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ADMIXTURES AND THEIR INTERACTIONS WITH HIGH RANGE
CALCIUM ALUMINATE CEMENT
by Thomas A. BIER, Alain MATHIEU, Bruno ESPINOSA, Christophe MARCELON
presented at the UNITECR congress, Japan; 1995.
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Abstract
Besides a careful choice of mineral raw materials, sophisticated applications of calcium
aluminate cements (CAC's) cannot be achieved without the use of admixtures.
Admixtures serve to control mostly the rheological behavior and setting (hardening)
characteristics.
In this paper, different concepts of admixtures are presented in order to influence given
properties such as flocculation, coulability and hardening.
The interactions of these admixtures with the CAC are shown by chemical as well as
rheological measurements. The results are discussed with respect to practical
applications.
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An overview about possible admixtures with
CAC's is given in /1/.
Introduction
As with all hydraulic binders, the chemical and
mineral composition and the fineness of a CAC
determine the rheology and reactivity of the
system. This means that setting time and flow
behavior of a refractory concrete or mortar
depend for a given water cement ratio only from
the constituents other than the cement. These
other constituents influence the rheological and
hardening properties of a concrete or mortar
more or less. Aggregates are normally
considered as inert, a larger influence being
attributed to reactive fine fillers. And the largest
influence is played nowadays by admixtures
which are on purpose added to tailor the above
mentioned properties. The raw material together
influence each other and constitute finally the
performance of the concrete. These interactions
have in the past very often been represented by a
triangle of interactions as shown in Figure 1.
CAC
Aggregate
General characteristics
(porosity)
Granulometry
Reactive
fillers
The major admixtures used in this study and
discussed in the paper are:
 Accelerators
 Retarders
 Plasticisers - Water reducing agents
Experimental approach
2
Conductivity of stirred suspensions
This technique, presented in recent publications
/2,3/, consists of measuring the electrical
conductivity of a stirred suspension of cement in
water at 20° C.
Conductivity is a very good picture of the ionic
concentration in water. The conductivity of the
suspension gives information on the dissolution /
nucleation / precipitation steps of the hydration
reaction of cement in water. A schematic picture
of conductivity and related properties is shown in
Figure 2, the above mentioned hydration steps
being represented as stages I, II and III.
4
I
Additives
Reactive phase
Figure 1 - Components influencing the properties in a
refractory concrete.
Amongst these components this paper deals with
the admixtures. Admixtures are defined
according to ASTM C125-88 as
"Materials other than water, aggregates, hydraulic
cement and fibre reinforcement used as
ingredients of concrete or mortar and added to
the batch immediately before or during its
mixing." and in the DBV - Manual :
"Are chemical substances added to fresh
concrete or mortar in order to influence certain
properties of the fresh and/or hardened concrete.
The quantities are so small that they don't have
to be taken into account for the calculation of the
mix design."
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II
III
Ca
3
Arbitrary Units
1
Conductivity
2
1
LOI
0
0
100
Time (min)
200
300
Figure 2 - Sketch of a conductivity curve for a calcium
alumina cement.
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Self Flow Value
The self flow has been determined for pastes of
pure cement and mixtures of cement and
reactive alumina.
The paste is mixed and then filled into a cylinder
(diameter = 30 mm, height = 50 mm) which sits
on a glass plate. The cylinder is then withdrawn,
the paste starts to flow under its own weight and
a "cake" with a much larger diameter is obtained.
The size of this diameter, expressed in mm, is a
measure of how well the paste flows. As an
indication : a flow of 80 mm represents a casting
grade consistency.
Strength after 6 hours
Setting time and strength after six hours
measured on 40 x 40 x 160 mm mortar prisms
can be used to evaluated the impact of
accelerators or retarders. Tests are carried on
mortars containing quartz sand (0-2 mm)
according to NF P 15 - 315.
3
Results
Accelerators
The most common, most effective accelerator for
alumina cements is lithium carbonate. Lithium
hydroxide acts more violent and is therefore
difficult to dose. However, for winter concreting it
might be appropriate.
0,025% Li2CO3
0,005% Li2CO3
0,1% Li2CO3
2,8
2,6
2,4
2,2
2,0
1,8
1,6
1,4
1,2
1,0
0
40
160
200
®
Figure 3 - Conductivity curves of Ciment Fondu with
different Li2CO3 concentrations.
With increasing Li2CO3 content the massive
precipitation (hydration) starts earlier and hence
the setting time starts earlier and the difference
between begin and end of set becomes smaller
(steeper decrease in conductivity upon massive
precipitation).
This translates in earlier setting times and a
faster hardening (smaller difference ? between
begin and end of set) as can be seen from the
following table I depicting mechanical properties
®
for accelerated Ciment Fondu systems.
Sample
Setting Time (min)
®
Curves of the dissolution and precipitation
®
behavior of Ciment Fondu and Li2CO3 mixes
have been presented and commented in the past
/2/. Figure 3 shows the influence of different
concentrations of lithium carbonate on the
®
precipitation behavior of Ciment Fondu .
80
120
Time (min)
Ciment Fondu
Compressive
Strength (MPa)
Set + 2 h
6h
Initial
Final
?
200
230
30
0
37,6
110
135
25
26,8
53,2
80
95
15
35,8
54,5
®
Ciment Fondu
+
0,005 Li2CO3
®
Ciment Fondu
+
0,01% Li2CO3
Table 1 - Comparison of macroscopic properties for
®
differently accelerated Ciment Fondu pastes.
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2,8
2,6
2,4
2,2
2,0
0,1% Li2CO3
2,8
2,4
2,2
2,0
1,8
1,6
0,1% Li2C03 + 0,1% CT
1,4
0,1% Li2CO3 + 0,2% CT
1,2
1,8
0,02% CT
0,1% CT
0,2% CT
1,6
1,4
0,1% CT
2,6
Conductivity (mS/cm)
Retarders
The action of a retarder for the same system as
in the paragraph above is shown in Figure 4 for
the use of tri-sodium citrate (CT).
1,0
0
40
80
120
Time (min)
160
200
Figure 5 - Conductivity curves for an accelerated
®
Ciment Fondu system with different CT contents.
1,2
1,0
0
50
100
150
200
250
300
350
Time (min)
Figure 4 - Influence of tri-sodium
®
conductivity curves of Ciment Fondu .
citrate
on
The contrary to Li2CO3 is observed : the time of
massive precipitation becomes longer the higher
the tri-sodium citrate content. Therefore the
setting times are prolonged.
But a second effect, not present with the
acceleration, can be observed. The dissolution of
Ca2+ and Al(OH)4- ions is much slower with
increasing citrate content. The apparition of an
'ear' (early precipitation of C2AH8) is even
suppressed with high CT concentrations. This
entrains an improved workability as we will see
later.
Retarders with accelerators
The combination of retarders and accelerators is
possible and recommended, because the two
effects do not necessarily cancel each other out
but intervene at different stages of the hydration.
Figure 5 gives an example for the use of Li2CO3
with CT.
The Li2CO3 accelerates in this case the
hardening (steep massive precipitation) but
shortens the setting time to much. The additional
use of CT adjusts the setting times without
interfering strongly with the hardening. As a
comparison the curve for the unique use of CT is
plotted into the same diagram.
Plasticisers
Cements pastes
Different industrially available plasticisers have
been tested. Figure 6 shows a comparison of CT,
trisodiumpolyphosphate (TPP), Darvan 7 S and
Melment F 10 in a Secar® 71 cement paste. The
action of the plasticisers is shown by the self flow
value.
200
DARVAN 7S
TPP
Melment F10
CT
100
0
0
1
2
3
Plasticiser Addition (%)
4
Figure 6 - Self flow values for a Secar® 71 cement
paste with different plasticiser additions.
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Different effects can be observed :




The following remarks become obvious :
Darvan 7S and Melment F 10 act in a similar
way : fairly high amounts are necessary (0,5
%) to obtain a plasticising effect. The self flow
value, however, which can be reached is
higher and less sensitive to dosage as for TPP
and CT.
CT acts much faster (around 0,1 %) but has a
very narrow range of dosage in order to obtain
optimum performance. This fluidifying effect is
a result of the slow down of ion dissolution as
seen in paragraph IV. 2.
TPP is between CT and the organic
plasticisers. It can be more easily dosed than
CT and good flow properties are reached with
reasonable amounts of TPP addition (around
0,3 to 0,5 %).
For both CT and TPP an over dosage leads to
a degradation of flow properties which has
been shown for the case of TPP in Secar® 71 /
silica fume systems earlier /3,4/.
The positive effects of Melment F10 (high flow
and little degradation) and CT (early flow) can be
combined by using both plasticisers. This is
shown in Figure 7 where the plasticisers
Melment, Darvan and Mighty are all used with a
0,2 % CT addition. For comparison the pure CT
flow curve is also given.



With all combinations of CT + plasticisers an
early self flow can be obtained.
The high sensitivity of CT to the amount
added does not exist for the combinations (no
degradation of flow with over dosage).
With certain plasticisers even higher early self
flow values can be obtained as compared to
CT or the plasticiser alone.
Cements / reactive alumina pastes
Interactions of fine reactive fillers like silica fume
or reactive alumina with CAC and their influence
on hydration behavior have been shown earlier
/4,5/. Plasticisers also react differently with such
a system as compared to a pure paste. Figure 8
shows self flow values for the case of TPP and
CT for a Secar® 71/reactive alumina system.
200
100
Citrate
Citrate - CAC/Alumina
TPP
TPP - CAC/Alumina
0
0,0
0,2
0,4
0,6
Plasticiser Addition (%)
0,8
®
Figure 8 - Comparison of pure Secar 71 cement
paste and cement /alumina paste for TPP and CT
additions.
200
100
Darvan 7S + 0.20% Citrate
Melment F10 + 0.20% Citrate
Mighty 100 + 0.20% Citrate
Citrate
0
0,0
0,2
0,4
0,6
Plasticiser Addition (%)
Figure 7 - Self flow values for Secar
pastes with 0,2 % CT and plasticisers.
0,8
®
71 cement
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Even without plasticiser addition the self flow is
enhanced for a cement/alumina systems. This is
in part due to an optimization of the particle size
distribution - an important parameter when
designing self or free flow castables /6/. This
good self flow can be enhanced, however, by the
addition of plasticisers like CT and TPP. An
additional positive effect is a decreased
sensibility to the amounts of plasticiser added.
The above mentioned beneficial effect of fine
fillers on granulometric curves and therefore flow
behavior is not the only factor. There is also
chemical interactions which contribute to the
plasticising effect : Figure 9 shows this for the
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use of Darvan 7S with and without the addition of
reactive alumina.

2,8
2,6
Conductivity (mS/cm)
plasticiser of the type TPP, Melment, Darvan,
Mighty or others.
2,4
2,2
2,0
It should be kept in mind that the control of
fluidity retards the systems which means that
a classical retarder like CT will play a double
role as well in the retarder/accelerator as in
the CT/plasticiser couple.
1,8
1,6
Darvan 7S
Darvan 7S + Alumina
1,4
5
1,2
Acknowledgements
1,0
0
100
Time (min)
200
300
Figure 9 - Conductivity curves for Secar® 71 cement
pastes with Darvan 7S and reactive alumina.
The addition of reactive alumina decreases the
dissolution of ions in the beginning which means
a better flow a fact demonstrated in Figure 8 for
CT and TPP.
4
Conclusions
The following conclusions can be drawn :

In order to optimize sophisticated refractory
concretes like LCC or SFC different
admixtures have to be combined besides the
careful choice of mineral raw materials.
These different admixtures are employed to
tailor various characteristics.

It is important that the active binder system is
taken into account. This means that the
admixtures have to be adapted to cement
plus fine reactive fillers and not only to the
cement.

Setting time and hardening are controlled by
a retarder/accelerator couple. The use of
both, seemingly contradictory admixtures,
assures a more comfortable control for
hydration kinetics.

The workability or flow - especially important
for SFC application - are best controlled by a
couple of CT and a plasticiser or super-
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The authors would like to thank all co-workers at
Lafarge Fondu International and Lafarge Coppée
Recherche who contributed to the studies which
led to this paper. Special thanks to Ms F. Kebli
and Mrs D. Gauthier who carried out most of the
work on self-flow and conductimetry.
6
Literature
/1/ J.D. Cox and J.H. Sharp ; "The use of
admixtures with calcium aluminate cements".
/2/ D. Sorrentino, J.P. Bayoux, R. Montgomery,
A. Mathieu and A. Capmas ; "The effect of
sodium gluconate and lithium carbonate on
the properties of calcium aluminate cements";
UNITECR 1991.
/3/ Th. A. Bier, A. Mathieu, B. Espinosa and
J.P. Bayoux ; "The use of conductimetry to
characterize the reactivity of calcium
aluminate cements" ; UNITECR 1993.
/4/ J.P. Bayoux,
C.M. George
and
J.P.
Letourneux ; "Theory and practice of fume
silica - aluminous cement interactions" - Part
II.
/5/ A. Mathieu, A. Capmas, J.P. Bayoux and
D. Richon ; "Calcium aluminate cement and
reactive alumina" ; UNITECR 1993.
/6/ A. Mathieu ; "Calcium aluminates in self
flowing castables" ; Bresilian Ceramic Association Aguas de Lindoia, Brazil, June 1995.