CORROSION STUDIES ON MICROSILICA ADDED

CEMENT IN MARINE ENVIRONMENT1

Nausha Asrar, Anees U. Malik, Shahreer Ahmed And Fadi S. Mujahed

Research & Development Center,

Saline Water Conversion Corporation P.O. Box # 8328, Al-Jubail 31951, Kingdom of Saudi Arabia.

SUMMARY In a hardened concrete structure, diffusion of oxygen, carbon dioxide, ions and moisture from the concrete/environment to the concrete/rebar interfaces take place through the pores, which result in the failure of the passivation provided by the alkalinity of the cement to the rebars. Microsilica is a mineral that improves the corrosion protection and strength of concrete by reducing the permeability of the concrete and forming a strength providing gel of calcium silicate hydrate (CSH). In order to study the corrosion behavior of microsilica in ASTM Type-I (ordinary portland cement, OPC), and ASTM Type-V (sulfate resistant cement, SRC), carbon steel reinforced concrete specimens of above cements containing 10% each of densified or undensified microsilica were exposed to 5% NaCl solution and seawater. Corrosion of rebars was monitored using Open Circuit Potential (OCP), Salt Fog and Immersion tests. Diffusion of ions into the concrete was studied by Salt Ingress, Rapid Chloride Permeability and Sulfate Resistant Tests. Compressive Strength of the concrete samples was also measured.

It has been noted that densified microsilica provides better corrosion protection to the rebars than the undensified one. Also, its protective effect appears to be dependent upon the type of cement and corrosive environment. In seawater, blending of microsilica with SRC suppresses the corrosion of rebars more than that of microsilica blended with OPC. An attempt has been made to explain the mechanism of corrosion provided by microsilica.

In view of the findings of the research work and behavior of microsilica in SRC and OPC cements, it has been concluded that blending of SRC with densified microsilica is a better option for controlling the rebar corrosion in concrete structures exposed to the seawater. Densified microsilica blended OPC, gives corrosion protection to rebars in chloride ion containing environment.

1 Issued as Technical Report No. TR 3804/ EVP 93001. A paper entitled � Corrosion Protection Performance of Microsilica Added Concretes in NaCl and Seawater Environments� was published in �Construction & Building Materials�, Vol. 13, pp. 213-219 (1999).

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1. INTRODUCTION

1.1 Reinforced Concrete

Reinforced concrete is the most commonly used material in construction industry.

There is no other material, which is utilized by human being in such large quantities

world wide as the concrete. The world consumption of concrete was over 3 billion

tones in 1990. The concrete, produced by mixing of cement, aggregate and water, has

the unique characteristics of withstanding weather and exposure conditions, high

strength and affordable cost. Uptill recent past, concrete was thought to be maintenance

free material until failures of the structures were reported from various parts of the

world. Some of the notable examples are the poor durability performance of bridge

decks in USA and Europe, and the deterioration of reinforced concrete structures in the

coastal regions of Arabian Gulf. The factors attributed to the failures include severe

environmental conditions, poor construction practices and improper choice of materials.

It is estimated that in 1989, $ 20 billions were needed for the rehabilitation of highway

structures in USA and more than 600 million pounds sterling in the UK [Maslehuddin

et al., 1990].

The deterioration of concrete structures in Gulf region is basically a corrosion problem,

which consequently resulted in loss of strength due to cracking of the structure. The

concrete structures in the Gulf coastal areas are under continuous exposure to ground

and atmosphere contaminated with salts. The salt has an easy ingress into the concrete

structure as a result of capillary action and high humidity conditions. The aggressively

of the atmosphere can be judged by the fact that the chloride and sulfate contents of

atmospheric air in Dhahran region (July- August) are about 490 times that in the air in a

typical marine atmosphere in the USA. The chloride content in the Arabian Gulf is

about 1.6 to 2 times as high as the seawater from the Mediterranean or the Atlantic

[Maslehuddin et al., 1990]. Due to increasing number of corrosion failure of reinforced

concrete structures in recent years, research activities are focused not only to

investigations on determining the cause of failure and finding the preventive measures

but also on service life prediction of concrete structures. The research on latter aspect

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would pave the way for maintenance and repairing of civil structures. Besides

experimental work which is based on accelerated durability testings [Ahmed et al.,

1997], mathematical modeling of service life prediction has been carried out [Marchand

et al., 1995, Mangat et al., 1994, Sharif et al., 1997].

The main casual factors responsible for concrete deterioration, in decreasing order of

importance are:

(i) Corrosion of reinforcement

(ii) Sulfate attack and salt weathering

(iii) Cracking due to environmental factor

1.2 Hydration of Cement

The setting and hardening of Portland cement is due to the hydration and hydrolysis of

its constituents, which are all anhydrous. The main constituents are: Tricalcium silicate

(C3S), Dicalcium silicate (C2S), Tricalcium aluminate (C3A), Tetracalcium alumino

ferrite (C4AF), CaSO4, MgO and CaO (free). Several different types of Portland cement

are available, these are classified in ASTM specification 150. Normal Portland cement

(Type I) is used as general purpose cement and Type V is sulfate-resisting cement, and

has not been generally used for structures susceptible to chloride infiltration. The

cement by itself is not cementitious only its hydration products are. On hydration, the

cement constituents give rise to hydrated calcium silicate or calcium silicate hydrate

obtained as poorly crystallized gel (3 CaO. 3SiO2 3H2 O) commonly known as

tobermorite gels. The hydration of cement involves releasing of substantial amount of

heat energy of the 4 major constituents, viz., C3S, C2S and C4AF release about 60-120

al/g of heat whereas C3A releases about 325 cal/g. The increased heat release is due to

exothermic reaction between C3A and gypsum.

When cement is gauged with water, the C3A, C3S and C4AF phases react very rapidly

and gauging water becomes saturated with Ca (OH)2 formed in the hydration reaction.

C2S phase hydrates rather slowly. The gypsum in the cement also dissolves rapidly, and

reacts with aluminate to give calcium sulpho aluminate, 3CaO.Al2O3. 3CaSO432H2O,

which also occurs in nature as ettringite. The important engineering properties of

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cement, viz., compressive strength and dimensional stability arises from the surface

properties and porosity of the cement gel. The principal constituent of this gel is

tobermotite, a calcium silicate hydrate (CSH) obtained by the hydration of C3S and C2S.

Ca(OH)2 (15-25% by weight of cement) formed during hydration does not contribute to

the strength and durability of the concrete rather it has diverse effects in the form of

retrogression in strength and stiffness of concrete. Inclusion of an additive like silica

fume can reduce the bad effect of Ca(OH)2. Microsilica is supposed to react with

Ca(OH)2 formed during hydration of cement and forms CSH. Thus the content of

Ca(OH)2 will decrease, while the content of CSH which provides strength and durability

will increase.

C2S and C3S constitute about 75% of the cement by weight. The ratio of C2S and C2S

plays an important role on the properties of cement. It is known that this ratio controls

the quantum of Ca(OH)2 in hydrated paste, as well as the rate of early stage of hydration

and strength development. Stochiometric calculations showed that hydration of C3S

produces about 2.2 times the Ca(OH)2 than the same amount of C2S. Liberation of

higher amounts of Ca(OH)2 is likely to increase sulfate attack due to several reasons.

The influence of C2S/C3S ratio on the sulfate resistance of cement has been studied

during laboratory tests on 2 cements which had fairly close C3A contents, 9.3% and

11.9% but widely differing C3S/C2S ratios of 2.57 and 7.88, respectively

[Rasheeduzzfar et al., 1990]. It was found that after 150 days of immersion in a Na2SO4

solution the deterioration in 7.88 C3S/C2S cement was 1.8 times more than 2.57

C3S/C2S cement. This type of attack is highly prevalent in the Middle East where a

sizable proportion of cement produced is type V, but has high C3S and C2S ratio.

1.3 Role of C3A in Chloride Binding

Study conducted on pore solution composition of hydrated paste of a high C3A cement

shows the strong beneficial influence of the C3A content of cement in the removal of

free chloride from the pore solution, which is critical for initiation of corrosion of steel

reinforcement embedded in steel [Rasheeduzzfar et al., 1991]. The C3A phase of

Portland cement has the ability to complex with the 'dissolvable chloride' resulting in

the formation of insoluble Friedal's salt (3CaO Al2O3 CaCl2 102HO) and possibly its

ferrite analogue (3CaOFeO3 CaCl2 10H2O). This combination of C3A phase with free

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chlorides in hydrated cement results in the reduction of corrosion inducing dissolvable

chloride in the pore solution, and hence in lowering the risk of corrosion. Chloride

analysis of the pore solutions extracted from mature hydrated cement pastes, fully

confirms a systematic increase in the removal of dissolvable chlorides from the pore

fluid, with an increase in the C3A content of cement [Rasheeduzzfar et al., 1990, 1991].

Holden et al., [1983] reported a 2.5 times higher diffusivity of chloride in hardened

cement made with low C3A Type V cement (C3A : 1.9%) compared to that made with a

high C3A Type I cement (C3A : 14.3%). It appears that the interactive effect of high

chloride complexability, and reduced chloride ion diffusivity of high C3A cement,

enables it to perform better in terms of corrosion protection of steel. It shows that the

current practice of indiscriminate use of Type V low C3A cement for reinforced

concrete superstructures by the construction industry in the Gulf countries is a mistake.

The superior performance of Type I over Type V in chloride environment has been

exemplified by a number of investigators [Monfore et al., 1960, Verbeck et al., 1968].

1.4 Performance of Blended Cements in Chloride and Sulfate Environment

The deterioration of concrete structures exposed to marine environment and in contact

with soil and/or contaminated ground water is quite a common phenomenon. The

sulfate ion reacts with hydration products of cement, namely C3A and Ca(OH)2

resulting in deterioration of concrete structure. In such environments, ASTM C150

Type V cement with a low C3A has been recommended.

In marine environment including saline subkha soil and ground water, there is

concomitant presence of chloride and sulphate; the role of chloride in durability

performance of concrete is not very well understood. The conjoint presence of Cl- and

SO4= in concrete particularly when it is fresh, influences reinforcement corrosion by two

mechanisms namely by a reduction in the chloride binding capacity of cements and by

decreasing the electrical resistivity of concrete. The rate of reinforcement corrosion in

the concrete specimens contaminated with Cl-+SO4= was on an average 1.4 to 2.0 times

that in the concrete specimens contaminated with only chloride [Maslehuddin et al.,

1996]. Studies were carried out on the effect of SO4=, and SO4

= and Cl- environments

on the expansion as well as reduction in strength of plain and blended cements [Al-

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Amoudi et al., 1995]. The results indicated hat the presence of chloride ions in the

sulfate environment mitigate the sulfate attack in plain and blended cements. The

deterioration related to the reaction of sulfate ions with the cement hydration products

was found to be more severe in specimens immersed in pure sulfate solution compared

with those immersed in the sulfate-chloride solution. The performance of flyash-

blended cement was found to be better than plain cement and cement blended with

silica fumes and blast furnace slag (BFS) in retarding sulfate attack. The higher

deterioration in silica fume and blast furnace slag cement mortar specimens was

attributed to the reaction of Mg++ cations with cementitious C-S-H Phase.

Blended cement formed by cement replacement with 20% silica fume, 30% flyash and

50% furnace slag showed significant superior strength and durability performance in

terms of rebar corrosion, sulfate resistance, chloride permeability and alkali silica

reactivity [Rasheeduzzafar 1992]. In another study, mortar specimens made with flyash

silica fumes and BFS were exposed to Na2SO4 and MgSO4 solutions. The results of the

study indicate that while performance of all blended cements, particularly those made

with silica fumes, was generally excellent yet their performance in MgSO4 solution was

not satisfactory [Amoudi et al., 1995]. Previous studies carried out by Rasheeduzzfar et

al., [1994] and Cohen et al., [1988] on performance of blended Portland cement in

magnesium sulfate and sodium sulfate solution arrived to the same conclusions.

The current practice of exclusive and indiscriminate use of Type V cement with very

low C3A content (<3%), by the construction industry is unwise. This is due to the fact

that corrosion of reinforcement and not the sulfate attack is the predominant mode of

deterioration. In environments characterized by the concomitant presence of high

chloride and sulfate concentrations, the structures would be concurrently subjected to

sulfate attack and chloride rebar corrosion. In such situations, the use of Type V cement

provides adequate protection against sulfate attack but would fail to remove free

chloride to any extent for the simple reason that up to 5% C3A in the cement is

preferentially consumed by 3% gypsum (CaSO4.2H2) typically added in all Portland

cements to regulate the time of set [Mehta 1978, Rasheeduzzafar et al., 1990]. A

possible approach to solve this problem of Cl- and SO4= concomitant existence is to use

a high C3A cement modified with a suitable admixture to provide sulfate resistance

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equivalent to a low C3A (Type V). While high C3A would take care of free Cl- by

complexing it, critical concentration admixture would lower down the C3A content

equivalent to that present in type V cement. Such cement would be simultaneously

resistant to sulfate attack and chloride induced reinforcement corrosion.

Another approach to address chloride - sulfate problem is to formulate high durability

performance modified cements by blending Type I cement with pozzolanic materials

such as silica fume, flyash and BFS. Results of the studies [Rasheeduzzafar et al.,

1992] conducted at KFUPM on blended OPC or Type I cements showed that silica

fume, flyash and BFS blended cement concretes performed 7 to 5 times better than plain

Type V (C3A : 2%) and 3 to 2 times better than Type I (C3A: 14%) cement concrete in

terms of time of initiation of corrosion. A significant conclusion arrives from this study

is that same blended high C3A cements performed 1.2 to 2 times better in terms of

sulfate resistance than the plain Type V (C3A:2%) cement.

1.5 Microsilica / Silica Fume

During the last three decades, some new Pozzolan materials have emerged in the

building industry as an off shoot of research aimed at energy conservation and strict

enforcement of pollution control measures to stop dispersing the materials into the

atmosphere. Silica fume (other names have been used are silica dust, condensed silica

fumes and microsilica) is one such Pozzolan, which has been used as a partial

replacement of Portland cement due to its versatile properties. The availability of high

range water-reducing admixtures (superplasticizers) has opened up new vistas for the

use of silica fume as part of the cementing material in concrete to produce very high

strength cement (> 100 Mpa/15,000 psi).

Silica fumes or microsilica is a by-product from the reduction of high purity quartz with

coal in electric arc furnaces in the manufacture of silicon and ferrosilicon alloys. The

fume, which has a high content of amorphous SiO2 and is consisted of very fine

spherical particles, is collected from the gases escaping from the furnaces. Silica fume

is also collected as a by-product in the production of other silicon alloys such as

ferrochrome, ferromanganese and ferrovanadium.

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Microsilica is predominantly silicon dioxide. Its prime characteristic is particle size

which would be as low as 0.2 micron, which is about 100 times smaller than Portland

cement grains. The extremely small grain size of microsilica is responsible for its high

reactivity with free lime in the concrete to form a strong and non-permeable paste. The

other important properties which established microsilica as a formidable building

material are its imperviousness to water, low permeability to chloride ion and resistant

to sulfate attack [Svenkerud 1991]. Table-1 shows typical composition of Elkam

Microsilica. Because of high surface area and high contents of amorphous silicon in

silica fumes, the latter acts as a highly active Pozzolan and reacts more quickly than

ordinary Pozzolans. The Pozzolanic reaction may begin as early as 2 days after cement

hydration [Mehta 1983, Sellervold et al.,1983] and the main Pozzolanic effect of silica

fume in concrete takes place between the ages of 3 and 28 days for curing at 20oC. The

presence of silica fumes provides increased internal cohesion of fresh concrete. It is

possible to design fluid silica fume concrete with essentially no bleeding or segregation.

As a result, local areas of weakness such as bleed water channels and voids under coarse

aggregate particles can be eliminated. The transition zone between cement paste and

coarse aggregate particles is an especially critical region in most concrete. It is

frequently the weakest part because of bleed-water voids, yet it is under the greatest

stress because of the elastic mismatch between the cement paste and the relatively stiff

aggregate material. The presence of microsilica brings reduction of bleeding in fresh

concrete and in consequences, significant improvements in the density of the transition

zone and in the mechanical behavior of hardened concrete. The strength of the

transition zone can be further enhanced by a Pozzolanic reaction. Cheng and Zhang

[1986] cite the Pozzolanic reaction as being responsible for the increased strength of

transition zone in silica fume concrete. Silica fume particles may also act as nucleation

sites for the products of cement-hydration reaction. Mehta [1987] pointed out that this

effect would cause grain refinement of the hydration, products. One might expect

"grain refinement" to lead improved mechanical behavior as it does in metal and ice.

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It has been shown by many investigators that the presence of microsilica in suitable

concentration in concrete dramatically reduces the permeability to chloride ion. With

water/cement ratio of 0.4 to 0.5 or lower, the microsilica - mixed concrete behaved

similar to latex modified concrete or polymer impregnated concrete [Perraton et al.,

1988]. Addition of microsilica also markedly lowers down the diffusion rates by a

factor of 3 or more [Elkem Microsilica, 1989]. A reduction by a factor of 3 indicates a

corresponding delay in the time of corrosion initiation. In a recent study [Toru et al.,

1991] investigated the influence of condensed silica fume (CSF) on the chloride

permeability and chloride corrosion of steel bars embedded on concrete is revealed. It

was confirmed that the small amount of CSF could effectively reduce the chloride

permeability and improve the protective function of concrete against the chloride

corrosion of steel bars, the most favorable ratio of silica fume in cement was about

10%. In another study [Manget et al., 1991], steel reinforcement electrodes embedded

in different matrices of concrete (flyash, BFS or microsilica) at water/cement + blend

ratio of 0.58 were exposed to simulated splash zone for ~ 600 days after initial curing in

air for 1 4 days. The corrosion potential and polarization resistance were monitored at

regular intervals to determine the state and rate of corrosion. The results show that

maximum protection against rebar corrosion is provided at 60% replacement of cement

by BFS and at 10-15% replacement by microsilica. The corrosion rates are more

sensitive to Cl-/OH- ratio. It has been reported that microsilica provides more protection

against corrosion especially when concrete is exposed to seawater or other aggressive

environments [El-Eissa et al., 1992]. The influence of BFS, silica fume and flyash

admixtures on the corrosion prevention properties as well as on the chloride prevention

properties of mortar were qualitatively investigated [Otsuk et al., 1991]. The mortar

specimens were exposed under wet (in salt water), dry or salt-water spray conditions.

The admixture were effective in improving the electrochemical and corrosion

prevention properties as well as permeability of mortar against chloride ion penetration.

The corrosion resistance of steel bars in concrete containing a combined admixture,

consisting of flyash or BFS blended with condensed silica fume, was studied in

solutions of 2% and 5% H2SO4 [Soeda et al., 1991]. The penetration depth of chloride

ion from 3% NaCl solution spray and carbonation depth from 5% CO2 gas were also

measured. The early strength of concrete with only flyash or BFS is lower than no

admixture concrete but it is improved by condensed silica fumes. The concrete

1.5.1 Chloride Ion Permeability

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containing the admixture performs better than the concrete without silica fumes with

respect to chemical resistance. Studies were carried out with mixes made of silica fumes

and type K cement and it was found that increased silica fume reduced the workability,

increased the strength significantly and dramatically reduced the expansion of

shrinkage-compensating concrete made with type K cement [Bayani et al., 1992]. The

silica-fume concrete made with type K cement can be considered a suitable material for

highway bridges.

The amount of C3A in cement plays a key role in the availability of chloride for

corrosion attack on rebar. Rasheeduzzafar et al., [1991] carried out a detailed study on

the reinforcement corrosion-resisting characteristics of silica fume blended cement

concrete in presence of varying C3A contents. The 4 plain cements had C3A contents of

2, 9 11 and 14% respectively to evaluate the effects of C3A factors on corrosion

resistance characteristics of plain as well as blended cements. Eight numbers of blended

cements were formulated in a manner that each of the four plain C3A cements had 10

and 20% cement replacements by silica fume. As expected, the results of accelerated

corrosion monitoring tests showed that the time of initiation of corrosion of

reinforcement is significantly influenced by C3A content. Silica fume blending of plain

cements by 10 and 20% partial replacement very significantly improves corrosion

resistance performance in terms of corrosion initiation time. On an average, 10 and

20% silica fume blended cements, respectively perform 3.45 and 3.75 times better than

the parent plain cements. Corrosion resistance of 9, 11 and 14% C3A cement blended

with 10% silica fume was found to be 5.12, 7.35 and 7.49 times better compared to the

performance of Type V 2% C3A cement commonly used in Middle East marine

environment. Hardly any perceptible advantage was observed by increasing silica fume

from 10 to 20% as cement replacement. The beneficial C3A - chloride complexing

effect was found to be operative in microsilica cements also, but on a reduced scale

compared to plain cements.

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Appropriate use of silica fumes improves both the mechanical characteristics and

durability of concrete. However, there is some disagreement among researchers as to

whether the mechanisms underlying these improvements depend primarily on physical

or pozzolonic nature of silica fume. Detwiler et al., [1989] used carbon black, which is

physically similar to silica fume but is not pozzolanic, to evaluate the relative

significance of physical and pozzolanic effects. Results show that at an age of 7 days,

the influence of silica fume on the compressive strength of concrete may be attributed

mainly to physical effects. But at the age of 28, both physical and chemical effects

become significant. Yogendran et al., [1987] studied the efficiency of silica fumes in

influencing the strength of high strength concrete at different water cementitious ratio

and dosage of silica fume. The results suggested that the optimum replacement of

cement by silica fume in high strength concrete (50 to 70 Mpa) at 28 days is 15%.

Furthermore, the effect of silica fume decreases with increasing cement content and

decreasing water-cement ratio. There is a difference of opinion regarding the reason

why silica fume increases the strength of concrete when it is used as a partial

replacement for cement. Some evidence supports the viewpoint that increase in strength

is due to an increase in the strength of cement paste constituent of concrete. However,

contradictory evidence shows no increase in strength of cement paste, but substantial

increase in concrete strength, when silica fume is used. The latter evidence is used to

support the theory that silica fumes strengthens concrete by strengthening the bond

between cement paste and aggregate. Studies were carried out by Xiafen Cong et al.,

[1992] using cement pastes, mortars and concrete with water-cementitious material ratio

ranging from 0.3 to 0.39. Mixes incorporate: (i) no admixtures (ii) a superplasticizer

only or (iii) silica fume and a superplasticizer. The results show that the replacement of

cement by silica fume and addition of a superplasticizer increases the strength of cement

paste which is responsible for increased compressive strength.

There is strong evidence to show that silica fume increases the homogeneity and

decreases the number of large pores in cement paste [Mehta et al., 1982, Feldman et al.,

1985] both of which would lead to a higher strength material. Work by Darwin et al.,

[1970] with cement paste and mortar supports the importance of the quality of paste in

controlling concrete strength.

1.5.2 Strength and Durability

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According to Bache [1981], the principal hydration effect of silica fume in concrete is

that of a filler which because of its fineness, can fix into spaces between cement grains

in the same way that sand fills the spaces between particles of coarse aggregates and

cement grains fill the spaces between sand grains. According to Delage and Aitcin

[1983] and Mehta et al., [1982], incorporation of silica fume in concrete essentially

eliminates pores between 500 and 0.5 microns in size and reduces the size of pores in

the 50 to 500 micron range.

The effect of CSF on the mechanical behavior of the steel-concrete bond, and on the

microstructure of the steel-cement paste transition zone was studied by several

investigators [GjΦrv et al., 1990; Bentur et al., 1987; Cheg-Yi et al., 1985]. The

addition of CSF up to 10% by weight of cement showed an improving effect on the

pullout strength, especially in the high compressive strength range of concrete. It also

affected the morphology and microstructure of steel-cement paste transition zone. Thus

aggregate matrix transition zone, which is usually porous and weak in normal concrete,

became very compact in presence of silica fume.

According to Goldman et al., [1989], the enhancement in concrete strength obtained by

the addition of silica fume is primarily due to the increased bond strength between

hydration products and coarse aggregate. The higher bond strength causes the aggregate

to act as�reinforcing filler'.

1.5.3 Hydration Mechanism

The effect of silica fumes on the hydration of type-K expansive cement was studied

[Holden et al., 1991] and it was found that in presence or absence of silica fumes,

expansion was associated with the formation of ettringite. Expansion continued as long

as ettringite was forming. Addition of microsilica accelerated the hydration of the paste

and also reduces the duration of expansion. Delayed expansion and therefore,

microcracking and degradation of the hardened expansive cement parts could be

prevented with the silica fume. The hydration of ordinary Portland cement was studied

by using silica fumes (5, 10, 15 and 20% by weight of cement) [Helmy et al., 1991]. It

was found that the addition of microsilica enhanced the hydration kinetics. It was also

found that 5% silica fume improved the mechanical properties of cement paste in tap

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water as well as in MgSO4 and Na2SO4 solutions. Electrical properties of hydrating

Portland cements with or without silica fumes were studied from 5 min to 90 days using

AC impedance spectroscopy [Christensen et al., 1992]. The bulk resistance of paste

increased with increasing silica fume contents and/or decreasing water content.

The compressive strength, bubble spacing factor, chloride ion permeability, and micro-

structural characteristics of silica fume concrete experimental side walk, placed in 1980

in Quebic province were examined [Lenard et al., 1992]. The hydrated cement paste,

even after the long field exposure under severe climatic conditions and regular deicing

salt applications, was relatively dense and unaffected for all but one sample, which

exhibited usually high degree of microcracking. The paste aggregates showed no

evidence of deterioration and the bond was still strong. There was no evidence of

leaching of cement components and the permeability was still very low. Hydrated DSP

cement pastes containing < 64 weight % silica fumes that reacted at < 120 oC were

studied using X-ray diffraction, Si-29 solid state nmr spectroscopy and TGA [Suu et al.,

1991]. The amounts of Ca(OH)2, unreacted silica fume and residual cement were

determined. The silica fume appeared to participate in an incipient pozzolanic reaction

without reducing the content of Ca(OH)2. The combined effect of polymer and super

plasticizer, using acrylic based polymers and naphthalene formaldehyde condensates

(superplasticizer) on cement mortar-containing silica fumes were studied [Xu et al.,

1990]. Condensed silica fume works as an antifoaming agent in the case of polymers

having air-entraining properties. Properties like setting time, water absorption, freezing

thaw resistance and dynamic Young modules were measured. Experimental and

simulation studies of the interfacial zone in concrete were carried out by making direct

comparison between results obtained using a 3 dimensional microstructural model and

those obtained experimentally on a similar set of mixes containing various amounts of

silica fumes [Bentz et al., 1992]. Based on this model and experimental results it is

suggested that the presence of silica fumes produces a more homogeneous

microstructure by balancing the Ca/Si molar ratio in the interfacial zone relative to that

in the bulk paste. Unlike in the ordinary Portland cement concrete, where this ratio

increases dramatically as the aggregate surface is approached. A comparative study of

various silica fumes as additives has been carried out by testing twenty silica fumes in

presence of two cement superplasticizers [DeLarrad et al., 1992]. It was possible to

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establish an empirical relationship showing the influence of the alkali content of the

silica fumes on its pozzolanic activity.

In view of the high applicability of microsilica in corrosion protection of rebars in

concrete, in the present work, study has been carried out to evaluate the chloride

permeability in ordinary portland cement (OPC) and sulfate resistant cement (SRC)

blended with Elkem Microsilica Grade 920. The influence of addition of microsilica on

compressive strength of the concrete has also been studied. In order to study the

corrosion of rebars at the rebar/concrete interface open circuit potential (OCP)

measurement has been carried out. These results may explain the destruction process of

concrete in seawater and chloride ion environments. Also, it will establish the effect of

blending of concretes with microsilica, on corrosion control of rebars.

2. OBJECTIVES

1. To compare the performance of ASTM Type-I and V when mixed with

microsilica.

2. To determine the influence of different constituents present in the hydrated

microsilica admixed concrete on some selective physico-chemical and mechanical

properties of concrete.

3. To determine the corrosion behavior of rebarred concrete containing microsilica

in chloride and seawater environment.

4. To study the chloride permeability in different microsilica added concrete mixes

and its influence on corrosion rates.

5. Evaluation of corrosion protection performance of microsilica containing concrete

by electrochemical method.

3. EXPERIMENTAL Concrete mixes were made using ASTM Type-I (OPC) and Type-V (SRC) cement with

and without microsilica. Microsilica used in this test program were of two types,

densified and undensified, and it was supplied by Saudi Al-Amar for Trading and

Industry, Dammam. The concrete mixes were batched and mixed by M/S Al-Bassam

Global Corp. at their readymix batch plant in Al-Jubail Industrial City. The mixes were

2 5 8 4

batched, mixed and sampled in the presence of M/S Saudi Al-Amar�s representative and

Corrosion Dept., RDC�s staff. The aggregates stockpiled at the batch plant were used in

the mixes. Coarse aggregates (3/4″ and 3/8″ nominal sizes) were batched under washed

and wet conditions.

3.1 Materials 3.1.1 Dry Undensified Microsilica

This form of silica was old �as produced� from the ferrosilicon or silicon metal bag

house, generally in bags. The undensified microsilica might be slightly agglomerated

and densified due to pneumatic handling, de-aeration in packaging, in addition to

pressure and vibration caused during transportation.

3.1.2 Dry Densified Microsilica

The air densified silica fume is processed by air injection into the bottom of the steel

storage silos creating turbulence and collision between silica fume particles. The

pressure-densified material is obtained by passing the uncompacted fume through a

device that compresses the individual particles together.

Both compacted forms are easily reversed and separated back to individual particles by

the shearing forces generated in concrete mixing. Composition of microsilica is given in

Table 1.

3.1.3 Cement

Ordinary Portland Cement (OPC), (ASTM Type-1) and Sulfate Resistant Cement (SRC)

(ASTM Type-V) were used in all concrete mixtures. Their chemical compositions are

given in Table 1.

2 5 8 5

To rebar the concrete samples, carbon steel rebars were cut in two sizes. Length of the

rebars for the samples to be tested electrochemically was kept 25 cm while for

destructive tests it was 15 cm. Diameter of rebars of both the tests was 14 mm. Rebars

were cut with the silicon carbide cutting wheel. Cutting speed was kept minimum and

cutting site was cooled properly to avoid heat generation which may affect the

microstructure of the steel and consequently corrosion behavior may change. Rebar

pieces were machined, polished, washed with ethyl alcohol, dried and weighed. After

weighing, 2.5 cm long portion at both ends were coated with coal tar epoxy. In case of

specimens to be tested electrochemically, 2.5 cm portion at the bottom and 12.5 cm

from the top of the rebar was coated. The ends of the rebars were coated in order to

keep the distance of exposed portion (10 cm long) of the rebar (inside the concrete)

equal from the outer surface of the concrete specimen. Rebars coated at the ends were

weighed again and fixed in the center of the moulds with the help of clamps provided at

both ends of the mould and concrete mixes were casted in.

3.2 Concrete Specimens Preparation

Five mixes designated as A, B, C, D and E were prepared. 10% each of densified and

undensified microsilica were mixed in OPC and SRC cements. Compositions of the

concrete mixes are given in Table�2. Seventy numbers of specimens were prepared from

each mix used in various tests and standard curing of 28 days was applied for the

specimens. These specimens were prepared at Al-Hoty-Stanger Ltd. Laboratories, Al-

Jubail in the presence of personnel of Corrosion Dept., R & D center, SWCC and Al-

Amar Co. Ltd.

3.2.1 Specific Gravity and Absorption Tests of Aggregates - ASTM C127/C128

Samples of sand and each individual size of coarse aggregate were collected from the

stockpiles prior to the first mix and tested for the specific gravities and absorptions. The

test results given in Table 3, were used to calculate the batch weights and total water

content of the concrete mixes.

3.1.4 Reinforcement Bars

2 5 8 6

Five mixes designated as A, B, C, D and E was prepared of OPC and SRC cement

adding densified and undensified microsilica. Coarse aggregates were batched in the

washed wet condition. Fresh concrete properties of each mix were recorded (Table�4).

Each mix contained of 0.075 m3 of concrete and 70 specimens of 152x76 mm were

casted for carrying out various tests. A steel reinforcement bar of 14 mm dia was fixed

in the center of the moulds after being cleaned and accurately weighed. These cylinders

were casted in two layers and vibrated for compaction. After casting, the specimens

were covered with wet cloth for 24 h prior to demoulding. After demoulding, the

specimens were cured in water at room temperature for a period of 28 days.

3.3 Rebar Corrosion Tests

Following tests were carried out at Corrosion Laboratories of the R & D Center to study

the corrosion behavior of rebar and rate of permeation of seawater and NaCl solution in

the concrete. In order to verify the results, in all the tests three specimens of each mix

were tested after every exposure period.

3.3.1 Salt Fog Test

In the salt fog chamber concrete specimens were placed in a way that all the specimens

were supported parallel to the principal direction of horizontal flow of fog. Specimens

were fixed separately on stainless steel sample holders. 5% sodium chloride solution

was atomized by compressed air, in the chamber. The temperature of the chamber was

kept at 38 °C.

Three specimens of each mix have been kept in the above mentioned conditions for 3,

9, 18 and 24 months period. After these durations, specimens were taken out from salt

fog chamber and were carefully observed for initiation of any crack. After this

observation, samples were cracked longitudinally and the rebar was investigated

microscopically for the formation of corrosion product on its surface.

3.2.2 Casting of the Test Specimens

2 5 8 7

In order to determine the chloride permeability trend in concrete during very long

exposure, salt ingress test was carried out for 800 days. Concrete specimens of all the

five types (as defined in Table 2) were immersed in 5% NaCl solution. Amount of

chloride ion ingressed in the concrete was determined by measuring the decrease in Cl-

ion concentration of the solution periodically. Kinetic of Cl- ion ingress was plotted and

its rate was calculated for different immersion periods.

3.3.3 Potential Measurement

Each concrete test specimen was partially immersed in 5% NaCl and seawater (Figure1)

of the composition given in Table 4. The potential readings were obtained by placing a

Saturated Calomel Electrode (SCE) firmly on concrete specimen. One of the two

terminals of a digital voltmeter was connected to the SCE, and the other was connected

to the exposed part of the reinforced steel bars to make a complete electrical circuit.

Readings were taken of three specimens and the average of the three readings was

computed as the potential of the reinforced steel bar at a fifteen days intervals. All of the

experiments were performed under free corrosion potential and at ambient temperature.

3.3.4 Immersion Test

The immersion tests were carried out at ambient temperature and open atmospheric

conditions. Concrete specimens of each mix were immersed vertically in 5% NaCl

solution and seawater. In these two media, specimens were dipped only 75% and 25%

of the area was always exposed to air. NaCl solution or seawater were regularly

replenished to maintain the level in the container. Three specimens of each mix were

taken out after an immersion period of 6, 12 and 24 months.

After taking out the specimens (from the seawater and NaCl solution), their surface

condition was inspected for cracks or chipping off problems. Lastly, specimens were

cracked longitudinally and the reinforced steel bars were inspected visually for the

extent of corrosion. Following tests were carried out at Al-Hoty- Stanger Laboratories, Al-Jubail. During

these tests specimens were tested in triplicate.

3.3.2 Salt Ingress Test

2 5 8 8

Cylindrical specimens of 12″ long and 6″ φ were lime water-cured for 1, 3, 7, 14, 28 and

56 days before carrying out the compressive strength test. These specimens were tested

in accordance with ASTM C-39 at the specific ages. Three specimens were tested at

each age and their average value was considered.

3.3.6 Rapid Chloride Permeability (RCP) Test The specimens for RCP testing of the concrete were conditioned as specified in

AASHTO T277. Two slices, each with a thickness of 50 mm, were cut from the

102x203 mm cylinders or cores, and their circumferences were coated with fiberglass

and resin. After an hour of air drying, they were kept under vacuum (pressure < 1 mm

Hg) for 3 hours, 1 hour more under vacuum with the specimens in de-aerated water, and

then left for 18 hours soaking in water at the atmospheric pressure. After this

conditioning, the specimens were placed in the testing cells. The testing consisted of

monitoring the amount of electrical current passed through the specimen, when a

potential difference of 60 V DC is maintained across the specimen for a period of 6

hours. In this test, chloride ions are forced to migrate out of NaCl solution subjected to a

negative charge, through the concrete, into a NaOH solution maintained at a positive

potential. The total charge passed, in columns, is used as an indicator of the resistance

of the concrete to the penetration of chloride ions.

3.3.7 Sulfate Resistance Test

The procedure (Test procedure TP F/2, Sulfate resistance of cementitious materials

using the flat prism method) was employed for this test with the following exceptions:

(i) The size of the prism was increased to 220x40x10 mm from 160x40x10

mm in order to accommodate a gauge length of 200 mm.

(ii) A demec gauge with gauge length of 200 mm was used to measure the

specimens to 0.002 mm accuracy.

(iii) The prism specimens were cast from mortar sieved out from plant batched

concrete mix. A standard test sieve of mesh size 4.75 mm (ASTM Mesh No.

4) was used to separate the mortar from the fresh concrete mix.

3.3.5 Compressive Strength Test

2 5 8 9

4. RESULTS AND DISCUSSION

4.1 Salt Fog Test

The extent of rebar corrosion in concrete specimens subjected to salt fog test was

monitored visually by sectioning them longitudinally into two parts. Visual observations

on the condition of rebar after 3, 9, 18 and 24 months of exposure are recorded in Table

5. These results indicate that the OPC and SRC specimens fail to protect the rebar from

corrosion after 18 months. However, addition of densified and undensified microsilica

appears to be effective in reducing the corrosion of rebars in both the type of cements.

But, it is more effective in the case of OPC cement, where no rebar corrosion was

detected even after 2 years of exposure, while in few specimens of densified microsilica

added SRC corrosion was observed after 2 years. During this test OPC blended with

densified and undensified microsilica were found equally protective in nature.

4.2 Electrochemical Test

The open circuit potential (OCP) Vs time data can be used with reasonable success to

interpret the susceptibility of the rebar to corrosion particularly localized corrosion

[Slater 1983]. A noble value OCP is ascribed to the reduction in the number of anodic

and cathodic sites which are formed by pores, defects, crevices etc. causing the surface

to be less reactive. On the contrary a shift of OCP in the negative direction generally

indicates a more reactive surface. Corrosion reaction taking place at the rebar-concrete

interface in a concrete specimen immersed in 5% NaCl solution and seawater was

monitored by OCP measurement of the rebars with time. The potential Vs time plots

shown in Figure 2 A and B indicate that the corrosion protection behavior of OPC and

SRC cement in 5% NaCl solution is very different from that found in seawater, which

has SO42+ and Mg2+ ions along with the Cl- ions. In 5% NaCl solution, rebar shows too

much rise in negative potential in OPC and SRC cement specimens as compared to the

densified microsilica (DMS) added concrete specimens. This confirms the effectiveness

of DMS in both the type of cement. However, undensified microsilica (UDMS) does not

appear to be effective in controlling the corrosion of rebar in OPC. In all the five types

of concrete mixes there is a linear rise in the negative potential up to 100 days. The

2 5 9 0

seawater corrosion of rebar in SRC specimens was found much lower as compared to in

OPC specimens. DMS further decreases the corrosion rate of rebars in SRC cement. On

the contrary, in the case of OPC cement after 100 days corrosion increases with the

addition of microsilica. This rise in the negative potential does not show parabolic

behavior up to 230 days. The negative potential of rebars in specimens of OPC with

UDMS was found much high than all the concrete mixes from fifth day of the

immersion till 230th day. On the basis of these results it appears that DMS, which does

supress the corrosion of rebars when added to OPC and SRC in chloride environment,

shows adverse effect on the corrosion protection behavior of OPC in seawater. UDMS

appears to be further harmful if added to OPC in seawater environment. The following

observations can be generalized from the above study:

(i) In chloride environment, plain OPC and SRC concretes show higher

negative potential indicating poor corrosion protection to the rebars as

compared to microsilica added concretes.

(ii) In seawater environment, OPC concrete specimens show much higher

negative potential than SRC specimens indicating poor corrosion protection

of OPC as compared to SRC.

(iii) In seawater environment blending of OPC concrete with densified or

undensified microsilica lowers down the potential to more negative values

indicating further deterioration in their protection behavior. In blended SRC

the potential values are much less negative indicating suppression of

corrosion in rebars.

A better corrosion protection behavior of SRC as compared to OPC in seawater

indicates that in the presence of chlorides, sulfate attack is retarded. Similar results have

been reported by Kind [1956], Yagonibali [1984] and Harrison [1990]. Kind [1956]

reported increased sulfate resistance in some cements in sulfate solutions containing

high chloride concentrations. He attributed this effect to:

2 5 9 1

(i) Increased solubility of calcium aluminate hydrate phases leading to calcium

sulfoaluminate crystallization, i.e. entringite, is formed in non-expansive

form.

(ii) Transformation of aluminate hydrate phases into chloroaluminate phases,

thereby reducing the quantity of entringite formed.

(iii) A decrease in lime concentration in the pore solution leading to the

conversion of the insoluble highly basic aluminate hydrate phases to soluble

low-basic compounds, forming entringite in the liquid phase.

Above explanation indicates that in cement, conversion of more C3A into soluble

ettringite, will decrease the alkalinity of the pore solution to much lower level as

compared to the cement which has comparatively lesser quantity of C3A. In view of the

above facts, in seawater the high corrosion of rebars in OPC having 8.5% C3A as

compared to SRC with only 3.5% C3A, can be assumed due to low alkalinity of pore

solution in OPC as compared to SRC.

A number of authors, such as GjΦru et al., [1979]; Neville [1969] and Al-Amoudi et al.,

[1995], have observed no significant difference between the performance of OPC and

SRC when both were exposed to a marine environment. All the above authors have

studied this effect on the mechanical strength of the concrete and not the corrosion

behavior of the rebars. However, the potential measurements results of the present

studies based on OCP show that the corrosion behavior of rebars in OPC differs from

SRC in seawater and chloride containing environments.

Comparatively high corrosion of rebars in OPC cement specimens on addition of

microsilica (Figure 2A) is attributed to the presence of Mg2+ cations associated with the

sulfate salts in seawater [Al-Amoudi et al., 1995]. The significant consumption of

portlandite [Ca(OH)2] by the pozzolanic reaction in microsilica blended cement causes

the Mg2+ cations to react directly with the calcium silicate hydrate (C-S-H) gel

converting it to a cohesion less, porous, reticulated magnesium silicate hydrate (M-S-H)

gel. The adverse effect of microsilica in MgSO4 environments has also been reported by

many investigators [Kind, 1962; Biczok, 1980; Lea et al., 1970; Rasheeduzzaffar et al.,

1994; Cohen et al., 1988; Bonen et al., 1992; Mangat et al., 1992 and Kalousek et al.,

2 5 9 2

1972]. In plain cements exposed to such a mixed sulfate environment, MgSO4 react

with Ca(OH)2 to form gypsum as shown in the following equations:

Ca(OH)2 + Na2SO4 + 2H2O → CaSO4. 2H2O + 2 NaOH (i)

Ca(OH)2 + MgSO4 + 2H2O → CaSO4.2H2O + Mg(OH)2 (ii) Since in microsilica blended cements Ca(OH)2 is consumed and C-S-H gel is formed,

MgSO4 reacts more directly with C-S-H gel in the following manner:

3CaO.2SiO2.xH2O + 3 MgSO4 + (10-x) H2O → 3 (CaSO4.2H2O) + 3 Mg(OH)2

+ 2SiO2H2O (iii)

4 Mg (OH)2 + SiO2 . nH2O → 4 MgO.SiO2.8.5H2O + (n-4.5) H2O (iv)

[Al-Amoudi et al., 1995] Thus, in the case of microsilica blended cements presence of more Ca(OH)2 in the pore

solution will lead to formation of more C-S-H which improves the strength and

corrosion protection behavior of concrete in chloride environment while on the contrary

will have adverse effect in the seawater environment due to presence of Mg2+ cations

which converts C-S-H to less protected M-S-H. It is known that the ratio of C3S/C2S

controls the quantum of Ca(OH)2 in hydrated cement, higher the C3S more the

liberation of Ca(OH)2 [Rasheeduzzaffar et al., 1990]. As the C3S/C2S ratio of OPC is

3.52 while that of SRC is 2.49, formation of more C-S-H in microsilica blended OPC

improves the corrosion protection in chloride environment (Figure 2B) while in

seawater it deteriorates (Figure 2A). 4.3 Immersion Test During visual inspection of the concrete specimens, immersed in seawater for one year,

signs of chipping off was observed in the specimens of OPC blended with microsilica

(Figures 3-6). It is important to mention that chipping was more in air-exposed portion

of the specimens as compared to the portion which was immersed in the seawater. The

surface condition of OPC specimens blended with UDMS and DMS (Figures 3 and 4)

indicates that the chipping problem has aggravated on blending with microsilica, while

this type of adverse effect of microsilica has not been observed on blending SRC with

2 5 9 3

microsilica (Figure 5). It appears that chipping of the concrete specimens occurs when

there is an irregular expansion in the concrete due to ettringite formation. On carrying out the destructive test by longitudinally cracking the concrete specimens

exposed to 5% NaCl or seawater for different durations, it was observed that the steel

bars in OPC cement blended with microsilica get more corroded when immersed in

seawater as compared to that in 5% NaCl solution environment (Table 7 and Figures 6

and 7). Table 7 reveals that SRC concrete are not very protective to rebars in 5% NaCl

solution, however, blending of this cement with microsilica does improve its corrosion

resistance. 4.4 Sulfate Resistance Test The data on expansion in OPC cement mortar specimens with and without microsilica

are plotted in Figure 8. The expansion in specimens of OPC immersed in the sulfate

solution was 0.020% after an exposure period of 336 days, compared with a value of

0.024% noted in specimens with undensified microsilica and 0.017% in specimens

blended with densified microsilica. The expansion data for specimens made with SRC

concrete are shown in Figure 9. These data indicate a trend similar to that exhibited by

the specimens made with OPC. But the major difference between the data of (Figures 8

and 9) is that SRC blending with DMS suppressed considerably the expansion,

however, OPC blending with the same grade of microsilica shows initially a slight

enhancement in expansion followed by virtually no change in expansion. Furthermore,

the expansion of blended OPC becomes lower C after 125 hours immersion. The

OPC blending with UDMS show clearly the enhancement of expansion on immersion in

sulfate solution. 4.5 Chloride Ion Ingress and Rapid Chloride Permeability (RCP) Tests Table 6 summarizes chloride ingress data in OPC and SRC concretes with and without

microsilica blending during Salt Ingress tests. Figure 10 shows kinetic of chloride

ingress in different types of concrete samples. Figures11 and 12 are bar diagrams

representing the effect of microsilica addition on the chloride ingress in different types

of concrete during Rapid Chloride Permeability tests. As it is evident from (Figures 10

and 11), chloride ion ingress measurement in all the five types of concrete mixes shows

the following behavior:

2 5 9 4

(i) In the initial 100 days chloride ingress in OPC is very fast, however, after this

duration, it slows down and there is very little increase in the Cl- ingress rate

(Table 6). (ii) In general Cl- ingress in SRC is very fast, however, slight decrease in the ingress

rate is observed after 400 days of immersion (Figure 10). (iii) Blending of microsilica with both the types of cement suppresses the ingress of

Cl- ions in the concrete, however, blending of densified microsilica in OPC

appears to be more effective than blending in SRC or blending of undensified

microsilica in OPC (Figure 11). Low chloride ingress in OPC as compared to the SRC seems to be due to high C3A

content in OPC as compared to SRC. The C3A phase of portland cement has the ability

to complex with the dissolvable chloride, resulting in the formation of insoluble

Friedle�s salt (3CaOAl2O3. CaCl2 + 10H2O). This combining of C3A phase with free

chlorides in hydrated cement, results in the reduction of the corrosion-inducing soluble

chlorides in the pore solution and also retards further ingress of chloride ion in the

concrete. These results are in consistent with the finding of Rasheeduzzafer et al.,

[1990, 1991]. Blending of microsilica to these portland cement further enhances the

chloride binding capacity of C3A by reducing the alkalinity of the pore solution which is

due to conversion of Ca(OH)2 in C-S-H. Also, it seems that the tiny particles (0.1-0.15

µm) of microsilica improves the packing of the system, which results in low

permeability of the chloride solution in the concrete. Low chloride permeability on

addition of microsilica has also been observed in RCP test carried out after a curing age

of 28 days (Figure 12). These results also show that at the age of 28 days, chloride

permeability rate in OPC and SRC are very high and are nearly equal, however,

blending of these cements with microsilica slows down the permeation of Cl- ions

drastically.

2 5 9 5

While studying the effect of any admixture on the corrosion protection behavior of

concrete, it is essential to see its effect on the compressive strength of concrete also. In

view of this, compressive strength of above mentioned concrete mixes were determined

after curing for different ages. Change in the compressive strength of all the five types

of concrete mixes with curing ages is shown in (Figure 13). This shows that the

compressive strength of all type of mixes gradually increases with curing time. OPC

which shows maximum strength after 1 day curing shows minimum strength after 56

days. SRC blended with densified microsilica shows highest strength at all the curing

ages after 3 days. Improvement in the compressive strength after blending the two types

of cement with densified and undensified microsilica at a standard curing age of 28 days

have been shown in (Figure 14A (in %) and 14.B in (psi). These Figures show that

blending of densified microsilica increases the compressive strength of SRC by 32%

(2020 psi) and of OPC by 23% (1400 psi) while undensified microsilica increases the

compressive strength of OPC by 10% (610 psi) only. Thus, densified microsilica

appears to be more effective in increasing the compressive strength of SRC than OPC.

5. CONCLUSIONS (i) Both OPC and SRC exhibit good corrosion resistance in seawater.

(ii) Blending of OPC with microsilica decreases the corrosion of rebars when the

concrete is exposed to a chloride atmosphere while it enhances the corrosion in

seawater.

(iii) Blending of SRC with microsilica suppresses the corrosion of rebars when the

concrete is exposed to seawater.

(iv) Microsilica causes drastic decrease in the permeability of chloride ion in OPC as

well as SRC concretes.

(v) Blending of microsilica increases the compressive strength of SRC concretes 10%

more than OPC concretes.

(vi) Densified microsilica appears to be a better admixture than undensified one.

4.6 Compressive Strength Test

2 5 9 6

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2 5 9 7

Table 1. Chemical Composition (%) of Cements and Elkem

Microsilica Grade 920

Constituent Microsilica Type V Cement

(SRC)

Type I Cement

(OPC)

Silicon dioxide 86-96 22.0 20.5

Aluminium oxide 1.0 4.2 5.6

Ferric oxide 0.1-1.5 5.1 3.8

Calcium oxide 0.1-0.5 64.3 64.4

Magnesium oxide 0.3-2.0 0.95 2.1

Sulfur trioxide 0.1-0.4 1.85 2.1

Loss on ignition 0.9 0.89 0.7

Potassium oxide 0.3-3.0 0.3 0.3

Sodium oxide 0.4-0.5 0.59 0.2

C3S - 54.6 56.7

C2S - 21.9 16.1

C3A - 3.5 8.5

C4AF - 12.9 11.6

OPC = Ordinary Portland Cement; SRC = Sulfate Resistant Cement

2 5 9 8

Table 2. Composition of the Concrete (per cubic meter) and Fresh Concrete Properties

Mix Designation A B C D E

Cement type OPC OPC OPC SRC SRC

Microsilica type - Undensified Densified Densified -

Cement kg/m3 385 350 350 350 385

Microsilica kg/m3 Nil 35 35 35 Nil

3/8″ agg. kg/m3 1160 1150 1150 1150 1160

Sand kg/m3 653 647 647 647 653

Free water lit/m3 150 150 150 150 150

Admixture SP6 lit/m3 6.0 6.0 6.0 6.0 6.0

Fresh Concrete Properties

Temperature, oC 28.3 27.7 26.0 27.5 27.4

Initial slump mm 230 240 230 200 230

Air content, % 2.3 2.0 2.3 2.7 2.5

Unit weight, kg/m3 2,388 2,380 2,399 2,367 2,400

OPC = Ordinary Portland Cement; SRC = Sulfate Resistant Cement

Table 3. Specific Gravity and Absorption Level of Sand and Aggregates

Aggregate Nominal Size 3/4″″″″ 3/8″″″″ Sand

Bulk Specific Gravity 2.501 2.544 2.625

S.S.D. Specific Gravity 2.557 2.598 2.637

Apparent Specific Gravity 2.650 2.689 2.655

Absorption (%) 2.3 2.1 0.4

2 5 9 9

Table 4. Composition of Gulf Seawater, Al-Jubail and Normal Seawater

Constituents Gulf Seawater, Al-Jubail

Normal Seawater

CATIONS (ppm)

Sodium, Na+ 13440 10780

Potassium, K+ 483 388

Calcium, Ca2+ 508 408

Magnesium, Mg2+ 1618 1297

Copper, Cu2+ 0.004 --

Iron, Fe3+ 0.008 --

Strontium, Sr2+ 1 1

Boron, B3+ 3 --

ANIONS (ppm)

Chloride, Cl- 24090 19360

Sulfate, SO42- 3384 2702

Bicarbonate, HCO3- 130 143

Bromide, Br - 83 66

Fluoride, F - 1 1.3

2 6 0 0

Table 5. Visual Observation of Corrosion of Rebars After Salt Fog Test

EXPOSURE PERIOD (MONTHS)

Mix Type 3 9 18 24

I II III I II III I II III I II III

OPC A A A A A A B B B B B B

OPC + UDMS A A A A A A A A A A A A

OPC + DMS A A A A A A A A A A A A

SRC + DMS A A A A A A A A A A B B

SRC A A - A A - B B - B B -

A = No Corrosion; B = Low Corrosion; C = Moderate Corrosion OPC = Ordinary Portland Cement; SRC = Sulfate Resistant Cement; DMS = Densified Microsilica; UDMS = Undensified Microsilica

Table 6. Chloride Ingress (ppm/day) in Microsilica Added Concrete Specimens

Immersion Period (Days) OPC

OPC + UDMS

OPC + DMS

SRC + DMS SRC

10 350 240 180 170 710

30 250 147 100 17 317

60 216 93 65 103 185

150 110 64 50 71 95

400 44 32 31 33 49

600 30 33 21 22 36

800 23 17 15 18 28

2 6 0 1

Table 7. Visual Inspection of Corrosion of Rebars After Immersion Test in 5% NaCl Solution and Seawater

Exposure Period (Months)

Concrete Test 6 12 24

Mix Type Media I Specimen II specimen III Specimen I Specimen II specimen III Specimen I Specimen II specimen III Specimen

OPC 5% NaCl Soln. A A A A A A B B B

Seawater A A A A A A A A A

OPC+UDMS 5% NaCl Soln. A A A A A A A A A

Seawater A A A B B B C C C

OPC+DMS 5% NaCl Soln. A A A A A A A A A

Seawater A A A A A A B B B

SRC 5% NaCl Soln. A A A B B C C B B

Seawater A A A A A A A A B

SRC+DMS 5% NaCl Soln. A A A A A A A A A

Seawater A A A A A A A A A

A = No corrosion; B = Low corrosion; C = Moderate corrosion; D = Severe Corrosion; OPC = Ordinary Portland Cement; SRC = Sulfate Resistant Cement; DMS = Densified Microsilica; UDMS = Undesified Microsilica

2 6 0 2

Figure 1. Concrete specimens immersed in 5% NaCl solution and seawater for open circuit potential measurement

2 4 6

2 6 0 3

-600

-500

-400

-300

-200

-100

0

0 50 100 150 200 250

Exposure Period (Days)

Pot

enta

il (m

V)

vs S

CE

OPCOPC+UDMSOPC+DMSSRC+DMSSRC

-700

-600

-500

-400

-300

-200

-100

0

0 50 100 150 200 250

Exposure Period (Days)

Pot

entia

l (m

V)

vs S

CE

OPCOPC+UDMSOPC+DMSSRC+DMSSRC

Figure 2. Kinetics of corrosion potentials of steel bars reinforced in different

concrete blends and immersed in [A] Seawater of Al-Jubail [B] 5% NaCl solution

DMS = Densified Microsilica UDMS = Undensified microsilica

OPC = Ordinary Portland Cement SRC = Sulfate Resistant Cement.

[A]

[B]

2 6 0 4

Figure 3. Surface condition of samples of OPC, blended with undensified microsilica, after an immersion test of 1 year in seawater

Figure 4. Surface condition of samples of OPC, blended with densified microsilica, after an immersion test of 1 year in seawater

2 6 0 5

Figure 5. Surface condition of samples of SRC, blended with densified

microsilica, after an immersion test of 1 year in seawater

Figure 6. Corrosion of carbon steel bars reinforced in samples of OPC, blended with densified microsilica, after an immersion test in

seawater for 1 year

2 6 0 6

Figure 7. Corrosion of carbon steel bars reinforced in samples of OPC, blended

with densified microsilica, after an immersion test in seawater for 1 year

2 6 0 7

0

0.005

0.01

0.015

0.02

0.025

0 50 100 150 200 250 300 350

�����������

EXPANSION, %

OPC(S)

OPC+UDMS (S)

OPC+DMS (S)

Figure 8. Expansion of microsilica added ordinary Portland cement

samples due to sulfate attack

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0 50 100 150 200 250 300 350

�����������

EXPANSION, %

SRC (S)

SRC+DM S (S)

Figure 9. Expansion of microsilica added sulfate resistant cement samples due to sulfate attack DMS = Densified Microsilica UDMS = Undensified microsilica

OPC = Ordinary Portland cement SRC = Sulfate Resistant Cement.

2 6 0 8

0.00

0.50

1.00

1.50

2.00

2.50

0 100 200 300 400 500 600 700 800

EXPOSURE PERIOD (DAYS)

CH

LOR

IDE

IN C

ON

CR

ET

E (

%)

OPCOPC+UDMSOPC+DMSSRC+DMSSRC

Figure 10. Kinetics of chloride ingress in different types of concrete sample.

SRC OPC SRC+DMS OPC+UDMS OPC+DMS0

0.5

1

1.5

2

2.5

CH

LOR

IDE

IN C

ON

CR

ET

E (

%)

SRC OPC SRC+DMS OPC+UDMS OPC+DMS

CONCRETE TYPE

Figure 11. Effect of microsilica addition on the chloride ingress in different types of concrete (A study of 800 days immersion in 5% NaCl solution)

DMS = Densified Microsilica UDMS = Undensified microsilica

OPC = Ordinary Portland Cement SRC = Sulfate Resistant Cement.

2 6 0 9

OPC OPC + UDMS OPC + DMS SRC + DMS SRC0

1000

2000

3000

4000

5000

CO

LOU

MB

S

OPC OPC + UDMS OPC + DMS SRC + DMS SRC

CONCRETE TYPE

Figure 12. Rapid Chloride Permeability test after 28 days.

DMS = Densified Microsilica UDMS = Undensified microsilica

OPC = Ordinary Portland Portland_Cement SRC = Sulfate Resistant Cement.

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

1 3 7 14 28 56

CURING TIME (DAYS)

CO

MP

RE

SS

IVE

ST

RE

NG

TH

(P

SI)

OPC

OPC+UDMS

OPC+DMS

SRC+DMS

SRC

Figure 13. Compressive strength of microsilica added OPC and SRC cement concrete specimens.

DMS = Densified Microsilica UDMS = Undensified microsilica

OPC = Ordinary Portland Portland_Cement SRC = Sulfate Resistant Cement.

2 6 1 0

0

5

10

15

20

25

30

35

OPC+UDMS OPC+DMS SRC+DMS

INC

RE

AS

E IN

CO

MP

RE

SS

IVE

ST

RE

NG

TH

(%

)

OPC+UDMS OPC+DMS SRC+DMS

6240 6240 6350

0

3000

6000

9000

CO

PR

ES

SIV

E S

TR

EN

GT

H (

PS

I)

OPC+UDMS OPC+DMS SRC+DMS

20201400

610

Figure 14. Compressive strength of different type of cements after blending with microsilica and curing for 28 days [A] increase in % [B] increase in psi.

DMS = Densified Microsilica UDMS = Undensified microsilica

OPC = Ordinary Portland Cement SRC = Sulfate Resistant Cement.

[A]

[A]

[B]

2 6 1 1

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