microsilica and nano silica in concrete mixtures

MICROSILICA AND NANO SILICA ON CONCRETE PROPERTIES

CONTENTS

1. INTRODUCTION 1

2. INVESTIGATIONS MADE ON MECHANICAL

3. PROPERTIES OF MICROSILICA CONCRETE 4

2.1 EXPERIMENTAL PROGRAMME 5

2.1.1 MATERIALS USED 5

2.1.2 CASTING AND CURING 7

2.1.3 TEST RESULTS 8

2.1.4 COMPRESSIVE STRENGTH TEST 8

2.1.5 SPLITTING TENSILE STRENGTH TEST 9

2.1.6 FLEXURAL STRENGTH TEST 10

3. INVESTIGATIONS MADE ON APPLICATION OF 13

NANO SILICA

3.1 PRODUCTION METHOD OF NANO SILICA 13

3.2 EFFECT OF NANO SILICA 14

3.3 APPLICATION OF NANO SILICA 16

4. INVESTIGATIONS MADE ON THE INFLUENCE OF 17

MICRO AND NANO SILICA ON CONCRETE

PERFORMANCE

4.1 MATERIALS USED AND TEST CONDUCTED 18

4.1 COMPRESSIVE STRENGTH 19

4.2 ELECTRICAL RESISTANCE 20

5. CONCLUSIONS 21

6. REFERENCES 23

DEPT. OF CIVIL ENGINEERING, U.V.C.E.


1. INTRODUCTION:

The construction industry uses concrete to a large extent. About 14 bln ton were used in

concrete is used in infrastructure and in buildings. It is composed of granular materials of different

sizes and the size range of the composed solid mix covers wide intervals. The overall grading of the

mix, containing particles from 300 nm to 32 mm determines the mix properties of the concrete. The

properties in fresh state (flow properties and workability) are for instance governed by the particle

size distribution (PSD), but also the properties of the concrete in hardened state, such as strength

and durability, are affected by the mix grading and resulting particle packing. One way to further

improve the packing is to increase the solid size range, e.g. by including particles with sizes below

300 nm. Possible materials which are currently available are limestone and silica fines likes silica

flavor (Sf), silica fume (SF) and nano-silica (nS). However, these products are synthesized in a

rather complex way, resulting in high purity and complex processes that make them non-feasible for

the construction industry.

In this new century, the technology of nano-structured material is developing at an

astonishing speed and will be applied extensively with many materials. Although cement is a

common building material, its main hydrate C–S–H gel is a natural nano-structured material [Qing,

Y., Zenan, Z., Deyu, K., Rongshen, C., 2007]. The mechanical and durability properties of concrete

are mainly dependent on the gradually refining structure of hardened cement paste and the gradually

improving paste–aggregate interface. Microsilica (silica fume) belongs to the category of highly

pozzolanic materials because it consists essentially of silica in non-crystalline form with a high

specific surface and thus exhibits great pozzolanic activity [Qing, Y., Zenan, Z., Deyu, K.,

Rongshen, C., 2007; Mitchell DRG, Hinczak I, Day RA., 1998]. A new pozzolanic material [Skarp,

U., and Sarkar, S.L, 2000. Collepardi, M., Ogoumah Olagot, J.J., , Skarp, U. and Troli, R ,2002

Collepardi, M., Collepardi, S., Skarp, U., Troli, R, 2002] produced synthetically, in form of water

emulsion of ultra-fine amorphous colloidal silica (UFACS), is available on the market and it

appears to be potentially better than silica fume for the higher content of amorphous silica (> 99%)

and the reduced size of its spherical particles (1-50 nm). Water permeability resistance and 28-days

compressive strength of concrete were improved by using nanosilica [Ji, T., 2005]. Addition of

nanosilica into high-strength concrete leads to an increase of both short-term strength and long-term

strength [Li,G., 2004]. In the present work, try have been done to assess the simultaneous effect of

nano and micro silica on concrete performances.

Microsilica is a mineral admixture composed of very fine solid glassy spheres of

silicondioxide (SiO2). Most microsilica particlesvare less than 1 micron (0.00004 inch) in

diameter,generally 50 to 100 times finer than average cement or fly ash particles.Frequently called



condensed silica fume, microsilicais a byproduct of the industrial manufacture of ferrosiliconand

metallic silicon in high-temperature electricarc furnaces. The ferrosilicon or silicon product is

drawnoff as a liquid from the bottom of the furnace. Vapor risingfrom the 2000-degree-C furnace

bed is oxidized, and as it cools condenses into particles which are trapped in huge cloth bags.

Processing the condensed fume to remove impurities and control particle size yields microsilica.

Microsilica in concrete contributes to strength and durability two ways:

As a pozzolan, microsilica provides a more uniform distribution and a greater volume of

hydration products.

As a filler, microsilica decreases the average size of

pores in the cement paste.

Mi c ro s i l i c a’s effectiveness as a pozzolan and a filler depends largely on its composition

and particle size which in turn depend on the design of the furnace and the composition of the raw

materials with which the furnaceis charged. At present there are no U.S. standard specifications for

the material or its applications. Dosages of microsilica used in concrete have typically been in the

range of 5 to 20 percent by weight of cement, but percentages as high as 40 have been reported.

Used as an admixture, microsilica can improve the p ro p e rties of both fresh and hardened

concrete. Used as a partial replacement for cement, microsilica can substitute for energy-consuming

cement without sacrifice of quality.

Now a days high performance concrete refers to the concrete that has uniaxial

compressive strength greater than normal concrete at same region. than the normal strength concrete

obtained in a particular region. This definition does not include a numerical value for compressive

strength indicating a transfer from a normal strength concrete to high strength concrete. In 1950’s,

concrete with a compressive strength of M35 MPa was considered as high strength concrete. In the

1990’s concrete with a compressive strength greater than 110MPa was used in developed countries.

However this numerical value (110MPa) could be considerably lower depending on the

characteristics of the local materials used for these concrete products. Report of ACI committee 363

in 1979 defined high-strength concrete as having compressive strength more than 41.37 MPa

(6000Psi).

High-strength and High-performance concrete are being widely used throughout the

world and to produce them it is necessary to reduce the water/binder ratio and increase the

binder content. High-strength concrete means good abrasion, impact and cavitation

resistance. Using High-strength concrete in structures today would result in economical

advantages. Most applications of high strength concrete to date have been in high-rise

buildings, long span bridges and some special structures. Major application of high strength

concrete in tall structures have been in columns and shear walls, which resulted in decreased



dead weight of the structures and increase in the amount of the rental floor space in the

lower stories.

2. According to the investigations made by V. Bhikshma∗a, K. Nitturkarb and Y.

Venkatesham(Department of Civil Engineering, University College of Engineering, Osmania

University (UCE,OU) , Hyderabad, India

Department of Civil Engineering, MVSR Engineering College Hyderabad, India

Department of Civil Engineering, UCE, OU, Hyderabad, India) respectively following results were

obtained. Their reports includes following contents:

In future, high range water reducing admixtures (super plasticiser) will open up new

possibilities for use of these materials as a part of cementing materials in concrete to

produce very high strengths, as some of them are more finer than cement. The brief

literature on the study has been presented in following text.

Hooton [1] investigated on influence of silica fume replacement of cement on physical

properties and resistance to sulphate attack, freezing and thawing, and alkali-silica

reactivity. He reported that the maximum 28-day compressive strength was obtained at 15% silica

fume replacement level at a w/b ratio of 0.35 with variable dosages of HRWRA. Prasad et al. [2]

has undertaken an investigation to study the effect of cement replacement with micro silica in the

production of High-strength concrete. Yogendran etal.[3] investigated on silica fume in High-

strength concrete at a constant water-binder ratio (w/b) of 0.34 and replacement percentages of 0 to

25, with varying dosages of HRWRA.The maximum 28-day compressive strength was obtained at

15% replacement level. Lewis [4] presented a broad overview on the production of micro silica,

effects of standardization of micro silica concrete-both in the fresh and hardened state. Bhanja., and

Gupta [5] reported and directed towards developing a better understanding of the isolated

contributions of silica fume concrete and determining its optimum content. Their study intended to

determine the contribution of silica fume on concrete over a wide range of w/c ratio ranging from

0.26 to 0.42 and cement replacement percentages from 0 to 30.

Tiwari and Momin [6] presented a research study carried out to improve the early age

compressive strength of Portland slag cement (PSC) with the help of silica fume. Silica fume from

three sources- one imported and two indigenous were used in various proportions to study their

effect on various properties of PSC.Venkatesh Babu and Natesan [7] Investigated on physico-

mechanical properties of High-performance concrete (HPC) mixes, with different replacement levels

of cement with condensed silica fume (CSF) of grade 960-D. Keeping some of the important points

of literature.



High-strength concrete of grades M40 and M50, the replacement levels of cement by

silicafume are selected as 0%, 3%, 6%, 9%, 12% and 15% for standard sizes of cubes, cylinders and

prisms for testing.

2.1 EXPERIMENTAL PROGRAMME

The experimental program was designed to compare the mechanical properties i.e,

compressive strength, flexural strength and splitting tensile strength of high strength concrete with

M40 and M50 grade of concrete and with different replacement levels of ordinary Portland cement

(ultra tech cement 53 grade) with silica fume or micro silica of 920-D.

The program consists of casting and testing a total of 144 specimens. The specimens of

standard cubes (150mmX150mmX150mm), standard cylinders of (150mm Dia X 300mm height)

and standard prisms of (100mmX100mmX500mm) were cast with and with out silica fume.

Universal testing machine was used to test all the specimens. In first series the specimens were cast

with M40 grade concrete with different replacement levels of cement as 0%, 3%, 6%, 9%, 12% and

15% with silica fume. And in the second series the same levels of replacement with M50 grade of

concrete were cast.

2.1.1 Materials Used

Ordinary Portland cement (Ultra tech cement) of 53 grade conforming to IS: 12269 and

locally available natural sand were used. Specific gravity and fineness modulus were found to be

2.53 and 2.73 respectively. Crushed granite stone chips (angular) of maximum size 20mm were used.

Specific gravity and fineness modulus were found to be 2.60 and 7.61 respectively. Potable water

was used for mixing and curing.

Silica fume (Grade 920-D) was obtained from “Elkem India private limited”, Mumbai,

India.

Super plasticizer by trade name Conplast SP-430 manufactured at Bangalore was used as

water reducing agent to achieve the required workability. It is available in brown liquid instantly

dispensable in water.

Physical properties of cement as per IS 4031 (Part-II)-1988, and silica fume as per IS 4031

(Part-II)-1999, tested at National Council for Cement and Building Materials. The experimental

program was designed to compare the mechanical properties i.e, compressive strength, flexural

strength and splitting tensile strength of high strength concrete with M40 and M50 grade of concrete

and with different replacement levels of ordinary Portland cement (ultra tech cement 53 grade) with

silica fume or micro silica of 920-D.

The program consists of casting and testing a total of 144 specimens. The specimens of

standard cubes (150mmX150mmX150mm), standard cylinders of (150mm Dia X 300mm height)







concrete were cast.

Physical properties of cement as per IS 4031 (Part-II)-1988, and silica fume as per IS4031

(Part-II)-1999, tested at National Council for Cement and Building Materials Hyderabad India, are

presented in Table 1.

Hyderabad India, are presented in Table 1.

Table 1. Physical properties of cement and silica fume

Designation Specific

gravity

Cement 3.15

Silica fume 2.27

Chemical properties of cement (as per IS 12269) and silica fume (as per ASTMC-99) tested

at Indian Institute of Chemical Technology, Hyderabad, India are presented in Table 2 and Table 3,

respectively.

Table 2. Chemical properties of cement



Table 3. Chemical properties of silica fume

Characteristics Specifications Result

(%by mass)

SiO2 % min 85.0 88.7

Moisture content % max 3.0 0.7

Loss on ignition

975c

% max 6.0 1.8

Carbon % max 2.5 0.9

>45 micron % max 10 0.2

Bulk density 500-700

Kg/m3

670

Two concrete mixes were designed to a compressive strengths of 40MPa and 50MPa with a

water-cementitious ratio of 0.36 and 0.30 respectively, as per IS code. In both the cases, the

Portland cement was replaced with silica fume by 0%, 3%,6%, 9%, 12%, and 15%. The water

reducing agent Conplast SP-430, 600 ml per 50kg of cement was added, to get thedesired

workability. The proportions of constituent materials i.e., cementitious material

(cement and silica fume), aggregates (coarse and fine), water and chemical admixture (super

-plasticizer) for two mixes are presented in Table 4.

Table 4. Proportions of Constituent materials of M40 and M50 Grade Concrete

Proportions of constituent

Grade of mix materials


Characteristics Result (%by mass)

Loss on ignition 1.95

Silica as (SiO2) 23.5

Alumina as (Al2O3) 4.42

Iron as (Fe2O3) 11.38


w/c ratio C F.A C.A

M40 0.36 1 0.92 2.82

M50 0.30 1 0.65 1.90

2.1.2 Casting and Curing of Test Specimens

The specimens of Standard cubes (150mm×150mm×150mm) 6 No.s, Standard prisms

(100mmX100mmX500mm) 6No.s and Standard cylinders (150mm diameterX300mm height) 6 No.s

were cast per a day, for 6 days. In all 72 specimens, cement was replaced by silica fume (RS-0, RS-3,

RS-6, RS-9, RS-12 and RS-15) with M40 mix case and 72 specimens with M50 mix case were cast.

Measured quantities of coarse aggregate and fine aggregate were spread out over

animpervious concrete floor. The dry ordinary Portland cement (ultra tech) and silica fume were

spread out on the aggregate and mixed thoroughly in dry state turning the mixture over and over

until uniformity of color was achieved. Water was measured exactly by weight, and super plasticiser

Conplast SP-430 (600ml per 50kg) was added to the water, 75% quantity of water was added to the

dry mix and it was thoroughly mixed to obtain homogeneous concrete. The time of mixing shall be

in 10-15 minutes.

2.1.3 TEST RESULTS:

The present investigation reports a part of a comprehensive study intended to determine the

contribution of silica fume on concrete mixes M40 and M50 with a w/c ratio of 0.36 and0.30 and

cement replacement levels from 0 to 15.

The optimum silica fume replacement level and strength improvement of high strength

concrete have been determined. The workability tests are presented in Table 5.

Table 5. Slump and compaction factor values of M40 and M50 grade concrete

M40 M50

Silica fume % Slump(mm) Slump(mm)

RS-3 45 40

RS-6 43 38



RS-9 41 37

RS-12 38 35

RS-15 35 32

2.1.4 Compressive Strength of Concrete

The test was carried out conforming to IS 516-1959 to obtain compressive strength of M40

and M50 grade of concrete. The compressive strength of High-strength concrete with OPC and silica

fume concrete at the age of 28 days is presented in Table 6. There is a significant improvement in the

strength of concrete because of the high pozzolanic nature of the silica fume and its void filling

ability. The compressive strength of the two mixes M40 and M50 at 28-days age, with replacement

of cement by silica fume (920-D) was increased gradually up to an optimum replacement level of

12% and then decreased. The maximum 28-day cube compressive strength of M40 grade with 12%

of silica fume was 61.20MPa, and of M50 grade with 12% silica fume was 68.66MPa.

The compressive strength of M40 grade concrete with partial replacement of 12% cement

by silica fume shows 16.37% greater, and of M50 grade with 12% replacement shows 20% greater,

than the controlled concrete.

The maximum compressive strength of concrete in combination with silica fume depends

on three parameters namely the replacement level, water cement ratio and chemical admixture. The

chemical admixture dosage plays a vital role in concrete to achieve the required workability at lower

w/c ratio.

Table 6. Twenty eight days compressive strength of concrete

Silica fume % Compressive strength( M P a )

M40 M50

RS-0 52.59 57.18

RS-3 54.18 57.63

RS-6 58.22 62.08



RS-9 60.74 62.81

RS-12 61.20 68.66

RS-15 58.50 63.50

Note: RS-Replacement of silica fume by weight of cement

2.1.5 Splitting Tensile Strength of Concrete

The test was carried out according to IS 5816- 1999 to obtain the splitting tensile strength of

M40 and M50 grade concrete. The test results of both the mixes were presented in the Table7

Table 7. Twenty eight days splitting tensile strength of concrete

Silica fume % Flexural strength ( MPa)

M40 M50

RS-3 5.11 5.14

RS-6 5.41 5.39

RS-9 5.78 5.7

RS-12 5.82 5.85

RS-15 5.58 5.68

As replacement level increases there is an increase in splitting tensile strength for both

M40 and M50 grades of concrete up to 12% replacement level, and beyond that level there is a

decrease in splitting tensile strength. The splitting tensile strength at 28-days age of curing of M40

and M50grade of concrete was 4.17MPa and 3.80MPa respectively. The splitting tensile strength of

both grades at 12% replacement, increased by about 36.06% and 20.63% respectively, when

compared to that of conventional concrete.

2.1.6Flexural Strength of Concrete



The tests were carried out conforming to IS 516-1959 to obtain the flexural strength of M40

and M50 grade concrete. Three standard prism specimens were cast for each replacement level and

tested under two-point loading. The experimental results of flexural strength with OPC for both the

mix cases are shown in Table 8.

Table 8. Twenty eight days flexural strength of concrete

Silica fume %

Flexural strength ( MPa)

M40 M50

RS-0 5 5.06

RS-3 5.11 5.14

RS-6 5.41 5.39

RS-9 5.78 5.7

RS-12 5.82 5.85

RS-15 5.58 5.68

The flexural strength at the age of 28- days of silica fume concrete continuously increased

with respect to controlled concrete and reached a maximum value of 12% replacement level for both

M40 and M50 grades concrete respectively. The maximum 28-day flexural strength of M40 and

M50 grades of concrete with 12% replacement of silica fume was 5.82MPa and 5.85MPa

respectively.

It can be concluded that the ultra-fine silica fume particles, which consist mainly of

amorphous silica, enhance the concrete strength by both pozzolanic and physical actions. The

results of the present investigation indicate that the percentage of silica fume contributing to the

mechanical properties is comparable or even more significant than that ofcontrol concrete.

The material used silica fume, slump and testing setup are presented in plates 1-5.



Typical stress-strain curves for M40 and M50 grades of concrete are presented in Figure 1-

The flexural strength at the age of 28- days of silica fume concrete continuously increased with

respect to controlled concrete and reached a maximum value of 12% replacement level for both M40 and

M50 grades concrete respectively.

The maximum 28-day flexural strength of M40 and M50 grades of concrete with 12% replacement

of silica fume was 5.82MPa and 5.85MPa respectively.



3. According to the investigations made by G.QUERCIA AND H.J.H. BROUWERS

(Materials Innovation Institute – M2i and 2Eindhoven University of Technology

Building and Physics, P.O. box 513, 5600 MB Eindhoven, The Netherlands) a special

type of nano-silica a new nano-silica is produced from olivine. This nS, as well as

commercially available nS, will be applied and tested. In addition, a mix design tool used for

self compacting concrete (SCC) will be modified to take into account particles in the size

range of 10 to 50 nm. The following results were obtained according to their studies and it is

as follows:



3.1 Production method of nS:

Nowadays there are different methods to produce nanosilica concrete. One of the

production methods is water route method at room temperature. In this process the starting materials

(mainly Na2SiO4 and organometallics like TMOS/TEOS) are added in a solvent, and then the pH of

the solution is changed, reaching the precipitation of silica gel. The produced gel is aged and filtered

to become a xerogel . This xerogel is dried and burned or dispersed again with stabilized agent (Na,

K, NH3, etc.) to produce a concentrated dispersion (20 to 40% solid content) suitable for use in

concrete industry .

An alternative production method is based on vaporization of silica between 1500 to 2000°C

by reducing quartz (SiO2) in an electric arc furnace. Furthermore, nS is produced as a byproduct of

the manufacture of silicon metals and ferro-silicon alloys, where it is collected by subsequent

condensation to fine particles in a cyclone. Nano-silica produced bythis method is a very fine

powder consisting of spherical particles or microspheres with a main diameter of 150 nm with high

specific surface area (15 to 25 m2/g).

Estevez et al. developed a biological method to produce a narrow and bimodal distribution

of nS from the digested humus of California red worms (between 55nm to 245nm depending of

calcination temperature). By means of this method, nanoparticles having a spherical shape with

88% process efficiency can be obtained. These particles were produced by feeding worms with rice

husk, biological waste material that contain 22% of SiO2.

Finally, nS can also be produced by precipitation method. In this method, nS is

precipitated from a solution at temperatures between 50 to 100 °C (precipitated silica). It was first

developed by Iller in 1954. This method uses different precursors like sodium silicates (Na2SiO3),

burned rice husk ash (RHA), semi-burned rice straw ash (SBRSA), magnesium silicate and others .

In addition, nano-silica (nS) is being developed via an alternative production route.

Basically, olivine and sulphuric acid are combined, whereby precipitated silica with extreme

fineness but agglomerate form is synthesized (nano-size with particles between 6 to 30 nm), and

even cheaper than contemporary micro-silica. The feasibility of this process has been proven in two

preceding PhD theses and published data .Currently, parallel PhD project ocuses on the process to

produce nS on industrial scale in large quantities for concrete production. Furthermore, the

combination of raw materials and process parameters on production will be examined.

3.2 Effect of nS addition in concrete and mortars: In concrete, the micro-silica (Sf

and SF) works on two levels. The first one is the chemical effect: the pozzolanic reaction of silica

with calcium hydroxide forms more CSH-gel at final stages. The second function is physical one,

because micro-silica is about 100 times smaller than cement. Micro-silica can fill the remaining

voids in the young and partially hydrated cement paste, increasing its final density. Some

researchers found that the addition of 1 kg of micro-silica permits a reduction of about 4 kg of



cement, and this can be higher if nS is used. Another possibility is to maintain the cement content at

a constant level but optimizing particle packing by using stone waste material to obtain a broad

PSD. Optimizing the PSD will increase the properties (strength, durability) of the concrete due to

the acceleration effect of nS in cement paste. Nano-silica addition in cement paste and concrete can

result in different effects. The accelerating effect in cement paste is well reported in the literature.

The main mechanism of this working principle is related to the high surface area of nS, because it

works as nucleation site for the precipitation of CSH gel.

However, according to Bjornstrom et al. it has not yet been determined whether the more

rapid hydration of cement in the presence of nS is due to its chemical reactivity upon dissolution

(pozzolanic activity) or to their considerable surface activity. Also the accelerating effect of nS

addition was established indirectly by measuring the viscosity change (rheology) of cement paste

and mortars. The viscosity test results shown that cement paste and mortar with nS addition needs

more water in order to keep the workability of the mixtures constant, also concluded that nS

exhibits stronger tendency for adsorption of ionic species in the aqueous medium and the formation

of agglomerates is expected. In the latter case, it is necessary to use a dispersing additive or

plasticizer to minimize this effect.

Ji studied the effect of nS addition on concrete water permeability and

microstructure. Different concrete mixes were evaluated incorporating nS particles of 10 to 20 nm

(s.s.a. of 160 m2/g), fly ash, gravel and plasticizer to obtain the same slump time as for normal

concrete and nS concrete. The test results show that nS can improve the microstructure and reduce

the water permeability of hardened concrete. Lin et al. demonstrated the effect of nS addition on

permeability of eco-concrete. They have shown with a mercury porosimetry test that the relative

permeability and pores sizes decrease with nS addition (1 and 2% bwoc). Decreasing permeability

in concrete with high fly ash content (50%) and similar nS concentrations (2% of nS power) was

reported by. Microstructural analysis of concrete by different electronic microscope techniques

(SEM, ESEM, TEM and others) revealed that the microstructure of the nS concrete is more uniform

and compact than for normal concrete. Ji demonstrated that nS can react with Ca(OH)2 crystals,

and reduce the size and amount of them, thus making the interfacial transition zone (ITZ) of

aggregates and binding cement paste denser. The nS particles fill the voids of the CSH-gel structure

and act as nucleus to tightly bond with CSH-gel particles. This means that nS application reduces

the calcium leaching rate of cement pastes and therefore increasing their durability .

The most reported effect of nS addition is the impact on the mechanical properties of

concrete and mortars. As it was explained before, the nS addition increases density, reduces

porosity, and improves the bond between cement matrix and aggregates. This produces concrete that

shows higher compressive and flexural strength. Also, it was observed that the nS effect depends on

the nature and production method (colloidal or dry powder). Even though the beneficial effect of nS



addition is reported, its concentration will be controlled at a maximum level of 5% to 10% bwoc,

depending on the author or reference. At high nS concentrations the autogenous shrinkage due to

self-desiccation increases, consequently resulting in higher Cracking potential. To avoid this effect,

high concentration of super plasticizer and water has to be added and appropriate curing methods

have to be applied.

The program consists of casting and testing a total of 144 specimens. The specimens

of standard cubes (150mmX150mmX150mm), standard cylinders of (150mm Dia X 300mm height)





concrete were cast.

3.3 Applications of nS

At present Sf, SF and nS, because of their price, are only used in the so-called high

performance concretes (HPC), eco-concretes and self compacting concretes (SSC). For the last types

of special concretes (eco-concrete and SCC), the application of these materials is a necessity. Also,

some explorative applications of nS in high performance well cementing slurries, specialized mortars

for rock-matching grouting, and gypsum particleboard [39] can be found, but nS is not used in

practice yet. The application of these concretes can be anywhere, both in infrastructure and in

buildings.

Nano-silica is applied in HPC and SCC concrete mainly as an anti-bleeding agent. It is also

added to increase the cohesiveness of concrete and to reduce the segregation tendency. Some

researchers found that the addition of colloidal ns (range 0 to 2% bwoc) causes a slight reduction in

the strength development of concretes with ground limestone, but does not affect the compressive

strength of mixtures with fly ash or ground fly ash (GFA). Similarly, Sari et al. used colloidal nS

(2% bwoc) to produce HPC concrete with compressive strength of 85 MPa, anti-bleeding properties,

high workability and short demolding times (10 h). Another application of nS well documented and

referred in several technical publications, is the use as additive in eco-concrete mixtures and tiles.

Eco-concretes are mixtures where cement is replaced by waste materials mainly sludge ash,

incinerated sludge ash, fly ash or others supplementary waste materials. One of the problems of these

mixtures is their low compressive strength and long setting period. This disadvantage is solved by

adding nS to eco-concrete mixes to obtain an accelerated setting and higher compressive strength.

Roddy et al. applied particulate nS in oil well cementing slurries in two specific ranges of particles

sizes, one between 5 to 50 nm, and a second between 5 and 30 nm. Also they used nS dry powders in

encapsulated form and concentrations of 5 to 15% bwoc. The respective test results for the slurries

demonstrate that the inclusion of nS reduces the setting time and increases the strength (compressive,



tensile, Young’s modulus and Poisson’s ratio) of the resulting cement in relation with other silica

components (amorphous 2.5 to 50 μm, crystalline 5 to 10 μm and colloidal suspension 20 nm types

silica) that were tested.

4. According to investigations made by M.Nilli, A.Ehsani and K.Shabani Civil Eng.,

Dept., Bu-Ali Sina University, Hamedan, I.R. Iran Eng., Research Institute of Jahad-

Agriculture Ministry, Tehran, I.R. Iran had conducted experiments by adding both

nanosilica and microsilica as partial replacement to cement by varying their percentages.

Experimental details and the various materials used in the experiment is as follows:

The mechanical and durability properties of concrete are mainly dependent on the gradually

refining structure of hardened cement paste and the gradually improving paste–aggregate interface.

Microsilica (silica fume) belongs to the category of highly pozzolanic materials because it consists

essentially of silica in non-crystalline form with a high specific surface and thus exhibits great

pozzolanic activity [Qing, Y., Zenan, Z., Deyu, K., Rongshen, C., 2007; Mitchell DRG, Hinczak I,

Day RA., 1998]. A new pozzolanic material [Skarp, U., and Sarkar, S.L, 2000. Collepardi, M.,

Ogoumah Olagot, J.J., , Skarp, U. and Troli, R ,2002 Collepardi, M., Collepardi, S., Skarp, U.,

Troli, R, 2002] produced synthetically, in form of water emulsion of ultra-fine amorphous colloidal

silica (UFACS), is available on the market and it appears to be potentially better than silica fume for

the higher content of amorphous silica (> 99%) and the reduced size of its spherical particles (1-50

nm). Water permeability resistance and 28-days compressive strength of concrete were improved by

using nanosilica [Ji, T., 2005]. Addition of nanosilica into high-strength concrete leads to an

increase of both short-term strength and long-term strength [Li,G., 2004]. In the present work, try

have been done to assess the simultaneous effect of nano and micro silica on concrete performances.

4.1 MATERIALS AND TESTING PROGRAM

Crushed stone, with 19 mm maximum nominal size, in two ranges of 5-10 and 10-19 with

relative density at saturated surface dry of 2.61 were used. Fineness modulus of sand and relative

density were 3.24 and of 2.56 respectively. Water absorption of fine and coarse aggregate is 3.09%

and 2%, respectively. Portland cement type 2, with a specific gravity of 3.11 and 3750 cm2/gr

surface area was used. A commercial carboxylic type plasticizer, (Gelenium 110M, BASF Co.), was

used to adjust workability of the fresh concrete. Silica fume, made by Semnan Ferro Alley factory

(IFC Co.), was used at 0%, 3%, 4.5%, 6% and 7.5% (by weight) as partial replacement of cement.

Colloidal nanosilica, made by Akzo Nobel Chemicals GmbH (Cembinder® 8) was also used at 0%,



1.5%, 3% and 4.5% (by weight) as partial replacement of cement. The characteristics of

cementitious materials are given in Table 1. Mix proportions of the concrete mixtures and results of

fresh concrete slump tests are given in Table 2. Water-cementitious material (w/cm) ratio of all

mixtures is constant and equal to 0.45. The colloidal nanosilica was mixed with Superplasticizer and

half of the mixing water. A pan mixer was used and the mixing procedures are as follows. At the

beginning, sand, cement, half of the mixing water and half of the admixture content were mixed for

1 minute. Then, the remaining water and admixture and also coarse aggregate were added into the

mixture and mixed for 2 minutes. Cube specimens (100×100×100 mm) were used for determination

of compressive strength development, electrical resistance development and capillary absorption.

The casting specimens were remolded after 24 hours and then were cured in water. Testing ages

were 3, 7, 28 and 91 days. Electrical resistance was measured via copper plates which were installed

in top and bottom of the saturated concrete specimen at the ages of testing. Capillary absorption test

was performed according to RILEM TC, CPC 11.2 (1982).

Table 1. Chemical Composition and Physical Properties of

Cementitious Material

Material Chemical composition

(%) and Physical properties

Cement Al2O3, 4.75; SiO2,

26.58; P2O3, 0.26; SO3, 7.74;

K2O, 0.76; CaO, 55.75; TiO2,

0.24; Cr2O3, 335ppm; MnO,

0.13; FeO, 3.83; SrO, 665ppm;

As2O3<144 ppm; CuO,

185ppm;

Microsilica SiO2, 85-95; C, 0.6-1.5;

Fe2O3, 0.4-2; CaO, 2-2.3;

Al2O3, 0.5-1.7; MgO, 0.1-0.9;

Colloidal Nanosilica Average primary particle

size: 50-60 nm; Specific surface

area (BET): 80 m2/g;

Solid content (SiO2-

content): 50 wt %; density: 1.4

g/cm3; Ph: 9.5; Viscosity: <15

cPS



4.2 EXPERIMENTAL RESULTS

4.2.1 Compressive strength

Fig. 1 shows the compressive strength development of concrete mixtures. The results show

that increasing in nanosilica content ,1.5% to 4.5% by weight, leads to an increase of compressive

strength at all stages. The results also indicate that the specimens which contain both nano and

micro silica, due to the high pozzolanic activity, have higher compressive strength than reference

ones. However, large quantities of nanosilica in the mixtures, due to agglomerate effect, don't lead

to increase compressive strength. As it is shown the highest compressive strength at the age of 28

days is corresponding to SF6, NS1.5 mixture.



The above figure represents the compressive strength obtained after 3 days , 7 days, 28 days and 91

days of water curing. The results show that increasing in nanosilica content from 1.5% to 4.5% by

weight, leads to an increase of compressive strength at all stages. It is found that for SF-6% and Ns-

1.5% 28 days compressive strength was found to be greater than other combination.

4.2.2 Electrical resistivity

The electrical resistance development of the specimens which was measured at the ages of 3,

7, 28 and 91 days is illustrated in Fig. 2. As it is shown, a considerable increase in electric resistance

of later ages of 91 days, compare to the early age result is observed. This is, of course, due to

hydration progress which occurred in the later ages. Similar to compressive strength results, the

maximum electrical resistance, at the ages of 28 days, is attained in the mixture that contains 6%

and 1.5% microsilica and nanosilica, respectively.

The resistivity of concrete is strongly dependent on the concrete quality and on the exposure

conditions. In concrete material with high electrical resistivity the corrosion process will be slow



compared to concrete with low resistivity in which the current can easily pass between anode and

cathode areas [Song, H. W., Saraswathy, V., 2007].

CONCLUSIONS

1. According to V. Bhikshma∗a, K. Nitturkarb and Y. Venkatesham

(Department of Civil Engineering, University College of Engineering, Osmania University

(UCE,OU) , Hyderabad, India

Department of Civil Engineering, MVSR Engineering College Hyderabad, India

Department of Civil Engineering, UCE, OU, Hyderabad, India)

Cement replacement upto 12% with silica fume leads to increase in compressive

strength, splitting tensile strength and flexural strength, for both M40 and M50 grades. Beyond 12%



there is a decrease in compressive strength, tensile strength and flexural strength for 28 days curing

period.

It is observed that the compressive strength, splitting tensile strength and flexural

strength of M40 grade concrete is increased by 16.37%, 36.06% and 16.40% respectively, and for

M50 grade concrete 20.20%, 20.63% and 15.61% respectively over controlled concrete.

There is an increase in Young’s modulus of concrete as silica fume content increases.

This increase is again up to a replacement level of 12%. The Young’s modulus at this replacement

level is Ec=32.19GPa, for M40 grade concrete which is 28.06% higher than conventional concrete.

There is a decrease in workability as the replacement level increases, and hence water

consumption will be more for higher replacements.

The ratio of cube strength to cylinder is found as 1.22 and 1.24 respectively for M40

and M50 grades, where as for the conventional concrete is1.20.

.The maximum replacement level of silica fume is 12% for M40 and M50 grades of

concrete.

2. According to G.QUERCIA AND H.J.H. BROUWERS (Materials Innovation Institute – M2i

and 2Eindhoven University of Technology Building and Physics, P.O. box 513, 5600 MB Eindhoven,

The Netherlands) A new nano-silica (nS) can be produced in high quantities and for low prices that

allows for a mass application in concrete. It may replace cement in the mix, which is the most costly

and environmentally unfriendly component in concrete. The use of nS makes concrete financially

more attractive and reduces the CO2 footprint of the produced concrete products. The nS will also

increase the product properties of the concrete: the workability and the properties in hardened state,

enabling the development of high performance concretes for extreme constructions.

3. According to M. Nili, A. Ehsani , and K. Shabani (Civil Eng., Dept., Bu-Ali Sina

University, Hamedan, I.R. Iran Eng., Research Institute of Jahad-Agriculture Ministry,

Tehran, I.R. Iran)

The highest compressive strength at the ages of 7 and 28 days was attained when the

mixtures contain 6% microsilica and 1.5% nanosilica.

A considerable increase in electric resistant of nano-micro silica specimens was

observed compare to reference ones and the highest value was corresponding to the specimens

which contain totally 7.5% nano and micro silica.

In general we can conclude that microsilica and nano-silica replace cement in some percentages

resuts in improved compressive strength, to quite an extent tensile strength is also improved. We



can also notice that incorporation of colloidal nano-silica and microsilica as partial replacements

of cement have got advantageous effect on overall concrete performance.

REFERENCES:

1. M. Nili, A. Ehsani , and K. Shabani, “ Influence of Nano-SiO2 and Microsilica on Concrete

Performance” proceedings of Second International conference on Sustainable Construction

Materials and technologies on June 28-30,2010.

2. G.QUERCIA AND H.J.H. BROUWERS “Application of nano-silica (nS) in concrete mixtures”

presented in 8th fib Symposium in Kgs. Lyngby, Denmark on June 20 – 23, 2010.

3. V. Bhikshma, K. Nitturkar and Y. Venkatesham “Investigation on Mechanical Properties of High

Strength Silica Fume Concrete” ASIAN JOURNAL OF CIVIL ENGINEERING (BUILDING AND

HOUSING) VOL. 10, NO. 3 (2009) PAGES 335-346


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