Synergistic Action of a Ternary System of
Portland Cement – Limestone – Silica Fume in
Concrete
J. Zelić, D. Jozić, and D. Krpan-Lisica
*
Abstract. Some experimental investigations on a synergistic action when a ternary system of Portland cement – silica fume – limestone is used in mortar or concrete are present in this paper. Standard laboratory tests with respect to the pore
size distribution, micromorphology, compressive strength and sulphate resistance
in both sodium and magnesium sulphate solutions were performed on mortars
made with 70% (by mass) of Portland cement (PC), type CEM II/B-S and 30% of
cement replacement materials consisted of various combination of fine ground
limestone filler (LF) and silica fume (SF). In addition to these ternary systems, binary blends, such as: PC-LF, as well as PC-SF, along with 100 % PC mortars,
were investigated for comparison. It is found that SF-blends reach higher compressive strengths than LF-blends for the same replacement of cement. When SF
was added together with LF, the mortars show considerable increase in the compressive strength and show a lower expansion than a control, sulphate-resisting
mortar, independent of the type of sulphate solutions, due to pore size refinement
microstructure of mortars.
1 Introduction
Concrete is a porous material with many different kinds of pores, ranging from the
air voids to the nanometre-scale pores produced by the cement-water chemical
reaction. Since, these nanoscale pores control the properties of the calciumsilicate-hydrates hydration products (C-S-H), which is the main "glue" that holds
concrete together, concrete is in some ways a nanoscale materials [1]. According
to literature data [2], the addition to concrete mixes of silica fume (also known as
condensed silica fume or microsilica), a by-product of the production of metallic
silicon or ferrosilicon alloys, with mean particles size over 100 nm, results in
high-performance concrete, in terms of better chemical resistance, higher strength
or better durability. The beneficial effects of adding silica fume (SF) are due to the
filling action of its fine particles in the pores and the formation of an additional
quantity of C-S-H by pozzolanic reaction between the SF and calcium hydroxide
J. Zelić, D. Jozić, and D. Krpan-Lisica*
Faculty of Chemical Technology, University of Split, Split, Croatia
e-mail: [email protected]
426
J. Zelić et al.
formed during cement hydration. Recently, a new pathway based on the addition
of the colloidal silica nanoparticles (nano-SiO2) to Portland cement has been explored. The research confirmed that the silica nanoparticles addition to Portland
cement has provided notably increase both the mechanical properties of cementbased materials [3,4], and the durability against the calcium leaching [5], respectively. Limestone is used in cement and concrete for various purposes, namely as a
raw material for clinker production and as coarse or fine aggregate. For long-time,
ground limestone has been considered as inert filler. During the last decades, however, studies have pointed that limestone filler (LF) addition to Portland cement
(PC) produces several effect on mechanism and kinetics of cement hydration;
thus, LF addition increases the hydration rate of PC, especially of C3S, at the early
ages, improves the particle packing of the cementitious system, provides new nucleation sites for calcium hydroxide, and produces the formation of calcium carboaluminates as results of the reaction between CaCO3 from limestone and C3A
from Portland clinker [6]. Today, modern cements often incorporate several mineral admixtures one of which is LF. European standard EN 197 identifies two
types of Portland limestone cements (PLC): Type II/A-L and Type II/B-L, containing 6-20% and 21-35% LF, respectively.
The aim of this paper was to investigate a synergistic action when ternary
system PC-SF-LF used in concrete. More specifically, an answer was sought for
the question of whether a suitable combination of LF and SF would improve the
properties of hardened concrete more than these materials would separately. It is
anticipated that the results and conclusions obtained here on mortars will be transferable to concretes [7].
2 Experimental
2.1 Materials
Commercial blended PC, Type CEM II/B-S 42.5N (supplied by Dalmacijacement,
Croatia), high purity LF (supplied by Konstruktor, Croatia) and SF (supplied by
Elkem Co., Norway) were used. The sulphate-resisting PC (supplied by Dalmacijacement, Croatia), Type CEM I-HS, was used as the control cement for the sulphate-resistance tests. Chemical composition and physical properties of the used
materials are given in Table1. Particle size distributions of the blended PC, LF and
SF were determined using the Coulter Counter method (methanol-LiCl solution),
as shown in Fig. 1. Salts MgSO4 x 7H2O, p.a., and Na2SO4 x 10H2O, p.a., were
used in preparation of sulphate solutions. The sulphate ion concentration in both
solutions was 2.5% and was kept constant during sulphate-resistance test.
Mortar mixes. Two kinds of mortar samples were prepared: (I) for mechanical
strength tests –prismatic specimens dimensions 40 x 40 x 160 mm, according to
HRN EN 196-1:2005; and (II) for the sulphate-resistance tests – prismatic specimens dimensions 25 x 25 x 350 mm, with two stainless-steel inserts cast into the
ends to facilitate accurate monitoring of the changes in length, according to the
ASTM C 452. Mix proportion and designation of mortars prepared are given in
Table 2. A different water-to-cementitious material ratio (w/cm) was used in each
Synergistic Action of a Ternary System of Portland Cement
427
Table 1 Chemical and physical properties of materials used
a
Cement I
b
Silica fume
Limestone
Cement II
89.68
1.15
1.58
1.40
1.45
0.17
2.99
1.83
0.66
0.32
0.20
53.45
0.64
43.23
-
21.61
3.71
4.49
62.94
1.88
1.73
0.76
0.80
-
-
-
55.65
19.98
2.23(max. 3.5)
13.64
-
2.26
2.70
3.26
4,200
13,000
3,021
-
Chemical composition, mass %:
SiO2
Al2O3
Fe2O3
CaO
MgO
SO3
Total Alkalis
Loss on Ignition
Carbon
21.03
6.56
2.68
58.10
2.57
2.54
1.02
1.04
-
Composition (Bogue), mass%:
C3S
C2S
C3A
C4AF
CaO (free)
62.10
17.60
11.20
8.20
0.90
Finenes:
3
Density, g/cm
3.18
2
Specific surface area (SSA), cm /g
Blaine
3,530
BET
11,000
180,000
a
Commercial Portland cement Type CEM II/B-S 42.5N
b
The sulphate-resisting Portland cement Type CEM I-HS
case in order to keep the mixture's fluidity constant. No superplasticizer was
added. After mixing, the cement mortars were cast into prismatic molds and compacted by vibration. The specimens were demolded after of 24 h at 90% relative
o
humidity and were then cured in water at 20 C until testing.
Table 2 Mix proportion and designation of mortars prepared for the study
Designation
Composition (mass%)
Cement I
Silica fume
Limestone
Cement II
w/cm
PO
PL15
PL15S8
PL15S15
PS8
PS15
a
SPC
100
85
77
70
92
85
0
0
15
15
15
0
0
0
0
0
0
0
0
0
100
0.57
0.57
0.61
0.65
0.60
0.65
0.53
0
0
8
15
8
15
0
428
J. Zelić et al.
100
Cumulative oversize, %
90
80
SF
70
PC
60
LF
50
40
30
20
10
0
0
10
20
30
40
50
60
70
Size, m icrom eter
Fig. 1 Particle size distribution of PC, SF and LF used
2.2 Test Methods
Mechanical properties. The mechanical properties of produced blend mortars (I
series) were determined by compressive strength measurements at ages of 3, 7, 14,
28, 90 and 120 days. The strength value was the average of three specimens.
Sulphate resistance. After 28-days curing in tap water, each blended mortars and
the control mortars based on the sulphate-resisting cement (II series) were individually immersed in a plastic tank containing both a sodium and magnesium sulphate solution, respectively. At 30, 60, 90, 120, 150, and 180 days of sulphate
immersion, the specimens were tested to determine the expansion of mortars.
The structure and particle shape of silica fume were identified using X-ray diffraction (RXD Philips PW 1010, with graphite monochromator) and transmission
electron microscopy (TEM Philips EM 301), respectively. The morphologies of
the hydration products were studied by using a scanning electron microscopy
(SEM Leitz AMR 1600T). At certain curing ages, mortars were crushed and
treated with acetone to stop the hydration. The fresh broken surface of mortars
was gold coated. The pore structure, both the pore volume and pore size distribution, in the 28-day-old hydrated mortars were determined by a mercury intrusion
porosimetry (MIP Carlo Erba series 200). The volume of mercury intrusion at the
maximum pressure was considered to be the total porosity. It is important to note
that only connected capillary pores can be reached by MIP.
3 Results and Discussion
The particle size distributions, physical and chemical properties of material used
are present in Fig. 1 and Table 1, respectively. The particles sharp and the amorphous nature of the SF used are shown in Fig. 2 (a) - (b), respectively. The results
Synergistic Action of a Ternary System of Portland Cement
429
reveal that SF, in form supplied, contains 89.68 mass% of silica, and consists of
amorphous and extremely fine spherical particles. Fig. 2 (a) also shows that SF exists almost completely in the form of fine spheres linked together into chains of
clusters (the particle agglomerate), rather than isolated spheres. The particles
range from fraction of 2 to 15 µm (Fig. 1), although silica fume has high nitrogen
BET SSA (Table 1). It is obviously that the reason for this phenomenon is that nitrogen can penetrate into space of original SF particle inside the agglomeration;
thus, the Coulter Counter method measures the agglomeration size while nitrogen
measures the original size [8].
Fig. 2 (a) TEM micrograph (magnification 130,000 x) and (b) X-ray diffraction curve of
the SF used
The derivate plots of cumulative pore volume of dV / d ln r versus pore radius, r for 28-day-old mortars are present in Fig. 3. At a 15 mass% replacement
of PC by the equal mass of LF, the derivate plot shows discontinuous pore structure with the first maximum of occurrence of pores radii at 0.02 µm, and second
one at 0.03 µm, respectively. As both PC and LF show similar characteristics (the
BET SAA, Table 1), their different behavior perhaps indicates the LF activity during cement hydration, which is responsible for the development of pore structure
with a pore size distribution covering many orders of magnitude. For mortars containing SF together with LF, a larger number of small pores is seen than in analogous mortar samples containing no LF. A discontinuous pore structure with two
derivate maximums; the first maximum at a pore radii of about 0.015 µm and the
second one, between 0.025-0.030 µm, are also perceptible for the binary PC-SF
(with 8 mass% of SF) and ternary the PC – SF – LF blends, respectively. According to Feldman [9], a discontinuous pore structure will result in a lower permeability, and better resistance to sulphate attack.
Figure 4 shows the total porosity of the 28-day-old mortars measured by MIP.
Comparing the porosity of the control PC mortar, all mortar samples show an increase of the total porosity with higher w/cm ratios. When SF in the amount of 8
and 15 mass% is added to the mortars containing LF the total porosity of mortars
first decreases (the PL15S8), and then the porosity increases (the PL15S15), respectively.
430
J. Zelić et al.
7000
PL15
6000
PO
PL15S8
5000
PS8
dV/dlnr
PL15S15
4000
PS15
3000
2000
1000
0
0,005
0,015
0,025
0,035
0,045
Pore radius, m icrom eter
Fig. 3 The derivate plots pore volume vs. pore radius
T otal pore volum e, mm 3/ g
60
50
40
Vp
30
20
10
0
PO
PS8
PS15
PL15
PL15S8
PL15S15
Sam ples
Fig. 4 Total pore volume in the 28-day-old mortars
Figure 5 presents the compressive strength of mortars for ages of 3, 7, 28, 90
and 120 days, respectively. The highest compressive strength shows PC mortar
containing 8 mass% of SF. The SF addition of 15 mass% increases the w/cm ratio
(Table 2) and, therefore, decreases the mortar strength due to the increase porosity
(Fig. 4). For the same replacement of PC, SF gives higher strength than LF. Added
Synergistic Action of a Ternary System of Portland Cement
431
70
Compressive strength, MPa
60
50
40
30
PO
PS8
PS15
PL15
PL15S8
PL15S15
20
10
0
0
50
100
150
Time, days
Fig. 5 Compressive strength developments of mortars studied
together with LF, SF increases the compressive strength, although the strength is
lower than that of mortars containing no LF. It appears that certain LF-SF combination can improve the strength of mortars more than LF alone.
Figure 6 (a)-(b) shows the SEM micrographs of the fracture surface of the 28day-old mortars containing SF alone (Fig. 6a), and SF together with LF (Fig. 6b).
The recticular network formation of C-S-H layer precipitated on the surface of silica grains and the calcium hydroxide hexagonal plates is detected for the PS8
sample (Fig. 6a). The mortar containing together 15 mass% of LF and 8 mass% of
SF (the PL15S8) does not exhibit recticular network features and shows occasional evidence of hexagonal plates (Fig. 6b). A distinct different between this
sample and the sample not containing LF (the PS8) is that its surface seems to be
consolidated and uniform, formed as an interconnected network.
Figures 7 and 8 show the length changes of mortars immersion in both sodium
(N) and magnesium (M) sulphate solutions for 30, 60, 120, 150, and 180 days, respectively. By comparing the expansion values of mortars exposed to the attack of
both sulphate solution, it can be observed that specimens not containing SF have
the higher initial expansion after 30 days of sulphate immersion, and the intensive
trend of breakdown; the P0 samples were disintegrated after only 90 days, while
the PL broke down after 150 days.
Mortars containing SF show better sulphate resistance, although they have larger initial total porosity associated with the higher w/cm ratios. These results indicate that pore size refinement caused by pozzolanic reaction has a beneficial effect
on the sulphate resistance. Other authors [10,11] came to similar conclusions. The
presence of brucite, Mg(OH)2 (which suggests a rapid attack by magnesium sulfate solution), detected in the SF-mortars after 120 days of immersion, had no
432
J. Zelić et al.
(a)
(b)
Fig. 6 SEM micrographs of the fracture surface of the 28-day-old PC mortars: (a) the PS8
sample, and (b) the PL15S8 sample
0,05
Expansion, %
0,04
30d
60d
0,03
90d
120d
0,02
150d
180d
0,01
)
(N
C
SP
N
)
PL
15
(N
)
PL
15
S8
N
L1
5S
15
N
PS
15
(
8(
N
)
PS
PO
(N
)
0
Age of sulphate im m ersion, days
Fig. 7 Length changes of mortars immersion in sodium sulphate solution
0,05
Expansion, %
0,04
30d
60d
0,03
90d
120d
0,02
150d
180d
0,01
C
(M
)
SP
PL
15
(M
)
PL
15
S8
M
L1
5S
15
M
15
(M
)
PS
)
8(
M
PS
PO
(M
)
0
Sam ples
Fig. 8 Length of mortar bars on immersion in magnesium sulphate solution
Synergistic Action of a Ternary System of Portland Cement
433
great effect on the reduction of compressive strengths [11]. It is obvious that the
effect of the pozzolanic reaction (reduction in permeability and refinement of the
pore structure) overcame this negative effect of the sulfate attack. The smallest
expansion, even smaller then controls mortars on the basis of sulphate-resisting
cement (SPC), independent of type of sulfate solution, is shown by the samples
containing 15 mass% of SF, and the SF-LF blends. Results from this work show
that the LF activity during cement hydration [6] in combination with the pore refinement caused by the pozzolanic activity of the SF addition may increase the
sulphate resistance of blended PC mortars or concrete.
4 Conclusions
The highest compressive strength is shown by PC mortar containing 8 mass% of
SF. The replacement of PC by 15 mass% of LF causes a significant reduction in
the compressive strength. Blending SF and LF simultaneously with PC is most effective on strength development, especially in the later ages, compared to a PC
alone. Both SF and LF additions used alone or in combination with PC, modify
the PC mortar microstructure. The 28-day-old PC-SF-LF mortars show a discontinuous structure with two derivate maximums: the first maximum at pore radii of
about 0.015 µm and the second one, between 0.025-0.030 µm, respectively. These
mortars are also characterised by a good sulphate resistance and show lower expansion than a control, sulphate-resistant mortar, regardless of the type of sulphate
solutions, due to refinement pore structure.
Acknowledgments. The authors would like to acknowledge the Commissioners of the
European Union for funding under the REINTRO Project (the 5FP), No. ICA2-CT-200210003.
References
1. Chong, K.P., Garboczi, E.J.: Smart and designer structural material systems. Prog.
Struct. Engng. Mater. 4, 417–430 (2002)
2. Malhotra, P.K., Monteiro, P.J.: Concrete, microstructure, properties and materials.
McGraw-Hill, New York (2006)
3. Li, G.: Properties of high-volume fly ash concrete incorporating nano-SiO2. Cem.
Concr. Res. 34, 1043–1049 (2004)
4. Dolado, J.S., Campillo, I., Erkizia, E., Ibáñez, J.A., Porro, A., Guerrero, A., Goñi, S.:
Effect of nanosilica additions on belite cement pastes held in sulfate solutions. J. Am.
Ceram. Soc. 90, 3972–3976 (2007)
5. Gaitero, J.J., Campillo, I., Guerrero, A.: Reduction of the calcium-leaching rate of cement paste by addition of silica nanoparticles. Cem. Concr. Res. 38, 1112–1118 (2008)
6. Irassar, E.F., González, M., Rahhal, V.: Sulphate resistance of type V cements with
limestone filler and natural pozzolana. Cem. Concr. Compos. 22, 361–368 (2000)
7. Popovics, S.: Portland cement - fly ash - silica fume systems in concrete. Adv. Cem.
Bas. Mat. 1, 83–91 (1993)
8. Cook, R.A., Hover, K.C.: Mercury porosimetry of hardened cement pastes. Cem.
Concr. Res. 29, 933–943 (1999)
434
J. Zelić et al.
9. Feldman, R.F.: Pore structure damage in blended cements caused by mercury intrusion. J. Am. Ceram. Soc. 67, 30–33 (1984)
10. Sideris, K.K., Savva, A.E., Papayianni, J.: Sulfate resistance and carbonation of plain
and blended cements. Cem. Concr. Compos. 28, 47–56 (2006)
11. Zelić, J., Radovanović, I., Jozić, D.: The effect of silica fume additions on the durability of Portland cement mortars exposed to magnesium sulfate attack. Mater.
Tehnol. 41, 91–94 (2007)