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MINERALISED CLINKER AND CEMENT
S Rasmussen
D Herfort
Aalborg Portland
Denmark
A B S T R A C T . Mineralised clinkers and the potential benefits of using cements based on
mineralised clinkers have been studied for many years. During the past two decades attention
has mainly focused on the benefits arising from the combined use of fluorine and S 0 . Until
recently, however, no producers of grey cement, have utilised this technology. The Danish
producer, Aalborg Portland has, since 1994, produced a cement based on mineralised clinker
as one of its main products for the domestic market. The clinker contains approximately 2%
S0 and 0.2% F. The combined mineralising action lowers burning temperatures in the kiln
by approximately 100°C, giving considerable reductions in emissions of NO in particular.
Cements ground from this clinker are more reactive with both higher early and 28 day
strengths than for otherwise identical non-mineralised cements. Where rapid strength devel­
opment is not desirable this technology can be used to partially replace the clinker by a mate­
rial such as limestone. In Denmark this is achieved by replacing 14% of the clinker with a
limestone rich dust giving a further reduction in NO emission in addition to a considerable
reduction in C 0 emission by weight of the cement produced.
3
3
x
x
2
Keywords: Mineralised cement, Clinker, Fluorine, Sulphate, Limestone filler.
M r Soren Rasmussen is Manager of the Project Service Department at Aalborg Portland.
His main interests include the properties of mineralised and composite cements, and the ef­
fects of admixtures.
M r D u n c a n Herfort is chief geologist at Aalborg Portland. His responsibilities include all
mineralogical aspects associated with raw material assessment, process and quality control,
and concrete analysis.
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60
Rasmussen, Herfort
INTRODUCTION
During recent decades the cement industry has had to face growing demands as a result of in­
creasing fuel costs, stricter emission limits, higher expectations regarding cement quality, and
in many cases increased competition. Reduced fuel consumption has mainly been achieved
by investing in more efficient kilns. Modern precalciner kilns, for example, use only half the
amount of fuel required by their wet process predecessors to produce a tonne of clinker, and
at much higher outputs. Further reductions in fuel consumption have been achieved by par­
tially replacing the clinker with pulverised fly ash, ground blast furnace slag, natural pozzo­
lans, limestone dust etc. The wide range of such composite cements produced in Europe is
exemplified by the long list of cement types specified in the European pre-standard, ENV
197.
Another way of reducing fuel consumption as well as emissions of C 0 and NO is by using a
mineraliser where this is not already present in the raw materials. The mineralisers which
have received most attention, particularly since the work done by Blue Circle in the 1970's
and early 1980's [1,2], are CaF and CaS0 , because of the marked improvements in clinker
combinability and strength development of the cement when these components are used in
combination. This technology has been successfully applied at Aalborg Portland since 1994,
with considerable savings in fuel consumption and improvement in cement quality [3].
2
2
x
4
MINERALISED CLINKER
Although at times used loosely, the term mineralisation in clinker production is defined in the
same way as it is in geology, i.e. it is the process of promoting mineral formation. A miner­
aliser is, therefore, the agent by which this is achieved, for example by lowering the tem­
perature of formation of a mineral through solid solution, or by increasing the content of the
melt, or changing the properties of the melt. Strictly speaking, then, a fluxing agent is also a
mineraliser if it promotes mineral formation. Not all fluxing agents are mineralisers, how­
ever, since some are known to inhibit the formation of clinker minerals. The term is conven­
tionally used to describe the minor components which promote the formation of alite. A1 0
and F e 0 are, therefore, not regarded as mineralisers in the production of Portland cement
clinker, because they occur as major components. For a comprehensive review of mineralis­
ers in Portland cement clinker, the reader is referred to Moir & Glasser [4]. Some of the ma­
jor trends are briefly described below.
2
2
3
3
The effects that transition metals have on the formation of alite varies considerably with no
clear relationship with their effect on the properties of the melt. Of the P-block elements, S
and P are known to inhibit the formation of alite, except where this is offset by sufficiently
high contents of other minor components, particularly F. The presence of halogens, notably F
and CI, are known to promote the formation of alite (or alinite in the case of CI). The effect
of the alkalis is strongly dependent on the degree of sulphurisation.
The above minor components are always present in cement but, apart from the alkalis and
sulphates, are usually at too low concentrations to have any significant effect. A combination
of sulphate and fluoride is known to have a particularly positive effect, although it is unusual
for the desired combination to occur naturally in the raw materials and/or fuel.
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Mineralised Clinker and Cement
61
A small number of producers, including Aalborg Portland have utilised this technology
through the controlled addition of CaS0 and CaF to the raw mix used to produce grey
clinker.
4
2
Two types of mineralised clinker are currently produced at Aalborg, both in its 5500 t/d semiwet, 2-stage separate line precalciner kiln, the details of which are published elsewhere [5,6].
The composition of the more normal clinker is given in Table 1, with S 0 and F contents of
1.9% and 0.18%, respectively. A high C S clinker is also produced in which the F content is
increased to 0.23% as shown in Table 2. Combination to between 1 and 2% free lime is
achieved at approximately 1350°C which is about 100°C below that for the non-mineralised
clinker. The bulk of the sulphate melt forms between 1000°C and 1200°C. Most of the alite
forms fairly rapidly over a narrow temperature interval between 1150°C and 1200°C before
any significant oxide melt formation begins. The rapid, early formation of alite is partly due
to its greater stability at lower temperatures, and partly to the high rate of diffusion of Ca
through the low viscosity sulphate melt. About 2/3 of the sulphate occurs in a readily water
soluble form, mainly as Ca-langbeinite, with the remainder incorporated in the silicate
phases, with the highest concentrations occurring in the belite phase which significantly in­
creases its hydraulic reactivity.
3
3
2+
Low temperatures for clinker formation and relatively high partial pressures of 0 prevent ex­
cessive volatilisation of the sulphates. Volatilisation of F is negligible. Ring formation and
other volatile related kiln build-ups are therefore limited. Direct fuel savings are marginal at
approximately 5%. Formation of NO in the kiln is reduced by up to 50%. Since the total
NO emission from this type of kiln is also dependent on fuel derived NO formed in the calciners, the overall reduction in emission of NO can vary from 10 to 30% depending on the
nitrogen content of the fuel.
2
x
x
x
x
MINERALISED CEMENT
The properties of cement based on all known mineralisers is beyond the scope of this paper
and only mineralised cement prepared from clinker containing controlled levels of S 0 and F
will be discussed. The properties of mineralised cement are no different from those of normal
Portland cement in terms of the dependency on the relative content of clinker minerals, fine­
ness, ratio of soluble sulphate to C A, etc. The only difference lies in the modified reactivity
of the silicate phases which gives longer setting times and faster strength development.
3
3
Setting Times
Setting times are increased with increased contents of F in the alite phase. Work by Renichi
Kondo et al [7] indicates that this is due to the precipitation of CaF on the surface of the alite
crystals during initial hydration. The relationship between setting time and the fluorine con­
tent of the mineralised cement produced at Aalborg shown in Figure 1 corresponds to 250
minutes/%F. The setting times were determined from pastes prepared to normal consistency,
according to EN-196, and do not necessarily correspond to the setting times of concretes
which often contain water reducing and air entraining additives.
2
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62
Rasmussen, Herfort
Studies at Aalborg Portland have shown that the difference in concrete setting times is ap­
proximately 30 minutes for the cements shown in Table 1 regardless of the type and dosage
of the admixture. In other words, even though the admixture may increase the setting time of
the concrete by up to 5 times that of the paste, the difference between concretes containing
the mineralised and non-mineralised cements remains constant at approximately 30 minutes.
In relative terms, then, the effect on concrete is not as great as would appear from the EN-196
setting times alone.
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
0.20
C O N T E N T OF FLUORINE IN C E M E N T , %
Figure 1 Relationship between initial setting time (EN-196) and
content of fluorine in cement
Compressive Strengths
Apart from the benefits on clinker production discussed above, the main advantage of miner­
alisation lies in the increased reactivity of the cement resulting in a more rapid strength de­
velopment. The increased reactivity is the result of increased solid solution of the mineralis­
ers, i.e. S and F in the silicate phases, along with an additional incorporation of Al which this
facilitates. An example of the enhanced strength development of mineralised cement com­
pared to a similar, but non-mineralised cement, is shown in Figure 2. The strengths plotted
are the average of two, and sometimes three separate EN-196 determinations. The chemical
and physical characteristics of these cements are shown in Table 1.
The higher 7 and 28 day strengths are mainly due to the increased reactivity of the belite
phase in the mineralised clinker. With the possible exception of the ferrite phase, which
makes little contribution to strength development, belite is normally the least reactive phase,
making relatively little contribution to 28 days strengths.
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Mineralised Clinker and Cement
63
In mineralised clinker, however, a far higher proportion of the belite has reacted at 28 days, to
the extent that increasing the ratio of C S to C S in the clinker (essentially lowering the LSF)
results in comparable or often higher 28 days strength.
2
3
80
10
I
1
1—
1
1
1—i
10
•
• i • i
1
100
TIME, days
Figure 2 Compressive strength of mortars with mineralised cement no MI and nonmineralised cement no NO
Table 1 Clinker and cement data
NON-MINERALISED
CEMENT, no. NO
MINERALISED
CEMENT, no. MI
CLINKER:
C S°
C S'>
CA
C AF
Free lime
Fluorine
S 0 in clinker
N a 0 eqv. acid sol.
%
%
%
%
%
%
%
%
52.9
20.1
6.1
11.6
1.36
0.06
0.90
0.59
54.1
20.0
6.2
10.8
0.98
0.18
1.90
0.56
CEMENT:
Fineness (Blaine)
45 urn residue
S 0 in cement
Limestone extender
m /kg
2
426
2.4
2.94
4.0
412
2.5
2.91
4.0
3
2
3
4
3
2
3
%
%
%
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64 Rasmussen, Herfort
1) Bogue composition corrected for S 0 and free lime
Although the early strengths, i.e. 1 and 2 day strengths, are essentially the same for the two
cements shown in Figure 2, higher early strengths can be achieved by increasing the LSF of
the mineralised clinker which is readily accomplished owing to its easier burnability.
Strength development for this type of cement is shown in Figure 3 for the cements labelled
MH420 and MH251 (ground to specific surface areas of 420 and 251 m /kg), with relevant
chemical and physical data shown in Table 2. The results show, among other things, that
very high early strengths can be achieved without grinding to excessive fineness. This allows
the addition of a relatively inactive filler such as limestone [8]. The strength development
obtained with a replacement of the clinker by 14% calcareous filler (cements MH420L and
MH251L) are also plotted in figure 3. The cement labelled MH420L in Table 2 has been
produced in Denmark since 1994, where it is the preferred cement for the pre-cast and con­
crete product producers. It is formally designated by ENV 197-1 as a PC 52.5 R, Portland
limestone cement (type II/A-L).
3
2
For the cements to comply with the upper limit of 62 MPa for ENV 197, class 42.5 cements,
the clinker fineness must be kept below 250 m /kg as shown by the strength plots in Figure 3.
The presence of a relatively high content of fine filler (MH251L) eliminates the risk of
bleeding which would normally be a problem at such a low fineness of the clinker. Com­
pared with conventional PC 42.5 cement, savings achieved on grinding would be at least
20%.
2
For both filler cements reductions in fuel consumption are more or less the same as the level
of clinker replacement, i.e. 14% in the examples shown. Reductions in NOx emissions are
estimated at slightly more than 30%.
80 ,
10 I
1
,
1
1
1
1
10
'
'
100
TIME, days
Figure 3 Compressive strength of mineralised cement with relatively high content of C S and
with varied fineness and content of limestone filler, mortar strength
determined according to EN-196
3
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Mineralised Clinker and Cement
65
Table 2 Clinker and cement data.
MINERALISED CEMENT
MH251
CLINKER
C S'>
CS
CA
C AF
Free lime
Fluorine
S 0 in clinker
N a 0 eqv. acid sol.
3
])
2
3
4
3
MH251L
MH420
MH420L
66.4
6.7
7.9
9.7
1.80
0.23
1.90
0.59
%
%
%
%
%
%
%
%
2
CEMENT
S 0 in cement
Fineness of clinker (Blaine)
45 um residue
Limestone filler
3
%
2
m /kg
%
%
3.01
251
19.0
0.0
2.92
251
17.3
14.0
3.04
420
2.1
0.0
2.95
420
2.0
14.0
1) Bogue composition corrected for S 0 and free lime
3
Durability
Experience in Denmark to date has been that concrete produced from mineralised cement is
equally durable to concrete produced from conventional Portland cements. This is docu­
mented in a report by the Danish Institute of Testing of Building Materials (DTI) [9], and in
publications by the present authors [10,11].
Despite having many properties in common with the other halogens, including chlorine, fluo­
rine does not cause corrosion of steel reinforcement due to the low solubility of CaF in pore
solutions found in concrete [12].
2
Another concern has been that clinker containing high sulphate contents may give rise to a
type of secondary ettringite formation which may lead to deleterious expansion in much the
same way as delayed ettringite formation (DEF). As noted in the section on clinker, however,
most of the sulphate present in the clinker is in a readily soluble form which contributes to
regulation of the initial set.
All but a negligible fraction of the remaining sulphate, which occurs in the unreacted sili­
cates, is available for any further ettringite formation, and even here an invariable excess of
A1 0 precludes any ettringite formation whatsoever. Numerous experimental studies
[10,13,14] have recently shown that expansion is unaffected by the level of sulphate in the
clinker.
2
3
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66 R a s m u s s e n , H e r f o r t
CONCLUSIONS
The production of mineralised clinker using controlled additions of fluorine and sulphates
lowers burning zone temperatures by approximately 100°C. Although this results in fuel
savings of only 5%, NO formed in the kiln is reduced by up to half, which in a precalciner
kiln typically equates to an overall reduction in NO emission of approximately 20%.
x
x
In terms of fuel consumption and C 0 emission the main advantage of producing mineralised
cement lies in its greater hydraulic reactivity which allows the replacement of clinker by C 0
neutral additions such as limestone.
2
2
REFERENCES
1.
MOIR, G, K. Mineralised High Alite Cements. World Cement, 1982, Vol. 13, No.
10, pp 374-382.
2.
MOIR, G, K. Improvements in the early strength properties of Portland cement. Phil
Trans. Lond., 1983, A310, pp 127-138.
3.
BORGHOLM, H, E, HERFORT, D, AND RASMUSSEN, S. A New Blended Ce­
ment Based on Mineralised Clinker. World Cement, 1995, Vol. 8, pp 27-33.
4.
MOIR, G, K. Effect of the Use of Mineralisers, Modifiers and Activators. 9th ICCC,
New Delhi, 1998, Vol. l,pp 125-152.
5.
BORGHOLM, H, E, AND NIELSEN, P,B. Ein neues Halbtrocken-Ofen-System fur
4000t/d im Aalborg Portland-Zementwerk. ZKG Inernatioanl, 1988, Vol. 41, No. 12,
pp 595-600.
6.
BORGHOLM, H, E. Comissioning the world's largest semi-dry process kiln system.
World Cement, 1989, Vol. 20, No. 3, pp 72-79.
7.
KONDO, R, DAIMON, M, SAKAI, E AND USHIYAMA, H. The Influence of Inor­
ganic Salts on the Hydration of Tricalcium Silicate. J. appl. Chem. Biotechnol. 1977,
27, pp 191-197.
8.
BORGHOLM, H,E, AND DAMTOFT, J, S. EUROPEAN PATENT, EP 0640 062
B l , 1993.
9.
PETERSEN, E, J, AND HAUGAARD, M. New Cement Types: Evaluation of the
Suitability of Basis-Cement (= Portland Limestone Cement Based on Mineralised
Clinker) in Concrete designed for Moderate and Aggressive Environments. DTI
Byggeteknisk Institut, March 21, 1994, In Danish.
Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved.
Mineralised Clinker and Cement
67
10.
HERFORT, D, RASMUSSEN, S, JONS, E, AND OSB^CK, B. Mineralogy and
Performance of Cement Based on High Sulphate Clinker. ASTM Symposium on In­
ternal Sulphate Attack on Cementitious Systems: Implications for Standards. In
press, Cement, Concrete and Aggregates.
11.
HERFORT, D, S0RENSEN, J AND COULTHARD, E. Mineralogy of Sulphate
Rich Clinker and the Potential for Internal Sulphate Attack. World Cement, 1997,
Vol. 28, No. 5,pp 77-85.
12.
ESCUDERO, M, L, AND MACIAS, A. Corrosion of Reinforcing Steel in Mortar of
Cement with CaF as a Minor Component. Cement and Concrete Research, 1995,
Vol. 25, No. 2, pp 376-386.
2
13.
KELHAM, S. Effects of Cement Composition and Hydration on Volume Stability of
Mortar. 10th ICCC, Gothenburg, 1997, Vol. 4, paper iv060.
14.
TENNIS, S, BHATTACHARJA, S. AND KLEMM, W. Ambient Temperature De­
layed Ettringite: A Reapraisal. ASTM Symposium on Internal Sulphate Attack on
Cementitious Systems: Implications for Standards. In press, Cement, Concrete and
Aggregates.
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