Effect of Cement on Silica Fume and Flyash

EFFECT OF FLY ASH AND SILICA FUME ON CONCRETE

1.0 INTRODUCTION

Earlier notion of using high amounts of cement for concrete has now changed on favour

of increased use of high amounts of mineral ad-mixtures and super plasticizers with reduced

amounts of cement and water in the concrete mixtures. Energy plays a crucial role in growth of

developing countries, like India. In context of low availability of non-recoverable energy sources

coupled with requirements of large quantities of energy to materials like cement, steel etc., the

importance of industrial wastes as building materials cannot be underestimated. In India about

110 million tones of fly ash has been produced by 68 major thermal power stations and are likely

to be doubled within next 10 years. It has been a published fact from research that waste

materials like fly ash; silica fume etc, through their use as construction materials can be

converted into meaningful wealth. Also, a partial replacement of cement with fly ash is desirable,

and indeed essential due to a variety of technical, economical and ecological reasons.

Researchers have reported that silica fume smaller in size and round shape fills the voids

between the coarser cement particles which may be otherwise occupied with water. A properly

proportional fly ash and silica fume in concrete mix improves properties of the concrete that may

not be achievable through the use of Portland cement alone. The resulting concrete mix becomes

strong, durable and economical and also eco-friendly as it utilizes an ecological hazardous

material.

B.V.BHOOMARADDI COLLEGE OF ENGINEERING & TECHNOLOGY HUBLI-31


2.0 MATERIALS 2.1 FLYASH (PULVERIZED-FUEL ASH)

Fly ash is a by-product of burning pulverized coal to generate electric power. There are

periodic variations in the operation of a power station can results in occasionally varying

properties of the fly ash. Differences in the fly as produced by different power stations. Even the

same power station will produce fly ash with varying properties if the coal used is non-uniform.

A simple picture of the behavior of concrete containing fly ash cannot be presented because fly

ash is to a single material of nearly constant composition

Figure show the Fly Ash

Fly ash depends on the chemical properties and the fineness of the Portland cement in the

mix. It is not surprising that there is no simple relation between the proportions of fly ash in the

B.V.BHOOMARADDI COLLEGE OF ENGINEERING & TECHNOLOGY HUBLI-31 2


total cementations material and the properties of the resulting concrete of otherwise fixed

proportions. Inevitable attempts to relate, by a single equation, the strength of concrete, even of

fixed proportions, to the various properties of fly ash such as fineness, residues of particles above

certain size, pozzolanic indices, carbon content, glass content, and chemical composition, have

been unsuccessful. Indeed, this situation is to be expected, given that no single equation can

predict the strength properties of Portland cements alone from their physical and chemical

properties.

The fly ash in concrete makes efficient use of product of hydration of cement such as

calcium hydroxide (C-H) which is otherwise a source of weakness in normal cement concrete

converts it into denser and stronger C-S-H compounds by pozzolanic reaction. The heat

generated during hydration initiates the pozzolanic reaction of fly ash.

2.2 SILICA FUME

Silica fume is a by-product of silicon or Ferro-Silica industry and is 100 times finer than

cement. A silica fume is also referred to as micro silica or condensed silica fumes. It consists of

amorphous silica and has high reactivity towards lime. The replacement level of silica fume is

generally low at about 10%. When SF is used in concrete mix, its introduction affects the

physical arrangement of the system, particularly near the aggregate surface where porosity exists.

Silica fume starts reacting at the early stage of hydration process. The pozzolanic action of silica

fume reduces substantially the quantity and size of “CH” crystals in hydrated cement paste. This

phenomenon along with low W/C ratio reduces the thickness of transition zones and thus the

preferential orientation of CH crystals is considerably reduced. All these result in more uniform,

stronger transition zone potential of micro cracking.



Figure shows the Silica fume

The specific gravity of silica fumes is generally 2.20 but it is very slightly higher when

the silica content is lower. This value can be compared with the specific gravity of Portland

cement, which is 3.15. the particles of silica fume are extremely fine, most of them having a

diameter ranging between 0.03 and 0.3m ; the median diameter is typically below 0.1m. The

specific surface of such fine particles cannot be determined using the Blaine method; nitrogen

adsorption indicates a specific surface of about 20000m2/kg, which is 13 to 20 times higher than

the specific surface of other pozzolanic materials, determined by the same method. Such fine

material as silica fume has a very low bulk density: 200 to 300kg/m3.

3.0 EFFECT OF FLY ASH ON CONCRETE

3.1 INFLUENCE OF FLY ASH ON PROPERTIES OF FRESH CONCRETE



For a constant workability, the reduction in the water demand of concrete due to fly ash is

usually between 5 and 15 percent by comparison with a Portland –cement –only mix having the

same cementations material content; the reduction is larger at higher water/cement rations.

A concrete mix containing fly ash is cohesive and has reduced bleeding capacity. The

mix can be suitable for pumping and for slip forming; finishing operations of fly ash concrete

and made easier.

The influence of fly ash on the properties of fresh concrete is linked to the shape of the

fly ash particles. Most of these are spherical and solid, but some of the large particles are hollow

spheres, know as cenospheres. The reduction in water demand of concrete caused by the

presence of fly ash is usually ascribed to their spherical shape, this being called a “ball bearing

effect”.

High carbon content in fly ash is that it adversely affects workability. Variation in carbon

content may also lead to earratic behavior with respect to air entrainment some air entraining

agent becoming adsorbed by the porous carbon.

3.2 STRENGTH DEVELOPMENT OF FLY ASH CONCRETE

The effect of chemical reactions, fly ash has a physical effect of improving the

microstructure of the hydrated cement paste. The main physical action is that of packing of the

fly ash particles at the interface of coarse aggregate particles, which are absent in the mortar used

in the test. The extend of packing depends both on the fly ash and on the cement used: better

packing is achieved with coarser Portland cement and with finer fly ash. One beneficial effect of



packing on strength is a reduction in the volume of entrapped air in the concrete. But the main

contribution of packing lies in a reduction in the volume of large capillary pores.

It is worth noting that the positive influence of the fineness of fly ash is coupled with it

spherical shape. Therefore, grinding of fly ash, although it increases fineness, may result in the

destruction of spherical particles with consequent increase in water demand of the mix due to the

irregular angular shape of the fly ash particles.

The beneficial influence of fly ash upon water demand does not extend beyond a fly ash

content of 20 percent by mass. An excessive content of fly ash is not beneficial from the point of

view of strength development either. The limiting content is probably around 30 percent by mass

of total cementitious material as can be seen from figure below.



Average values of strength of concrete cylinder moist cured at 23oC are shown in table

above. All the mixes had a total cementations material content of 307kg/m3 with 25 percent

content of fly ash by mass of total cementations material. The water/cement ratio was 0.4 to 0.45,

and the mixes had a slump of 75mm. The same table gives the strength of a Portland-cement

only concrete with the same cement content and the same water/cement ratio. It is worth adding

that maximum size of aggregate was 9.5mm so that the beneficial effect of fly ash with respect

to packing around the coarse aggregate particles was smaller than would be the case with

conventional concrete; therein may lie the explanation of apparently limited effect of fly ash on

strength.

3.3 DURABILITY ASPECTS

Although an early reason for the use of the various cementations materials in concrete

was their influence on the rate of development of heat and of strength, even more important is

their influence on the resistance of concrete to chemical attack which is the consequence not only

of the chemical nature of the hydrated attack, which is the consequence not only of the chemical

nature of the hydrated cement paste but also of its microstructure. It is no exaggeration to say

that the cementations materials have a major influence on all aspects of durability related to the

transport of attacking agent through concrete. The cementations materials considered are finer

than Portland cement and therefore, improve particle packing, so that provided adequate wet

curing is applied, their presence reduces permeability.



Even though the use of fly ash reduced permeability, it allows faster carbonation. The

increase in the rate of carbonation is greater when fly ash is used with Portland blast furnace

cement. The enhanced carbonation need not necessarily be large in practice when mixes with

proper mix proportions are used. Also, carbonation may reduce the permeability, but not when

both fly ash and ggb are present in the mix. Good resistance to freezing and thawing without air

entrainment was found in concretes (with a water/cement ratio of 0.27 and super plasticizer)

containing class C fly ash representing 20 to 35 percent of the mass of the total cementations

material, and silica fume (10 percent on the same basis). Likewise, good resistance to sulfate

attack was observed with class C fly ash contents up to 50 percent and 10 percent of silica fumes.

Control of the alkali-silica reaction is specialized topic in which a detailed knowledge of

the aggregate to be used is necessary. However, the beneficial effects of the incorporation of fly

ash (about 30to40 percent by mass) in the blended cement should be noted. These materials

contain only a small amount of water-soluble alkalis so that, at a given content of cementations

material which includes Portland cement with high alkali content, the presence of fly ash in the

blended cement reduces the total alkali content in the mix. Thus, the use of these materials may

obviate the need for low-alkali cement but the absence of expansive reactions should be verified

by test.

The beneficial effects of the inclusion of silica fume in steam-cured concrete at 65o C

upon its penetrability by chlorides was confirmed by Campbell and Detwiler. For significant

improvement, the minimum silica fume content was 10 percent in Portland-cement-only

concrete, but 7.5percent was highly effective in mixes containing 30 to 40 percent of ggbs in the

total cementations material. It may be added that curing Portland-cement-only concrete at 50 o C

was found to result in increased penetrability by chlorides.

4.0 EFFECT OF SILICA FUME ON CONCRETE

4.1 INFLUENCE OF SILICA FUME ON PROPERTIES OF RESH CONCRETE



The very large surface area of the particles of silica fume, which have to be wetted,

increases the water demand, so that, in mixes with a low water/cement ratio, it necessary to use a

superplasticizer. In this way, it is possible to maintain both the required water/cement ratio and

the necessary workability.

The effectiveness of superplasticizer in enhanced by the presence of silica fume. For

instance, in mixes with a slump of 120mm, a given dosage of superplasticizer was found to

reduce the water demand by 10 kg/m3 in a Portland-cement-only concrete. The same dosage

maintained the slump when the silica fume content was 10percent by mass of cementations

material. Without the super plasticizer, the water demand due to the inclusion of silica fume in

the mix would have risen by 40 kg/m3. It can therefore, be seem that the use of both silica fume

and a suitable superplasticizer is beneficial and makes it possible to use low water/cement ratios

at a given workability. The lower water/cement ratio results in an increase in strength which is

larger than would be expected solely from the pozzolanic action of silica fume. However, in

relative terms, the effect of the lower water/cement ratio upon strength is smaller than the overall

direct effect of silica fume.



At this stage, it may be useful to note that the pattern of the relation between compressive

strength and the water/cementations material ratio is the same for concretes with and without

silica fume but at the same ration, concrete with silica fume has a higher strength. Example of

the relation between the 28 day compressive strength of 100mm cubes and the water

cementations material are show in figure above. The same figure also showed the relation for

concrete containing Portland cement only.

The presence of silica fume affects significantly the properties of fresh concrete. The mix

is strongly cohesive and, in consequence, there is very little bleeding, or even more. The reduced

bleeding can lead to plastic shrinkage cracking under drying conditions, unless preventive

measures are taken. On other hand, voids caused by trapped bleed water are absent.



The cohesive character of the mix affects the slump so that, for both mixes equally to be

capable of compaction, the mix with silica fume needs a slump 25 to 50mm. higher than a mix

containing Portland cement only. Mixes with a very high content of cementations material tend

to be ‘sticky’ and do not easily allow the slump test inappropriate and that the flow test is

preferable. The ‘sticky’ nature should not be misinterpreted: as soon as vibration is applied the

mix becomes mobile. However in order to avoid and excessively sticky mix, it is recommended

that the water content should not be lower than 150 kg/m3 when the fine aggregate is angular in

shape, or not lower than 130 kg/m3 when rounded fine aggregate is used.

The cohesiveness of concrete containing silica fume makes it satisfactory for pumping

and for underwater concreting as well as for use as flowing concrete. Entrained air remains

stable, but an increase dosage of the air-entraining admixture is required because of the high

fineness of silica fume. In addition, there are problems in obtaining a suitable air-void system

when super plasticizers are used.

There are no reports of incompatibility of silica fume with admixtures in general. It is

useful to observe that the retarding effect of lignosulfonate based admixtures is smaller when

silica fume is present in the mix. Consequently larger dosages of these admixtures can be used

without causing an excessive retardation.

STRENGTH DEVELOPMENT OF THE PORTLAND CEMENT-SILICA FUME

SYSTEM

In addition to the pozzolanic reaction between the amorphous silica in silica fume and

calcium hydroxide produced by the hydration of Portland cement, silica fume contributes to the

progress of hydration of the latter material. This silica fume contributes to the progress of

hydration of the latter material. This contribution arises from the extreme fineness of the silica



fume particles which provide nucleation sites for calcium hydroxide. Thus, early strength

development takes place.

Silica fume dissolves in a saturated solution of calcium hydroxide within a few minutes.

Therefore as soon as enough Portland cement has hydrated to result in saturation of the pore

water with calcium hydroxide, calcium silicate hydrate is formed on the surface of the silica

fume particles. Their reaction proceeds, initially, at a high rate For example, when the mass of

silica fume was 10percent of the total mass of cementations material, one-half of the silica fume

was observed to react in 1 day; and two-thirds during the first 3 days. However, subsequent

reaction was very slow, only three-quarters of the silica have hydrated at 90 days.

The acceleration of hydration processes by silica fume occurs also when, in addition to

Portland cement, ggbs is present in the mix.

A consequence of the rapidity of the early reactions in concretes containing silica fume is

that the development of heat of hydration in such concretes may be as high as when rapid-

hardening Portland cement is used alone.

The behavior of concrete with silica fume beyond the age of about 3 months depends on

the moisture conditions under which the concrete is stored. Up to the age of 31/2 years, test

showed a small increase in compressive strength of wet stored concretes with 10 percent of silica

fume by (by mass of total cementations material) and water/cement ratios of 0.25, 0.3 and 0.4.

Under dry storage conditions, retrogression of strength typically up to 12 percent below the peak

value at about 3 months was observed in test on laboratory specimens. However, the strength of

concrete containing silica fume, determined on cores up to 10 years old, clearly show no

retrogression of strength. This finding is of importance because the behavior of test specimens in

which moisture gradient exist may be misleading.



One consequence of the high early reactivity of silica fume is that the mix water is

rapidly used up; in other words, self-desiccation takes place. At the same time, the dense

microstructure of the hydrated cement paste makes it difficult for water from outside, if

available, to penetrate toward the unhydrated remnants of Portland cement of silica fume

particles. In consequence, strength development ceases much earlier than with Portland cement

alone; some experimental date are shown in table below from which it can be seen that there was

no increase in strength beyond 56 days. The date of tablerefer to mixes with a total content of

cementations material of 400kg/m3, sulfate-resisting Portland cement, silica fume content of

10,15 and 20 percent by mass of total cementations material, and a water/cement ratio of 0.36 the

concrete specimens were maintained under moist conditions.

The contribution of silica fume to the early strength development(up to about 7 days) is

probably through improvement in packing, that is, action as a filler and improvement of the

interface zone with the aggregate. The bond of the hydrated cement paste with aggregate,

especially the larger particles, is greatly improved, allowing the aggregate better to participate in



stress transfer. Some contrary arguments about the role of silica fume have been advanced, but

they are likely to reflect specific test conditions rather than intrinsic behavior.

The contribution of a given amount of silica fume to the strength of concrete arising from

packing and interface effects should remain constant with time. This is unlike the effect of

pozzolanic activity which continues to take place. Indeed, at a fixed content of silica fume, the

increase in strength of concrete between 7 and 28 days was found to be independent of the value

of the 7 day strength. The contribution of silica fume to strength at say, 28 days should, however,

increase with an increase in the content of silica fume in the mix. This was found to be the case

for concrete with 28-days strength between approximately 20 and 80 MPa the increase in

strength was 7MPa for a 10 percent content, and 16MPa for 20 percent content of silica fume.

4.2 DURABILITY OF CONCRETE CONTAINING SILICA FUME

The importance of adequate curing of concrete containing silica fume from the stand

point of reactions of hydration. As far as durability is concerned, we should note that a

consequence of more advanced hydration is a reduced permeability, adequate curing is of

particular importance. Generally, for concretes of equal strength, the reduction in permeability

due to a longer period of curing is greater in concrete containing silica fume than in Portland-

cement-only concrete.

The desirable minimum curing period depends, among other things, on temperature,

which in the field may be subject to considerable variation. Low temperature slows down the

hydration reactions involving silica fume even more than is the case in Portland –cement-only

concretes. However, upon a subsequent rise in temperature, the usual reactions take place and the

accelerating effect of a higher temperature is greater than in the case of Portland cement alone.

Also the harmful effects of higher temperature on pore structure are smaller in the presence of

silica fume. It is important to note that carbonation is particularly adversely affected by

inadequate curing.



The influence of silica fume upon the permeability of concrete is grater than is indicated

by test on hydrated cement paste because, in the former case, silica fume reduces the

permeability of the transition zone around the aggregate particles, as well as the permeability of

the bulk paste. The influence of silica fume upon permeability of concrete is very large: a 5

percent content of silica fume was reported by Khayat and Aitcin to reduce the coefficient of

permeability by 3 orders of magnitude. Thus, in relative terms, the influence of silica fume upon

permeability is much larger than upon compressive strength.

A consequence of reduced permeability is a grater resistance to the ingress of chloride

ions. Even using Portland cements with C3A contents up to as much as 14 percent, the presence

of 5 to 10 percent of silica fume in the total cementations material greatly slows down the ingress

of chloride ions into concrete. The reduction in the diffusivity of chlorides due to the presence of

silica fume in hydrated cement is larger at water/cementations material ratios greater than 0.4

than at extremely low values of the water/cementations material ratio; in the latter case the

hydrated cement paste has a very low diffusivity, event without silica fume.

The sulfate resistance of concrete containing silica fume is good, partly because of lower

permeability, and partly in consequence of a lower content of calcium hydroxide and of alumina,

which have become incorporated in C-S-H. Test on solutions of magnesium, sodium and calcium

chlorides.

With respect to resistance to freezing and thawing, some investigators reported a poor

resistance of air-entrained concrete congaing silica fume as compared with Portland cement only

concrete. A possible explanation is that with adequate entrained air content, the concrete. A

possible explanation is that with adequate entrained air content, the concrete containing silica

fume had larger air-void content, the concrete containing silica fume had larger air void spacing

factor and at the same time, the dense structure of the hydrated cement paste prevented the

movement of water. On the other hand, other investigators found a good resistance of concretes

containing silica fume to freezing and thawing and also to scaling by de-icing agents. Experience

with structures in situ gave variable results.



Resolution of this conflict in reports on performance would require a detailed knowledge

of test procedures used, including the maturity and moisture condition of the concretes at the

time of the test. Indeed, influence of silica fume upon resistance to freezing and thawing is

complex. After a period of moist curing the pore size in the hydrated cement paste becomes

smaller in the pore size in the hydrated cement paste becomes smaller in the pore size in the

hydrated cement paste becomes smaller in the pore size in consequence, the freezing point of

pore water is reduced . In the interior of concrete, self desiccation is likely to have reduced the

water content below the critical level of saturation so that freezing would not cause damage.

The preceding discussion show that generalizations about the influence of silica fume on

the resistance of concrete to freezing and thawing, and even more so to scaling by de-icing

agents are not possible: much depends on the particular concrete used, on its treatment prior to

freezing and thawing, and on the rapidity of temperature changes. It is, therefore not surprising

that many publications of present conflicting results, and there would be little value in reviewing

them. For practical purposes, the only conclusion which can be drawn is that it is necessary to

test any concrete which it is proposed to use, and the test results have to be interpreted in the

light of the expected conditions of exposure.

Because silica fume reduces the alkali content in the pore water, the pH of pore water

becomes lowered. Tests on mature cement pastes made with Portland cement with a very

alkalinity (pH of 13.9) have shown a reduction in the value of pH caused by the inclusion in the

mix of 10 percent of silica fume to be 0.5: 20percent of silica fume reduced the value of pH by 1,

Event with the last named reduction, the value of pH was 12.9 Havdahl and Justnes confirmed

that the ph stays 12.5. Thus the alkalinity is adequately high for the protection of reinforcing

steel from corrosion.

The presence of silica fume in concrete has a beneficial influence upon resistance to

abrasion because in the absence of bleeding, no weak top layer is formed and also because of a

better bond between the hydrated cement paste and the coarse aggregate; differential wear and

loosening of particles do not therefore, occur. Shrinkage of concrete congaing silica fume is

somewhat larger, typically 15 percent than in Portland-cement-only concrete.



5.0 CONCLUSION:

Use of fly ash as a partial replacement for Portland cement is generally limited to Class F

fly ashes. It can replace up to 30% by mass of Portland cement, and can add to the concrete’s

final strength and increase its chemical resistance and durability. Recently concrete mix design

for partial cement replacement with High Volume Fly Ash (50 % cement replacement) has been

developed. For Roller Compacted Concrete (RCC)[used in dam construction] replacement values

of 70% have been achieved with POZZOCRETE (processed fly ash) at the Ghatghar Dam

project in Maharashtra, India. Due to the spherical shape of fly ash particles, it can also increase

workability of cement while reducing water demand. The replacement of Portland cement with

fly ash also reduces the greenhouse gas foot print of concrete, as the production of one ton of

Portland cement produces approximately one ton of CO2. Since the worldwide production of

Portland cement is expected to reach nearly 2 billion tons by 2010, replacement of 30% of this

amount by fly ash could dramatically reduce global carbon emissions

It has been found that silica fume improves compressive strength, bond strength, and

abrasion resistance. The improvements in concrete properties from addition of silica fume stem

from both the mechanical improvements resulting from addition of a very fine powder to the

cement paste mix as well as from pozzolanic reactions between the silica fume and free calcium

hydroxide in the paste.

Addition of silica fume also reduces the permeability of concrete to chloride ions, which

protects concrete's reinforcing steel from corrosion, especially in chloride-rich environments

such as those of northern roadways and runways (because of the use of deicing salts) and

saltwater bridges.


http://en.wikipedia.org/wiki/Deicing

http://en.wikipedia.org/wiki/Corrosion

http://en.wikipedia.org/wiki/Rebar

http://en.wikipedia.org/wiki/Chloride

http://en.wikipedia.org/wiki/Permeability_(fluid)

http://en.wikipedia.org/wiki/Calcium_hydroxide

http://en.wikipedia.org/wiki/Calcium_hydroxide

http://en.wikipedia.org/wiki/Abrasion

http://en.wikipedia.org/wiki/Bond_strength

http://en.wikipedia.org/wiki/Compressive_strength

http://en.wikipedia.org/wiki/Carbon_dioxide

http://en.wikipedia.org/wiki/Greenhouse_gas


6.0 REFERENCES 1. A.M Neville, Properties of Concrete 2. M.S.Shetty, Concrete Technology

` 3. http://www.wikipedia.org/ 4. http://www.fhwa.dot.gov/infrastructure/materialsgrp/silica.htm

5. http://www.fhwa.dot.gov/infrastructure/materialsgrp/flyash.htm


http://www.fhwa.dot.gov/infrastructure/materialsgrp/flyash.htm

http://www.fhwa.dot.gov/infrastructure/materialsgrp/silica.htm

http://www.wikipedia.org/

Documents

Effect of Cement on Silica Fume and Flyash