Note: Descriptions are shown in the official language in which they were submitted.
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WO 98/57905 PCT/GB98l01758
AQUEOUS SUSPENSIONS OF METAKAOLIN AND A METHOD OF PRODUCING CEMENT1TIOUS
COMPOSITIONS
The present invention relates to aqueous
suspensions of inorganic particulate materials.
Aqueous suspensions or slurries of so-called
silica fume comprising silica particles are known for
incorporation together with hydraulic cement in
settable cementitious compositions for use in building,
construction and like industries. Such suspensions
provide a ready mixed product which can be easily
transported and added to the other ingredients required
to form the cementitious composition. The silica
present is known to react chemically with basic
material such as lime present as a product of the
hydration of the cement which assists setting and
hardening. The inclusion of the silica as an additive
in the cementitious composition improves the properties
of the composition, especially its compressive strength
when it is allowed to set and harden.
Preparation and use of silica fume suspensions or
slurries is not ideal. Thorough and intensive mixing
is required to produce the suspension. Complete
dispersion of the silica particles may be difficult or
impossible to achieve and the formation of reactive
aggregates or agglomerates may result. Such aggregates
- can reduce the effectiveness of the silica content of
the suspension and can, seriously, lead to gellation,
expansion and cracking in the cementitious material
when allowed to set and harden. Additionally, the
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maximum' dose of the silica particles which can be
incorporated in a practical cementitious composition is
limited to about 10 weight per cent (based on the dry
weight of cement present).
According to the present invention in a first
aspect an aqueous suspension suitable for use in the
production of cementitious compositions includes, prior
to production of a cementitious composition, an aqueous
medium and a particulate inorganic material suspended
I0 in the aqueous medium, the particulate inorganic
material comprising metakaolin and being substantially
lime-free and having a pozzolanic activity, the
suspension having a pH of at least 7.5, the suspension
being in a stable form having a rheology which allows
it to be flowable as a wet slurry and wherein the
suspension includes a thickening agent which inhibits
sedimentation of the inorganic particulate material in
the suspension. The flowability of the suspension
allows it to be pumped, stirred, metered etc. and to be
transported to a user site for addition to a
cementitious composition at the user site.
Metakaolin having a substantial pozzolanic
activity is known per se for use as an additive to
cementitious compositions. However, such material is
known and used in a dry powdered form. Use of such
material in an aqueous suspension as in accordance with
the first aspect of the present invention has not
previously been known for use in such an application.
In GB-A-2294259 it is envisaged that addition of water
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to metakaolin produces a material having the appearance
of a dry powdered product. However, this prior
specification does not contemplate the production of a
' wet, stable, non-sedimenting flowable slurry of
metakaolin as described herein.
Calcined kaolin pigments are used in low
concentrations in aqueous suspensions in the paper
industry. Such pigments are quite different in several
respects from metakaolin to be used for the inorganic
particulate material used in the compositions of the
present invention. Such differences may be summarised
as follows.
Calcined kaolin pigments are produced by calcining
kaolin at temperatures substantially greater than
1000°C, eg 1050°C to 1150°C. On cooling after
calcining at these higher temperatures,
recrystallisation in crystal phases quite different
from the original occurs. Such pigments have no
substantial pozzolanic activity and would not therefore
be useful as additives for use in cementitious
materials as described herein.
In contrast, metakaolin is produced by calcining
kaolin at lower temperatures. The kaolin is
dehydroxylated and the structure becomes amorphous but
no recrystallisation occurs. Metakaolin may be
~ characterised as the product of calcining kaolin
between the temperature of 400°C at which
dehydroxylation starts to occur and 980°C at which an
exothermic transition occurs. Desirably, the
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metakaolin to be employed in the suspension according
to the first aspect of the present invention contains
less than 5% more particularly less than 1%, especially
less than 0.50, by weight of chemically bound water (as
measured as loss of weight by ignition at 1000°C for 2
hours ) .
Metakaolin when formed into an aqueous suspension
by addition to water or an aqueous solution normally
has a pH of between 6 and 7. When the solids
concentration (on a dry weight basis) of such a
suspension is 40 per cent or more, especially 50 per
cent or more, the viscosity of the suspension is unduly
high, eg greater than 2000 mPa.s. However, we have
found unexpectedly and beneficially that if the pH of
the suspension is increased by addition of a basic
substance the viscosity can be reduced to a level
whereby the suspension is usable in applications
similar to those in which silica fume suspensions are
used. Furthermore, incorporation in the suspension of
a thickening agent allows the suspension to retain a
suitable stable, non-sedimenting form which benefits
its suitability for transport to and use by a user.
Desirably, the pH of the suspension according to
the first aspect of the present invention is in the
range 7.5 to 9.5 especially in the range 8.5 to 9Ø
The pH of the suspension may be adjusted by simple
addition of a suitable alkali or base other than lime
which does not substantially react with the metakaolin
present. The alkali or base is preferably a material
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which is readily available commercially, has minimal
effect on the end use of the aqueous suspension and
gives suitable pH adjusting effect for minimum dose.
The alkali or base may be selected from hydroxides and
conjugate base salts of alkali metals, eg hydroxides,
silicates, or carbonates of sodium or potassium.
Sodium hydroxide in dry or solution form is preferred
as pH adjuster.
The amount of alkali or base added will depend
upon the particular alkali or base used and its
concentration. In general, active alkali amounts of
much less than 1% are likely to be required to achieve
the required pH. For example, to achieve a pH in the
desired range 8.5 to 9.0, about 0.2% by weight of
sodium hydroxide added in dry form is required (based
on the dry weight of inorganic particulate material).
Desirably, the aqueous suspension according to the
first aspect of the present invention has a solids
concentration of at least 40 per cent by weight,
desirably at least 50 per cent by weight although it
may be 60 per cent or more, based on the dry weight of
solids (the inorganic particulate material) contained
in the aqueous medium. The maximum solids
concentration will be that at which the suspension is
still a wet, flowable, stable slurry which may be
- determined by experimentation. The maximum solids
concentration of the suspension may be about 65% by
weight or in some cases even from 70% to 80% by weight.
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We have found that if a wet flowable slurry of an
inorganic particulate material comprising metakaolin is
produced, it is necessary to avoid sedimentation of the
material, otherwise the suspension when obtained by a
user requires substantial reworking or is not useful at
all. The suspension according to the first aspect of
the present invention is such that substantial visible
sedimentation of the inorganic particulate material
does not occur in the aqueous medium. Potential
sedimentation is beneficially deterred by incorporation
in the aqueous suspension of a thickening agent, eg in
a concentration of not more than l0% by weight on a dry
weight basis. Desirably, no sedimentation occurs in
the suspension for at least two weeks after its
preparation or, at least, if a limited degree of
sedimentation occurs in this period, it is easily
reworked, eg by simple mechanical agitation and/or
pumping and/or mixing. Desirably, the amount of
thickening agent employed is such that no gellation
occurs in the suspension for a period of at least two
weeks or, at least, if a limited degree of gellation
occurs it is easily broken down by simple mechanical
agitation and/or pumping and/or mixing.
Suitable thickening agents which may be
incorporated in the aqueous suspension according to the
first aspect to deter sedimentation (without
substantial gellation) include materials known as
thickening/gelling agents especially for use in aqueous
media, eg selected from hydrophilic polymers such as
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polysaccharides, eg starches or gums, eg xanthan gum or
guar gum, alginates, cellulose ethers, modified
celluloses such as carboxy methyl cellulose and the
" like known to make an. aqueous phase more viscous,
absorbent clays such as attapulgite and smectite clays,
eg bentonite, which absorb water and give a thickening
(water-reducing) effect, ultrafine siliceous
particulate materials, eg silica, which form a
siliceous polymer in the aqueous phase thus increasing
viscosity, particulate minerals, eg hydrotalcite,
having a charged (cationic) mineral surface which
interacts with particle surfaces of the inorganic
particulate material to create a network-like structure
which limits sedimentation. Gums such as xanthan gum
or guar gum are preferred.
As noted earlier, the amount of thickening agent
employed is preferably such that substantial gellation
does not occur in the suspension or occurs only to an
extent that any gel formed can be broken down by simple
mechanical agitation. The maximum amount of thickening
agent depends on the agent and the suspension to which
it is applied. The maximum amount may be determined by
experimentation. Generally, the maximum amount will be
not greater than 5% by weight, in many cases less than
2% by weight. Where the thickening agent comprises a
- gum, the amount of gum employed (on a dry weight basis)
is preferably in the range 0.05% to 0.5% of the
suspension by weight.
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_g_
In ASTM C 618, a pozzolan is defined as a
siliceous material or siliceous and aluminous material
which, in itself, possesses little or no cementitious
value but which will, in finely divided form in the
presence of moisture, react chemically with calcium
hydroxide at ordinary temperature to form compounds
possessing cementitious properties. The reactivity is
determined using the known Chapelle test wherein a
dilute slurry of the pozzolan is reacted with an excess
of calcium hydroxide at a temperature of 95°C for 15
hours. After this period, the unreacted calcium
hydroxide is measured by titration.
Preferably the pozzolanic activity of the
inorganic particulate material employed in the
suspensions of the invention is such that the
reactivity of the material with calcium hydroxide is at
least 100 mg of calcium hydroxide per gramme,
especially at least 400 mg of calcium hydroxide per
gramme. Preferably, the inorganic particulate material
having a reactivity with calcium hydroxide of at least
700 mg calcium hydroxide per gramme.
Production of metakaolin having the required
pozzolanic activity is known, eg as described in the
article by Walters, G V and Jones, T R, 2nd
International Conference on Durability of Concrete,
Canada, ed V H Malhotra, 1991, pp 941-953.
Essentially, to produce a suitable metakaolin,
kaolin is calcined for a period of at least a few
minutes, eg between a few minutes and up to 10 hours,
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at a temperature in the temperature range defined
earlier, eg in the range 600°C to 950°C, eg 650°C to
850°C.
The particles of the inorganic particulate
material employed in the suspension according to the
first aspect of the present invention may have the
following particle size properties:
not less than 70 per cent having a particle size
(equivalent spherical diameter or "esd" as measured by
sedimentation) less than lO~Cm;
not less than 40 per cent having a particle size less
than 5~,m; and
not less than 20 per cent having a particle size less
than 3~cm.
We have found that aqueous suspensions in
accordance with the first aspect of the present
invention unexpectedly and beneficially can be fluid,
eg have a viscosity of 1000 mPa.s or less, over a
reasonable period of time eg 7 days or more, often 10
days or more, eg two weeks or more, and can, as noted
above, therefore be flowable and pumpable and can be
stirred and/or mixed and conveniently transported and
used as a flowable ready mixed additive for addition to
cementitious or cement-forming materials and concrete-
making compositions and the like. Such suspensions may
advantageously be prepared and used without the use of
added dispersing agents, although dispersing agents may
optionally be employed if desired. Desirably, the
kaolin from which the metakaolin employed in the
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suspension according to the first aspect of the
invention is produced is relatively pure, ie has an
impurity metallic oxide content (ie other than A120)
and SiOz) of less than 10 weight per cent desirably
less than 6 weight per cent with an alkali metal oxide
content of less than 3 weight per cent. Such
suspensions can be suitable for delivery to and use at
a user site without the user encountering serious
problems, eg substantial deposit of solid material at
the base of the storage tank in which the delivered
suspension is contained, the deposit being difficult to
rehomogenise, or strong gellation giving difficulty in
stirring or pumping.
The aqueous suspension according to the first
IS aspect of the present invention may show the following
viscosity properties:
(i) an initial (makedown) viscosity of <500 mPa.s to
aid makedown;
(ii) a viscosity increase which is minimal, eg not more
than 500 mPa.s, over the first three days after
makedown;
(iii) at all ages of up to at least one month after
makedown a viscosity of not more than 1500 mPa.s.
Preferably, the viscosity of the suspension is not
greater than 1000 mPa.s over a one week period,
preferably over a two week period, after makedown where
the solids concentration of the suspension is at least
60 per cent by weight and is not greater than 700 mPa.s
over a one week period, preferably over a two week
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period, after makedown where the solids concentration
of the suspension is at least 50 per cent by weight.
In this specification, references to viscosity of
' a suspension or slurry are to be understood as meaning
viscosity as measured at ambient temperature (20°C)
using a Brookfield Viscometer operating with a spindle
speed of 100 rpm.
In the preparation of aqueous suspensions
according to the first aspect of the present invention
it is preferred that the suspension is made fluid by pH
adjustment prior to addition of thickening agent.
However, partial or complete pH adjustment may be made
during or after addition of thickening agent. A
preferred sequence of addition of ingredients to water,
eg in an efficient mixer, is pH adjuster followed by
metakaolin containing inorganic particulate material
and then thickener.
Surprisingly and beneficially, aqueous suspensions
in accordance with the first aspect of the present
invention provide excellent materials suitable for use
in known applications for aqueous suspensions of
pozzolanic particulate materials, eg as additives to
cementitious compositions or as additives to alkali
activators to form cements and cement-containing
products. Pozzolanic particulate materials for
additives to cementitious compositions or to form
cements or cement products are preferred by users in
some applications to be in slurry rather than dry form,
eg because this offers ease of handling and ease of
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mixing. Such suspensions in accordance with the first
aspect of the present invention can, as noted above, be
produced in high solids form, eg having a solids
content of 40 weight per cent or more, eg 50 per cent
or more, and they can be flowable, pumpable, stirrable
and transportable and can show no sedimentation and no
or little gellation for at least ten days, even two
weeks or more after preparation. Furthermore, they may
be stable to excessive temperature variations, eg over
the range -50°C to +100°C, and may show no substantial
dilatancy of the particles of the inorganic particulate
material in the suspension.
The suspensions according to the first aspect of
the present invention may be prepared more easily than
known silica fume suspensions for use in the same
applications. The suspensions according to the
invention require thorough but less intensive mixing
and show less tendency to produce aggregates or
agglomerates than silica fume suspensions. This allows
the suspensions according to the first aspect of the
present invention to be used over a wider range of
concentrations than those of silica fume to give a
wider range of potential beneficial effects in
cementitious compositions, cements and cement-
containing products.
Additives in the various known classes of
materials incorporated in aqueous particulate
suspensions for use by addition to cementitious
compositions or formation of cements or cement-
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containing products may be incorporated in the
suspension according to the first aspect. Such
optional additives include, for example,
' (i) dispersing agents, eg water soluble polymers such
as polycarboxylates, eg polyacrylic acids and their
salts, lignosulphonate salts and sulphonated melamine
or naphthalene formaldehydes and also alkali silicates;
(ii) water repellents, eg stearates of calcium, zinc or
aluminium or other chemicals with hydrophobic imparting
properties;
(iii) other inorganic particulate material having
pozzolanic properties, eg fine siliceous materials;
(iv) other additives deemed appropriate or necessary
for the final product application, eg micaceous
materials and other mineral extenders or property
modifiers;
(v) colouring agents, eg pigments such as iron oxides;
(vi) shrinkage control agents;
(vii) fibres, whiskers, rods, strands, foils, beads and
the like to play a part in the cementitious material
produced from the suspension, eg as a shrinkage control
agent and/or as a tensile strength improver; such
additives may be made of materials known for this
application, eg fibres may be made of glass and/or
thermoplastic materials.
(viii) biocides.
Desirably, in the suspension according to the
first aspect of the present invention, the metakaolin
forms at least 70 per cent by weight, eg at least 85
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per cent by weight of the particulate solids present in
the suspension.
The suspensions according to the first aspect of
the present invention may be employed in the same known
applications as silica fume suspensions. Thus, the
suspensions according to the first aspect of the
present invention may be employed to form settable
cementitious compositions in known materials
incorporating hydraulic cements, eg for use in the
following applications:
(a) concrete (optionally including metal or other
reinforcements) - ready mixed;
(b) concrete (optionally including metal or other
reinforcements) - preset;
(c) paving products - slabs, bricks, pavers;
(d) tiles - eg for roofs or floors;
(e) renders - cement and/or lime based materials;
(f) mortars - jointing, grouting and sealant
materials;
(g) sprayed concrete/mortars/render;
(h) flooring systems including self-levelling systems;
(i) fibre reinforced systems/composites;
(j) oil-well cement (class G and H portland cement)
applications.
The suspension according to the first aspect of
the present invention may also be added to alkaline
materials known as alkali activators to form hardenable
cementitious compositions or cements. Such alkali
activators are known per se in the art, eg for use to
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form alkali activated slags, and will in general be
_ added in greater amounts than that previously added to
establish the pH of the suspension to facilitate
' flowability etc. as described hereinbefore. For
example, the relative amounts by weight of metakaolin
to alkali activator present in the cementitious
composition formed may be in the range of from 1% to
99% especially 30% to 70%. The following materials are
suitable for use as alkali activators for forming
cementitious compositions in this way: hydroxides and
conjugate base salts of alkali metals especially sodium
and potassium hydroxide and sodium and potassium
silicate.
The cementitious compositions formed by use of
alkali activators may be employed in any of the
applications as described hereinbefore (for
compositions including an hydraulic cement) although
the production of paving products, tiles and the like
is likely to be a particularly suitable application for
such compositions.
The settable cementitious composition may
incorporate other known optional additives, eg:
(i) any of the additives specified above for optional
addition to the aqueous suspension according to the
first aspect of the present invention;
(ii) commercially available chemicals to retard or
accelerate setting of cement;
(iii) chemicals to entrain air in cement or composites
thereof to impart freeze-thaw resistance;
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(iv) chemicals such as calcium nitrate to inhibit
corrosion of materials employed to reinforce cement
composites, eg steel employed to reinforce concrete.
Hardenable cementitious compositions, eg concrete
compositions, produced by addition of suspensions
according to the first aspect of the present invention
to cement or cement forming material, eg alkali
activator, and optionally other ingredients, eg sand
and aggregate to form concrete, show various advantages
over those produced using known silica fume
suspensions. The presence of aggregates and
agglomerates in the composition can be avoided thereby
avoiding the problems due to such aggregates and
agglomerates described hereinbefore. The inorganic
particulate material of the suspension according to the
first aspect of the present invention may be
incorporated at higher dose levels, eg up to about 250
on a dry weight basis of the settable cementitious
composition to be prepared. Additionally, the
pozzolanic activity of metakaolin can be greater than
that of silica fume and this allows better bonding into
the settable cementitious composition thereby providing
improved mechanical (compressive) strength when the
composition eventually sets and hardens. Furthermore,
the hardened material (compared with materials formed
from silica fume suspensions) can have a better
capacity to resist chemical degradation. It can have a
better capacity to resist ingress of contaminants such
as chlorides because it can provide a hardened material
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having a more limited pore structure and it can also
have an improved chemical resistance resulting from a
better conversion of chemically reactive calcium
hydroxide.
According to the present invention in a second
aspect there is provided a method of producing a
settable cementitious composition which includes
addition of a pozzolanic material to an hydraulic
cement or an alkali activator in the presence of water
wherein the pozzolanic material comprises an aqueous
suspension according to the first aspect.
Water may be added to the cement and/or alkali
activator prior to and/or during and/or after addition
of the said suspension to the hydraulic cement and/or
alkali activator.
The settable composition may, depending on the
intended use, include further additives, eg ingredients
and reinforcements etc. as described hereinbefore to
form concrete or reinforced concrete.
The preparation and properties of aqueous
suspensions embodying the present invention will now be
described with reference to the following illustrative
Examples which are given by way of example only.
Example 1
A slurry was prepared by addition of a metakaolin
to a dilute (lime-free) solution of sodium hydroxide to
give a solids content on a dry weight basis of 60 per
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cent. The metakaolin employed had the following
properties:
(i) about 12% by weight of particles having an esd of
greater than 10/Cm;
(ii) about 37% by weight of particles having an esd of
less than 2~Cm;
( iii ) surface area 14m2 . g-1;
(iv) Si02 content 52wt%;
(v) A1203 content 4lwt % ;
(vi) content of other metallic oxides 5.5wt%;
(vii) pozzolanic reactivity (Chapelle Test) 1050mg
Ca(OH)2 per gramme.
The suspension was thoroughly mixed after which
the pH was adjusted to 9.4 (as measured by a pH meter)
by further sodium hydroxide addition after which 0.125
per cent (on a dry weight basis) of xanthan gum
thickening agent sold under the trade name Kelzan was
added. Also 200 parts per million by weight of a
isothioazolinone biocide (sold under the trade name
MetaSol 150 by Calgon Corporation, Pittsburgh, USA) was
added. Further thorough mixing was applied. The
resulting slurry S1 was found to have the viscosity
properties listed in the Table 1 as follows.
Table 1
Initial viscosity 300 mPa.s
Viscosity after 1 hour 350 mPa.s
Viscosity after 24 hours 450 mPa.s
Viscosity after 72 hours 650 mPa.s
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In addition, no sedimentation of the particulate
material contained in the slurry S1 took place within a
period of 14 days after production (makedown) of the
slurry S1. A limited amount of gelling in the slurry
S1 occurred after 6 days following production but the
gelling could easily be reversed by simple stirring.
Example 2
A slurry S2 was produced in a manner similar to
that given in Example 1 except that the metakaolin
employed had the following properties:
(i) about 4% by weight of particles having an esd
greater than 10~.m;
(ii) about 58% by weight of particles having an esd
less than 2~Cm;
(iii) surface area 15m2,g-1;
(iv) Si02 content 55wt%;
(v) A1203 content 40wt%;
(vi) content of other metallic oxides 3.6wt%;
(vii) pozzolanic reactivity (Chapelle test) 1050mg
Ca(OH)2 per gramme;
and the final pH of the slurry S2 was 8.4.
The final slurry S2 produced showed the following
properties:
(i) an initial viscosity of 150 mPa.s;
(ii) no sedimentation in the period of 21 days after
production;
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(iii) limited easily reversible gelling in the period
of 6 days to 21 days after production.
Example 3
Samples of the slurry S1 prepared in Example 1
were added to standard portland cement whereby loo by
weight and 20o by weight of the solids present
respectively in the composition formed consisted of the
metakaolin present. Water was added to give a standard
consistency as determined by the well known VICAT test
(BS EN 196-3:1995) thereby forming cement slurries C1
and C2 respectively. Further cement slurries C3 and C4
respectively having the same compositions as C1 and C2
were made using dry metakaolin instead of the slurries
S1 and S2. The initial setting time and the final
setting time of each of the cement compositions C1 to
C4 was measured. The results obtained are given in
Table 2 as follows:
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Table 2
Cement Metakaolin Initial Final
composition form setting time setting time
slurry (minutes) (minutes)
C1 Slurry 140 185
C2 Slurry 130 185
C3 Powder 125 180
C4 Powder 110 180
As is seen in Table 2 the metakaolin form, ie
slurry or dry powder form, does not significantly
S affect the final setting time of the cement
composition.
Example 4
Slurry S1 produced as in Example 1 and slurry S2
produced as in Example 2 were each separately added
together with 0.5% (by dry weight relative to the
solids present) biocide as used in Example 1 to
standard portland cement and water to give respectively
cement compositions C5 and C6. 15 per cent by weight
of the cementitious binder present in each case was
metakaolin. A similar composition C7 comprising only
portland cement (ie no metakaolin included as part of
the binder composition) was prepared as a control for
comparative purposes. The compositions C5, C6 and C7
were each allowed to set.
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The following properties of the resulting hardened
materials produced by setting of compositions C5 to C7
were measured in a known way:
(i) compressive strength and density 7 days after
production of the settable composition;
(ii) compressive strength and density 28 days after
production of the settable composition;
(iii) compaction factor (workability/fluidity) of the
still fluid composition.
The results obtained are listed in Table 3 as
follows:
Table 3
Cement Compressive Density Compaction
composition Strength (kg/m3) factor
- slurry (MPa)
7 28 7 28
days days days days
C5 61.9 75.0 2399 2386 0.96
C6 61.5 75.8 2408 2385 0.94
C7 52.0 68.0 2400 2410 0.96
(control)
The results given in Table 3 show that the
addition of a slurry of a metakaolin to a composition
of portland cement and water improves the compressive
strength of the resulting hardened material without
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significantly affecting its density and compaction
factor.
Example 5
A sample C5a of cement composition C5 prepared as
in Example 4 was cooled to a temperature of about -5°C
and remained at this temperature for a period of 72
hours. The temperature of the sample was then allowed
to rise naturally to ambient temperature (20°C). The
compressive strength and density after 7 hours and
after 28 hours were measured. The results obtained are
shown in Table 4 below.
A further sample C5b of composition C5 prepared as
in Example 4 was heated to a temperature of 90°C for a
period of 24 hours. The sample was then allowed to
cool slowly to ambient temperature (20°C). The same
properties as measured for sample C5a were measured and
the results are shown in Table 4 as follows.
In Table 4, the results obtained for properties
of the sample of composition C5 measured as in Example
4 are repeated. This sample, which was not subjected
to temperature cycling, is referred to as C5c in Table
4.
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Table 4
Cement Thermal Compressive Density
composition treatment strength (kg/m3)
(MPa)
7 28 7 28
days days days days
C5a Freezing/ 65.8 79.1 2401 2395
defrosting
C5b Heating/ 65.0 81.2 2401 2401
cooling
C5c None 61.9 75.0 2399 2386
The results given in Table 4 show that the aqueous
suspension of metakaolin shows no undesirable effects
after prolonged heating or cooling/freezing.
Example 6
A metakaolin slurry was prepared as in Example 1.
A silica fume slurry of known composition having the
same solids concentration as the metakaolin slurry was
also prepared for comparison. Various cement and
concrete compositions were made from the silica fume
slurry and separately from the metakaolin slurry (each
composition formed from silica fume slurry having a
corresponding composition formed from metakaolin
slurry), by adding the slurry in each case to portland
cement forming 85 to 95 per cent by weight of the
solids present in each case and water forming 50 per
i
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cent by dry weight of the solids present and thoroughly
mixing the compositions produced.
Photomicrographs of the hardened products in each
w
case were taken. For the compositions produced using
the silica fume slurry the photomicrographs showed
discrete agglomerates present which were identified as
agglomerates of silica fume particles. In contrast,
the corresponding compositions produced using the
metakaolin slurry showed no distinct agglomerates.
Example 7
Concrete compositions were made using the
metakaolin and silica fume slurries as produced in
Example 6, to each concrete composition having the
following properties:
binder: metakaolin or silica fume (15% weight) plus
portland cement;
water: 50% by dry weight of binder;
aggregate: present with the binder in an aggregate to
binder weight ratio of 4.65 to 1.
The compressive strength of the concrete produced
in each case was measured after 7 days and after 28
days. The results which were obtained are as shown in
Table 5 as follows.
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Table 5
Pozzolanic additive Compressive strength
in concrete (MPa)
composition
after '7 days after 28 days
metakaolin 62 86
silica fume 55 73
As seen in Table 5 superior compressive strength
concrete is obtained by use of metakaolin slurry
instead of conventionally used silica fume slurry.
