understanding the powdered silica fume
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a research on silica fumeTRANSCRIPT
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Understanding the Powdered Silica Fume
Syed Ali Rizwan1, Gert Schmidt2 and Thomas A. Bier2
Abstract
The powdered silica fume has been used quite often in the developed world for making high
performance concrete (HPC) and self-compacting concrete (SCC). However there is a trend in
using silica fume to achieve higher concrete strength in developing countries without knowing its
complete characteristics, mix proportions, aggregate grading and environmental conditions
during placements.
Silica fume seems to be an interesting pozzolanic mineral admixture known for improving the
concrete microstructure and its anti-bleed properties. However the underlying mechanism is still
not very clear. An attempt is made to provide some information regarding this characteristic of
powdered silica fume. It appears that the role of carbon particles and that of agglomerated silica
fume particles may be the underlying mechanism regarding the anti-bleed properties of silica
fume.
1. NUST Institute of Civil Engineering (NICE), National University of Sciences &
Technology (NUST), Islamabad, Pakistan. [email protected], Member ACI
Committees 237, 234 and 232.
2. IKGB, Technical University Freiberg, Germany. [email protected],
Key Words: Silica fume, high-performance concrete, mercury intrusion porosimetry,
Superplasticizer and shear rate.
Introduction
The modern concrete systems including HPC and SCC usually possess high powder content with
low w/cm ratio. In order to make them environment friendly and to avoid related problems,
usually a part of cement is replaced with secondary raw materials (SRMs) and especially with
silica fume (SF) which seems quite popular amongst construction technologists especially in
developing countries. The addition of SF increases the rate of early hydration due to release of
OH- ions and alkalis into pore water fluid which provides ability to provide nucleation sites to
products of hydration sites for CSH and ettringite(1). SF increases hydration of C3S and C3A in
early hours of hydration (1). SF tends to affect the pattern of crystallization and degree of
orientation of CH crystals during first few days of cement hydration (1). Rao (1) states that at
30% SF addition false setting of cement paste was observed. Highly crystalline portlandite (CH),
and amorphous CSH are formed in the hydration of Portland cement(PC) and the hydrated
cement paste consists of approximately 70%CSH, 20% CH , 7% sulphoaluminates and 3%
secondary phases (2). Cement paste containing SF produces amorphous CSH gel with high
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density and low Ca/Si ratio. (2). It has been reported that in a mixture of 70%PC and 30% SF ,
CH disappeared entirely(2). CH also decreases when SF and FA are used together (2). Silica
Fume , because of its extreme fineness, penetrates between cement grains and decreases water-
cement ratio in the given volume(3).There seem to be co-relations between some secondary
constituents of silica fumes(C, Al2O3, Fe2O3, MgO and Alkalis) , the contents of which increase
as the quantity of silica (SiO2) and after analysis it can be found that only carbon and alkalis are
the important components determining the strength development (3) which proposes the
following relations for two different series.
Fc = 173.8 -2.11C (1)
Fc= 194.7 2.81 C (2)
Where Fc (kN) is the mean compressive force at failure for 40x40 mm specimens and C is the
percentage of carbon in the silica fume. For the flow time increases with increase in the Carbon
content and density decreases with the increase in flow time (3). Literature states that a reduced
porosity and the presence of many more hollow shell pores (1-15 microns) are present during
early hydration(but persist later on as well) of cement with SF due to cement grain dissolution in
the gel when SF replaces a part of cement. These pores appear to be connected to continuous
capillary pore system by means of smaller gel pores (4). In order to utilize SF effectively in
concrete, its characteristics must be known in order to avoid running into a very big mess later on
during and after construction. There may be problems related to temperature rise, Autogenous
shrinkage and possible early cracking. It has been found (6) that the Autogenous shrinkage in
concrete with low water-cement ratio is the major factor for early age cracking. SRMs and
especially SF have a significant impact on the ability of concrete to resist the penetration of
chloride ions, resisting alkali-silica reaction and other deleterious actions which are related
mostly to the quality of microstructure (7)
The above discussions force the reader to think about the following questions.
1. Why/how silica fume acts as anti-bleed SRM?
2. Which constituents of silica fume may be responsible?
3. Why there is intense heat liberation in the calorimetric investigations during the early
hydration stages?
4. What is the role of silica fume on the fresh and hardened properties of concrete and
especially HPC/SCC?
It may be possible that the role of carbon and that of agglomerated silica fume particles is
responsible for this phenomenon. Carbon can always be found in the silica fume. The origin of
carbon comes from the production process shown in Fig.1
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Fig 1. Manufacture Process of Silica Fume.
Carbon Content in Silica Fume
Different types of carbon containing materials perform as reducing agents (coal, brown coal,
wood charcoal etc.). The carbon particles are much bigger than the silica fume particles (see figs.
2(a) to 2(E) and are irregular in shape and these seem to contain a morphology which seems to
encourage the intake of fluids. The carbon not only influences the color of SF, but also its
content, size and the origin of its particles seem to have an effect on the properties of silica fume.
Also a nearly white silica fume contains certain content of carbon which seems to influence the
surface reactivity of the silica fume (10, 11).The carbon content of the silica fume is an indicator
of the state of aggregation of the grains, with a high carbon content showing a coarser granularity
and the grading governing the flow time (depends on air content as well) of mortar. It may be
possible that such carbon particles accommodate mercury/water during MIP and mixing
respectively. During setting process, alkalis released by cement and silica fume control
hardening kinetics of the mixture and high alkalis favor cement hydration while opposite may be
expected for SF hydration. In nut shell, the carbon content of a silica fume, shown by its color, is
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a good indicator of its quality i.e. low carbon content imparts a light grey color in SF particles
and translates into more fluidity and higher strengths (3). Study on Pure SRMs indicated that
significant amount of mercury gets intruded in the SF sample as evident from the MIP
measurements (5, 8).
It is known to materials engineers that 95% of SF particles have a size less than 1 micron, a bulk
density in the range of 130-430 kg/m3 for as produced micro silica, a specific gravity of around
2.2 and BET area of 13000-30,000 m2/kg. SF usually accelerates the C3S, C2S and C4AF
hydrations while it reduces bleeding and segregation, generates heat of hydration even in the
replacement mode (optimal replacement of 10%).It may be kept in mind that HPC /SCC
containing SF suffer higher strength losses due to temperature. SF decreases thermal
conductivity and creep strains while it increases plastic and Autogenous shrinkages.
Experimental
SEM and MIP techniques were used to investigate the characteristics of silica fume particles.
Table A in appendix shows the properties of a typical silica fume. It had LOI of 1.2%, free
Carbon 0.6%, pH of 7.5 primary particles 0.1-0.3 microns with secondary agglomerates of size
greater than micron were 30%, greater than 10 microns of 5% and greater than 45 microns of
1.5%. The particle size distribution and properties vary but only slightly for different types of
silica fume grades like 971 U, 920D and 968 U of Elkem.
The Characterization of carbon particles in SF
Fig.2 (a) Carbon Particle in Silica Fume Fig. 2(b) Another type of Carbon Particle in
Silica Fume
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Fig.2 (C) Another Type of carbon Particle in
silica fume.
Fig 2 (D) Another form of carbon particle in
silica fume.
Fig. 2 (E) Another form of carbon particle in silica fume
Fi.2 Different types, shapes and surface morphologies of carbon particles found in Silica Fume
To look into the anti-bleed properties and anti-segregation properties of SF, it was thought to
perform MIP test on various powders including SF. Fig 3(a) shows the partial MIP diagram
while Fig. 3(b) shows the cumulative diagram of MIP on powders. Fig.3(c) shows the SF particle
morphology obtained using SEM technique. When doing SF particle characterization by Laser
granulometry, the average size (D50) of SF particles determined (around 6-14 microns) is that of
the primary group of fused SF particles shown in Fig.4 and not that of single particle which is in
nanometers.
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Fig.3(a) Partial Diagram of
SRMs (Rizwan 2006)
Fig.3(b)Cumulative Diagram
of SRMs (Rizwan 2006)
Fig.3(c)Silica Fume (Rizwan
2006)
Fig 3(a) and 3 (b) are interesting and form the basis of thinking process. In fig 3(a), at 57.94 nm
average pore radius of SF particles, about 5.507 cc/g of mercury is intruded. The question is
where such an amount of mercury is accommodated in SF particles? Then in Fig 3(b) mercury
intrusion in SF particles starts at 2374.9 nm where 6.65% mercury is intruded. Then almost
smooth mercury intrusion takes place up to SF particle size of 69.94 nm wherein 66.41%
mercury has intruded. This smooth intrusion may be due to some kind of connectivity between
particles, due to small gaps and in the carbon particles in the primary group of SF particles. Fig.
4 shows TEM pictures of the primary SF group.
Fig.4 TEM image of silica fume primary group of particles ( See their fusion and connection)
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Thereafter comes the rising part in Fig 3(b) indicating connectivity due to possible breaking of
small sized SF particles near applied pressures of about 350 MPA plu. This part terminates at
43.9 nm particle size wherein about 89.5% mercury is intruded and just after that it appears that
due to excessive applied pressure during MIP, the SF particles start breaking resulting into
almost entire per cent mercury intrusion. It is obvious from Fig.4 that it is almost impossible to
separate the SF particles in the primary group even after the application of high shear rates and
therefore the particle size (D50) of SF as given by the Laser granulometry is not that of single SF
particle but it is of the primary group of agglomerated SF particles. The actual idea of the particle
size may best be obtained either by SEM or TEM process. There is some percent of carbon in
almost all SF samples and this is given in the technical data sheet of the manufacturer and is
annexed in this paper. Carbon particles are irregular in shape and quite massive in size.
Table 1 gives the physical and chemical properties of powders.
Table No 1. Physical and Chemical Composition of Powders
Parameters CEM I 42.5 R SF
Specific gravity
Particle size (d50),m BET surface Areas, m2/g
Chemical Analysis
Loss on ignition
Silicon Dioxide
Aluminum Oxide (wt, %)
Ferric Oxide
Calcium Oxide
Magnesium Oxide
Sulfur Trioxide
Sodium Oxide
Potassium Oxide
3.1552
18.42
1.098
2.75
19.17
5.21
2.39
61.12
2.78
3.30
1.25
1.01
2.3560
12.16
20.457
1.6
95%
0.2
0.05
0.25
0.4
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0.1
1.2
It can be seen that the Table 1 indicates the size of SF as 12.16 microns which the size of
inseparable SF particles in the primary group.
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Fig 5(a) TEM picture showing SF Powder-
showing circular particles of varying sizes.
Fig 5(b) TEM picture showing SF powder-
Primary groups are seen
Figures 5(a) and (b) are TEM representations of SF particles. Several properties of SF including
the particle size, their connectivity and the morphology can be seen. These figures indicate that
small particles tend to stick to a relatively larger particle within the primary group. The particles
are round and of smooth surface.
Fig.6(a) Shrinkage Response of SCP mixes. Fig 6(b) Calorimetric Response of SCP mixes.
Fig. 6(a) shows the early shrinkage response of two SCP formulations. It can be seen that cement
paste containing 10% replacement of SF shows much faster and higher shrinkage than the pure
self-compacting paste at almost similar Vicat setting times. This shrinkage is thought to be
brought about by the consumption and uptake of water by the SF particles indicating that water is
being held somewhere or is being consumed at a faster pace in the cement formulation
containing SF. This may be due to reduced effective water-cement ratio which decreases the
distance between cement grains. This idea is strengthened in the fig. 6(b) which shows the
Calorimetry curves of self-compacting paste systems (SCP). It can be seen that SCP system
containing 10% replacement SF shows much higher and earlier peak with reduced dormant
Interconnection
between SF
Particles
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period. The dots on curves are Vicat final setting times of respective formulations. Such a setting
time (time-zero) sits either at the peak or start of acceleration period. Such systems were tested
for flow, strength and microstructure and the details are available in (8).However Fig 6 clearly
shows that when SF replaces 10% CEM I, the hydration kinetics are increased with increased
heat released observed in the second peak despite clinker dilution. This phenomenon may be
related to consumption of water within and around SF particles and its possible uptake by the
carbon particles. Reduced dormant period is also visible. However at the end of 72 hours
Calorimetry all systems look more or less the same wherein SCP systems using SF shows
slightly reduced heat released. The SF particles offer nucleation sites, produce physical packing
effect and chemical pozzolanic effect. All these mechanisms work simultaneously as the
hydration proceeds and result in improved microstructure which in turn adds to strength and
durability against concrete deleterious mechanisms.
Safety Concerns
The available published data is limited and indicates that silica fume does not pose a health
hazard due to its extreme fineness and the nature of silica involved. However caution should be
exercised in using this material. The U.S occupational Safety and Health Agency (OSHA)
prescribes a permissible exposure limit (PEL) of 15 mg/ m3 of total dust. The American
conference of Governmental Industrial Hygienists has established a threshold limit value (TLV)
of 10 mg/ m3 of the total dust (9).
Results and Discussion
In order to simply get enhanced concrete strength by using SF due to its filler, pozzolanic and
nucleation effects, it is unwise to use it in such formulations wherein no attention has been paid
to the improvement in the packing density of aggregate phase. After seeing the SEM/TEM and
MIP results of SF and carbon particles it may be suggested that antibleed properties may be the
result of presence of carbon particles and that of the space between SF agglomerates. Moreover
it must be remembered that SF increases early heat release in fresh state of cementitious systems
(which is due to cement hydration and SF hydration) so it may not be used in concretes to be
poured in hot conditions otherwise the system can crack to do early age cracking etc. There seem
to be numerous unknown factors which also contribute to heat buildup and water uptake by SF
particles. Silica fume possesses some carbon content which is large irregular particles. Some
people suggest that SF particles are hollow circular but it could not be established in this work. It
is very seldom that SF particles will be broken. In TEM pictures chains of bigger and smaller
particles can be seen. Within the small pores existing between the agglomerates of bigger
(primary) and smaller (secondary) interconnected penetrating and inseparable particles, mercury
or water can be accommodated .In general water has the tendency to be around the SF particles.
In fresh state SF particles hold/uptake some of the mixing water, thereby reducing the
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workability and creating slump retention. Usually addition of SF as cement replacement
increases the water demand of the cementitious systems incorporating it. In hardened state it
improves the strength of the cementitious systems by pore refinement effect.
Concluding Remarks
SF should be used with good judgment in HPC/SCC placements especially where aggregate
phase has not been packed and especially in hot climates. In order to utilize the full potential of
HPC/SCC packing concepts may also be applied to aggregate phase and then binder phase may
be packed by SF for optimal response. The increased mercury intrusion and anti-bleed properties
of HPC/SCC seem to be due to the presence of carbon particles and to the presence of space
within agglomerated SF particle groups. The faster water consumption/uptake is also confirmed
in the shrinkage and calorimetric measurements.
Acknowledgements
The authors are grateful to the laboratory staff of IKGB, Technical University Freiberg,
Germany for their co-operation in carrying out tests.
References
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containg fly ash and silica fume, Cement and Concrete Research 32 (2002) 1131-1132
(3) De. Larrard, F, Gorse, J.F and Puch, C., Comparative study of various silica fumes as
additives in high-performance cementitious materials, materials & Structures ,
1992,25,265-272.
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presence of hollow-shell pores, Cement and Concrete Research 29 (1999) 133-142
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shrinkage and pore structure of cement paste with mineral admixtures, Construction and
Building Materials 24(2010) 1855-1860.
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7. Thomas, M.D.A et al., The Effect of supplementary cementitious materials on chloride
ion binding in hardened cement paste, Cement and concrete research 42 (2012) 1-7.
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(9). Mahotra, V.M., Fly-ash, Slag, Silica Fume and Rice-Husk ash in Concrete: A review,
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APPENDIX
Fig. A SEM Image of SF Particles at 30,000 Magnification Manufacturer data
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Table A. A Typical Chemical Data sheet provided by the Manufacturer
Table A (Continued). Chemical Analysis of typical SF as given by the Manufacturer-