Pozzolanic materials - PFA and silica fume
Main references:

Neville A et al "Concrete Technology", revised reprint 1990, Longman Scientific
and Technical. (Chapter 2).
Pozzolanic materials
A pozzolanic material or a pozzolan, is a siliceous or siliceous and aluminous material
which in itself has little or no cementitious value but will chemically react with lime
(liberated from hydrating cement) at ordinary temperature to form compounds
possessing cementitious properties.
There are many types of pozzolans including volcanic ash sometimes referred to as
original pozzolan or natural pozzolan, silica fume (SF), pulverized fuel ash (PFA) also
called fly ash, ground granulated blast-furnace slag (GGBS) and others.
In this lecture we shall study two most commonly used pozzolan for concrete: PFA
and SF.
Pulverized fuel ash (PFA)
PFA is fine powder collected from coal-burning electric power plants. In Hong Kong
PFA became available in large quantity since mid 1980s when the power plans
replace oil by coal as their primary fuel. About one million tones of PAF collected
each year.
Physical and chemical properties of PFA
PFA produced from different power plants or at one plant with different coal sources
may have different colours. Particle size and shape characteristics of PFA are
dependent on the source and uniformity of the coal, the degree of pulverization prior
to burning, the combustion environment, uniformity of combustion, and the type of
collection system used. The majority of PFA are glassy, solid or hollow, and spherical
in shape. The fineness of PFA is the same order as ordinary Portland cement. Specific
gravity of solid PFA particle normally ranges from 2.2 to 2.8.
PFA collected from Castle Peak Power stations is spherical in shape having a particle
diameter ranging from 1 to 200 m.
PFA contains mainly silica and alumina. The following table list the chemical
composition of local PFA, UK PFA and typical ordinary Portland cement.
ASTM C618-87 classify PFA (fly ash) in to two lasses, namely Class F and Class C.
Generally, Class F consists of less than 10% of CaO while Class C contains more than
10% CaO.
Chemical compound
SiO2 %
Al2O3 %
Fe2O3 %
CaO %
MgO %
TiO %
K2O %
Na2O %
SO3
Castle Peak
45-80
18-30
1-6
0.5-8
0.5-3
0.5-1.5
<1
<1
0-2
Typical UK
45-51
24-32
7-11
1-5
1-4
0.8-1.1
3-4.5
0.9-1.7
0.3-1.3
Typical OPC
18-25
4-7
1-4
63-68
0.5-3.5
0.2-0.8
3
Pozolanic reaction of PFA
Main compounds formed from hydration of cement are hydrated calcium silicate and
calcium hydroxide. Calcium hydroxide is water-soluble and has no cementitious
value. Calcium hydroxide may come out of the concrete with moisture, leaving voids
in concrete. PFA chemically combine with calcium hydroxides and other soluble
alkalis such as potassium and sodium hydroxides to produce calcium-silicate-hydrate
(C-S-H). The C-S-H strengthens cement paste and filling the voids improving
impermeability of concrete.
Influence of PFA on Concrete
The description of the influences of PFA on properties of concrete in this section is
based on researches conducted in Hong Kong using local PFA and commonly adopted
concrete mixes. The research findings in Hong Kong on local PFA concrete is
generally in line with research outcomes elsewhere, although different conclusions
exist, such as the sensitivity to curing condition.
a. Effect of PFA on properties of fresh concrete
Workability: Generally, adding PFA in a concrete mix as a replacement of cement
will increase the slump value of the mix or, if the same slump value is to be
maintained, reduce the water demand. In this case, however, due to the slow reaction
of PFA and the reduction of cement content, early strength of the concrete will suffer.
In order to maintain workability and at the same time to raise early age strength, water
reducing agent or superplasticiser is usually added to the mix so that water content is
reduced and so is the W/C ratio. Alternatively, cement may be added to the mix to
maintain required W/C ratio and achieve required early age strength. In this case the
heat of cement hydration and alkali content of the mix may be higher than the mix
using water reducing agent.
Stability: The term stability here refers to the properties concerning bleeding,
cohesion and segregation of fresh concrete. It is generally reported that PFA concrete
shows improved cohesiveness and finishibility, reduced bleeding and segregation.
b. Effect of PFA on strength development
Strength development: Strength development due to Pozzolanic reaction in PFA
concrete is a relatively slow process compared to that due to OPC hydration in
conventional OPC concrete. Strength development due to OPC hydration start from a
few hours after casting, while strength development due to pozzolans can only be
observed after 1-7 days. For conventional OPC concrete, 28-day strength is about
90% of its final strength while for PFA concrete 28-day strength is only about 70-80
% of its final strength. Strength growth in PFA concrete depends on dosage of PFA,
type of cement, curing conditions etc. If PFA concrete is compared with conventional
OPC concrete on the basis of percentage of its 28-day strength, the general trend is
that the early relative strength (before 28 days) of PFA concrete is lower, and the
relative strength after 28 days is higher than conventional OPC concrete.
c. Effects of W/C ratio, temperature and curing on PFA concrete
W/C ratio: The trend that increased W/C ratio results in decreased 28-day strength
applies to both conventional OPC concrete and PFA concrete. For a given W/C ratio,
however, 28-day strength of PFA concrete is generally lower than control concrete
without PFA as cement replacement.
Low temperature: Low curing temperature delays the development of strength. This
effect is greater for PFA concrete than for OPC concrete. In an investigation on Hong
Kong PFA concrete, it was found that for concrete without PFA, a curing temperature
as low as 10C (for the first 3 days and then cured at about 20C) does not have
significant influence on 28-day strength compared to the same concrete cured at
standard temperature of 27 C. For PFA concrete, however, the 28-day strength of
PFA concrete cured at 10C is significantly lower than the strength of the same
concrete cured at 27C. The effect of low temperature on strength of PFA concrete
reduces with time, and the full strength of the concrete eventually develops at a later
time.
Higher temperature: The final strength of a concrete reaches its maximum when the
concrete is cured at an "optimum temperature". For conventional OPC concrete,
optimum temperature is normally about say 10C. Curing temperature above or below
the optimum will result in a reduced final strength of the concrete. For PFA concrete,
the optimum temperature appears to be much higher than for concrete without PFA.
For example, an investigation on Hong Kong PFA concrete, at a curing temperature
of 50C, the 90-day strength of PFA concrete is not reduced compared to 90-day
strength of the same concrete cured at lower temperature. At higher curing
temperature greater than 50C and up to 75C, 90-day strength of PFA concrete is
reduced compared to the 90-day strength of the concrete cured at standard
temperature of 27C. But the reduction is much less than that for ordinary OPC
concrete. The above investigation indicates that PFA concrete can tolerate higher
temperature than Conventional OPC concrete.
Impermeability: PFA concrete has a better impermeability than comparable concrete
without PFA. Calcium hydroxide liberated by hydrating cement is water-soluble and
may be leached out of hardened concrete, leaving voids for the ingress of water. Due
to pozzolanic reaction, PFA chemically combine with calcium, potassium, and
sodium hydroxides to produce calcium-silicate-hydrate (C-S-H), thus reducing the
risk of leaching calcium hydroxide.
Hong Kong regulations concerning PFA
In Hong Kong, a PFA replacement up to 25% of cement content is permitted by the
General Specification for Civil Engineering Works.
Cost of PFA concrete in Hong Kong
The material cost of PFA concrete in Hong Kong is close to that of normal OPC
concrete without PFA. In 1994, material cost for OPC concrete is about HK$284
Per cubic meter for grade 30 concrete, HK$313 for grade 45 concrete. PFA concrete
cost is HK$3-8 less than OPC concrete.
Silica fume
Silica fume is a by-product resulting from the production of silicon or ferrosilicon
alloys or other silicon alloys. Silica fume is light or dark gray in colour containing
high content of amorphous silicon dioxide. Silica fume powder as collected from
waste gasses without further treatment is some times referred to as undensified silica
fume to distinguish it with other forms of treated silica fume. Undensified silicon
fume consists of very fine vitreous spherical particles with average diameter about
0.1m, which is 100 times smaller than the average cement particle. The undensified
silica fume is almost as fine as cigarette ash and the bulk density is only about 200 300 kg/m3 and relative density of typical silica fume particle is 2.2 to 2.5. Because the
extreme fineness and high silicon content, silica fume is a highly effective pozzolan.
Types of silica fume
The extreme fineness and low loose density create handling problem. For this reason
silica fume is normally condensed or slurrified before delivered to the end users.
Condensed silica fume is the product of further treatment of undensified silica fume,
in which fine undensified silica fume powder is condensed into small spheres about
0.5 - 1 mm in diameter. The bulk density of condensed silica fume is increased to
about 600kg/m3. In the past, condensed silica fume is also referred to as densified
silica fume, microsilica, silica powder or pelletized silica fume, etc. The widely
accepted name now is condensed silica fume. In practice, condensed silica fume is
often added into the concrete mix in a slurry form which is made by mixing the solid
fume with water before adding to the whole mix. Silica fume may also added to
concrete mix in its dry solid form.
Slurrified silica fume is a thick liquid produced by blending silica fume powder with
an ordinary water and water reducing agent.
Condensed silica fume is easy to transport and handle comparing to other forms of
silica fume. Slurrified silica fume can directly added in to concrete but difficult to
store and transportation cost is relatively high.
Brief history of silica fume concrete
The first known tests on the silica fume concrete (SFC) were in the early 1950's at the
Norwegian Institute of Technology. At the same time SFC was employed in a tunnel
project in Oslo alum shale region. However the world-wide investigation and practical
use of silica fume was not starrted until 1970s when a large amount of silica fume was
collected as a results of introduction of far stricter environmental legislation in many
countries. When silica fume was first introduced in concrete industry as cement
replacement and was usually for economic purpose. As research work progress and
with better knowledge about SFC, also because the increased price, silica fume is now
often used as an effective additive to produce a better quality concrete. High strength
SFC up to 300 Mpa have been used in some countries; calcium nitrate attack was
effectively reduced by applying silica fume in concrete fertilizer storage silos; CSF
has been used in repairing a dam stilling basin for suitable abrasion erosion resistance;
silica fume has been employed as essential additives to prevent alkali-silica reaction.
Chemical composition of silica fume
Chemical composition of SF varies depending on the nature of the product from the
manufacture process of which the SF is collected. The main constituent material in SF
is silica (SiO2), the content of which is normally over 90%. The following table lists a
chemical analysis of a commercially available silica fume.
SiO2
Al2O3
Fe2O3
CaO
MgO
K2O
Na2O
92.85
0.61
0.94
0.39
1.58
0.87
0.5
Fundamental performance of silica fume in concrete
The influence of SF on the properties of concrete may be explained by two
fundamental performance of SF, i.e. pozzolanic reaction and microfiller effect.
Pozzolanic reaction of SF is the reaction with calcium hydroxide produced by the
cement hydration. This reaction has been shown by researchers by measuring the
CaOH content in a cement-SF mortars or paste. CaOH content reduces with
increasing SF content. The reaction forms more aggregate-binding gel and at the same
time reduce detrimental product of cement hydration, CaOH. The net effect is an
increase in overall strength and durability.
Pozzolanic reaction in concrete or mortar is generally observed after 3 - 7 days.
Diameter of SF particle is about 0.1 m, 100 times smaller than cement particles. The
extreme fineness allows SF to fill the microscopic voids between cement particles.
The micro filer effect is credited with greatly reduced permeability and improved
paste-to-aggregate bond of SFC compared with conventional concrete.
Properties of fresh concrete
Workability - water demand: Use of SF in concrete usually increases water demand.
The increased water demand causes an increase in water to cement ratio and could
negate the benefits of adding silica fume. For this reason the SFC normally
incorporates a water reducing agent or superplasticiser.
Stability: SFC is more cohesive than convention concrete. This is true for SFCs both
with and without superplasticiser. Increased cohesiveness reduces the chance for
bleeding and segregation. The increased cohesiveness, however, increase the required
compaction energy.
Plastic shrinkage: Increased cohesiveness of SFC encourages the potentiality of
plastic shrinkage and cracking which appears when the bleeding water cannot
compensate for the water loss on the surface by evaporation. It has recommended that
under conditions of fast evaporation, curing measures should be taken immediately
after placing the concrete.
Strength development
Strength development is a major concern in the production of concrete incorporating
pozzolan materials. Like PFA concrete, SFC shows a two-stage strength development
pattern: before and after the commencement of the pozzolanic reaction of SF. The
pozzolanic reaction of SF takes place at between 3 - 7 days at about 20C. At higher
temperature the reaction could be accelerated. At a temperature of 5C, pozzolanic
reaction could be delayed for a month or even longer.
However the absolute early age strength of SFC incorporating medium dosage of
silica fume as a cement replacement is not lower than the concrete without SF
replacement. This is due to the effect of micro filler action. If the comparison of early
age strength is based on the percentage of 28-day strength, SFC is generally lower
than conventional concrete.
Properties of hardened concrete
Compressive strength: High strength concrete with a cube compressive strength
around 100MPa can be easily achieved by incorporating SF with suitable water
reducing agent and suitable aggregates.
With constant W/C ratio, compressive strength of SFC is normally higher than
conventional concrete. Researches indicate that the shape of W/C to strength curve of
SFC is similar to conventional concrete but shifted to a higher level. Optimum dosage
of silica depends on many factors including type of water reducing agent and type of
cement. It can be determined using trial mixes and, 10% of SF by weight of cement is
a good starting point.
Impermeability: Impermeability of SFC is improved compare with similar concrete
without SF.
Questions
Why water reducing agent or superplasticiser are normally used in concrete
containing silica fume.