ANNUAL TRANSACTIONS OF THE NORDIC RHEOLOGY SOCIETY, VOL. 17, 2009
ABSTRACT
Fast progress of present-day concrete
practice brings development of new
technologies applying new types of special
concrete even in heavy conditions. Self-
compacting concrete ranks among relatively
new types of concrete. This concrete is
characterized by very good physico-
mechanical properties as well as high level
of flowing property and long workability of
fresh concrete. Cement matrix always
contains certain amount of fine additions
and, to reach desired rheological properties,
it is modified by superplasticizers, mostly
polycarboxylate based. To be safe in
practical use, it is necessary to test
rheological properties of modified cement
matrix within the interval of at least 90
minutes in laboratory conditions.
The paper describes the research of
different dosage of meta kaolin used as
active addition and its influence on final
rheological properties of fresh cement paste
and resulting physico-mechanical properties
of hardened cement stone.
Meta kaolin has become a popular
partial replacement of cement in
construction industry in the last few years.
Meta kaolin has pozzolanic properties,
which is the reason of its positive effect on
final properties of concrete. Pozzolanic
properties cause reaction of active
components with calcium hydroxide and
formation of binding phases of following
types: C-S-H gel, C
4
AH
13
, C
3
AH
6
and
C
2
ASH
8
.
INTRODUCTION
Use of Metakaolin in construction
industry as partial replacement of cement
started in the 1960’s and the interest in this
material has considerably increased in
recent years. Metakaolin has pozzolanic
properties bringing positive effects on
resulting properties of concrete. Pozzolanic
properties cause chemical reaction of active
components with calcium hydroxide
(portlandite), which is formed as a product
of cement hydration. This reaction leads to
formation of binding phases of following
types: C-S-H gel, C
4
AH
13
, C
3
AH
6
, and
C
2
ASH
8
.
The paper deals with the research of
different dosage of metakaolin used as
active additive and its influence on
rheological properties of fresh cement paste
and resulting physico-mechanical properties
of hardened cement stone. Results of
experiments with cement suspension imply
similar behavior of input components in
concrete.
METAKAOLIN
Metakaolin is white, amorphous, highly
reactive aluminiumsilicate pozzolan forming
stabile hydrates after mixing with lime stone
in water and providing mortar with
hydraulic properties. Heating up of clay
with kaolinite Al
2
O
3
.2SiO
2
.2H
2
O as the
Study of Meta Kaolin Influence on Rheological Properties of Cement Mortars
Rudolf Hela
1
and Jana Maršálová
1
1
Brno University of Technology, Faculty of Civil Engineering, Institute of Technology of
Building Materials and Components, Veveri 95, 662 37 BRNO Czech Republic
basic mineral component to the temperature
of 500 °C - 600 °C causes loss of structural
water with the result of deformation of
crystalline structure of kaolinite and
formation of an unhydrated reactive form –
so-called metakaolinite.
Action of metakaolin in concrete
Metakaolin is usually added to concrete
in amount of 5 – 15% by weight of cement.
Addition of metakaolin causes increase of
mechanical strength, enhancement of long-
term strengths, decrease of permeability,
porosity, reduction of efflorescence,
increase of resistance to soluble chemicals
like sulphates, chlorides and acids.
Addition of metakaolin decreases
workability of fresh concrete mix. This
disadvantage can be reduced by
superplasticizers. However, rheological
properties of fresh concrete mix depend on
the type of superplasticizer. Generally,
polycarboxylate based superplasticizers
have better influence on workability than
poly naphthalene/melamine sulfonates.
Worse workability of concrete mix caused
by metakaolin can also be adjusted by
addition of fly ash.
Content of metakaolin in concrete
decreases permeability and rate of
penetration of damaging ions because of
refinement of structure of pores of cement
stone. Higher dose of metakaolin in concrete
increases the proportion of pores with
diameter up to 0.02 μm. At the same time,
the volume of capillary pores of the size
0.05 – 0.1 μm causing higher permeability,
is lower.
Appropriate use of metakaolin can
bring considerable increase of resulting
strengths, in particular in the initial stages of
hardening. Values of compressive strength
of concrete with metakaolin after 28 days
can be higher by 20%. Increase of resulting
strengths is caused in particular by
following aspects: immediate action of
metakaolin as filler, speeding up of cement
hydration during the first 24 hours and the
above-mentioned pozzolanic reaction.
Addition of metakaolin as partial
replacement of cement contributes to higher
compactness of arrangement of concrete
components, which increases flowability of
mastic cement, enhances mechanical bond
and improves adhesion between cement
paste and aggregate.
METHODS OF EXPERIMENTAL WORK
This paper deals with the research of
influence of metakaolin on rheological
properties of cement suspension.
Knowledge found in experiments can be
used for designing and production of
concrete with special properties.
Experiments tested influence of metakaolin
on rheological behavior of cement pastes
and resulting physico-mechanical
properties. Rheological properties of cement
paste were tested on the Rheotest apparatus.
Physico-mechanical properties, in particular
strengths after 48 hours and 28 days, were
tested after hardening of modified mixes.
Physico-mechanical properties and
chemical composition of metakaolin are
stated in Tables 1 and 2.
Six mix-designs were designed and
experimentally tested. Cement used was
CEM I 42.5 R, constant proportion.
Polycarboxylate based superplasticizer was
used in amount of 1.2% a 1.8% by weight of
cement. Amount of metakaolin was 5%,
10% and 15% by weight of cement. W/c
ratio was constant for all mix-designs
(0.395).
Table 1. Physico-mechanical properties of
metakaolin.
Properties Value
Mean size of particle D50 (μm)
3 - 5
Maximal grain size (mm) 1,0
Specific activity 226Ra (Bq/kg) 100
Table 2. Chemical composition of metakaolin.
Chemical composition (%)
SiO
2
Al
2
O
3
Fe
2
O
3
MgO TiO
2
K
2
O + Na
2
O
CaO Loss by
annealing
51-53 40-42 1,2-1,4 0,30-0,45 0,80 1,35-1,45 0,25-0,30 3,5
Table 3. Average values of viscosity of cement paste with CEM I 42.5 R with addition of
metakaolin and admixture.
Superplasticizer - 1,2% Superplasticizer - 1,8%
viscosity (Pa.s) viscosity (Pa.s)
Additive
Dosage
0
min
30
min
60
min
90
min
120
min
0 min 30
min
60
min
90
min
120
min
5% 804,9 157,8 134,1 126,2 102,6 489,2 189,4 134,1 110,5 86,8
10% 1041,5 292,0 244,6 228,8 228,8 962,7 268,3 205,2 173,6 149,9
Metakaolin
15% 1468,0 552,4 315,6 465,6 599,7 946,9 449,8 355,1 323,5 347,2
Table 4. Compressive strength of cement paste – Cement CEM I 42.5 R, metakaolin,
additive.
Reference mix-design
cement + water (w = 0,395)
Superplasticizer - 1,2% Superplasticizer - 1,8%
Compressive strength
R
b
[MPa]
Compressive strength
R
b
[MPa]
Compressive strength
R
b
[MPa]
48 hours 28 days
Additive
Dosage
48 hours 28 days 48 hours 28 days
5% 29,9 38,9 30,2 43,1
10% 33,1 44,3 33,7 45,4
31,0 43,4
Metakaoli
n
15% 34,2 49,3 35,1 50,0
TEST RESULTS
Measured values of viscosity of tested
mix-designs are stated in Table 3 and
graphically represented in Fig. 1.
Fig. 1 shows decrease of viscosity in
time. This phenomenon is apparent in
particular for suspension with metakaolin in
amount of 5% and 10%. Curves
representing development of viscosity in
time from 0 to 120 min were similarly
decreasing for all mix-designs with addition
of tested additive in amount of less then or
equal to 10%. Curve of viscosity of cement
suspension with metakaolin in amount of
15% showed increase at the end of tested
period.
Figure 1. Development of viscosity in time.
Compressive strengths after 48 hours
and 28 days of standardized maturing were
determined for all tested mix-designs.
Values of compressive strengths are stated
in Table 4 and Fig. 2.
Figure 2. Development of compressive
strength after 48 hours, 28 days.
Fig. 2 shows values of compressive
strength of samples 48 hours and 28 days
old. The results of measurement prove that
compressive strength considerably increases
even after 48 hours. The value of
compressive strength grew proportionally to
amount of metakaolin and superplasticizer.
Compressive strength of mix with
metakaolin after 28 days was higher by 10%
to 15% compared to reference samples.
CONCLUSIONS
This paper describes influence of
metakaolin used as partial replacement of
cement on behavior of cement based
suspense – rheological properties of fresh
mix and strength characteristics of cement
stone. Knowledge found by research of
modified cement paste imply behavior of
fresh and hardened concrete. On the basis of
measurements it can be concluded that:
• Higher amount of superplasticizer
increases workability of fresh mix.
Higher addition of metakaolin also
enhances workability. Dosage of 15% of
metakaolin causes decrease of
workability of suspension in time.
Increasing amount of percentual
proportion of metakaolin in concrete
mix seems to require higher dosage of
superplasticizer to ensure longer period
of workability.
• Addition of metakaolin increases also
final strength of cement stone.
Compressive strength was growing with
higher dosage of additive. Since the
amount of 15% metakaolin results in
loss of viscosity in time, it seems
appropriate to use dosage of 10% by
volume of cement.
ACKNOWLEDGEMENTS
This paper was supported by project
VV CEZ MSMT 0021630511, DT2, A
MPO CR Impuls FI-IM5/016.
REFERENCES
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superplasticizers and raw admixtures”,
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