribution, the rate of cooling the cement clinker, surface area, the presence of admixtures, the temperature etc. which results a complex hydration process. Due to the complexity of cement hydration, it is appropriate to examine the reactions of clinker phases separately.

C-S-H gel which is the main binding material which binds the sand and aggregate particles together in concrete is formed by reaction of both C35 and C2S with water. The reactions are summarized in Equation (2.5) and (2.6). C3S is much more reactive than C2S under normal temperature conditions. Thus, C3S contributes more to early strengths. However, the hydration of C2S is much slower.

2C3S + 6H → C3S2H3 + 3CH (2.5)

Tricalcium Water C-S-H Gel Calcium

Silicate Hydroxide

2C2S + 4H → C3S2H3 + CH (2) (2.6)

Dicalcium Water C-S-H Gel Calcium

Silicate Hydroxide

The calcium silicate hydrate (C-S-H) gel, which is responsible for the cement paste strength, setting and hardening, occupies a high percent age of the total solids in a cement paste.

The other major components of Portland cement clinker, tricalcium aluminate and calcium aluminoferrite also react with water. Since these clinker minerals also react with gypsum, their hydration chemistry is more complicated.

The hydration reaction of C3A with water given in Equation 2.7 is very rapid, but C3A has not any contribution to strength of the cement. However, hydration of C3A has an adverse effect on the durability of cement. Therefore, gypsum is added to retard the hydration of C3A.

C3A + 3CŠH2 + 26H → C6AŠ3H32 (2.7)

Tricalcium Gypsum Water Ettringite

Aluminate

Although the gypsum is consumed entirely if C3A still remains, another reaction given in Equation 2.8 takes place:

2C3A + C6AŠ3H32 + 4H → 3C4AŠH12 (2.8)

Tricalcium Ettringite Water Calcium Alumino

Aluminate Monosulfo Hydrate

The hydration reaction of C4AF, which has a slower reaction rate compared to hydration of C3A, contributes less to strength of Portland cement. The reaction of C4AF with water is as follows:

C4AF + 3CSH2 + 21H → C6(A,F) S3H32 + (A,F)H3 (2.9)

Tetracalcium Gypsum Water Calcium (Iron, Aluminum)

Aluminoferrite Trisulfo Aluminate Hydroxide

Although the gypsum is consumed entirely, if there is still C3A, then the following reaction occurs:

C4AF + C6(A,F) S 3H32 + 7H → 3C4(A,F) SH12 + (A,F)H3 (6)

Tetracalcium Calcium Water Calcium (Iron, Aluminium)

Aluminoferrite Trisulfo Monosulfo Hydroxide

Aluminate Aluminate

Main Constituents of Cement

The main cementitious material in concrete is cement. However, to decrease the cost, to improve the concrete performance and to produce more environmental friendly products several supplementary cementitious materials, which are generally natural minerals or by-products of some other industrial processes, are used in cement.

Thus, cement consists of different materials which are homogenous in composition. The main constituents of cement other than clinker listed in TS EN 197-1 are as follows:

Pozollanic materials

Natural Pozzolana (P)

Artificial Pozzolana

Silica fume (D)

Granulated Blast Furnace Slag (S)

Fly Ash (V, W)

Siliceous fly ash (V)

Calcareous fly ash (W)

Burnt shale (T)

Limestone (L, LL)

The percentages of these materials in cements vary from cement type to cement type and depending on the application and the properties of concrete desired. However, TS EN 197-1 states that the composition of the cements conform this standard shall be in the limits given in Table 2.3.

Table 2.3 Cement Composition Percentages According to TS EN 197-1.

Pozollanic materials

Although, pozzolanic materials do not harden in themselves when mixed with water, they exhibit cementitious properties when combined with calcium hydroxide at normal ambient temperature.

Silisium dioxide (SiO2) and Aluminium oxide (Al2O3) are the major compounds in pozzolanas. The minor compounds in pozzolanas are iron oxides (Fe2O3) and others.

Natural Pozzolana (P, Q)

Materials originated from volcanic eruption are usually called as natural pozzolanas [Advances in cement technology: chemistry, manufacture and testing, S. N. Ghosh]. According to TS EN 197-1, there are two types of natural pozzolana, namely natural and natural calcined pozzolanas denoted by P and Q, respectively.

Artificial Pozzolana

Artificial pozzolans are the by-products of various thermal treatments, such as burnt shale, silica fume, fly ash, slags, etc.

Silica fume (D)

Silica fume, also called condensed silica fume and micro silica, is a finely divided residue resulting from the production of elemental silicon or ferrosilicon alloys that is carried from the furnace by exhaust gases. [Concrete Construction Engineering HandBook, Edward G. Nawy]

Granulated Blast Furnace Slag (S)

In the production of iron, iron ore is smelted in a blast furnace. During this process, molten iron that collects in the bottom of the furnace and liquid iron blast furnace slag floating on the pool of iron, are periodically tapped from the furnace at a temperature of 1400-1500⁰C. [Lea's chemistry of cement and concrete Peter C. Hewlett] Granulated blast furnace slag is made by rapid cooling of a slag melt which contains at least two-thirds by mass of glassy slag and has hydraulic properties.

TS EN 197-1 states that granulated blast furnace slag composition shall has at least two-thirds by mass of the sum of calcium oxide (CaO), magnesium oxide (MgO) and silicon dioxide (SiO2). The rest of the composition is Aluminium oxide (Al2O3) together with small amounts of other compounds. Also, (CaO + MgO)/(SiO2) ratio by mass shall exceed 1,0. [TS EN 197-1, Part 1: Composition, specifications and conformity criteria for common cements, European Committee for standardization, Brussels, 2000]

Fly Ash (V, W)

Fly ash is a finely divided residue that results from the combustion of pulverized coal and is carried from the combustion chamber of the furnace by exhaust gases. Commercially available fly ash is a by-product of thermal power plants. [Concrete Construction Engineering HandBook. Edward G. Nawy]

TS EN 197-1 divided fly ashes into two groups namely, siliceous fly ash and calcareous fly ash.

Siliceous fly ash (V)

Siliceous fly ash, a fine powder of mostly spherical particles having Pozzolanic properties, consists mainly of reactive silicon dioxide (SiO2) and Aluminium oxide (Al2O3). [TS EN 197-1, Part 1: Composition, specifications and conformity criteria for common cements, European Committee for standardization, Brussels, 2000]

Calcareous fly ash (W)

Calcareous fly ash, a fine powder having both hydraulic and/or Pozzolanic properties, consists mainly of reactive calcium oxide (CaO), reactive silicon dioxide (SiO2) and aluminum oxide (Al2O3). [TS EN 197-1, Part 1: Composition, specifications and conformity criteria for common cements, European Committee for standardization, Brussels, 2000]

Burnt shale (T)

Burnt shale which is produced by burning of oil shale in fluidized bed furnace between 600 and 800⁰C is another cementitious constituent used in cement production. Burnt shale is composed of clinker phases, mainly dicalcium silicate and monocalcium aluminate.

Limestone (L, LL)

Limestone, a sedimentary rock, consists mainly of calcium carbonate; the most stable form is calcite. Limestone often contains Mg, Al and Fe combined as carbonates and silicates.

TS EN 197-1 states that to use limestone as a constituent in cement, calcium oxide content at least 75% by mass.

Moreover, limestone is divided into two groups in TS EN 197-1 according to its Total Organic Carbon (TOC) content. If TOC value does not exceed 0,20 % by mass, the limestone is demonstrated with LL. If TOC value does not exceed 0,50 % by mass, then the limestone is demonstrated with L.

Effects of the Mineral Additives on Mortar and Concrete Properties

As mentioned before, minerals additives influences the properties of both cement and concrete. The following presents the effects of main constituent of cement on water requirement, workability and strength.

Water Requirement

The amount of mixing water required for a specified consistency of a mortar or concrete is called as water requirement of cement mortar or concrete. Adding excess or less amount of water can lead to adverse results on the strength of cement mortar or concrete. Therefore, it is required to determine how much water is sufficient for the cement mortar or concrete.

Cementitious materials other than clinker have different impacts on the water requirement of cement mortar or concrete since they have different particle size, shape etc. For example, Natural pozzolans have significant effect on water demand of concrete. According to Vu et al results, since the natural pozzolans increase the specific surface area, the water requirement of cement containing natural pozzolans has higher water requirement compared to ordinary Portland cement. Vu, D., Stroeven, P., and Bui, V. 2001. "Strength and durability aspects of calcined caolin-blended portland cement mortar and concrete." Cem. Concr. Compos., 23, 471-478. The same effect, increase in water requirement, is also observed when clinker is replaced with silica fume in cement.

However, for a given slump, water requirement of a cement containing fly ash is less than the water requirement of a Portland cement. Although the dosage of fly ash increases the water reduction increases, not all fly ash does the same effect on mortar. According to Brink and Halstead findings (R.H. Brink and W.J. Halstead, Studies relating to the testing of fly ash for use in concrete, Proc. Am. Soc. Testing Mats. 56 (1956) (1956), pp. 1161-1206) as the carbon content of the fly ash increases the water demand increases.

Workability

Workability is defined as the easiness of the concrete mixing, handling, compacting, placing and finishing. Water content of concrete has an important effect on workability. There are several factors affecting workability such as quantity and characteristics of cementing materials, and amount of water etc.

According to Pan et al study, the lubricant effect and morphology improvement on cement mortar or concrete of natural pozzolans increase with the increase in fineness. As a result, natural pozzolans improve the consistency and the workability of the concrete.[ Influence of the fineness of sewage sludge ash on the mortar properties Shih-Cheng Pana, Dyi-Hwa Tsenga, , , Chih-Chiang Leea and Chau Leeb ]

Li Yijin et al studied on usage of fly ashes having different fineness as a cementitious material replacing the clinker in cement and replacing cement in concrete. They found out that fly ash improves the workability of cement mortar or concrete due to their spherical shape causing ball bearing" effect. [Admixtures and Ground Slag for Concrete 1990; ACI Comm. 226 1987c The effect of fly ash on the fluidity of cement paste, mortar and concrete Li Yijin, Zhou Shiqiong, Yin Jian, and Gao Yingli]

Also, as Peter stated, the water requirement of concrete containing ground granulated blast furnace slag decreases with the increase in the amount of ground granulated blast furnace slag. [Special Concretes - Workability and Mixing Peter Bartos]

Thus, as the volume of fine particles increase which leading to decrease in interference of aggregate particle, for a given slump, fly ash, slag and burnt shale improve the workability of concrete.

Strength

Supplementary cementitious materials such as fly ash, ground granulated blast furnace slag, burnt shale and silica fume contribute to the strength gain of concrete. However, the characteristics of the supplementary materials and replacement level limit them for the strength gain of concrete. For example, pozzolanic reactivity of the fly ash is one of the limiting parameter [Fly ash in concrete: properties and performance edited by K. Wesche June 13th 1991].

In addition to cementitious materials used, test type is another factor affecting the strength. Because as the size of the specimen, moisture content of the specimen, the rate of loading and type of test machine change, the strength result changes.

Factor Affecting Concrete Strength

Concrete is a composite material consisting of mainly cement, water, coarse and fine aggregates and chemical admixtures. Several complex reactions taking place when these materials are mixed result strength gain of the concrete. There are several factors affecting the strength of concrete.

As mentioned above, since the concrete is a mixture of cement, water, coarse and fine aggregate and chemical admixtures, properties of each of these materials have an influence on the strength of concrete. For example, as

Importance of Flow

Flow test, which depends on especially the water to cement ratio and on various aspects of the cement such as fineness, flocculation, and rate of hydration reactions, gives an idea about the consistency of a cement mortar or a fresh concrete and it [Significance of tests and properties of concrete and concrete-making materials, Joseph F. Lamond,J. H. Pielert].

Consistency is an important parameter for the concrete workability. In addition, as Joseph F. Lamond and Pielert stated, by using a standard consistency, i.e. using a standard flow, errors because of consolidation or bleeding in samples will be avoided.

Inter-Laboratory Test Evaluation

An inter-laboratory test is carried out by a representative number of participating laboratories repeatedly within each participating laboratory on identical samples under agreed conditions.

There are three main objectives for inter-laboratory testing:

Proficiency Testing

Certification of Materials

Test Method Validation

The precision, which is a fundamental characteristic of a test method, is the degree to which the repeated tests under the same conditions show the same results.

In this study, since an inter-laboratory test is an appropriate procedure to measure the precision of a test method, an inter-laboratory test is applied to test the precision of the test method prescribed in TS EN 196-1.

Assessment of Inter-Laboratory Test Results

In the inter-laboratory test method carried out in this study, the assessment of the test results is carried out in accordance with TS EN 196-1 and Normal Gauss Distribution.

Acceptance of Test Results

Acceptance of test results was determined according to TS EN 196-1. For each mould, if there is any result showing 10% deviance from the mean of the six results, then one of these results is discarded. Then, the remaining five results are averaged. If again, there is any result showing 10% deviance from the mean of the five results, then all of the results are discarded.

Omission of Outliers

Since the unjustified minimization of the extreme values results in an impression of the performance of the test method, the extreme values called outliers for each data set were omitted.

To determine the outliers of the rest of the data sets, Grubb's Test was applied. Since the Grubb's test is valid for data set which is normally distributed, normality of the rest of the data was checked with a computer program called SPSS.

In SPSS, normality is checked by Kolmogorov-Smirnov Test which serves as a goodness of fit test to a normal distribution.

The hypotheses used in this test are:

HO: there is no difference between the distribution of the data set and a normal one

HA: there is a difference between the distribution of the data set and normal

The P-value is provided by SPSS, if it is below 0.05, then the data set is not normally distributed.

In Grubb's Test, by ranking the data set, the smallest and the largest values are determined. Then, the mean and the standard deviation are calculated. Depending on the suspicion of a possible outlier of a value, one of the following equations is used [Basic statistics and pharmaceutical statistical applications, James E. De Muth]:

(2.5)

(2.6)

Then, the calculated T values are compared with the critical value given in Table 2.

Table 2.3 Critical Values for Grubb's Test

n

gcrit

α = 0.05

gcrit

α = 0.01

 

n

gcrit

α = 0.05

gcrit

α = 0.01

 

n

gcrit

α = 0.05

gcrit

α = 0.01

3

1.1531

1.1546

 

15

2.4090

2.7049

 

80

3.1319

3.5208

4

1.4625

1.4925

 

16

2.4433

2.7470

 

90

3.1733

3.5632

5

1.6714

1.7489

 

17

2.4748

2.7854

 

100

3.2095

3.6002

6

1.8221

1.9442

 

18

2.5040

2.8208

 

120

3.2706

3.6619

7

1.9381

2.0973

 

19

2.5312

2.8535

 

140

3.3208

3.7121

8

2.0317

2.2208

 

20

2.5566

2.8838

 

160

3.3633

3.7542

9

2.1096

2.3231

 

25

2.6629

3.0086

 

180

3.4001

3.7904

10

2.1761

2.4097

 

30

2.7451

3.1029

 

200

3.4324

3.8220

11

2.2339

2.4843

 

40

2.8675

3.2395

 

300

3.5525

3.9385

12

2.2850

2.5494

 

50

2.9570

3.3366

 

400

3.6339

4.0166

13

2.3305

2.6070

 

60

3.0269

3.4111

 

500

3.6952

4.0749

14

2.3717

2.6585

 

70

3.0839

3.4710

 

600

3.7442

4.1214

Evaluation of the Test Results

After finding out the outliers as described above, the evaluation of the test results were performed. Since sample size was so small that the t-test was used to determine the confidence interval.

According to the number of data sets which are included in the assessment, t value corresponding to 95% confidence for two-tail is chosen from t-tables given in Table 2.3.

Then, 95% confidence interval is calculated with the equation given in 2.8.

(2.8)

Table 2.4 t-Table

The Degree

of Freedom

Cumulative Probability

For two-tails

0.10

0.05

0.025

0.01

1

6.314

12.71

31.82

63.66

2

2.920

4.303

6.965

9.925

3

2.353

3.182

4.541

5.841

4

2.132

2.776

3.747

4.604

5

2.015

2.571

3.365

4.032

6

1.943

2.447

3.143

3.707

7

1.895

2.365

2.998

3.499

8

1.860

2.306

2.896

3.355

9

1.833

2.262

2.821

3.250

10

1.812

2.228

2.764

3.169

11

1.796

2.201

2.718

3.106

12

1.782

2.179

2.681

3.055

13

1.771

2.160

2.650

3.012

14

1.761

2.145

2.624

2.977

15

1.753

2.131

2.602

2.947

In addition to determination of confidence interval, calculation of repeatability and the reproducibility of the test results are play an important role upon an inter-laboratory test evaluation. As Guslicov et al mentioned, for 20 years, the progress of the standard deviation of repeatability and reproducibility and coefficient of variance have given an idea on the progress of the inter-laboratory tests.

It is observed that during these periods, as the development of standards and the interpretation and applicability of the standards increase, the coefficient of variance decreases. Moreover, Guslicov et al concluded that there is an improvement in the participants' studied on the test method.

Also, as stated in TS EN 196-1 and it can be seen on Figure 2.4, for 28-day strength, for the experienced laboratories under the conditions defined in TS EN 196-1, the repeatability in terms of coefficient of variance is expected to be less than 6%. Besides, it is also stated in TS EN 196-1 that the repeatability in terms of coefficient of variance is expected to be in between 1% or 3%.

Figure 2.4 Progress of the coefficient of variance for 1, 2 and 28-day compressive strength [Guslicov G, Coarna M, Pop A, Vlad C, Vlad N Accred Qual Assur (2009) 14:541-546 Consideration concerning interlaboratory test for cement in the last 20 years]

Thus, for the obtained results, the standard deviation of repeatability and reproducibility are calculated as given in equations 2.9 and 2.11, respectively, which are defined in ISO 5725-2 [ INTERNATIONAL STANDARD IS0 5725-211994].

(2.9)

(2.10)

(2.11)

(2.12)

Where p denotes the total number of laboratories participating in the inter-laboratory experiment, i denotes the number of a particular laboratory, j denotes the mold number, n denotes the number of test results obtained in one laboratory at one mold, denotes the arithmetic mean of the test results, denotes the grand mean of the test results.

Comparison of the Test Results by Using Kruskal-Wallis Test Method

Kruskal-Wallis Test is a statistical test method in which three or more data sets can be compared whether data samples belong to the identical population or not.

The hypotheses used in this test are:

HO: the samples are from identical populations.

HA the samples are not from identical populations.

The P-value, i.e. the Asymptotic Significance, is provided by SPSS. If it is below 0.05, then the samples are not from the same population.

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