Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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RAPID-SETTING CEMENTITIOUS COMPOSITION
The present invention relates to rapid-setting cement compositions and more
particularly to the use of certain chemical compounds as quick-setting agents
in the
formulation of such compositions.
Rapid-setting cement compositions are desirable and widely-used materials,
particularly
in applications known as lining and relining construction where a continuous
layer of
modest thickness of a cement composition is applied to a surface. A common
procedure
in which rapid-setting cement compositions are used is "shotcreting", the
application of
concrete to a surface by spraying from a nozzle. Other applications for rapid-
setting cement
compositions include cement plugging compounds and quick-setting mortars. In
all such
usages, the cement composition must set quickly in order to retain its
position as applied.
The particular quick-setting agent used in rapid-setting cement compositions
will
influence a number of performance criteria of interest to the industry. Not
only must the
agent effect a rapid setting within certain time limits, but this rapid
setting is also desirably
achieved with economically-attractive amounts of a quick-setting agent. In
addition, the
rapid development of strength properties, which does not arise simply because
of the rapid
setting, is an important performance criterion of a quick-setting agent. The
ability to
continue early strength development to a high final strength is also a
desirable objective.
Quick-setting agents have traditionally been strongly alkaline solutions of
silicates and
aluminates, the aluminates tending to be better than the silicates in
providing early strength
development. Both, however, are caustic in nature, and this caustic nature
gives rise to
health and environmental concerns.
It is an objective in the cement art to provide alternatives to current quick-
setting
2 5 agents and particularly to provide non-caustic quick-setting agents which
provide rapid early
strength development and other desired properties.
In accordance with the invention, it has been found that water-soluble salts
of citric
acid, malic acid (hydroxybutanedioc acid) and citramalic acid (2-hydroxy-2-
methylbutanedioc acid), in particular alkali metal salts, are especially
useful as quick-setting
3 0 agents in cementitious compositions. This is a surprising discovery as
such acids and their
alkali metal salts are well known in the art as cement retarders.
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The invention therefore provides in one major aspect thereof rapid-setting
cementitious
compositions comprising hydraulic cement and at least 4% by weight of the
cement of a
water-soluble salt of citric acid, malic or citramalic acid or mixture
thereof. These
compositions will be applied in admixture with sufficient water for hydraulic
setting of the
cement. Both very quick setting and rapid and good strength development are
obtained
with the invention.
In another embodiment, the compositions of the present invention consist
essentially
of hydraulic cement and at least 4% by weight of the cement of a water-soluble
salt of citric
acid, malic or citramalic acid or mixture thereof.
A salt of citric acid, malic or citramalic acid is, for the purposes of this
invention,
defined as an acid molecule with all of its acid groups in salt form, i.e. a
tri-salt in the case
of citric acid and a di-salt in the case of malic or citramalic acid. The
preferred water-
soluble salt forms are the alkali metal salts, particularly the sodium,
potassium and lithium
salts. The particularly preferred quick-setting agents are sodium citrate,
potassium citrate,
disodium malate, disodium citramalate, dipotassium malate and dipotassium
citramalate and
mixtures thereof, with potassium citrate, dipotassium malate or dipotassium
citramalate
being most preferred. However, lithium salts may be preferred where the alkali-
silica
reaction is a concern, since it is well known that lithium salts inhibit the
alkali-silica
reaction.
The citric acid, malic and citramalic acid salts used in the invention may be
readily
prepared, or commercially-available varieties may be used. The acid salts are
preferably
added to the cementitious compositions as aqueous solutions, generally at
concentrations
of about 50% by weight of salt, although the concentration is not critical.
For the less
soluble salts, e.g. lithium salts, the concentrations may be less than 50% by
weight. In
general, at least 4% by weight of water-soluble citrate, malate or
citramalate, or mixture
thereof, based on the weight of the hydraulic cement, is required to attain
the quick-setting
action achievable with this invention. When a citrate, malate or citramalate
is employed
in amounts of 4% or more, it will rapidly accelerate the setting of the
cement. Substantially
greater amounts up to 20% by weight or more may be employed, depending upon
the final
formulation, objectives and conditions of use. Desirably, at least 5% water-
soluble citrate,
malate or citramalate based on the weight of the cement is employed, with
amounts greater
than 10% being unnecessary in most applications. Preferably, the amount of
citrate, malate
or citramalate is in the range of 5.0 to 8.5%, more usually 5.5 to 8.0%, based
on the
weight of the hydraulic cement. However, the skilled person can readily
ascertain the
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appropriate material and quantity in any given set of circumstances.
In general, the water-soluble citrates, malates and citramalates, depending
largely on
the amount employed, can be used to provide quick setting as desired in a
variety of
applications. Hence, an initial setting time of not greater than 20 minutes by
ASTM
Method C266 can readily be achieved. Setting times in the range of 10 seconds
to 5
minutes, preferably 30 seconds to 5 minutes, as are common in shotcreting
applications, are
easily targeted by the addition of water-soluble citrates, malates or
citramalates in
accordance with the invention. Again, this is well within the skill of the
art.
The hydraulic cement employed in the compositions of the invention may be
selected
from the wide range of commercially-available calcium alumino-silicate mineral
blends,
which when mixed with water will set to form a hard product. The most common
type of
hydraulic cement is portland cement, i.e. types I-IV portland cement, and for
purposes of
the present invention, portland cement is the preferred hydraulic cement.
In addition to cement, the compositions of the invention may include other
ingredients
commonly present in quick-setting cement compositions and other cement
additives which
do not substantially interfere with the quick-setting ability of the
compositions of the
invention. A common ingredient of quick-setting cement composition is
aggregate,
especially fine aggregate such as sand and the like. Pea gravel aggregates
(maximum
diameter of about 0.95cm (0.375 inch)) and the like may also be used in
shotcrete
applications with suitable nozzles. Coarse aggregates may also be used in
quick-setting
applications. In typical applications, the amount of aggregate, e.g. fine
aggregate such as
silica sand, will be present in a weight ratio to the cement of from 0.5:1 to
6:1, more
usually from 1.2:1 to 5:1, and particularly from 1.5:1 to 4.5:1. Lighter
weight aggregates
such as perlite and the like may also be used. A variety of other materials
not classed as
2 5 aggregates may also be included. Examples are silica fume, silicones and
fibers such as
glass and steel fibers. Agents to provide cohesion or bonding of the applied
cement
composition, such as polyvinyl alcohols, may also be included.
The amount of water to be included in the quick-setting compositions of the
invention
will vary depending upon the particular application and other known factors,
but is always
sufficient for the hydraulic setting of the cement component. The amount of
water used in
shotcrete application is usually controlled to avoid undesired fluidity.
Hence, the amount
of water in shotcrete compositions will usually vary from 20% to 60%, more
usually 30%
to 45%, by weight of the hydraulic cement component. Since the water-soluble
citrates,
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malates and citramalates have been found to have a solids dispersing effect,
the amount of
water may be less than that otherwise indicated for particular applications.
In other
applications, greater amounts of water than used in shotcreting may be
employed to provide
desired fluidity. Other cement additives to maintain fluidity may also be used
but in
general the compositions will achieve a rapid initial set in not greater than
20 minutes,
preferably no more than 15 minutes under the influence of the water-soluble
citrate, malate
or citramalate when employed in amounts of 4% by weight of the cement or
greater.
The rapid-setting compositions and method of the invention are particularly
useful in
shotcrete applications. Shotcreting can be carried out using a dry mix cement
composition
(the dry mix system) or a wet mix cement composition (the wet mix system), as
described
below.
a) The dry mix system is one in which dry-mixed cement and aggregate are
pneumatically conveyed through a delivery hose, with water and a quick-set
agent generally
being added in the vicinity of the nozzle assembly or upstream of the nozzle
just prior to
passage into the nozzle assembly (the quick-setting agent can be added at the
point of
mixing instead of vicinal to the nozzle, but this is relatively rare). The
mixture is applied
on to the requisite surface in the vicinity of the nozzle, by pneumatically
projecting it out
from the nozzle under pressure, e.g. by means of compressed air. With the dry
mix system
it is difficult to control the quantity of water added and therefore to
control the water to
cement ratio (hereinafter referred to as the W/C ratio). The method has the
additional
drawback that dust is generated.
b) The wet mix system is a system in which the cement and aggregate are mixed
in
the presence of water and the mixture pumped through a delivery hose to a
nozzle and then
pneumatically projected out of the nozzle, a quick-setting agent being added
within the
nozzle assembly or upstream of the nozzle, just prior to passage into the
nozzle. There are
also examples of this system where the cement composition can be pumped,
rather than
pneumatically conveyed, to a point midway between the point of mixing and the
nozzle in
the delivery hose followed by pneumatic conveying between this midpoint and
nozzle. The
system allows good control of W/C ratio and generates little dust.
Pressures employed in the pneumatic delivery of cement compositions in
shotcreting
generally range from 60 to 140 psi (413-965 KPa), and are more usually 80 to
120 psi
(551-827KPa).
A particularly useful method of employing the malic or citramalic acid-based
quick-setting agents of the present invention in shotcreting is to add less
than an
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accelerating amount of a water-soluble salt of malic or citramalic acid
initially upon
batching of the cementitious composition and then to add an additional
accelerating amount
of the water-soluble salt of malic acid or citramalic acid in the vicinity of
the nozzle, such as
is described in United States Patent No. 5,634,972. If desired, a rheology
modifying
5 material capable of imparting thixotropic properties to the cementitious
composition may be
added as described in the aforementioned US Patent No. 5,634,972.
The following examples merely demonstrate the invention and are not intended
to limit
the same. In the examples, setting time and compressive strength measurements
conform
to American Society for Testing and Materials Methods C 266 and C 109,
respectively. All
measurements were made at room temperature. In the examples, set times are
determined
from the time the citrate, malate or citramalate is mixed with the cement.
EXAMPLE I
The mix procedure simulates the time scale of wet-mix shotcreting, such that
the
mortar is premixed and the shotcrete accelerator, added at the nozzle, is
mixed with the
mortar only for a brief moment.
In a plastic bag, 3000 g of mortar mix (dry ingredients), which consists of
590 g of
ordinaiy portland cement, 30 g of silica fume, and 2380 g of silica sand, is
blended.
Approximately 75% of tL total of 206.5 g of mixing water is placed in the
mixing bowl of
a HobattTM lab mixer. The blended dry ingredients are then added. The mixer is
started on
low speed and the mortar is mixed for two minutes. During this time, the 35.4
g of
potassium citrate (6% by weight of cement) is dissolved in the remaining mix
water. This
is added into the mixing bowl at the conclusion of the two minute initial mix
cycle, and the
mortar is mixed for an additional minute. The test specimens are then cast
immediately.
Setting time and compressive strength measurements conform to American Society
for
Testing and Materials Methods C 266 and C 109, respectively, with the
following results.
Setting Time @ 16 C+/-0.5 C(61 F+/-1 F)
Final: 7 minutes
Compressive Strength @ 16 C(61 F)
MPa (PSI)
1 hour 5(730)
4 hour 12 (1709)
1 day 17 (2404)
7 day 25 (3647)
28 day 38 (5433)
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EXAMPLE 2
Example 1 is repeated except that the experiment is conducted at a temperature
of
51.3 F and the amount of potassium citrate is increased to 8% by weight of the
cement.
Setting time and strength properties by ASTM C266 and C 109, respectively, are
as follows.
Setting Time @0.5oC+/-0.5 C(51 F+/-1 F)
Final: 5 minutes
Compressive Strength @ 16 C(61 F)
MPa (PSI)
1 hour 5 (707)
4 hour 7 (1060)
1 day 19 (2742)
7 day 30 (4256)
28 day 41 (5856)
EXAMPLE 3
In cement paste experiments (100 g. portland cement and 35 g. of deionized
water),
5% of potassium citrate and 5% of lithium citrate (based on the weight of the
cement) are
individually evaluated with following set times recorded by a VicatTM needle
apparatus.
Citrate Initial Set Final Set
potassium citrate 240 seconds 320 seconds
lithium citrate 200 seconds 280 seconds
EXAMPLE 4
Following the general procedure of Example 1, a shotcrete mortar mix is
prepared to
contain 122 g. of Ashgrove Type I portland cement, 220 g. of silica sand, 40
g. of water
and 7 g. of potassium citrate. The sand and cement are mixed together and then
mixed with
a solution of the citrate in the water. The resulting composition has an
initial set of 30
seconds (ASTM C266) and a 1 hour compressive strength of 1800 psi (ASTM C109).
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EXAMPLE 5
In a plastic bag, 2775 g of mortar mix (dry ingredients), which consists of
740 g of
ordinary portland cement and 2035 g of silica sand, is blended together.
Approximately 50%
of a total of 333 g of mixing water is placed in the mixing bowl of a Hobart
lab mixer.
The blended dry ingredients are then added. 44.4 g of dipotassium malate (6%
by weight
of cement) is dissolved in the remaining mix water and added into the mixing
bowl. The
mortar is then mixed for three minutes. The test specimens are then cast
immediately and
determined to have the following properties:
Initial setting time: 7 minutes
Final setting time: 9 minutes
Compressive Strength PSI (MPa):
1 hour 673 (4.64)
4 hour 1525 (10.52)
1 day 2739 (18.89)
7 day 4411 (30.42)
EXAMPLE 6
A mortar mix prepared in accordance with ASTM C109 is prepared to contain 8.5%
dipotassium malate based on the weight of the cement at a water to cement
ratio of 0.45
and is determined to have the following properties:
Initial setting time: 4 minutes
Final setting time: 6.5 minutes
Compressive Strength PSI (MPa):
1 hour 863 ( 5.95)
4 hour 1840 (12.69)
1 day 3040 (20.97)
7 day 4081 (28.14)
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EXAMPLE 7
A mortar mix is prepared in accordance with ASTM C109 and contains 8% sodium
citramalate based on the weight of cement at a water to cement ratio of 0.40
and is
determined to have the following set times:
Initial set: 3 minutes
Final set: 5 minutes
