MICROSILICA AND NANO SILICA ON CONCRETE PROPERTIES

MICROSILICA AND NANO SILICA ON CONCRETE PROPERTIES

Influence of micosilica and nanosilica on concrete properties

SEMINAR REPORTSUBMITTED TO BANGALORE UNIVERSITY IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE AWARD OF THE DEGREE OF

MASTER OF ENGINEERING CIVILBy

Swathi.s.vI SEM me-pre stressed concrete

Under the guidance of

PROF h. sharada baiFACULTY OF ENGINEERING- CIVIL,BANGALORE UNIVERSITY,JNANABHARATHI,BANGALORE- 560056.UNIVERSITY VISVESVARAYA COLLEGE OF ENGINEERINGFACULTY OF CIVIL ENGINEERING

CERTIFICATE

THIS TO CERTIFY THAT THE SEMINAR WORK ENTITLEDInfluence of micosilica and nanosilica on concrete properties is a bonafide study carried out by

NAME: swathi.s.v REG NO: 11gamc4007

In partial fulfillment for the award of degree of MASTER of Engineering by Bangalore University, Bangalore during the year 2011-2012.

Dr. H. N. RAMESH Dr. H. SHARADA BAI Professor and chairman, GUIDE, PROFFESSOR IN CIVIL ENGINEERING, FACULTY OF ENGINEERING-CIVIL,FACULTY OF ENGINEERING-CIVIL, BANGALORE UNIVERSITY,BANGALORE UNIVERSITY, JNANABHARATHI CAMPUS, JNANABHARATHI CAMPUS, BANGALORE 560056BANGALORE - 560056

CONTENTS1. INTRODUCTION 12. INVESTIGATIONS MADE ON MECHANICAL 3. PROPERTIES OF MICROSILICA CONCRETE42.1 EXPERIMENTAL PROGRAMME 52.1.1 MATERIALS USED5 2.1.2 CASTING AND CURING72.1.3 TEST RESULTS82.1.4 COMPRESSIVE STRENGTH TEST8 2.1.5 SPLITTING TENSILE STRENGTH TEST92.1.6 FLEXURAL STRENGTH TEST103. INVESTIGATIONS MADE ON APPLICATION OF 13NANO SILICA3.1 PRODUCTION METHOD OF NANO SILICA133.2 EFFECT OF NANO SILICA143.3 APPLICATION OF NANO SILICA164. INVESTIGATIONS MADE ON THE INFLUENCE OF17MICRO AND NANO SILICA ON CONCRETE PERFORMANCE4.1 MATERIALS USED AND TEST CONDUCTED184.1 COMPRESSIVE STRENGTH194.2 ELECTRICAL RESISTANCE 205. CONCLUSIONS216. REFERENCES23

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 CSH 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 pasteaggregate 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 by- product 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 ofpores in the cement paste.Mi c ro s i l i c as 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 1950s, concrete with a compressive strength of M35 MPa was considered as high strength concrete. In the 1990s 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 theworld and to produce them it is necessary to reduce the water/binder ratio and increase thebinder content. High-strength concrete means good abrasion, impact and cavitationresistance. Using High-strength concrete in structures today would result in economicaladvantages. Most applications of high strength concrete to date have been in high-risebuildings, long span bridges and some special structures. Major application of high strengthconcrete in tall structures have been in columns and shear walls, which resulted in decreaseddead weight of the structures and increase in the amount of the rental floor space in thelower stories.

2. According to the investigations made by V. Bhikshmaa, K. Nitturkarb and Y. Venkatesham(Department of Civil Engineering, University College of Engineering, Osmania University (UCE,OU) , Hyderabad, IndiaDepartment of Civil Engineering, MVSR Engineering College Hyderabad, IndiaDepartment 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 newpossibilities for use of these materials as a part of cementing materials in concrete toproduce very high strengths, as some of them are more finer than cement. The briefliterature 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-silicareactivity. 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 agecompressive 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 PROGRAMMEThe 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 ofstandard 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 UsedOrdinary Portland cement (Ultra tech cement) of 53 grade conforming to IS: 12269 andlocally 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 aswater 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 ofstandard 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.

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 fumeDesignation Specificgravity

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

CharacteristicsResult (%by mass)

Loss on ignition1.95

Silica as (SiO2)23.5

Alumina as (Al2O3)4.42

Iron as (Fe2O3)11.38

Table 3. Chemical properties of silica fume CharacteristicsSpecificationsResult

(%by mass)

SiO2% min 85.088.7

Moisture content% max 3.00.7

Loss on ignition 975c% max 6.01.8

Carbon% max 2.50.9

>45 micron% max 100.2

Bulk density500-700 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 constituentGrade of mixmaterials w/c ratio C F.A C.A

M40 0.36 1 0.92 2.82M50 0.30 1 0.65 1.902.1.2Casting and Curing of Test SpecimensThe specimens of Standard cubes (150mm150mm150mm) 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.

2.1.3TEST 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-941 37

RS-12 38 35

RS-15 35 32

2.1.4Compressive Strength of ConcreteThe test was carried out conforming to IS 516-1959 to obtain compressive strength of M40and 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% cementby 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 dependson 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 concreteSilica fume % Compressive strength( M Pa )

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 ConcreteThe 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 bothM40 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 ConcreteThe 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 TechnologyBuilding 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 2000C 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 ofconcrete 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) 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.3.3 Applications of nSAt 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, Youngs modulus and Poissons ratio) of the resulting cement in relation with other silica components (amorphous 2.5 to 50 m, crystalline 5 to 10 m 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 pasteaggregate 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 (100100100 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