S.Kaprielov, А.Sheynfeld, H.Kardumian, V.Dondukov
Characteristies of the structure and properties of highstrength concrete, containing multicomponent modifiers including silica fume, fly ash and metakaolin
Mass production of high-strength concrete in Russia involves using multicomponent
powder-like modifiers on organic-mineral basis of MB series (MB-01 and МБ-50С).
These are composite materials, the mineral part of which includes silica fume or a mix
of silica fume (SF) and fly ash (FA) in various proportion, and an organic part consisting of superplasticizer (SP) on the basis of naphthalene-formaldehyde polycondensate,
and setting retarder (SR) on the basis of phosphorus-organic product.
Previous research demonstrated that varying the proportions of above mentioned components and the dosages of the modifiers can lead to high-strength fine-grained concrete
with elastic modulus and creep comparable to that of heavy-weight granite aggregate
concrete of equal compressive strength grade [1]. However, obtained even though the
fine-grained concrete with improved E-modulus and creep value shrinks more relative
to a heavy-weight concrete.
This has initiated the development of a new modifier in which SF and FA are partially
or completely replaced by metakaolin (MK) and calcium sulfate. Having a property to
strengthen the structure of cement paste, the mixture of metakaolin and calcium sulfate
in fact acts as a sulfoaluminate type expansive compound (EC) due to the formation of
ettringite [2]. Naturally, this influences the cement paste structure, which differs from
that of a well investigated system, containing SF and FA, and has a number of distinct
features.
This provides an opportunity to control the shrinkage of concrete.
Experiment
The goal of the experiments was to determine a correlation between the amount of EC
in a modifier (and, consequently, of EC content in cement systems) and parameters of
cement paste structure and concrete properties.
Portland cement “PC500”, conforming to Russian Standard GOST 10178 quartz sand
with fineness modulus of 2.5, granite coarse aggregate with fractions of 5 to 20 mm
were used.
Three types of concrete modifier of different composition:
 with a mineral part consisting of SF and FA mix only, and an organic part, consisting of SF in amount of 6% of modifier total mass (MB 6-50C hereinafter in the
text);
 with a mineral part consisting of SF, FA and EC, and an organic part of SP in the
amount of 6% of modifiers total mass (Embelit 6-50 hereinafter in the text);
 with a mineral part consisting of EC only, and an organic part, consisting of SP in
the amount of 6% (Embelit 6-100 hereinafter) and 10% of modifiers total mass
(Embelit 10-100 hereinafter in the text).
2
Equal dosage (25% cement mass) of three different types of modifiers with mineral part
of various composition was added to cement systems. All modifiers contained the same
amount of superplasticizer. One of those included a mineral part consisting of SF and
FA only, and second and third included mineral parts consisting of 50 and 100% EC in
place of SF and FA, respectively.
The phase composition (balance between crystalline hydrates) and porosity of cement
paste, as well as concrete strength, modulus of elasticity, expansion-shrinkage and creep
were studied.
In addition the studies involved research of the influence of different dosages of one
modifier with maximum EC content on cement paste characteristics and concrete above
mentioned properties.
The water-cementitious material ratio (W/CM) of all cement paste samples was 0.22;
for the one of fine-grained and heavy-weight concrete samples the W/CM was 0.28.
The mixture proportions and properties of the concretes are presented in Table 1.
Test methods
Activity factors of modifiers (such as compressive strength and cone flow of the mixture, liner expansion, selfstress) were studied in accordance with to technical specifications.
Strength and cone flow factors were determined by comparison between samples made
out of mixture with 1:3 cement-to-sand ratio with equal water content, in which 10% of
cement was replaced by the different modifiers.
Linear expansion and self-stress factors were determined by comparison between samples made out from a 1:1 sand-to-cement ratio mixture, with equal water content, in
which 10% of cement was replaced with a modifier. The value of each factor is expressed as a percentage of the same factor value of the reference sample prepared without modifier.
Parameters of the cement paste structure were studied through a series of methods.
Porosity within the range from 1ּ10-³ up to 1ּ10³ m was determined using mutually
complementary methods of small angular X-ray refraction, nuclear magnetic resonance,
mercury porosimetry and optical method.
The degree of hydration of cement and the content of crystalline hydrates in cement
paste (phase composition) were studied by X-ray phase analysis (XPA) and differential
thermal analysis (DTA) and by scanning electron microscopy (SEM).
Properties of concrete mixtures and concrete were evaluated in accordance with Russian
standards.
All cement paste and concrete samples were cured at a temperature of 202 оС at different relative humidity (RH): 7 days at RH of 100% and from 7 to 120 days – at 60%.
Results
Structure of cement paste
In Table 2 and Fig.1а the cement paste porosity data and information on alteration of
strength and phase composition are presented.
The degree of cement hydration as well as cement paste strength is practically the same
in all samples: at 90 days these values are 61-64% and 131-135 МPа, accordingly.
Table 1
Characteristics of concrete mixtures
Mixture proportion, kg/m3
Modifier
№
1.
2.
3.
4.
5.
Brand
МB 6-50С
Embelit 6-50
Embelit 6-100
Embelit 10-100
Embelit 6-100
Components, %
SF+FA
EC
SP
Dosage,
% cement
94
47
0
0
0
0
47
94
90
94
6
6
6
10
6
25
25
25
12.5
25
cement
modifier
sand
c.aggregate
water
Slump,
cm
Entrained
air, %
W/CМ
622
615
623
687
500
156
153
154
87
125
1254
1250
1255
1246
585
1005
220
217
220
219
177
21
24
22
23
22
5.5
6.5
5.5
7.0
2.3
0.28
0.28
0.28
0.28
0.28
Table 2
Porosity of the cement pastes at 28 days
№
Brand
submicropores
(50Å<d0,1m)
micropores
(0.1 m <d20 m)
macropores
(20 м<d1000 m)
МB 6-50С
Embelit 6-50
Embelit 6-100
Embelit 10-100
14.20
16.56
18.46
18.20
1.50
1.38
1.25
1.61
5.60
2.50
3.00
2.00
31.80
33.55
37.73
33.71
Capillary pores, %
Components, %
SF+FA
1.
2.
3.
4.
94
47
0
0
EC
0
47
94
90
Technological pores,
%
Total porosity,
%
(10Å<d1000
m)
Modifier
SP
Dosage,
% cement
6
6
6
10
25
25
25
12.5
Gel pores
(10Å<d50Å)
10.50
13.11
15.02
11.90
Table 3
Concrete deformations characteristics
Modifier
№
Brand
Components, %
EC
1.
МB 6-50С
94
0
2.
Embelit 6-50
47
47
3.
Embelit 6-100
0
94
4.
Embelit 10-100
0
90
5.
Embelit 6-100
0
94
----------------------------------------------------* (-) – shrinkage; (+) – expansion
SF+FA
SP
Dosage,
% cement
6
6
6
10
6
25
25
25
12.5
25
E-modulus,
GPa
39
38
42
39
46
Poisson’s
ratio
0.18
0.18
0.19
0.18
0.20
Expansion-shrinkage*)
at … days, %
7
+0.012
+0.051
+0.102
+0.070
+0.102
28
-0.031
+0.007
+0.061
+0.010
+0.075
120
-0.039
+0.001
+0.060
+0.004
+0.075
Creep,
МПа-1×106
26.4
30.3
36.9
19.0
16.0
Self-stress at … days,
MPa
7
0.9
2.1
1.0
1.3
28
0.3
1.1
0.5
0.7
120
0.2
1.0
0.4
0.6
b) Portlandite content
7
140
Ca(OH) 2 , mass. %
Compressive strength, MPa
a) Compressive strength
150
90 days
130
120
28 days
110
0
50
EC content in mineral part, %
6
5
4
28 days
3
100
90 days
2
1
0
c) Relative C-S-H(I) content
50
EC content in mineral part, %
100
d) Relative ettringite content
1.25
Ettringite, arbit rary units
CSH (I) arbitrary units
90 days
1.0
90 days
0.75
28 days
2.0
1.5
28 days
1.0
0.5
0
50
EC content in mineral part, %
100
0
100
50
EC content in mineral part, %
Fig.1. Influence of EC content in modifier’s mineral part on compressive strength
and phase composition of cement paste
Increase of EC quantity in a modifier’s mineral part caused a significant increase in
portlandite concentration (Fig.1b), whereas the concentration of calcium silicate hydrates of С-S-Н(I) type decreased insignificantly (Fig.1c).
X-ray analysis results in all curing periods show the presence of ettringite. Calcium
monosulfoaluminate is not present. Basic reflection change kinetics of C S H and
C3A(C S )3H31 demonstrate that gypsum binding processes and formation of ettringite in
cement paste are interconnected and completed by the age of 3 days.
Increase in EC concentration in the modifier mineral part from 0 up to 100% increased
ettringite amount at 28 and 90 days by 1.3 to 2 times (Fig.1d). This is in agreement with
the following observation: decrease in Embelit dosage from 25% to 12.5% (accordingly
decrease in EC amount in cement system) causes insignificant decrease in ettringite
concentration.
Complex research of pore-size distribution in cement pastes demonstrated, that an addition of EC to modifier increased total porosity of cement paste (from 31.80% to
37.73%). Therefore the balance between gel (10Å<d50Å), capillary (50Å<d20 m)
and technological (20 m<d1000 m) pores was changed.
Increase of EC quantity in mineral part of modifier up to 100% (consequently EC increase in cement system) decreased macropores volume of technological origin and significantly increase gel pore and submicropore volumes (Table 2, samples 1, 2, 3). In
contrary, a decrease of modifier Embelit dosage twice and, consequently, a decrease in
the EC concentration in the cement system decreased gel pores volume (sample 4).
Strength and deformation properties of concrete
Concrete curing kinetics does not depend on EC quantity in a modifier mineral part. The
strength of different samples with MB-50C and Embelit in all curing periods does not
differ by more than 10%. All concrete samples at 28 days had approximately equal cubic and prism compressive strength: 98 to 105 МPа and 74 to 84 МPа, respectively.
Based on this parameter all concrete may be graded as В80 class.
The flexural and axial tensile strength of samples containing modifiers with and without
EC are differ in a very narrow range 5.3…6.4 МPа and 3.0…3.9 МPа, respectively.
Nevertheless, attention should be paid to the following: at 28 to 180 days a significant
increase of flexural strength (28…57%) and axial tensile strength (36…67%) is observed. This fact was finded against the background of less significant (not greater than
11%) increase in compressive strength occurred within the same period.
Variation of EC quantity in a modifier mineral part from 0 to 100% resulted in expansion of all samples instead of shrinkage (Table 3, mixtures 1 to 3) and practically did
not influence modulus of elasticity (38.0 to 42.0 GPа), Poisson’s ratio (0.18 to 0.19) and
creep (26.410-6 to 36.910-6 МPа-1).
A fine-grained concrete with МБ 6-50С without EC when added at 25% of cement mass
after 7 days of curing at RH=100% caused insignificant swelling. Thereafter at
RH=60% the concrete was fully compensated by shrinkage deformation at 120 days
(Fig.2 and 3а).
%
Expansion (+)
+0.12
+0.10
concrete with coarse aggregate containing EMBELIT6-100 (25%C)
+0.08
fine-grained concrete containing EMBELIT 6-100 (25%C
+0.06
+0.04
Shr inkage (-)
+0.02
0
-0.02
-0.04
fine-grained concrete
containing EMBELIT 10-100 (12.5%C)
fine-grained concrete
containing EMBELIT 6-50 (25%C)
fine-grained concrete containing MB 6-50C (25%C)
1 3
7
RH=100%
14
60
28
RH=6 0%
120
Age, days
Fig.2. Expansion-shrinkage of cement systems with modifiers containing EC
In contrast, fine-grained concrete with modifiers containing EC in the mineral part in
the amount of 50% (Embelit 6-50) and 100% (Embelit 10-100) after 7 days of curing at
RH=100% showed expansion deformation, equaled to 0.05% and 0.07%, respectively.
After curing in dry air conditions, residual expansion deformation at 120 days were
0.001% and 0.004%, respectively. These samples can therefore be classified as concrete with compensated shrinkage (Fig.2 and 3а).
Fine-grained concrete containing modifier Embelit 10-100 (with maximum concentration of EC) in the dosage of 25% cement weight after 7 days of curing at RH=100%
showed expansion deformation – 0.1%, which after curing in dry air conditions
(RH=60%) after 120 days were 0.06%. This means that this concrete could be classified
as expansive cement concrete (Fig.2 and 3а).
The self-stress value of samples of high-strength fine-grained concrete with modifiers
Embelit 6-50 and Embelit 6-100 after 7 days of water curing was 0.9 МPа and 2.1 МPа,
respectively. Studies of residual self-stressing of samples of fine-grained concrete with
modifiers Embelit with expansive compound showed that sample preservation under
dry air conditions (RH=60%) after 7 days of water curing (RH=100%) caused only partial loss in self-stressing which quickly stabilized at 30 day period and remained at levels of 0.3 МPа and 1.0 МPа (Fig.3b).
Comparison between fine-grained and heavy-weight concrete with modifier Embelit 6100 (maximum EC content) in the amount of 25% cement mass demonstrated, that at
practically the same modulus of elasticity (42,0…46,0 GPа), Poisson’s ratio
(0.19…0.20) and residual expansion deformation (0.060…0.075%), concrete with
coarse aggregate has lower creep (Table 3, samples 3 and 5) and self-stressing value
(Fig.3b).
Analysis of results obtained at this research stage demonstrates that using the modifier
Embelit, with EC in its mineral part, can lead to high-strength fine-grained concrete
with the same deformation parameters as a granite coarse aggregate heavy-weight concrete with similar compressive strength.
а)
b)
0.14
3
Expansion, %
0.12
0.10
0.06
0.04
2
Shrinkage, %
0.02
Selfstressig, MPа
1
0.08
2
1
1
2
0
-0 .02
-0 .04
0
50
100
EC content in modifiers mineral part, %
0
50
100
EC content in modifiers mineral part, %
Fig.3. Expansion-shrinkage (a) and self-stressing (b) of high-strength fine-grained
concrete vs EC quantity in mineral part of modifier
1 – curing at 7 days at RH=100%
2 – curing at 7 days at RH=100% and 113 days at RH=60%
Discusion
Addition of different amounts of metakaolin-based expansive compound to organic
mineral modifiers helps to compensate shrinkage or to obtain expansion instead of
shrinkage, whereas all other properties of concrete mixtures and concrete remain unchanged.
The base for obtaining the above mentioned properties is change of the balance between
crystalline hydrates of cement paste: concentration of microcrystalline ettringite and
portlandite doubles, low basic calcium silicate hydrates C-S-H(I) type insignificantly
(by 10-12%) decreases.
It was observed that there are variations in consistent patterns of changes in phase composition of high-strength cement paste with modifiers MB-50C and Embelit. If with
MB-50C, containing in mineral part SF and FA only the main factor responsible for
creating a high-strength structure is the increase in C-S-H(I) and decrease in portlandite
concentrations, than with addition of Embelit containing EC, mechanism of a high
strength structure formation is based on generation of number of types of ettringite and
low basic calcium silicate hydrates, and also on specific pore-size distribution.
An attempt will be made to discuss the correlation between concrete properties and cement paste structure parameters, using a well known structure classification based on
degree of dispersivity. Presence of large amount of dispersed tightly-bounded crystals
of calcium sulfoaluminate hydrates and hydrosilicates C-S-H(I) type on the surface of
unreacted minerals of cement causes an increase in gel pore volume by 3…5%, which
fall into a so-called overmolecular dispersity category (10 Å…50 Å).
One of the sulfoluminate hydrates varieties – a microcrystalline ettringite being a part of
gel pores has a high specific surface and bind large volumes of water, and therefore has
a greater expansion effect than large needle-shaped crystals of ettringite.
Most part of the hydration new formations, including larger crystals of ettringite belong
to another – submicroscopic dispersity level (0.05…0.1 m). To this level one may attribute submicrocapillaries that are predominantly responsible for gas and diffusion
permeability of cement paste. The volume of those capillaries increases by 2-4%.
The presence of sub-microcrystals show that in the cement paste with Embelit modifier
there are conditions for the development of various types of ettringite – finely dispersed
and in a form of larger needle-shaped crystals, which is the basis for stability of the
expansive cement system.
Dispersity of new growths predetermined similar low levels of microcapillars volume
(1.2…1.5%) as well as of structure defects such as microcracks, ranging between
0.1…20 m, affecting strength and water permeability of cement paste.
Decreased amount or absence of ultra fine particles of SF in mineral part of Embelit
modifiers causes a decrease in the viscosity of the system and results in a significant
decrease in technological defects and pores, ranging 20…1000 m, creating a macroscopic dispersity level of cement paste.
Observed changes of the cement paste structure are confirmed by the results obtained
during studies of strength and deformation parameters of fine-grained and heavy-weight
concrete with multicomponent organic-mineral modifiers.
Conclusions
1. A high-strength structure of cement paste with Embelit modifier, containing metakaolin-based expansive compound, has two main features: presence of different types
of ettringite tightly-bounded to low basic calcium hydrosilicates C-S-H(I) type, as well
as specific pore-size distribution character.
Ettringite is present mostly in the form of gel-like and microcrystalline new growths,
which being in dispersion form is one of the main factors providing stability of the system.
The porosity of cement paste is characterized by a decrease in macropore volume from
20 to 1000 m in size and an increase in gel pore and micropore volume of 10…1000 Å
in size.
2. Variation in the amount of expansive compound in the mineral part of modifier as
well as in the modifier dosage in the system, provides control of expansion-shrinkage
deformations to get high strength fine-grained concrete out of high-slump mixtures with
improved deformation properties: increased modulus of elasticity and decreased creep
factor, with compensated shrinkage or expansion.
3. Optimal dosage of Embelit modifier in terms of strength and deformation parameters
of concrete is 10 to 12% of cement mass.
4. Using complex organic mineral modifiers containing EC on the base of metakaolin
can lead to fine-grained concrete with deformation parameters, comparable to those of
heavy-weight granite coarse aggregate-based concrete with similar compressive strength
parameters.
References
1. Kaprielov S., Karpenko N., Sheinfeld А., Kouznetsov Е. Influence of multicomponent modifier containing silica fume, fly ash, superplasticizer and air-entraining agent
on structure and deformability of high-strength concrete. Seventh CANMET/ACI International Conference on Superplasticizers and обжег chemical admixtures in concrete.
Berlin, Germany, 2003, р.р.99-107.
2. Kardumian Н., Kaprielov S. Shrinkage Controlling of Self Compacting HighStrength Concrete. 15 Internationale Baustofftagung (IBAUSIL). Weimar, Deutschland,
2003. Band 2, рр.513-523.