M. FRIDRICHOVÁ et al.: OPTIMIZING THE REACTIVITY OF A RAW-MATERIAL MIXTURE FOR ...
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OPTIMIZING THE REACTIVITY OF A RAW-MATERIAL
MIXTURE FOR PORTLAND CLINKER FIRING
OPTIMIZIRANJE REAKTIVNOSTI ME[ANICE SUROVIN PRI
@GANJU PORTLAND KLINKERJA
Marcela Fridrichová, Dominik Gazdi~, Karel Dvoøák, Radek Magrla
Brno University of Technology, Faculty of Civil Engineering, Veveøí 331/95, 602 00 Brno, Czech Republic
magrla.r@fce.vutbr.cz
Prejem rokopisa – received: 2015-07-01; sprejem za objavo – accepted for publication: 2016-03-16
doi:10.17222/mit.2015.187
On the basis of long-term chemical, mineralogical and granulometric analyses it was concluded that concrete recycled materials
are usable for the production of Portland cement clinkers. However, they should be combined with high-calcium limestone,
because even the finest undersize fraction has a relatively low CaO content, usually not exceeding 10 % of mass fractions.
Furthermore, it was concluded that batching recycled materials with high-calcium limestone in a ratio of about 1:3 can produce
Portland cement clinker of the same quality as common commercial clinker. However, there is an issue with the preparation of
the cement clinker in the very low reactivity of the raw-material mixture containing high-calcium limestone and an increased
content of silica in the concrete component. In order to improve the reactivity of the raw-material mixture modifications with
fluxes for clinker production were proposed. An undersize fraction of recycled concrete of 0-8 mm and high-calcium limestone
were used to prepare the raw-material mixtures. Gypsum, CaSO4·2H2O, fluorite, CaF2, and sodium fluorosilicate, Na2SiF6 were
chosen as suitable additives. The prepared samples of raw-material mixtures were assayed for reactivity during the cement
clinker firing process using the kinetic method. After the evaluation of the obtained results, a sample modified with the most
effective grinding aid was burned in an operation rotary kiln. The effectiveness of the modifications was evaluated based on the
phase composition of the burned cement clinker and by determining the technological properties.
Keywords: Portland clinker, high-calcium limestone, concrete recycled materials, gypsum, fluorite, sodium fluorosilicate
Na osnovi dolgotrajnih kemijskih, mineralo{kih in granulometrijskih analiz je mogo~e zaklju~iti, da so reciklirani betonski
materiali uporabni za izdelavo Portland cementnih klinkerjev. Vendar pa morajo biti kombinirani z apnencem z visoko
vsebnostjo kalcija, ker ima tudi najdrobnej{a frakcija relativno majhno vsebnost CaO, obi~ajno pod 10 masnih %. Poleg tega je
mogo~e zaklju~iti, da so serije recikliranih materialov z apnencem z visoko vsebnostjo kalcija v razmerju okrog 1:3, primerne
za izdelavo Portland cementnega klinkerja, enake kvalitete kot je obi~ajen komercialni klinker. Vendar pa je vpra{anje, kako
pripraviti cementni klinker pri nizki reaktivnosti me{anice surovin, ki vsebuje apnenec z visoko vsebnostjo kalcija in pove~ano
vsebnost kremena v betonski komponenti. Da bi izbolj{ali reaktivnost me{anice surovin za proizvodnjo klinkerja, so bile
predlagane modifikacije me{anice s talili. Najdrobnej{a frakcija recikliranega betona od 0–8 mm in apnenec z veliko vsebnostjo
kalcija sta bili uporabljeni za pripravo me{anice surovin. Mavec, CaSO4·2H2O, fluorit, CaF2 in natrijev fluorosilikat, Na2SiF6 so
bili izbrani kot primerni dodatki. Pripravljeni vzorci me{anice surovin so bili z uporabo kineti~ne metode preizku{eni na
reaktivnost med postopkom `ganja cementnega klinkerja. Po oceni dobljenih rezultatov je bil vzorec, modificiran z najbolj
u~inkovitim bru{enjem, `gan v rotacijski pe~i. U~inkovitost modifikacije je bila ocenjena na osnovi sestave faz `ganega
cementnega klinkerja in z dolo~itvijo tehnolo{kih lastnosti.
Klju~ne besede: Portland klinker, apnenec z veliko vsebnostjo kalcija, reciklirani betoni, mavec, fluorit, natrijev fluorosilikat
1 INTRODUCTION
The basic raw material used in the cement industry is
limestone. It is a rock that together with dolomite con-
stitutes four fifths of all the sediments on the Earth’s
surface. For this reason its deposits may appear virtually
inexhaustible. In spite of this, the overview of limestone
mining in the second half of the 20th century alone
indicates that these large industrially-useable deposits
are soon to shrink substantially with the current accele-
rating rate of mining. However, seeing as modern society
produces a large amount of waste, such as FBC ashes
and demolition waste, the possibility arises to process
these as secondary raw-material resources for the pro-
duction of building materials. As far as demolition waste
is concerned, they are used mainly in the production of
recycled materials that are well-established today as
secondary aggregate. Only their finest, undersize frac-
tion, typically from 0 mm to 8 mm, does not find
sufficient use. It is this finest fraction that contains the
highest content of the original cement mortar whose che-
mical composition can approach that of raw materials for
clinker burning. Based on long-term, chemical-mineralo-
gical analyses at our institute, the conclusion was
reached that the recycled materials are applicable in the
burning of Portland clinkers. However, they must be
combined with high-calcium limestone. Based on
laboratory and pilot plant burning in a model rotary kiln,
the optimal batching ratio was determined at 1 : 3. This
batching delivers Portland clinker of the same quality as
common commercial clinker. Nevertheless, the issue
with obtaining clinker in this way is in the very low
reactivity of the raw-material mixture containing
high-calcium limestone and increased silica content in
Materiali in tehnologije / Materials and technology 51 (2017) 2, 219–223 219
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
UDK 66.017:544.424.2:662.613.12 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(2)219(2017)
the concrete component. For this reason it is necessary to
address the improvement of reactivity of a mixture
designed this way.1–4
One option is to use fluxes or mineralizers. These
substances have the capacity to intensify the rate of
clinker formation and have positive effects on the proper-
ties of the clinker.5,6 Fluorides, sulfates, carbonates,
chlorides, alkaline and metal oxides or various com-
binations have been widely studies and their favorable
effect on the rate of clinkerization was reported.7–12
2 METHODOLOGY
The recycled material was made with recycled con-
crete of the undersize fraction of 0-8 mm and high-cal-
cium limestone; their chemical analysis is in Table 1.
The undersize fraction of recycled concrete was sorted in
a ball mill 15 w/% remainder on a 0.09 mm screen. The
high-calcium limestone was sorted in a similar way. Both
the basic raw materials were then batched at the ratio
recycled concrete : high-calcium limestone 1 : 3; this
ratio was previously determined to be optimal. After
batching and thorough homogenisation of both raw
materials, ca. 100 g samples of recycled materials were
prepared, modified by the chosen additives; i.e. gypsum,
dosed at concentration of 1, 2, and 3 w/%, fluorite, tested
at concentrations of (0.5, 1, 1.5 and 2) w/%, and sodium
fluorosilicate dosed at (0.1, 0.5 and 1) w/%. The pre-
pared samples were then tested for reactivity during
clinker firing using the kinetic method. The effectiveness
of the modifications was determined on the basis of the
phase composition of the clinker.
Table 1: Chemical composition of input components
Tabela 1: Kemijska sestava vhodnih sestavin
Component
Component content (%)
Recycled concrete
0–8 mm Limestone
SiO2 63.29 1.02
TiO2 0.38 0.05
Al2O3 9.87 0.57
Fe2O3 2.59 0.20
MnO 0.07 0.01
MgO 1.56 0.34
CaO 9.39 54.70
Na2O 1,72 0.01
K2O 3.38 0.13
loss on ignition 7.39 42.99
SO3 0.43 0.05
Total 100.07 100.07
After the reactivity had been evaluated the clinker
was burned in a laboratory model rotary kiln in an
amount of 5 kg from the raw material of the highest
reached reactivity. After burning, the clinker was put to
phase analysis by means of microscopy point integration
and X-ray diffraction analysis. Then it was sorted
together with 5 w/% gypsum in a ball mill into cement at
a specific surface of 375 m2·kg–1. The cement thus
produced was tested for basic technological properties,
i.e. granulometry, initial and final setting time and
compressive and flexural strengths.
3 REACTIVITY DETERMINATION
The kinetic method of determining raw-material
reactivity permits a separated evaluation of the influence
of mineral resources and the influence of the means of
raw-material preparation on the rate of clinker creation
in the clinkering zone of the rotary kiln. The basis of the
kinetic method is isothermal burning of the clinker at
1430 °C and its quantitative phase analysis by means of
microscopy. Based on this analysis the rate constant K of
the reaction producing alite (C + C2S = C3S) is calculated
from the kinetic Equation (1):
K . t = [1 - 3(1 - )]2 (1)
where:
 ….. conversion level CaO to C3S
t ….. time of isothermal burning
K ….. rate constant
For the reactivity of raw material Rm it follows
Equation (2):
Rm = K / Ks (2)
where: Ks ….. rate constant of comparative raw mate-
rial.
The reactivity of the raw material Rm thus expressed
constitutes the relative rate of isothermal formation of
clinker from the raw material at a medium temperature in
the clinkering zone of a rotary kiln. It includes the in-
fluence of all reactivity factors, which divided into
uninfluenced factors of the raw resource (mineralogical
composition and microstructure, the content of secon-
dary oxides, etc.) and influenced factors of the raw-ma-
terial preparation (composition, granulometry, homo-
geneity). The effect of the uninfluenced factors on
reactivity can be determined during the standardization
of the influenced factors, i.e., the preparation of raw
materials with the same chemical and granulometric
compositions. Because the practical implementation of
this process is very difficult, it is carried out by
calculation based on a knowledge of the mechanism and
the kinetics of clinker production. In doing so, the results
of the phase analysis of the determination Rm are used,
supplemented by a screen analysis of the raw material.
Thus, the following applies for the reactivity of the
raw-material resource:
Rz = k / ks (3)
where:
k ….. rate constant of raw the material with stan-
dardized parameters of preparation
ks ….. rate constant of comparative raw material with
standardized parameters of preparation
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220 Materiali in tehnologije / Materials and technology 51 (2017) 2, 219–223
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The results of the determination of the phase com-
position and reactivity are shown in Table 2, Table 3 and
Table 4 for samples of the raw material with the indi-
vidual modification additives. Besides these there are
values determined for the reference sample of the raw
material without additives.
Table 2: Raw material with added gypsum
Tabela 2: Surovina z dodatkom mavca
Raw
material
Content of phase (w/%)
Reference
sample
1 %
gypsum
2 %
gypsum
3 %
gypsum
C3S 25.6 25.0 21.5 19.5
C2S 48.7 49.1 51.8 53.8
MH 17.2 16.6 17.4 16.9
Cfree 8.5 9.3 9.2 9.8
pore 35.6 37.5 37.8 38.0
C3Srov 61.5 64.2 60.4 60.8
C2Srov 21.3 19.2 22.4 22.3
Reactivity
Rm 0.18 0.15 0.12 0.10
Rz 0.25 0.23 0.18 0.14
Table 3: Raw material with added CaF2
Tabela 3: Surovina z dodatkom CaF2
Raw
material
Content of phase (w/%)
Reference
sample
0.5 %
CaF2
1 %
CaF2
1.5 %
CaF2
2 %
CaF2
C3S 25.6 35.2 38.4 40.8 42.2
C2S 48.7 42.3 38.7 39.2 36.6
MH 17.2 15.7 16.3 15.0 15.7
Cfree 8.5 6.8 6.6 5.0 5.5
pore 35.6 35.4 38.3 36.1 39.0
C3Srov 61.5 63.9 66.2 61.9 65.4
C2Srov 21.3 20.4 17.5 23.1 18.9
Reactivity
Rm 0.18 0.36 0.42 0.60 0.57
Rz 0.25 0.55 0.63 0.97 0.90
Table 4: Raw material with added Na2SiF6
Tabela 4: Surovina z dodatkom Na2SiF6
Raw
material
Content of phase (w/%)
Reference
sample
0.1 %
Na2SiF6
0.5 %
Na2SiF6
1 %
Na2SiF6
C3S 25.6 30.9 36.5 40.2
C2S 48.7 45.3 42.4 38.1
MH 17.2 16.9 16.5 16.3
Cfree 8.5 6.9 4.6 5.4
pore 35.6 34.3 36.8 39.0
C3Srov 61.5 60.0 55.9 63.0
C2Srov 21.3 23.1 27.6 20.7
Reactivity
Rm 0.18 0.30 0.59 0.55
Rz 0.25 0.43 0.89 0.85
The results presented in the tables show that the
reactivity of the raw material of the reference non-
modified sample is very low. The modification by
gypsum influenced the reactivity of the raw material in
the opposite way to what was assumed. Not only did it
not increase the reactivity, it significantly reduced it at
gypsum concentration of 2 w/%. Furthermore, an
increased SO3 content in the raw material caused an
adverse increase of belite content at the expense of alite.
On the other hand, fluorite had a very good influence on
the reactivity of the raw material. With an increased
content of this additive the values of the reactivity
increased and the maximum was reached at 15 w/%
dose. At the same time CaF2 had a positive influence on
the formation of alite and as its content increased, so did
the alite content in the clinker. Na2SiF6 also had a
positive effect on the reactivity of the raw material. The
maximum values of the effectiveness were between 0.5
and 1 w/%, but the maximum values were a little lower
than in the case of fluorite use. For a good reactivity of
the raw material, the modification by fluorite with a
concentration of 1.5 w/% was determined to be the
optimum one and thus the raw material of this com-
position was used in further testing. It should be noted
that this modification was very successful because
reactivity afforded Rm = 0.59, respectively Rz = 0.89 is
higher than, e.g., the reactivity of the raw material from
the Mokra cement factory with its values Rm = 0.52, Rz =
0.51.
4 PILOT PLANT BURNING
For pilot-plant burning the raw material with the
additive 1.5 w/% CaF2 was converted into the form of
compacted pieces by means of its homogenization with
1 % dextrin and water and subsequent processing using a
plodder machine. The thus prepared and dried pieces
were put in a model rotary kiln where they were burned
at 1550 °C. Cooling in the conditions of this burning was
performed by sudden free fall of the clinker in the air out
of the heating end of the kiln.
A qualitative representation of each phase of the
burnt clinker was observed by means of X-ray diffraction
analysis. A dominant content of alite was identified as
well as the presence of belite was apparent only through
diffraction dhkl = 0.2403 nm, diffractive lines C3A and
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MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 1: X-ray diffraction pattern of clinker: A – alite, B – belite, C –
C3A
Slika 1: Rentgenogram klinkerja: A – alit, B – belit, C – C3A
C4AF were featureless, see the X-ray diffraction pattern
in Figure 1.
The phase analysis carried out by means of micro-
scopic point integration of the burnt sample identified it
as a clinker with a dominant content of alite, a relatively
high content of belite and C3A, and, on the other hand, a
reduced amount brown millerite a slight remainder of
free CaO (Figure 2). Alite formed hypautomorphic to
automorphic crystals with a high degree of mutual
coalescence. The average size of the alite crystals ranged
from 25 μm to 30 μm, while there were plentiful inclu-
sions of alite in belite. Belite formed the usual oval
agglomerates of grains with an average size of 15 μm to
20 μm. Free CaO occurred only sporadically in the form
of clusters of globular grains in alite areas. Quantitative
phase composition of the burnt clinker is in Table 5.
Table 5: Phase composition of burnt clinker determined microscopi-
cally
Tabela 5: Sestava faz `ganega klinkerja, dolo~ena z mikroskopom
Phase Content of phase (w/%)
C3S 55.6
C2S 25.9
C3A 12.1
C4AF 5.8
CaOfree 0.6
Total 100.0
5 TECHNOLOGICAL PROPERTIES OF THE
BURNT CLINKER
The pilot-plant burnt clinker was milled with 5 w/%
gypsum into cement with a specific surface of 375
m2 kg–1. Then it was tested for technological properties;
the results together with the results of the properties of
the reference cement are shown in Table 6. The refe-
rence cement was prepared using the same procedure as
above with commercial clinker collected in the cement
factory Mokra.
Table 6: Technological properties of alite cement
Tabela 6: Tehnolo{ke lastnosti alit cementa
Observed property Referencesample Alite cement
Specific surface Blaine (m2 kg–1) 375 375
Water-cement ratio (%) 26.1 29.0
Initial setting time (h min) 3:05 2:05
Final setting time (h min ) 6:45 5:00
Compressive strength (MPa)
1 d 23.2 28.6
3 d 45.5 54.8
7 d 46.6 63.8
28 d 56.1 76.5
Flexural strength (MPa)
1 d 6.6 8.5
3 d 9.1 13.6
7 d 10.6 15.9
28 d 11.6 16.9
The results shown in the table indicate that at the
same specific surface of the two compared cements, the
reference cement showed a slightly lower value of
water-cement ratio. The difference is probably connected
with a slightly different phase composition of both
clinkers. It is also evident that the tested cement, com-
pared to the reference, has a shorter initial and setting
time. This difference is related to the phase composition
of the pilot-plant burnt clinker, respectively, with a rela-
tively high proportion of C3A. The compressive strength
values of the tested cement were after all the observed
hydration times higher than the reference sample’s
strength on average by 30 %, and even the flexural
strength was higher by 50 %. The main cause of this
phenomenon can probably be considered to be the higher
hydraulicity of the burnt clinker, given its modification
by fluorite.
6 CONCLUSIONS
The insufficiency of preparation and pilot-plant pro-
duction of Portland clinker based on recycled concrete
and high-calcium limestone, which lay in the low reac-
tivity of the raw material, has been removed by modi-
fying with fluorite. At a dose of 1.5 w/% of this additive
in the raw material reactivity has improved so much that
it even exceeded that of the normal raw material from the
cement factory Mokra. It can be concluded that recycled
concrete materials are utilizable for Portland cement
burning in combination with high-calcium limestone.
Because the resources of high-calcium limestone can
also be found in existing waste materials, such as sugar
factory sludge etc., their combination may represent not
only the economic benefits, but particularly in the future,
a contribution to the transition to non-waste technology
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Figure 2: Microstructure of clinker: 1 – alite, 2 – belite, 3 – C3A and
C4AF, 4 – pore
Slika 2: Mikrostruktura klinkerja; 1 – alit, 2 – belit, 3 – C3A in C4AF,
4 – pore
too, and thus contribute to the protection of raw-material
resources.
Acknowledgment
This work was financially supported by project
number: GA14-32942S "Effect of fluidized bed ash on
the thermodynamic stability of hydraulic binders" and
project No. LO1408 "AdMaS UP – Advanced Materials,
Structures and Technologies", supported by Ministry of
Education, Youth and Sports under the "National Sus-
tainability Programme I".
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