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. syedalirizwan@hotmail.com, Member ACI

Committees 237, 234 and 232.

2. IKGB, Technical University Freiberg, Germany. gert.schmidt@ikgb.tu-freiberg.de,

Thomas.bier@ikgb.tu-freiberg.de

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

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

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

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

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.

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)

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

-

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.

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

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

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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APPENDIX

Fig. A SEM Image of SF Particles at 30,000 Magnification Manufacturer data

Table A. A Typical Chemical Data sheet provided by the Manufacturer

Table A (Continued). Chemical Analysis of typical SF as given by the Manufacturer-