Silica Fume: Its Role in Cement and Concrete Technology

Arnon Chaipanich*

Department of Physics, Faculty of Science, Chiang Mai University, Chiang Mai 50200,

Thailand

*Correspondending author. E-mail: arnon@chiangmai.ac.th

ABSTRACT

The role of silica fume as a cementitious by-product is reported, in terms of its historical and experimental aspects. Silica fume is found to be ≈100nm in size and therefore

is the finest component ever to be used to make cements or concrete so far. Its average

particle size when compared to that of Portland cement (≈10–15μm) is almost 100 times

smaller in terms of the diameter size of the particle. Evidently, enhanced compressive strength

from the use of silica fume was found. This is thought to be due to improved densification

of the cement matrix.

Key words: Silica fume, By-products, Strength

INTRODUCTION

Since the development of Portland cement by Joseph Aspdin in 1824, there have been

a number of developments through out the 20th century from the use of pozzolanic by-products in order to cut down the fuel cost and CO2 emission in the manufacturing process. At the

same time, turning waste to by-products, reusing of materials help reduce waste from industries, thus moving towards sustainable construction. The particle size of Portland cement

(PC) itself is ≈10–15 μm where fly ash and ground granulated blast-furnace slag, two common by-products used through out the world, are also of around 10 μm (Neville, 1995; Malhotra

and Mehta, 1996). Silica fume is a by-product from the ferrosilicon industry. It was first used

in concrete in the early 1950’s at the Norwegian Institute of Technology but its beneficial

effects on concrete properties were not realized until early 1980’s where silica fume was

started to be used in concrete structures such as in the New Tjorn cable-stayed bridge in

Sweden and in Kinzua dam in the USA (Federation International de la Precontriant, 1988;

Khayat and Aitcin, 1993). Although its technical advantages is now being recognized more

so than ever before, only a small percentage of the current supply of silica fume is being used

as a mineral admixture in the cement and concrete industries (Malhotra and Mehta, 1996).

One of the attractive characteristics of silica fume is its particle size which was reported to be

approximately 100–500nm (Malhotra and Mehta, 1996; Federation International de la

Precontriant, 1988).

Furthermore, it is known that cement improvement (also enhance the performance of

concrete) depends on both physical and chemical properties, improved physical characteristics such as the use of finer particles (higher surface area) nonetheless has advantages in its

own right in terms of filling the cement matrix, densifying the structure, resulting in higher

strength and at the same time allowing faster chemical reactions to occur. In this work, physical characterization such as particle size and morphology of silica fume were carried out,

using a Scanning Electron Microscope. Furthermore, the effect of silica fume on the

compressive strength was also investigated.
MATERIALS AND METHODS

Powder characteristics such as particle size, morphology and elements were determined,

using a Scanning Electron Microscope (SEM; JEOL JSM-840A) and Energy Dispersive

X-ray spectrometer. For compressive strength test, concrete of 100x100x100 mm cubes were

cast, using a water to binder ratio of 0.45 (cement content = 410kg/m3). Silica fume was used

to replace Portland cement by direct weight at 5, 10, 15 and 20%. Natural sand and crushed

coarse aggregates (20 mm maximum) were used for all mixes. A superplastizing admixture

to BS 5075: part 3 was used to maintain a nominal slump of 75 mm. The samples were

demoulded and cured in water at 20°C for a specified period (1 to 28 days). After curing,

concrete specimens were tested for compressive strength in accordance with BS 1881: part

116.

RESULTS AND DISCUSSION

Silica fume was found to consist of essentially SiO2 at 95.3% (Table 1). When tested

using EDS technique, it can also be seen that the main elements were Si and O (Figure 1).

The Si-O structure is said to be a disordered structure resulting as the product of solidification or consolidation from a fused material (Malhotra and Mehta, 1996). From Figure 2, it

can be seen that silica fume is round in shape with its typical particle size of ≈ 100nm.

Table 1. Chemical characteristic of silica fume.

Oxide %

SiO2 95.3

Al2O3 0.65

Fe2O3 0.28

CaO 0.27

MgO 0.41

K2O 0.77

Na2O 0.26

Figure 1. Energy Dispersive X-ray of silica fume.

K

O

Mg

Si

K

K

Full Scale 1275 cts Cursor: 0.000 keV keV

Spectrum 1
Figure 2. Scanning Electron Micrograph of silica fume particles.

The effect of silica fume on compressive strength of concrete can be seen in Figures 3

and 4, where Figure 3 shows the compressive strength development up to 28 days and Figure

4 shows the relative strength results compared to Portland cement concrete. It was found that

at very early strength (1–2 days), the strength of silica fume concrete was slightly less than

that of PC concrete. However, at later strength (from 7 days on), the compressive strength

was found to increase significantly and with increasing level of silica fume used in the mixes.

Greatest increase in strength can be seen in the concrete mix with silica fume at 20% and

approximately 35% increase was found at 28 days. The significant gain in strength is thought

to be due to the very high pozzolanic reactivity of silica fume once calcium hydroxide

became available from Portland cement hydration, thus resulting in a denser structure of the

cement matrix. Enhanced concrete strength from the use of silica fume has also been reported

to be due to stronger bonding in the inter-facial zone between the cement paste and aggregates (Sarkar, 1991).

Silica fume is also known as micro silica since its first being put on the market by

Elkem, a Norwegian leading ferrosilicon producer, as the word ‘micro’ was more common

then. Now moving on in the 21st century, perhaps being in the ‘nanotechnology’ era, silica

fume may now be referred to as nanosilica. Regardless of the trade name, its submicron size

has so far made silica fume the most superior in strength enhancement to concrete structures.

Figure 3. Compressive strength of silica fume concrete at w/c=0.45.

90

80

70

60

50

40

30

20

10

0

Strength, MPa

AGE, days

0 7 14 21 28

PC

SF5

SF10

SF15

SF20
Figure 4. Relative strength of silica fume concrete to PC control concrete.

CONCLUSION

Silica fume is found to be ≈100nm in size and therefore is the finest component ever to

be used to make cements or concrete so far. Its average particle size when compared to that of

Portland cement (≈10–15 μm) is almost 100 times smaller in term of the diameter size of the

particle. Enhanced compressive strength from the use of silica fume at 28 days was found.

The compressive strength was found to increase significantly with increasing level of silica

fume used in the mixes. Greatest increase in strength can be seen in the concrete mix with

silica fume at 20%. This is thought to be due to improved densification of the cement matrix

and also to stronger bonding in the inter-facial zone between the cement paste and aggregates.

ACKNOWLEDGEMENTS

The author is grateful to the Thailand Research Fund for funding this research.

REFERENCES

Federation International de la Precontriant 1988. State of the Art Report. Condensed silica

fume in concrete. Thomas Telford.

Khayat, K. H., and P. C. Aitcin 1993. Silica fume-A unique supplementary cementitious

material. Mineral Admixtures in Cement and Concrete. In S.N. Ghosh (ed) ABI Books

Private Limited Vol. 4 : 227–265.

Malhotra, V. M. and P. K. Mehta 1996. Pozzolanic and cementitious materials. Advances in

Concrete Technology, Gordon and Breach Publishers, Vol. 1.

Neville, A. M. 1995. Properties of concrete. Fourth Ed. Longman Scientific & Technical.

Sarkar, S. L. 1991. High strength concrete and its microstructure. Progress in Cement and

Concrete, Cement and Concrete Science &Technology. Vol.1, Part I. In S.N. Ghosh

(ed).

140

130

120

110

100

90

80

Relative Strength %

MIX

PC SF5 SF10 SF15 SF20

1

2

7

14

28