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1037
The concept of differential thermal analysis (DTA) came from Le Chatelier’s pioneering work in
1887 on the heat of decomposition of clays and limestones. Since then the application of DTA has
become widespread for material characterization and evaluation. Particularly during the last two
decades, DTA has found diverse applications in the cement industry. The ability to detect minor chemical
changes and provide relevant analytical information for portland cement, its raw materials, and its
hydration products has made DTA one of the foremost analytical methods, along with X-ray diffraction
(XRD), X-ray fluorescence (XRF), and scanning electron microscopy (SEM). The technique is conven-
ient, fast, and accurate, and provides information that is not readily available from other techniques. In
addition, it has special applications in monitoring thermal changes in simulations of cement kiln opera-
tion. Thermal analysis instrumentation frequently used for such applications is shown in Figure 8.4.1. In
addition to the DTA, the following thermoanalytical techniques (Table 8.4.1) are frequently used in
analysis pertaining to cement science and manufacturing.
Figure 8.4.1. DSC and TGA equipment frequently used by the cement industry.
*Senior Scientist, Construction Technology Laboratories, Inc., 5400 Old Orchard Road, Skokie, Illinois, U.S.A.**Senior Principal Scientist, Construction Technology Laboratories, Inc.,5400 Old Orchard Road, Skokie, Illinois, U.S.A.
Chapter 8.4
by Javed I. Bhatty* and F. M. Miller**
Application of Thermal
Analysis in CementManufacturing
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Raw Materials in Cement Manufacturing
Raw materials for cement manufacture include all the types of materials listed in Table 8.4.2. Of
these, the lime sources, usually carbonates, are the most important. Only a representative sampling
of some of the most widely used raw materials are shown here.
Innovations in Portland Cement Manufacturing 1038
Table 8.4.1. Thermoanalytical Techniques Frequently Used in Cement Analysis
Technique used Abbreviation Common applications
Thermogravimetric TGA Monitors loss of weight when material decom-
Analysis decomposes as a function of temperature
Differential DTG Examines rate of weight change to increase pre-
Thermogravimetry cision of identifying temperatures at which weight
changes occur and separations of multiple
weight change events
Derivative Differential DDTA Permits elucidation of rate changes in thermal
Thermal Analysis processes and deconvolution of complex patterns
Differential Scanning DSC Quantification of heat increments associated
Calorimetry with thermal changes
Table 8.4.2. Raw Materials in Cement Manufacture (Ramachandran, 1969;
Kosmatka, 2001)
Materials Sources
Carbonates Limestone, chalk, marble, sea shell, marl
Aluminosilicates Clay, shale, fly ash, and slate
Corrective materials Sand and sandstones, bauxite, iron ore, laterite, mill scale,
diatomaceous earth
Mineralizers CaF 2, Na2SiF 6, fluorspar/gypsum
Set regulator Gypsum, anhydrite
Hydraulic blending materials Natural pozzolanic rocks, blast-furnace slag, fly ash, silica
fume, metakaolin
Reviews by Mackenzie (1964), Ramachandran (1969), Barta (1972), Ben-Dor (1983), and Bhatty
(1993) have covered several aspects of these applications.
The annual worldwide production of portland cement is estimated to be more than 1.6 billion
metric tons (PCA, 2002), which requires more than 2.4 billion metric tons of raw materials and
some 320 million metric tons of fuel. The major cement producers are given in Table 8.4.3; the
Republic of China is by far the dominant producer of portland cement, followed by India, USA,
and Japan.
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Limestone and clay are the most common sources of lime (calcium oxide) and silica respectively,
required in cement making. Secondary sources of lime such as chalk, marl, and shell deposits are
also used when available. Other sources of silica are silt, shale, quartz sand, and fly ash. The pres-
ence of alumina and iron-bearing minerals in the raw mix is also required to ensure the presence
of some liquid phase in the burning zone, as well as essential for clinker nodulization and for
improving the burnability of the raw mix. The dual
goals of raw mix design are to provide a compositionthat is burnable and to produce cement that is
consistent with market demands. The composition
corresponds roughly to the range of values shown in
Table 8.4.4, leading to the compound compositions
shown in Table 8.4.5. Thermal analysis has been
frequently employed in characterizing raw materials
to ensure compositional quality and detect the pres-
ence of any deleterious impurities therein.
1039Application of Thermal Analysis in Cement Manufacturing
Table 8.4.3. Major Portland Cement Producers in the World (PCA, 2002)
Country Annual production (x 1000 metric tons)
Republic of China 573,000
India 90,000USA 88,000
Japan 80,000
South Korea 48,000
Brazil 40,000
Germany 38,000
Italy 36,000
Turkey 34,000
Total World 1,606,000
Oxides Wt, %
CaO 60 – 67
SiO2 17 – 25
Al2O3 3 – 8
Fe2O3 0.5 – 6.0
MgO 0.1 – 4.0
Alkalis 0.2 – 1.3
SO3 2 – 4
Table 8.4.4. Typical Oxide Analysis
of Cement
Table 8.4.5. Typical Compound Composition of Ordinary Portland Cement
Chemical CementChemical name formulae notation
Tricalcium silicate 3CaO·SiO2 C3S*
Dicalcium silicate 2CaO·SiO2 C2S
Tricalcium aluminate 3CaO ·Al2O3 C3A
Tetracalcium aluminoferrite 4CaO ·Al2O3 · Fe2O3 C4AF
Calcium sulfate dihydrate (gypsum) CaSO4 ·2H2O CS–
H2
*Cement chemist notations: C = CaO, S = SiO2, A = Al2O3, F = Fe2O3, S–
= SO3, H = H2O
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Tricalcium silicate (C3S) is the major phase with a significant impact on the setting and early
strength properties of cement, whereas tricalcium aluminate (C3A), being the most reactive
compound, has the highest initial impact on the early hydration behavior of cement. Thermal
analysis has routinely been used in monitoring the hydration characteristics of the individualcement components, as discussed in a later section of this chapter.
CHARACTERIZATION OF MAJOR RAW MATERIALS
Raw materials of high purity and uniform composition are required to ensure the production of
quality cement. The use of thermal analysis gives critical information on the mineralogical and
compound composition of the raw materials used for making cement clinker. Particularly, the
application of DSC and TGA techniques in the qualitative analysis of these materials is useful
because it can detect the presence of many deleterious substances prior to the preparation of rawfeed in cement manufacture. Deleterious components in the raw materials are always minimized to
avoid serious process upsets or production of cement of variable composition and unpredictable
properties.
In cement manufacture, limestone is usually the principal raw material, whereas clay, shale, sand,
or sandstone provide silica and alumina. Iron oxide or mill scale represent the iron sources
normally necessary to augment the liquid phase or control the C3A content during clinker forma-
tion. The materials are proportioned and blended to prepare the kiln feed that is fired to produce
clinker.
Since the formation of limestones is usually the result of the deposit of shells of sea creatures in
ancient times, there may be some residual organic matter present. Knowing the presence of such
organic matter and its approximate abundance can be a major advantage in planning strategies
designed to deal with environmental regulations. This is another area in which the application of
thermal analyses can be of significant value.
Limestone
It is critical that limestone (CaCO3) is of adequate quality to permit proper raw feed formulation.
The presence of excessive Mg contamination in limestone often leads to inferior clinker that forms
potentially less durable cements. DTA techniques can detect impurity in a limestone, enabling
adjustments to be made in the formulation of the raw mix. DTA plots for limestone, dolomite, and
iron-bearing limestone used in cement raw feed are given in Figure 8.4.2. The x-axis shows the
temperature to which the sample is subjected, and the y-axis shows the heat change, H, endother-
mic (heat consumption) or exothermic (heat release) during the analysis.
Innovations in Portland Cement Manufacturing 1040
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An endothermic peak between 850°C and 950°C results from the decomposition of CaCO3 in air
with the evolution of CO2 and is accompanied by a weight loss. The presence of notable amounts
of MgCO3 as a contaminant in limestone, however, produces an additional endothermic peak.Dolomite exhibits two endothermic peaks. The first peak around 750°C is caused by the decompo-
sition of MgCO3, and the second peak around 900°C is caused by the decomposition of CaCO3. If
dolomite is present as a contaminant, the peaks will be shifted slightly (Ramachandran, 1969). In
carbonate rocks, dolomite can be estimated by using DTA, even in amounts as low as 0.3%
(Bensted, 1978). Pure magnesite, MgCO3, is identified by an endothermic peak around 700°C due
to its decomposition and release of CO2, whereas brucite-bearing limestones show an endothermic
effect around 475°C, as a result of dehydroxylation; brucite is the mineral name of magnesium
hydroxide, Mg(OH)2. Iron-bearing limestones show an additional endothermic peak between
350°C and 400°C possibly attributable to dehydration of hydrated iron oxides. The presence of
quartz in limestone is determined, and can be quantified, by a reversible endotherm at 575°C
corresponding to the transformation of to quartz.
Clays
Clays, shales, slates, and schists provide ingredients such as SiO2, Al2O3, Fe2O3, and alkalies that take
part in the formation of the essential phases – silicates, aluminates, and the melts in cement clinker.
The most desirable clay minerals for cement making are kaolinite, which is a hydrous aluminum sili-
cate Al4Si4O10(OH)8, and low-alkali montmorillonite, essentially Al4Si8O20(OH)4·nH2O with the
substitution of Mg for part of Al (Read, 1970). These clays contain mainly Al2O3 and SiO2, with a low
alkali content, and also satisfy other requirements in formulating the kiln feed mix. Kaolin is pre-
ferred over pure quartz because kaolin is already naturally finely divided and easy to burn, whereas
1041Application of Thermal Analysis in Cement Manufacturing
0 300 600 900 1200 1500
H e a t c h a n g e ,
∆ H
Temperature, °C
Limestone
CaCO3
CaCO3
CaCO3
MgCO3
Dolomite
Iron-bearinglimestone
Figure 8.4.2. DTA plots showing typical peaks for important carbonaceous raw materials usedin cement manufacture.
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the use of quartz involves expensive and abrasive grinding and may require extra energy for burning.
Natural clays may require less kiln thermal energy due to the presence of some natural fluxes.
DTA has been effectively used in identifying clay minerals, as reported by Mehta (1964) andRamachandran (1969). Typical DTA plots of several clay minerals – kaolinite, montmorillonite,
illite, chlorite, and gibbsite – are given in Figure 8.4.3. A highly crystalline kaolinite mineral shows
a characteristic endothermic peak due to dehydroxylation between 550°C and 600°C, and an
exothermic peak around 1000°C due to phase transition (devitrification of glass) in forming
mullite or γ -Al2O3 nucleation. A poorly crystalline kaolinite often shows an additional small
endothermic peak at about 100°C due to loss of free water.
Montmorillonite clays definitely show an endothermic peak around 100°C as a result of dehydra-
tion, followed by another peak between 600°C and 700°C due to loss of lattice water. Another
endothermic peak noted at about 900°C is due to lattice breakdown, and a final exothermic peak at
955°C is due to a phase transition in which a spinel is formed.
The thermal analyses are typically quick to identify the presence of any unwanted inorganic and
organic contaminations in clays that would otherwise potentially cause processing, operational,
and environmental problems during cement manufacture. The presence of quartz in clay miner-
als is determined by DTA. Since the endothermic peak of to quartz is overlapped by the de-
hydroxylation peak, it can be observed without interference in the exotherm, during cooling,caused by the transformation of to quartz.
Innovations in Portland Cement Manufacturing 1042
∆ H
0 200 400 600 800 1000 1200 1400
Temperature, °C
Kaolinite
Illite
Montmorillonite
Chlorite
Gibbsite
Figure 8.4.3. DTA plots for typical clays used in cement manufacture.
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Shale
As mentioned above, shale is also a common material which, like clay, can be used as a silica,
alumina, and iron source in cement manufacture. Thermal analysis has been frequently used in
characterizing the mineral composition of shale. Since shale has a mineralogical composition very
similar to many clays, the thermal analysis results are similar. In some plants, the terms “shale” and
“clay” are essentially interchangeable. Other argillaceous materials of interest are the slates and
schists. They also provide the necessary silicates, aluminates, and some iron for the kiln feed mix
design.
Sand
Most sands are largely quartz. The principal thermoanalytical feature of quartz is the aforemen-
tioned to transition at 575°C, which can be quantified. The quartz content of raw materials, or
potential raw materials, is very important with respect to raw mix burnability; characterization
using thermal analysis is a useful adjunct to a microscopical characterization.
Iron Ore
Thermal analysis can be used with iron-bearing materials to characterize reduced iron species and
hydrates. Reduced iron species will oxidize when heated in a DSC in air; this exotherm can be
quantified to determine how much reduced iron is present. The endotherm resulting from the
dehydration of hydrates such as goethite, or decarbonation of limonites, can be used to quantify
the amount of such mineral phases in the iron ore of interest. Thermal analyses (TGA and DSC)
can also determine the amount of moisture and the temperature at which it is released from iron
hydrates to assess their impact on the raw feed thermal behavior, especially during their passage
through the preheater stages.
Mill Scale
Mill scale is usually principally iron metal, together with small amounts of iron oxides. The mode
of generation of this material often dictates that it contain variable amounts of oils, which are
organic and can cause emissions of hydrocarbons or carbon monoxide from the stack. DSC is a
powerful tool in identifying the quantity of any such oil, and the temperature at which it is
volatilized and/or burned. This information will aid plant personnel in deciding whether a given
mill scale source is compatible with plant operations and/or environmental requirements.
Gypsum
Gypsum is added to clinker during the finish milling, primarily to regulate the setting properties of
cement. Thermal analysis is often used to determine the presence of hemihydrate (also known as
plaster) in gypsum. It is critical to know the degree of conversion of gypsum to plaster, as an excess
of plaster may cause the phenomenon known as false set, while too little dehydration can cause
1043Application of Thermal Analysis in Cement Manufacturing
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quick setting behavior. In the field, inadequate amounts of gypsum, or especially plaster, in a
cement to be used with fly ash and/or admixtures in concrete can cause erratic setting and strength
development. A typical gypsum DTA plot is shown in Figure 8.4.4 for a crucible with a pinhole in
the lid. Dehydration of gypsum gives rise to an endothermic peak around 150°C (decomposition of gypsum to plaster) and another at 200°C (dehydration of plaster to soluble anhydrite,-CaSO4).
Innovations in Portland Cement Manufacturing 1044
∆ H
20 70 120 170 220 270 320 370
Temperature,°C
Plaster
Gypsum
Figure 8.4.4. Typical DTA plot for gypsum.
CHARACTERIZATION OF AUXILIARY RAW MATERIALS
In recent years, a variety of alternative materials have also been used as partial replacement of the
raw kiln feed components in cement manufacturing. For instance, fly ash, owing to its chemical
similarities with clay and shale, is being used as a replacement source of SiO2, Al2O3, and even
Fe2O3. Fly ashes frequently contain unburned carbon that should be quantified prior to their usein the kiln feed. Additionally, the ashes may also have organic impurities that need to be identified.
Thermal techniques have successfully identified these contaminants and quantified their potential
effect on the kiln operation and the emission profile. The method permits early identification of
the problems and rectification prior to formulating a kiln feed design.
Fly Ash
Lately, fly ashes rich in unburned carbon are being considered as economical materials having
additional benefits in supplementing fuel. Applications of thermal analysis (both TGA and DSC)have contributed greatly to characterizing the intrinsic fuel value of such ashes. The techniques
have also been used to determine the existence (if any) of volatile organics and their temperature
of release in the cement kiln, helping to avoid any emission problems. Figure 8.4.5 shows a DSC
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plot of a fly ash rich in unburned carbon. A large exotherm around 600°C confirms release of heat
attributed to carbon. The vertical axis is heat released in J/g.
1045Application of Thermal Analysis in Cement Manufacturing
∆ H
50 150 250 350 450
Heat released bycombustion of
residual carbon
550 650 750 850
Temperature, °C
Figure 8.4.5. DSC plot for fly ash with significant unburned carbon content.
Cement Kiln Dust
Cement kiln dust (CKD) is typically generated at about 5% of the total cement production at a
cement plant. Usually, if rich in alkali, the CKD has been disposed of in nearby landfills (often
quarried-out sections of the limestone quarry). Whenever possible, it is recycled as a supplement
to kiln feed. CKD is typically a calcined or partially calcined raw feed. Reuse of calcined CKD
conserves energy, since that increment of limestone does not have to be calcined subsequently.
Thermal analyses (especially TGA) are ideally suited to quantify the degree of calcination andamount of lime present in the CKD. The technique is usually supplemented with XRF to deter-
mine the quantity of alkali or sulfur along with other species to ensure a proper raw mix design for
reuse of CKD as a kiln feed supplement. The presence of excessively high alkalies can lead to high-
alkali cement, which is not favored by concrete makers owing to potential alkali-silica reaction and
related durability problems.
Other auxilliary materials of interest as components of cement raw feed that have been character-
ized by thermal analyses include sewage sludge ash, municipal ash, foundry sand, copper slag, and
catalytic fines.
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PREDICTING EMISSIONS FROM RAW MATERIALS
Ever more stringent environmental regulations have made it necessary for cement plants, especially
in the environmentally conscious countries of the European and North American regions, to inves-
tigate their raw materials for potential contributions to stack emissions. Thermal analysis tech-
niques can be, and in many situations have been, applied to identify and avoid high-organic sections
of the limestone quarry, or to evaluate the feasibility of using alternative raw materials such as high-
carbon fly ash or oil-contaminated soils. DSC is a particularly valuable tool for this purpose.
The limestone quarry shown in Figure 8.4.6 is an example of a deposit where it is critical to know
the composition and properties of the deposit at each location through its depth. Thermal tech-
niques can help identify locations that may be problematic during pyroprocessing and can thus help
determine the best possible use of the quarry; the same reasoning can apply for shale deposits, asshown in Figure 8.4.7.
Shkolnik and Miller (1996) developed techniques for using DSC to predict emissions due to
organic contaminants in cement raw materials, and discuss quarry development and raw material
substitution that take advantage of thermal analysis techniques. These tests can be carried out in
an atmosphere of simulated flue gas to mimic actual kiln process conditions. They conducted case
studies involving potential emission problems and 1) identified areas in limestone quarries with
the potential for organic emissions, 2) compared regions in the shale overburden for emission
potential, 3) compared the DSC results on kiln feed with those of the constituent raw material to
identify “the culprit” in high-organic emissions, and 4) evaluated the potential for emission
changes when raw materials are changed.
Innovations in Portland Cement Manufacturing 1046
Figure 8.4.6. Limestone quarry showing layers of material that may have chemical variation as well as organic contaminants.
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DSC plots of selected limestone and shale samples acquired from various depth locations of the
same quarries are given in Figures 8.4.8 and 8.4.9. Exothermic peaks below 350°C indicate the
presence of organic species that could be released earlier in the back end of the kiln and would
most likely report to stack emissions, either as hydrocarbons or as CO. Deeper limestone has much
higher peaks at lower temperatures (around 350°C and less), and would therefore be expected to
represent greater emission problems. Likewise, shale 1 would be expected to experience more
severe emission problems owing to its low temperature peaks.
1047Application of Thermal Analysis in Cement Manufacturing
Figure 8.4.7. Shale deposit with overburden – organic impurities with potential environmentalsignificance.
∆ H
50 150 250 350 450 550 650Temperature, °C
Limestone: 6m deep
Limestone: 6 – 12m deep
Low temperature peaks
Figure 8.4.8. DSC plots showing organic impurities with potential emission problems in lime-stones. Low temperature peaks suggest presence of organics with potential emission-relatedimpact.
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Innovations in Portland Cement Manufacturing 1048
CLINKERING REACTIONS
DTA or DSC can readily be used to closely simulate the conditions existing in the rotary kiln
during clinkering (Pope and Judd, 1977). The level and rate of heating of the samples as well as the
atmospheric conditions in DTA or DSC can within limits be adjusted to those of the kiln, thereby
confirming these techniques (and also TGA) as powerful tools for characterizing not only the raw
materials but also the finished product.
The formation of clinker results from a series of thermochemical reactions that take place when the
kiln feed is heated through different stages in a rotary kiln. Since thermal analysis can be used to
determine the heat generated or absorbed in chemical reactions, it is also appropriate for following
the clinkering process. As is already known, some of the reactions involved during clinkering areendothermic while others are exothermic. Ignoring all process heat losses, the net theoretical require-
ment of heat for converting kiln feed to 1 kg of clinker is close to 420 Kcal. With respect to minerals
formation, the sequence of processes during gradual heating of a kiln feed, as simulated by a DTA
run (Gouda, 1981) is shown in Figure 8.4.10.
The plot clearly shows the stage of dehydration caused by the loss of moisture in the raw feed at
about 400°C, followed by decarbonation at 740°C and 850°C resulting from the decomposition of
dolomite and limestone involving loss of carbon dioxide. This is followed by devitrification of
clay-derived glass, exothermic belite formation between 1100°C-1270°C, liquid formation between
∆ H
50 150 250 350 450 550 650
Temperature, °C
Shale 1
Shale 2
Low temperature peaks
Figure 8.4.9. DSC plots showing organic impurities in shales, with potential emission impact.
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1250°C-1300°C, and alite formation above this temperature. The region beyond 1300°C may also
reflect the crystal growth. If the sample is subsequently cooled, the exotherm attributable to the
crystallization of melt may be seen. In certain cases, it may also be possible to discern small
endotherms attributable to the volatilization of alkali and/or sulfur compounds.
The major reactions and the heat exchange involved during the clinkering of kiln feed, as reported
by Hondoo (1999), are summarized in Table 8.4.6.
1049Application of Thermal Analysis in Cement Manufacturing
∆ H
Temperature, °C
Crystal growth
Belite
D o l o m i t e
d e c a r b o n a t i o n 7 4 0 ° C
L i m e s t o n e
d e c a r b o n a t i o n 8 5 0 ° C
B e l i t e f o r m
a t i o n
1 2 7 0 °
C
M e l t i n g 1 2 8
0 ° C
Ferrite
FerriteAluminate
Dehydration and decarbonation Clinkerization
D e h y d r o x y l a t i o n
4 5 0 ° C
Alite
Figure 8.4.10. DTA plot showing a sequence of phase formation in a typical cement raw feed.
Table 8.4.6. Reactions and Heat Exchange During Clinkering (Hondoo, 1999)
Temperature, °C Clinkering reactions Heat exchange
100 Dehydration, loss of free moisture Endothermic
450-500 Dehydroxylation, release of bound water Endothermic
850-950 Decarbonation, release of CO2 from limestone, Endothermic
dolomite, carbonates
>900 Cr ystallization of dehydrated clay products Exothermic
900-1200 Reaction between lime and clay products, and Exothermic
formation of belite
1250-1280 Melt formation and liquid phases aluminates and Endothermic
ferrites
>1280 Further formation of liquid phase, alite formation and Endothermic
completion of clinkering, possible alkali/sulfur overall
volatilization
Cooling Crystallization of melt Exothermic
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High-temperature DTA can be used to determine the temperature of clinker melt formation, belite
formation, alite formation, and crystallization of the clinker interstitial phase, as well as the evapora-
tion of volatile materials such as alkali salts. This technique is particularly valuable when applied to
exercises optimizing raw mix design for burnability and fuel economy. For example, if a kiln feedcontains two types of silica (quartz and clay mineral derivation), belite may form in two different
temperature ranges. The clay-derived silica may react at much lower temperatures than the quartz.
Such a raw material mix might generate two separate belite exotherms. These could have a significant
influence on the desired flame profile to obtain optimum clinkering. An example of a DTA for a
white clinker production process is shown in the Figure 8.4.11 (Chrom´ y, 1974). Typical exothermic
and endothermic peaks identified during the clinkering of a clinker raw feed are given in Table 8.4.7.
Innovations in Portland Cement Manufacturing 1050
12501200 1300 1400 15001350 1450
Temperature, °C
1360°C1380°C
1290°C
H
Figure 8.4.11. DSC plot for a white cement clinker feed.
Table 8.4.7. Typical Peaks During Clinkering of Raw Feed
Temperature, °C Peak type Phase formation
1290 Exothermic Belite Formation
1360 Endothermic α‘ → α Transition of Belite, and
1380 Endothermic melt formation
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Burnability of Raw Feed
Burnability is the ease with which a raw mix is converted to clinker when subjected to firing. The re-
quired clinkering temperature and resulting heat requirement are the parameters that determine the
burnability of a given raw mix. Goswami and Panda (1985) used thermal analysis to study the burn-
ability of a given cement raw feed. The technique detects differences in the burnability of raw feeds in
question. DTA plots of three different raw feeds A, B, and C, are shown in Figure 8.4.12.
1051Application of Thermal Analysis in Cement Manufacturing
∆ H
0 500 1000750250 1250 1500
Temperature, °C
A
B
C
Decarbonation
peaks
Alite peaks
Figure 8.4.12. DTA plots for raw mixes showing variation in burnability.
As can be seen, the position and intensity of endothermic peaks due to alite formation (around
1370°C) are critical in determining the burnability of the raw feeds. Raw feed A appears to have
better burnability because alite formation with this feed takes place at a lower temperature of 1350°C as compared to 1375°C for both B and C feeds. The data also suggest that the alite forma-
tion reaction occurs more easily for feed A, and that the heat requirement for raw feed A to convert
to clinker may be lower than either B or C.
Role of Mineralizers on Clinkering
Mineralizers and also fluxes are sometimes employed to conserve energy in cement manufacturing.
Fluxes lower the temperature at which the liquid phase is formed; alumina and iron oxide are the
most commonly used fluxes. Mineralizers enhance the clinkering reaction by promoting solid andliquid state sintering and lowering the minimum temperature of stability of alite; fluorides and
fluosilicate compounds are known mineralizers.
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Ampian and Flint (1973) have used DTA and TG techniques to monitor the effects of mineralizers
such as fluosilicates on the formation of C3S from C2S and free lime. They estimated the extent of
these effects by measuring the free lime content from the corresponding reactions and substantiat-
ing the results using X-ray diffraction data.
The technique of DTA has also been used by Viswanath and Ghosh (1983) to study the effect of
mineralizers on the burnability of a cement plant raw mix. Fluoride, calcium fluorides, and phosph-
ogypsum were used as mineralizers for a hard-burning raw mix, with a lime saturation factor (LSF)
of 84 and alumina to iron ratio (A/F) of 1.35. The DTA plots are shown in Figure 8.4.13a-d.
Innovations in Portland Cement Manufacturing 1052
∆ H
Temperature, °C(a)
Plant raw mix1330°C
1355°C
1300°C
1270°C
∆ H
Temperature, °C(b)
1140°C
1180°C
1260°C
1230°C
Phosphogypsum(0.5% SO3)
Figure 8.4.13a-b. Effects of mineralizers on a raw mix.
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TROUBLESHOOTING IN KILN OPERATION
Kiln Feed Composition
Chen (1982) used DTA and TG to troubleshoot problems pertaining to the clinker-burning process
for some raw materials containing minor constituents such as K, Na, S, Cl, and F. These are poten-
tially troublesome constituents, for they tend to form volatile or low-melting salts such as CaF2,
1053Application of Thermal Analysis in Cement Manufacturing
∆ H
Temperature, °C(c)
1345°C
1315°C
1280°C
1230°C1165°C
0.2% Calcium fluoride
∆ H
Temperature, °C(d)
1140°C
1230°C1320°C
0.4% Fluoride
Figure 8.4.13c-d. Effects of mineralizers on a raw mix.
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KCl, K2SO4, Na2SO4, and CaSO4-K2SO4 double salts. In addition, mixtures of these salts may melt
at especially low temperatures. As a result, they condense on lower-temperature refractory surfaces
and also cause plugging problems in suspension preheaters. Chen (1982) also emphasized the appli-
cation of DTA in the identification of spurrite (2CaO· SiO2 ·CaCO3), a needle-like compound thatforms during the clinker-burning process and adversely affects the raw meal flowability. Janko
(1985) examined and evaluated the composition of ring formation in the kiln and quantified
spurrite formation using the TGA and SEM techniques. Figure 8.4.14 shows thermal analysis peaks
for both calcite from recarbonation of lime and spurrite formed in a kiln ring.
Innovations in Portland Cement Manufacturing 1054
∆ H
W e i g h t l o s s ,
%
400
-8
-6
-4
-2
0
2
4
6
600 800 1000 1200
Temperature, °C
TGA plot
DSC plot
760°CCalcite
800°CSpurrite
Correspondingweight loss
Figure 8.4.14. DSC and TGA plots showing calcite and spurrite peaks with corresponding weight losses.
Preheater/Precalciner Performance
Thermal analysis can be of use in monitoring preheater/precalciner performance in a cement oper-
ation. By subjecting the feed from individual stages of the preheater to DSC/TG analyses, the
degree of calcination can be quantified from the weight loss due to decarbonation of uncalcined
limestone in the kiln feed. The data can be used to establish the temperature profile of the
preheater. The same approach can also be used for product from the calciner, to monitor its
performance by quantifying the degree of calcination.
Figure 8.4.15 shows two hot meals from the fourth stage of a preheater at a cement plant, taken at
two separate occasions. The peaks and weight loss clearly indicate improved calcination in the
second sample relative to the first sample: i.e., A is more calcined than B.
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1055Application of Thermal Analysis in Cement Manufacturing
Kiln Buildups
The formation of many buildups in rotary kilns results from the volatile components contained in
the raw materials. The alkalies, typically derived from the minerals illite, orthoclase, muscovite,
sodalite, and/or scapolite, generate volatile alkali sulfates and chlorides in the hottest part of the kiln.
The volatiles are swept together with the combustion gases toward the colder regions of the kiln
system where they condense. The buildups are highly undesirable, as they may narrow the feed inlet,
increase pressure drop through the preheater, reduce gas flow, and ultimately require shutdown to
remove the accretions, thereby increasing kiln downtime and causing serious production losses.
Thermal analyses (DTA/TGA) have been routinely used by Hondoo and Agharwal (1989) to identify
the mineralogical composition of the buildups and thereby elucidate the underlying reaction mech-
anism. The buildups were taken from different stages of a dry kiln plant with cyclone preheaters and
subjected to DTA analysis. The samples of buildups were taken from 1) the blades of the induced
draft fan, 2) top of the stage-4 cyclone, 3) bottom of the stage-4 cyclone, and 4) coatings from the
calcination zone. The DTA plots are shown in Figure 8.4.16. A summary of data interpretation is
given in Table 8.4.8.
W e i g h t l o s s ,
%
0
-12
-10
-8
-6
-4
-2
0
200 400 800600 1000
Temperature, °C
Hot meal B
Hot meal A
Figure 8.4.15. DTA plots of two separate hot meals A and B taken from fourth stage of preheater.
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Innovations in Portland Cement Manufacturing 1056
∆ H
0 500 1000 1500 2000
Temperature, °C
Stage-4cyclone (top)
Stage-4cyclone (bottom)
Calcinationcoating
Fan blades
Sample location Composition and evaluation of buildupsBuildup from draft fan blades Decarbonation peak at low temperature indicates probability
of lime that has been recarbonated. Hard nature of the mate-
rial indicates gaps in loose buildup have been filled by recar-
bonation. Small high temperature peaks may be associated
with clinker melt formation or alkalies.
Buildup from stage-4 cyclone (top) Sharp endotherm at 740°C is probably due to calcination of
recarbonated lime; the shoulder probably represents calcina-
tion of calcite, and the peak at 860°C may be spurrite. The
high temperature peak at 1180°C may be related to salt melts.
Buildup from stage-4 cyclone The low temperature shoulder on the main peak is recarbon-
(bottom) ated lime; the main peak may be combined CaCO3 andspurrite. The high temperature “blip” at 1320°C is likely
clinker melt.
Coatings from calcination zone Two small peaks at 1235°C and 1320°C suggest the
presence of sulfoaluminates and formation of clinker melt;
high sulfur content may make the coating fairly hard in nature.
Figure 8.4.16. DTA plots showing buildups from different locations of cement operation.
Table 8.4.8. Mechanism of Kiln Buildups from DTA Data (Hondoo and Agharwal, 1989)
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1057Application of Thermal Analysis in Cement Manufacturing
Clinker Storage and Weathering
TGA and DSC have also been routinely used to estimate the degree of weathering of clinker when
stored in the open, or in closed storage that might have been accidentally exposed to humidity. The
technique promptly detects the moisture, and determines the degree of hydration and carbonation
the clinker might have undergone upon such exposure. Severe weathering can have adverse affects
on the hydration and setting properties of the resulting cements. It can significantly reduce the
strength and durability of products made from the cement as it is prematurely hydrated prior to
finish milling. The methods assess hydration prior to grinding to determine the potential damage
to the engineering properties of cement. As a rule, strengths will not be severely curtailed by
carbonation. However, the low temperature peaks may indicate the presence of components that
may strongly inhibit strength development of the clinker in cement.
Securing a representative sample of the weathered clinker is a critical part of this evaluation.
Figure 8.4.17 shows anticipated DTA plots for weathered and stored clinkers. Peaks around 100°C,
500°C, and 750°C correspond to the release of:
• free moisture and moisture associated with C-S-H or ettringite formed during weathering
• free and bound moisture from dehydroxylation of Ca(OH)2
• CO2 from decarbonation of calcium carbonate formed by reacting with atmospheric carbon
dioxide
Application of TGA can provide the weight losses associated with the TGA peaks to permit properidentification of dissociation temperatures. The DTA plot can also be differentiated to reveal
DDTA plot that permits precise identification of temperatures at which associated thermal effects
∆ H
0 200 400 600 800 1000
Temperature, °C
Weathered clinker
Stored clinker
Free andbound moisture
Dehydroxylation Decarbonation
Figure 8.4.17. DSC plots for weathered and stored clinkers.
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occur. The appearance of peaks and the associated weight losses in weathered clinker as compared
to the original clinker gives a relatively better measure of the damage to the clinker from improper
storage or prolonged weathering.
Finish Milling and Gypsum/Plaster Formation
Thermal analysis has been found extremely useful for quantifying the effects of temperature in the
finish mill with regard to gypsum conversion to plaster. As stated earlier, the purpose of addition
of gypsum (CaSO4 ·2H2O) to clinker during finish grinding is to control the setting of cement. It is
normal, and desirable, for some of the gypsum to be dehydrated in the mill during finish grinding.
However, excessively hot clinker feed to the ball mill coupled with inadequate airflow through the
mill, heat generated due to friction from ball milling, and a low humidity system can cause
overheating that converts too much of the gypsum to hemihydrate, also known as plaster(CaSO4 ·1 ⁄ 2H2O), and to soluble anhydrite (γ -CaSO4). The presence of an excessive amount of
plaster or soluble anhydrite can cause false set of cement, attributable to gypsum crystals precipi-
tating from the paste. Although this stiffness can usually be dispersed by remixing, the addition of
extra water to disperse the system can lead to poor strength development, and an imbalance in the
proper gypsum/plaster ratio can lead to problems related to cement-admixture compatibility.
The application of DSC in determining the conversion of gypsum to plaster has proven
extremely useful. The endothermic peaks from dehydration of gypsum, plaster, and syngenite
(CaSO4· K2SO4 ·H2O) occur around 155°C, 195°C, and 260°C respectively, when a cement sample is
gradually heated. These temperatures apply to DSC runs where the crucible lid has been punctured
with a small hole; the separation between gypsum and plaster endotherms is greatly improved with
this technique. DSC plots in Figure 8.4.18 show a cement with appropriate proportions of gypsum
and plaster, whereas Figure 8.4.19 shows a cement with excessive conversion of gypsum to plaster,
together with the formation of syngenite.
Innovations in Portland Cement Manufacturing 1058
∆ H
20 70 170120 220 320270 370
Temperature, °C
Gypsum
Plaster
Figure 8.4.18. DSC plot showing gypsum and plaster in a finished ground cement.
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Silo Set
If cement transferred into a silo is very hot, and if the exterior walls of the silo are cool, water
transfer can occur from the hot silo interior to the cooler exterior, forming syngenite, and creating
the phenomenon known as “silo set.” Syngenite (CaSO4 ·K2SO4 ·H2O) is a needle-like crystallinesubstance that can hinder cement flowability significantly. Thermal analysis is also helpful in
discovering and quantifying the syngenite in cement. This material is formed by the transfer of
water from gypsum to potassium sulfate, often occurring in cement silos during storage. Figure
8.4.19 has already shown the existence of syngenite in cement.
Soiled and /or Contaminated Cement Shipments
Occasionally, a shipment of cement can be inadvertently exposed to moisture due to water leakage
or flooding during shipment. Thermal analysis is a useful tool for quantifying the damage causedby such exposure. The method uses the dehydration, dehydroxylation, and decarbonation peaks to
measure the degree of hydration and assess the damage. This method can be used to supplement
information from other techniques to more accurately assess the full extent of damage.
CHARACTERIZATION OF CEMENT HYDRATION
Thermal analysis techniques have routinely been used to determine the hydration characteristics of
cements. The application is not limited to portland cements, but applies to other cementitious
materials as well. Blended cements, hydraulic cements, and other specialty cements has also beenevaluated beneficially with these techniques to determine the major hydration products when
cement is reacted with water and cured over time. In a typical portland cement-water paste, the
following reaction products are formed from the major cement phases during hydration:
1059Application of Thermal Analysis in Cement Manufacturing
∆ H
20 70 170120 220 320 370270 420
Temperature, °C
Gypsum
Plaster
Syngenite
Figure 8.4.19. DSC plot showing gypsum, plaster, and syngenite in a finished ground cement.
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Innovations in Portland Cement Manufacturing 1060
2C3S + 6H2O¡ 3Ca(OH)2 + “C3S2H3” (1)(calcium-silicate-hydrate)
2C2S + 4H2O¡ Ca(OH)2 + “C3S2H3” (2)
C3A + 3CaSO4 ·2H2O + 26H2O¡
C6AÆ3H32 (3)(Ettringite)
C4AF + 3CaSO4·2H2O + 2Ca(OH)2 + 24H2O¡ C6(A,F)Æ3H32 (4)(Iron-substituted ettringite)
Generally, C3S2H3 is calcium-silicate-hydrate of an approximate composition, and Ca(OH)2 is
calcium hydroxide. In common cement notations, C3S2H3 and Ca(OH)2 are also termed C-S-H
and CH respectively. The rate of reaction and the formation of the hydration products are depend-
ent upon the composition of the cement. These determine most of the physical properties of the
hydrated system, such as the heat of hydration, setting times, tendency toward abnormal stiffening,admixture compatibility, compressive strength, and durability-related properties such as expansion
and alkali-silica reactivity.
Thermal analysis can provide both qualitative and quantitative information on the hydration char-
acteristics of cement, based on the rate of formation of the hydration products. The data can be
related to the composition and quality of cement to project its potential engineering behavior.
Hydration of Calcium Silicates (C3S and C2S)
C3S and C2S are the two major phases in portland cement. Upon hydration they produce C-S-H
and Ca(OH)2 according to Equations (1) and (2) given above. C-S-H is the principal product that
contributes to the cementitious characteristic of cement paste. The amount of Ca(OH)2, denoted
as CH, can be close to 20% by mass in a fully hydrated cement paste. TGA can conveniently quan-
tify the amount of both C-S-H and CH formed in a paste. Such data can then be used to estimate
chemically bound water and the degree of cement hydration.
Typical DSC plots of hydrated cement pastes showing to C-S-H and CH peaks resulting from C3S
and C2S hydration are in Figure 8.4.20 (Sha, 1999). There are two primary endothermic peaks. The
peak around 115°C results from the dehydration of C-S-H phase. The peak around 480°C is due to
dehydration of CH. Another peak around 750°C is due to the decarbonation of any CaCO3 formed
due to reaction between calcium hydroxide and atmospheric carbon dioxide.
There are also several secondary peaks in the DSC plot, primarily due to the presence of aluminate
phases (discussed separately in later sections). The peak around 140°C is due to decomposition of
ettringite; the peak at 165°C denotes decomposition of iron-substituted ettringite or carboalumi-
nate; the peak around 195°C is due to the decomposition of monosulfoaluminate; and the peak at
390°C may be due to the decomposition of hydrated iron oxides. A summary of the peak locations
and the corresponding hydration products is given in Table 8.4.9.
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1061Application of Thermal Analysis in Cement Manufacturing
∆ H
0 400200 800600 1000
Temperature, °C
45 Days
Hydrated iron oxide
17 Days
CSH
Ca(OH)2
CaCO3
Monosulfate
Ettringite
Figure 8.4.20. DSC plots of cement pastes hydrated for different curing times.
Hydration product Peak °C
Physically bound water 105
C-S-H 115
Ettringite 140
Iron substituted ettringite 165
Monosulfoaluminate 195
Hydrated iron oxides 390
Calcium hydroxide 480
Calcium carbonate 750
Table 8.4.9. Peak Locations of
Hydrating Cement PasteGenerally, the peak temperatures vary with the tech-
niques used, but their positions relative to each
other remain unchanged. It will be noted that the
endotherms for C-S-H and ettringite are close
together, and separation may become an issue. If so,
the sample can be treated with acetone and heated
to 45°C for 4 hours prior to testing. This treatment
eliminates the C-S-H endotherm, allowing an
interference-free determination of the ettringite
endotherm.
Estimation of Chemically Bound Water
The amount of chemically bound water present in a cement-water system is directly related to the
degree of hydration and the setting and strength properties of the paste. As indicated earlier, ther-
mal analysis is a useful tool for estimating the bound water in a hydrating cement. Thermo-
gravimetry may be especially useful here. From the TGA plots the amount of bound water in
hydrating cement has been calculated by El-Jazairi and Illston (1977, 1980) by accounting for the
weight losses during dehydration and dehydroxylation periods. The actual water loss between
105°C and 1000°C is given by the sum of dehydration (Ldh) and dehydroxylation (Ldx) losses. Thewater loss, however, is less than the total ignition loss, which also includes the decarbonation loss
(Ldc) in the temperature region 750°C-1000°C. From these values, estimation of bound water can
be carried out by using Equation (5):
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Innovations in Portland Cement Manufacturing 1062
Table 8.4.10. Estimated Calcium Hydroxide in Cement Paste (Bhatty and Reid, 1985)
Curing time, Chemically bound Calcium hydroxide, Degree of days water, wt. % wt. % hydration, %
1 6.28 4.93 29.90
3 9.64 9.86 45.95
7 11.64 13.15 55.43
28 15.36 17.26 73.14
120 16.53 18.91 78.21
200 17.55 20.55 83.57
Optimization of Cement Blends
Pozzolans such as fly ash, silica fume, metakaolin, or ground granulated blast furnace slag are
added to portland cement to produce blended cements. The pozzolan is added to react with the
generated CH to form C-S-H during hydration. As mentioned above, thermal techniques such as
TGA and DSC are extremely useful in determining the amount of CH formed in a fully hydrated
cement; this data enables estimation of the optimum amount of pozzolan required in blended
Chemically bound water = Ldh + Ldx + 0.41Ldc (5)
where the factor 0.41 corrects for the water loss equivalent to that of decarbonation, assuming that
the carbonate is formed by carbon dioxide reaction with calcium hydroxide from hydration. Aselection of calculations of chemically-bound water and its relationship to the degree of hydration
of cement paste is shown in Table 8.4.10.
Estimation of Calcium Hydroxide and Degree of Hydration
Quantifying the amount of calcium hydroxide (CH) in a cement paste is vital in determining the
degree of hydration and the associated strength characteristics. Determination of CH can also be
important for estimating the appropriate amount of pozzolanic addition needed for a given
cement water system (see the section below). Methods of CH determination are many, includingreferences to thermal analysis. According to El-Jazairi and Illston (1977, 1980) the amount of CH is
estimated by taking into account both the dehydroxylation and decarbonation losses during the
TGA run of a neat cement paste by using Equation (6):
Calcium hydroxide (CH) = 4.11Ldx + 1.68Ldc (6)
The factors 4.11 and 1.68 are stoichiometric factors for hydroxylation and decarbonation, respec-
tively. The first term of Equation (6) gives the amount of free CH formed during hydration. The
second term of the equation refers to the carbonate formed from carbon dioxide reaction with CH.Calculated values of CH for a Type I cement are also given in Table 8.4.10 (Bhatty and Reid, 1985).
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cements. Full hydration of a cement paste can be achieved by accelerating the process at elevated
temperature and pressure.
In DTA or DSC patterns, comparing hydration products of the portland cement with those of theblended cement, the C-S-H peak will be enhanced and the calcium hydroxide peak correspond-
ingly diminished, as demonstrated in Figure 8.4.21.
1063Application of Thermal Analysis in Cement Manufacturing
∆ H
0 400200 800600 1000
Temperature, °C
Blended cement
Portland cement
C-S-Hpeaks
CH peaks
Figure 8.4.21. DSC plots for pastes made with portland vs. blended cements.
Hydration of Tricalcium Aluminate (C3A)
Tricalcium aluminate (C3A) is the most reactive phase in cement and is therefore the first to react
with water. Gypsum is added to cement during finish milling to control the setting properties of
cement to avoid flash set. In the presence of dissolved gypsum, C3A begins a rapid reaction to formettringite C3A·3CÆ ·H32 as follows:
C3A + 3CÆ ·H2 + 26H→ C3A·3CÆ ·H32 (7)(Gypsum) (Ettringite)
After all the gypsum is used up, the ettringite reacts with unreacted C3A to form monosulfate
according to the following equation:
2C3A + C3A· CÆ ·H32 + 4H→
3(C3A·CÆ ·H12) (8)(Ettringite) (Monosulfate)
Study of these two forms of calcium sulfoaluminate hydrates has been of interest for their role in
cement chemistry which has been investigated for many years. DSC, sometimes in conjunction
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with X-ray diffraction, has been an invaluable tool in quantifying ettringite and other sulfoalumi-
nate hydrates in cement hydration products.
False Setting and Flash Setting of Cement
Thermal analysis is now frequently used in detecting the potential for both false and flash setting
in portland cements (Theisen, 1983; Shkolnik, 2002). False setting results from abnormal early
stiffening of cement pastes due to the rehydration to gypsum of the more soluble hemihydrate in
cement. The hemihydrate results from gypsum calcination during the finish milling of clinker.
Temperature during grinding can rise enough (close to 150°C) to cause the gypsum, CaSO4 ·2H2O
(or CÆ · H2) to partially dehydrate to calcium sulfate hemihydrate; at elevated temperatures
gypsum can convert to γ -anhydrite according to the following equations:
CÆ ·H2→ CÆ ·H1/2 (-hemihydrate or plaster) at 150°C (9)
CÆ ·H2→ CÆ ·H (γ -anhydrite) below 300°C (10)
In a cement paste, the hemihydrate rehydrates back to gypsum and forms a semi-rigid crystalline
matrix. As earlier described, DTA/DSC analysis of cement gives peaks for dehydration of gypsum
and hemihydrate at 160°C and 210°C respectively. Using the peak heights and the areas under the
peaks, the proportions of gypsum and hemihydrate in cement are quantified. The data on gypsum
and hemihydrate are used to assess the potential for false set.
Similarly, any potential for flash set in cement can be estimated by the amount of gypsum or plas-
ter present. Flash set occurs due to lack of available sulfate to properly control the hydration of
C3A. Using the same DTA/DSC and the quantification method, any lack of gypsum and/or plaster
in cement is quantified, and the potential for flash set can be assessed. X-ray fluorescence (XRF) is
a technique to determine total sulfate in cement and can be used to supplement DTA/DSC data
(Broton, 2002). Figure 8.4.22 shows DSC plots of a normal cement and one with a false setting
tendency. The distribution of sulfate forms in the cements and their actual false set behavior as
determined with the mortar false set test (ASTM C 359 specification) to verify the findings are
shown in Table 8.4.11 below.
Innovations in Portland Cement Manufacturing 1064
Table 8.4.11. DSC and ASTM C 359 Data for Normal and False Set Cements
Cement types Normal cement False setting cement
Sulfate (wt. %) per DSC test
Gypsum 1.7 0.2
Plaster 1.6 1.7
Syngenite 1.1 0.7
ASTM C 359 – Mortar false set penetration test
Initial penetration 50 50
5 min 50 10
8 min 50 2
11 min 50 1
Remix 50 50
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Hydration of Tetracalcium Aluminoferrite (C4AF)
The ferrite phase in cement reacts with water at a slower rate than the C3A in a manner that has not
always been clearly understood. In a cement paste C4AF reacts with lime and gypsum to form iron-
substituted ettringite; it may also yield FH3 and normal ettringite. The reaction takes place as follows:
C4AF + CH + 3CÆ ·H2 + 25H→ C3(A,F)·3CÆ ·H32 (10)(Gypsum) (Iron-substituted ettringite)
C4AF + 3CÆH2 + 30H→ C6AÆ 3H32 + FH3 + CH (11)
When the supply of gypsum for reaction is exhausted, the ettringite transforms to monosulfate of the type C3(A,F)·CÆH12. DTA and TGA techniques have also been used to monitor the hydration
of the ferrite phase.
CONCLUSIONS
Thermal analysis techniques, TGA, DTA, and DSC in particular, have found multiple uses in study-
ing the fundamental characteristics of cement and cementitious materials. Lately, thermal tech-
niques have found diverse applications in the analyses of cement raw materials for improving
processing parameters at the plant, such as formulating the optimum kiln feed mix design, assess-ing potential environmental compliance, and monitoring calcination behavior during the clinker-
ing operation. The techniques have also found vital applications in clinker and cement
characterization. The processes of cement hydration, setting, and strength properties have been
1065Application of Thermal Analysis in Cement Manufacturing
∆ H
0 200100 300 400
Temperature, °C
Syngenite
Plaster
Gypsum
Normal cement
False setting cement
Figure 8.4.22. Cements showing normal and false setting tendencies.
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monitored successfully and improved by their applications. Thermal techniques are being
employed routinely for troubleshooting material, operational, and product problems in cement
manufacturing with the objectives of improving the process of cement manufacturing and the effi-
cacy of the present techniques for cement evaluation.
REFERENCES
Ampian, S. G., and Flint, E. P., “Effect of Silicofluorides on the Formation of Calcium Silicates,Aluminates and Aluminoferrite,” Bulletin of American Ceramic Society Vol. 52, 1973, pages 604-609.
Barta, R., in Differential Thermal Analysis, Vol. 2, Ed. Mackenzie, R. C., Academic Press, London,U.K., 1972, 207 pages.
Ben-Dor, L., “Thermal Methods,” Advances in Cement Technology, Ed. Ghosh, S. N., Pergamon,
Oxford, UK, 1983, pages 673-710.Bensted, J., “Early Hydration of Portland Cement in Accelerating Media,” Tonind-Ztg., Vol. 102,1978, pages 544-545.
Bhatty, J. I., “Application of Thermal Analysis to Problems in Cement Chemistry,” Treatise onAnalytical Chemistry, John Wiley & Sons, Inc., Part 1, Vol. 13, Chapter 6, 1993, pages 355-396.
Bhatty, J. I., and Reid, K.J., “Use of Thermal Analysis in the Hydration Studies of a Type I PortlandCement Produced from Mineral Tailings,” Thermochimica Acta, Vol. 91, 1985, page 95.
Broton, D., Personal Communications, Construction Technology Laboratories, Inc., Skokie,
Illinois, 2002.Chen, H., “Thermal Analysis,” 7th International Conference on Thermal Analysis, Ed. Miller, B.,Chichester, UK: Wiley Press, Vol. 2, 1982, page 1303.
Chrom´ y, S., “Mechanism of White Cement Formation,” 6th International Congress on Chemistry of Cement, Moscow, Vol. 3, 1974, pages 268-284.
El-Jazairi, B., and J. M. Illston, “A Simultaneous Semi-Isothermal Method of Thermogravimetry and Derivative Thermogravimetry and Its Application to Cement Pastes,” Cement and Concrete Research, Vol. 7, 1977, pages 247-257.
El-Jazairi, B., and J. M. Illston, “The Hydration of Cement Paste Using The Semi-IsothermalMethod of Derivative Thermogravimetry,” Cement and Concrete Research, Vol. 10, 1980,pages 361-366.
Goswami, G., and Panda, D., “Application of Microscopy, XRD and DTA in Study of Cement MixBurnability,” 7th International Conference on Cement Microcopy, Texas, March 1985, pages 81-96.
Gouda, G. R., “Designing an Pyroprocessing System for Cement,” 16th International Cement Seminar, Rock Products, Chicago, 1981, pages 51-61.
Handoo, S. K., and Agarwal, S., “Application of DTA to Study Typical Build-Ups in Rotary Kiln,”7th National Symposium on Thermal Analysis, Shrinagar, VIII 5, 1989, pages 338-391.
Handoo, S. K., “Thermoanalytical Techniques,” Progress in Cement and Concrete: Modernizationand Technology Upgrading in Cement Plants, Eds. Ghosh S.N. and Kumar, K., Akademia BooksInternational, New Delhi, India, Vol. 5, 1999, pages 126-153.
Innovations in Portland Cement Manufacturing 1066
5/9/2018 57522707-Chap-8-4[1]-Application of Thermal Analysis in Cement Mnaufacturing - slidepdf.com
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Janko, A., “Additional Estimation of the Ring Formed in a Clinker Burning Rotary Kiln by Meansof Thermogravimetric Analysis (TGA) and Electron Microscopy (SEM),” 7th International Conference on Cement Microcopy, Texas, March 1985, pages 56-66.
Kosmatka, S., Portland, Blended, and Other Hydraulic Cements, IS004, 2001, Portland CementAssociation, Skokie, Illinois, U.S.A.
Le Chatelier, H., Bull. Soc. Franc. Miner. Vol. 10, 1887, page 204.
Mackenzie, R. C., “Differential Thermal Analysis,” The Chemistry of Cements, Ed., Taylor, H. F. W.,Academic Press, London, Vol. 2, pages 271-288, 1964.
Maki, I., and K. Kato, “Phase Identification of Alite in Portland Cement Clinker,” Cement and Concrete Research, Vol. 12, 1982, pages 93-100.
Mehta, P. K., “Analytical Techniques for improving Quality Control in Portland Cement Plants,”
Pit and Quarry, 1964, pages 141-145.Pope, M. I., and M. D. Judd, Differential Thermal Analysis, Heyden, London, 1977.
Ramachandran, V. S., Application of Differential Thermal Analysis in Cement Chemistry, ChemicalPublishing Company, New York, 1969, 308 pages.
Read, H. H., Rutley's Elements of Mineralogy, 26th Edition, Thomas Murby, London, 1970.
Sha, W., O’Neill, E. A, and Guo, Z., “Differential Scanning Calorimetry Study of Ordinary PortlandCement,” Cement and Concrete Research, Vol. 29, 1999, pages 1487-1489.
Shkolnik, E., Personal Communications, Construction Technology Laboratories, Inc., Skokie,
Illinois, 2002.
Shkolnik, E., and Miller, F. M., “Differential Scanning Calorimetry for Determining the Volatility and Combustibility of Cement Raw Meal Organic Matter,” World Cement, May 1996, pages 81-87.
Theisen, K., “Relationship Between Gypsum Dehydration and Strength Development in PortlandCement,” Zem-Kalk-Gips, Vol. 10, 1983, pages 571-577.
PCA, U.S. Cement Industry Facts Sheet, 2002 Edition, Portland Cement Association, Skokie, Illinois,U.S.A, 2002.
Viswanath, V. N., and Ghosh, S. N., “Mineralizers and Fluxes in Clinkerization,” Advances in
Cement Technology, Ed. Ghosh, S.N., 1983, pages 177-202.
1067Application of Thermal Analysis in Cement Manufacturing