Note: Descriptions are shown in the official language in which they were submitted.
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Controlled Reactivity Quicklime And Method
For Producing Aerated Autoclaved Concrete
Field of the Invention
This invention relates generally to a process for producing foamed or cellular
concrete molded articles by subjecting a molded article obtained from a slurry
of
quick-stiffening cement compound to high temperature and high pressure curing
in
which a controlled reactivity quicklime component is utilized in forming the
slurry.
Description of the Prior Art
The production of low density aerated autoclaved concrete is well established.
Aerated autoclaved concrete is manufactured by mixing a silica rich material
such as
fine ground sand or fly ash, cement, a sulfate source such as gypsum,
quicklime, a
rising agent such as aluminum powder, and water. In a first chemical reaction,
the
quicklime reacts with the water to form heat and calcium hydroxide. The
calcium
hydroxide, in turn, reacts with the water and aluminum powder to form hydrogen
gas
which expands the concrete mix to about twice its original volume, or more.
Similar
to bread rising, the mix expands into a porous mass.
After expansion has occurred, the porous mass is cut to a desired size and
shape and is placed in an autoclave to build strength, rigidity and durability
with the
cement component serving to harden the mass. The autoclave is an airtight
chamber
that is filled with pressurized steam. During the autoclaving process, which
is the
formation of C-S-H gel tobermorite, typically 10-12 hours, a second chemical
reaction
occurs that gives the highly porous material its strength, rigidity and
durability.
The rate of reaction, or reactivity, of the quicklime with water and the
subsequent reaction of calcium hydroxide and water with aluminum powder in the
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first chemical reaction is critical to the development of the required
characteristics of
the final product. In particular, a controlled reactivity quicklime is
necessary for the
development of uniform cell structure within the porous mass.
At the present time, the reactivity of quicklime used in producing aerated
autoclaved concrete is controlled or varied by varying the calcination
parameters of
the manufacturing operation which produces quicklime itself. By altering the
temperature of calcination, the duration of calcination, and the type of
calciner used,
quicklime can be manufactured with a reactivity in a range from highly
reactive for
light-burned quicklime, to slightly reactive for hard-burned quicklime. This
method
to control the reactivity of quicklime for use in aerated autoclaved concrete
requires
a significant amount of time to set up and is effective only when producing
large
quantity of quicklime with a particular reactivity. In addition, variations in
the quality
of the quicklime can have adverse effects on the quality of the aerated
autoclaved
concrete.
A need exists, therefore, for an improved controlled reactivity quicklime
which
is useful in producing aerated autoclaved concrete.
A need exists for such a controlled reactivity quicklime which does not depend
upon the calcination process itself or varying the parameters of such process.
A need exists for a controlled reactivity quicklime which can be fine tuned to
produce a variety of quicklime reactivities quickly and economically, even in
small
quantities. -
A need exists for a chemical modifier to produce a controlled reactivity
quicklime having a particular reactivity for a particular end use.
A need exists for a chemical modifier to produce a controlled reactivity
quicklime which can be used to either pretreat the quicklime prior to use in
other
processes or which can be added directly to a slurry of the quicklime and
water and
other ingredients.
A need exists for a chemically modified quicklime with a controlled reactivity
which does not have adverse effects on the quality of the ultimate aerated
autoclaved
concrete which is produced.
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Summary of the Invention
The present invention discloses an improved method for the production of
aerated autoclaved concrete in which the properties of the aerated autoclaved
concrete are controlled or varied by controlling or varying the reactivity of
the
quicklime component of the aerated autoclaved concrete mix. The present
invention also discloses a method of producing an improved quicklime, for use
in
aerated autoclaved concrete, with a desired reactivity. The reactivity of the
quicklime can be altered by the addition of certain chemical modifiers either
prior to
or simultaneously with the mixing of the aerated autoclaved concrete
components.
Alteration of the reactivity of the quicklime produces corresponding changes
in the
properties of the aerated autoclaved concrete. A decrease in the reactivity of
the
quicklime generally produces desirable changes in the properties of the
aerated
autoclaved concrete, such as a more uniform cell structure, lower density,
higher
strength and higher durability. The method of the present invention allows for
the
production of aerated autoclaved concrete of selected properties, without
modification to conventional calcination processes and independent of the
variability
and quality of the quicklime.
In accordance with one aspect of the present invention there is provided a
method of manufacturing an aerated autoclaved concrete material, the method
comprising the steps of: preparing a quick-stiffening mixture by combining a
silica
containing material, modified quicklime, a rising agent, gypsum, cement and
water;
depositing the quick-stiffening mixture into a mold and allowing it to form a
stiffened
body; removing the stiffened body from the mold and placing the stiffened body
in
an autoclave station in which the stiffened body is steam cured at elevated
temperature and pressure; wherein the modified quicklime which is used to form
the
quick-stiffening mixture is modified with a chemical modifier, wherein the
chemical
modifier is selected from the group consisting of water, glycerol,
polyacrylates,
phosphoric acid, carboxylates, sucrose and mixtures thereof to provide a
desired
degree of chemical reactivity in the quick-stiffening mixture; wherein the
chemical
modifier is used to pretreat the quicklime prior to the addition of modified
quicklime
to the quick-stiffening mixture; and wherein the relative reactivity of the
quicklime is
controlled by adding the chemical modifier in an amount sufficient to slow the
initial
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second heat rise of the quicklime to approximately 5 C during subsequent
slaking as measured according to ASTM C-110.
In accordance with another aspect of the present invention there is provided
a method of manufacturing an aerated autoclaved concrete material, the method
5 comprising the steps of: preparing a quick-stiffening mixture by combining a
silica
containing material, modified quicklime, a rising agent, gypsum, cement and
water in
a slaking operation; depositing the quick-stiffening mixture into a mold and
allowing
it to form a stiffened body; removing the stiffened body from the mold and
placing
the stiffened body in an autoclave station in which the stiffened body is
steam cured
10 at elevated temperature and pressure; wherein the modified quicklime which
is used
to form the quick-stiffening mixture is modified with a chemical modifier,
wherein the
chemical modifier is selected from the group consisting of glycerol,
polyacrylates,
phosphoric acid, carboxylates, sucrose and mixtures thereof to provide a
desired
degree of chemical reactivity in the quick-stiffening mixture; wherein the
quicklime is
further modified by the addition of water which is added to the quicklime in a
controlled manner prior to addition of the quicklime to the quick-stiffening
mixture in
the slaking operation, the addition of water serving to create a surface
reaction
which slows down the subsequent slaking reaction and also reduces any tendency
of the quicklime to agglomerate; and wherein the relative reactivity of the
quicklime
is controlled by adding the chemical modifier in an amount sufficient to slow
the
initial 10 second heat rise of the quicklime to approximately 5 C during
subsequent
slaking as measured according to ASTM C-110.
In the method of manufacturing an aerated autoclaved concrete material, a
quick-stiffening mixture is prepared by combining a silica rich material,
quicklime, a
sulfate source such as gypsum, a rising agent, cement and water. The mixture
is
deposited into a mold and is allowed to form a stiffened body. The stiffened
body is
removed from the mold and is placed in an autoclave station in which it is
steam
cured at elevated temperature and pressure. The quicklime which is used to
form
the quick-stiffening mixture is modified with a chemical modifier to provide a
desired
degree of chemical reactivity in the quick-stiffening mixture.
Preferably, the chemical modifier is selected from the group consisting of
glycerol, glycols, lignosulfonates, amines and polyacrylates, metal sulfates,
gypsum,
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sulfuric acid, phosphoric acid, carboxylates, sucrose and mixtures thereof.
Most
preferably, the chemical modifier is selected from the group consisting of
sulfuric
acid, gypsum, alkali and alkaline earth metal lignosulfonates, glycerol,
ethylene
glycol, diethylene glycol, triethylene glycol, monoethylene amine, diethylene
amine,
triethanolamine, polyacrylates, water and mixtures thereof. Examples of
suitable
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polyacrylates include the alkali metal salts of polyacrylic acid, for example
sodium
polyacrylate (SPAL) and potassium polyacrylate.
Additional objects, features and advantages will be apparent in the written
description which follows.
Brief Description of the Drawings
Figure 1 is a graph of exotherm curves showing the reactivity of quicklime
with
various chemical additives over a time period of 0 to 10 minutes;
Figure 2 is a graph of exotherm curves similar to Figure 1 with the same
chemical additives being used to modify the reactivity of the quicklime with
the heat
rise being shown over a time period of 0 to 70 minutes;
Figure 3 is another graph of exotherm curves showing heat rise versus time
where the reactivity of the quicklime has been modified by the addition of a
controlled amount of water and the addition of a controlled amount of sulfuric
acid;
Figure 4 is a graph of exotherm curves similar to Figure 3 but with a
different
time scale;
Figure 5 is a graph of exotherm curves showing the reactivity of quicklime in
the presence of gypsum with various chemical additives being used to alter the
reactivity of the quicklime;
Figure 6 is a graph of exotherm curves similar to Figure 5 but on a different
time scale;
Figures 7-10 are exotherm curves of heat rise versus time showing the affect
of various amine chemical modifiers on the reactivity of the quicklime; and
Figure 11 is a simplified view of a prior art plant layout for the production
of
aerated autoclaved concrete materials.
Detailed Description of the Invention
Aerated autoclaved concrete, as well as similar types of porous steam cured
lightweight concrete, e.g., foam concrete, is produced in a well known and
traditional
manner. Any of several available silica containing materials, such as sand,
fly ash or
similar materials, is mixed with quicklime, a sulfate source such as gypsum,
cement,
a rising agent and water. In the case of aerated concrete, the most common
rising
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agent is aluminum powder. When these materials are homogenized and brought
into
close contact, the quickiime (Ca0) reacts with water and the silica (Si02)
material and
forms a type of calcium silicate hydrate mass at the same time that the
aluminum
powder reacts with the water and calcium hydroxide which is formed to develop
hydrogen gas. The evolution of the hydrogen gas gives the mass a
macroporosity.
This pore formation results in an increase in the volume of the mass from a
limited
initial volume to a relatively large volume, e.g. twice the initial volume.
In practice, the slurry of components are mixed together to form a quick-
stiffening mixture which is placed into a mold from a suitable mixing device.
After
rising, the mass is allowed to stiffen in the mold during a suitable
stiffening time.
During this stiffening time, a semi-plastic body is formed which has a
relatively low
strength but which is sufficiently stiff to form a green body without support
from the
mold which can be transported on its own. As soon as this stiffness occurs,
the
body is typically removed from the mold and is cut or divided into a suitable
shape
or number of pieces, the resulting shapes being suitable for use in the
building
industry. The divided sections of the stiffened body are placed in an
autoclaving
station where they are steam cured at high pressure and temperature, e.g., 170-
200 C, in order to obtain a prerequisite strength. The body can then be
transported
from the autoclaving station to an unloading station in which the elements of
the
body are separated from each other and are packaged and/or transported to a
desired
location.
During the reaction between the water, quicklime silica and aluminum, heat is
evolved. It is desirable to control this heat evolution since too fast a
temperature
increase in the mass produces difficulties in controlling the rising process
and
produces other undesirable effects such as lack of uniform pore size in the
ultimate
mass. Hard burnt lime, which reacts slowly with limited development of heat
has
been widely used for the manufacture of aerated autoclaved concrete because of
its
slower reactivity. The use of a hard-burned quicklime has a number of
disadvantages
including the fact that hard-burnt lime is considerably more expensive than
soft burnt,
highly reactive lime of the type used, e.g., in the steel industry. Another
inconvenience is that the hard burnt lime gives a relatively long stiffening
time which
slows the manufacturing process. Also, as has been mentioned, controlling the
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reactivity of the quicklime by controlling the calcination parameters during
the
calcining process requires a significant amount of time to set up and is
effective only
when producing large quantities of quicklime for a particular reactivity.
Aerated autoclaved concrete (AAC) weighs less than about two-thirds of
conventional concrete and has thermal insulation values of up to R-1.6 per
inch, more
than twice that of typical concrete. It can be created in varying densities,
depending
upon the mixture of components and can be tailored to specific designs. Very
low
density ACC, for example, has a low compressive strength but a high insulation
value. A typical 8 x 8 x 16 inch block, with a density of about 30 pounds per
cubic
foot offers a compressive strength of more than 500 pounds per square inch and
a
R- value of approximately 10.
The low weight and high insulation value of AAC, coupled with its relatively
high strength allows it to function as an excellent structural insulating
system. Unlike
traditional concrete, which usually requires external insulation, ACC can be
used in
moderate climates without external insulation in low rise commercial,
industrial and
residential buildings. The material's lower weight also makes it easier to
transport,
lift and assemble than typical pre-cast concrete elements. The material's fire
resistance and high degree of sound absorption also make it useful for
interior
building materials.
With these factors in mind, the present invention provides an improved
controlled reactivity quicklime for use in an aerated autoclaved concrete
production
method. In one embodiment of the present invention, quicklime is calcined in a
rotary
kiln by conventional methods. The quicklime is used in preparing a quick-
stiffening
mixture by combining a silica containing material, quicklime, a sulfate
source, a rising
agent, cement and water. The slurry or mixture is deposited into a mold and is
allowed to form a stiffened body. The stiffened body is removed from the mold
and
is placed in an autoclaved station in which it is steam cured at elevated
temperature
and pressure. The quicklime which is used to form the quick-stiffening mixture
is
modified with a chemical modifier to provide a desired degree of chemical
reactivity
in the quick-stiffening mixture.
Preferably, the silica rich material is selected from the group consisting of
sand,
fly ash, mine tailings and combinations thereof. The cement can conveniently
be
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selected from the group consisting of ordinary Portland cement, high alumina
cement,
gypsum cement and blends of two or more thereof. The preferred rising agent is
aluminum powder. The chemical modifier is preferably selected from the group
consisting of glycerol, glycols, lignosulfonates, amines, polyacrylate type
materials,
alkali and alkaline earth metal sulfates, gypsum, sulfuric acid, phosphoric
acid,
carboxylates, sucrose, water and mixtures thereof. Examples of the metal
sulfates
include calcium sulfate and sodium sulfate. Examples of suitable polyacrylate
type
materials include the alkali metal salts of polyacrylic acid, for example
sodium
polyacrylate (SPAL) and potassium polyacrylate. However, due to the relative
low
cost and availability of sodium polyacrylate, it is preferred. Other materials
such as
co-polymers of polyacrylate are also effective. A commercially available
polyacrylate
is "DISPEX N40VT"', available from Allied Colloids. Other anionic
polyelectrolytes such
as polycarboxylic acid, polyphosphoric acid, copolymers of polycarboxylic acid
and
polyphosphoric acid and the alkali metal salts thereof may also be useful.
Most preferably, the chemical modifier is selected from the group consisting
of sulfuric acid, gypsum, water, alkali and alkaline earth metal
lignosulfonates,
glycerol, ethylene glycol, diethylene glycol, triethylene glycol, monoethylene
amine,
diethylene amine, triethanolamine, polyacrylates and mixtures thereof. The
chemical
modifier is used in the range from about 0.1 to 5% by weight, preferably about
0.1
to 2% by weight, most preferably about 0.5 to 1 % by weight of quicklime,
based
upon the particular modifier chosen.
Thus, in a typical example, a chemical modifier such as a selected
lignosulfonate is utilized in an amount from about 1 to 5% by weight based on
the
total weight of the quicklime present and is added to the quick-stiffening
mixture to
act as- a chemical modifier and decrease the reactivity of the quicklime.
Example
preferred lignosulfonates include, for example, sodium lignosulfonate and
calcium
lignosulfonate. The quicklime reacts at a selected rate with the water, as
determined
by the type and amount of chemical additive present, to form heat and calcium
hydroxide. The calcium hydroxide, in turn, reacts with the aluminum powder to
form
hydrogen gas which expands the concrete mixture into a porous mass.
In another embodiment of the invention, the chemical modifier is premixed with
the quicklime prior to introduction into the mixture of fine ground sand (or
similar
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materials such as fly ash or mine tailings), cement, aluminum powder or water.
For
example, a preferred chemical additive or modifier is a polyacrylate
dispersant used
in an amount ranging from about 1 to 5% by weight based on the total weight of
quicklime present.
In order to evaluate the reactivity of the chemically modified quicklime,
industry
standard tests such as those published by the American Society of Testing
Materials
or modifications thereof can be utilized. According to a modified ASTM C-1 10
test
procedure, the rise in heat of a given quantity of quicklime in a given
quantity of
water with respect to time under standard conditions is measured. In the tests
which
follow, 150 grams of quicklime were mixed with 600 grams of water under
controlled
conditions and a 40 C temperature rise in 4 to 10 minutes was measured.
Referring now to Figure 1 and Figure 2 in the drawings, the exotherm curve
designated as "Control" represents the reactivity of conventional quicklime
calcined
in a rotary kiln with no chemical additive. The exotherm curve designated as
"2%
additive A" represents the reactivity of conventional quicklime with a
chemical
additive of 2% sodium polyacrylate. The exotherm curve designated as "4%
additive
A" represents the reactivity of conventional quicklime with a chemical
additive of 4%
sodium polyacrylate. The exotherm curve designated as "1 % additive B"
represents
the reactivity of conventional quicklime with a chemical additive of 1 %
sodium
lignosulfonate. The exotherm curve designated as "4% additive B" represents
the
reactivity of conventional quicklime with a chemical additive of 4% sodium
lignosulfonate. In Figure 1, each of the exotherm curves representing the
reactivity
of conventional quicklime with a chemical additive reflects a decreased
reactivity from
the conventional quicklime with no additive. The greatest effect is
represented by
the exotherm curve for "4% additive B". Figure 2 in the drawings illustrates
the
same exotherm curves of Figure 1 over an extended period of time such that
each of
the four exotherm curves representing quicklime with chemical additives has
leveled
off.
Figures 3-5 are generally similar to Figure 1 previously discussed except that
the graphs represent the exotherm curves of quicklime which has been
chemically
modified with triethanolamine in various concentrations.
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Figure 6 is another exotherm curve, generally similar to the curves previously
discussed, in which heat rise versus time is graphed for a chemically modified
quicklime in which the modifying additive is glycerol.
The data presented in Figures 3-6 shows that triethanolamine is a very good
candidate for controlling reactivity of the quicklime. At low levels, it
produces a
controlled reaction which can be predicted dependably. At these levels, it
also acts
as a grinding aid, eliminating or limiting the need to add water or other
organics
which lubricate and reduce static during grinding operations. The glycerol,
while
effective in controlling reactivity as indicated in Figure 6, is not as cost
effective at
the indicated levels but does provide an acceptable degree of control over the
reactivity of the quicklime.
In addition to having a quicklime which is more slowly reactive as measured
by ASTM type standards, it is also desirable that the quicklime not form
larger sized
agglomerated particles during the mixing reaction. This is important because
the
aerated concrete mixture will typically contain significant amounts of gypsum
(calcium sulfate). The presence of gypsum during the slaking of quicklime is
known
to produce large agglomerated particles of calcium hydroxide. These large
agglomerated particles are detrimental to the formation of high quality
aerated
autoclaved concrete.
Another aspect of the present invention is the discovery that the addition of
a very small amount of water to the quicklime in a controlled manner to create
a
surface reaction will not only slow down the subsequent slaking reaction but
will also
significantly reduce the quicklime's agglomeration tendency.
Turning to Figure 7, the normal quicklime control had a 40 C temperature rise
(called T60) in less than 1 minute, but more importantly it had a very rapid
temperature rise in the first few seconds. As shown from Figure 3, the
quicklime has
a 15 to 20 C temperature rise in 10 seconds. The agglomeration of this
material
during slaking is measured by the percent of material greater than 90 microns
is only
3.2%. When this control quicklime is slaked in the presence of a large amount
of
gypsum, as would be present in an aerated autoclaved concrete mixture, the
time to
achieve a 40 C temperature rise (T60) increases to 3 minutes (Figure 9).
However,
the initial 10 seconds of the reaction remains about the same with a 15 to 20
C
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temperature increase (Figure 10). the agglomeration of this quicklime in the
presence
of gypsum increases to 80%.
By pre-treating the quicklime with the addition of 2% water in a controlled
manner, the T60 increases to approximately 1.5 minutes (Figure 7). By "in a
controlled manner" is meant that the water is sprayed or dribbled on a
conveyor belt
as the quicklime is passing to, for example, a ball mill or a roller mill at
about, four
example 15-20 tons per hour. A spray bar or other means can be used to provide
an
even distribution as the quicklime moves along the conveyor belt. Although the
T60
increased to approximately 1.5 minutes in Figure 7, the initial 10 seconds
only have
a 5 C temperature increase (Figure 8) and the agglomeration decreases slightly
to
approximately 1 %. When the 2% water treated quicklime is tested with gypsum
the
T60 increases to 5 minutes (Figure 9). As shown in Figure 10, the first 10
seconds
of the reaction do not change with the temperature increasing by 5 C. However,
the
agglomeration tendency decreases significantly to 38%.
If, instead of using a 2% water spray, a 10% solution of sulfuric acid is used
(i.e., adding 2.22% of a 10% sulfuric acid to the quicklime as a
pretreatment), a
material is produced which not only has the slow reaction in the normal
slaking test
(T60 of 2.6 minutes) but which also has a low agglomeration tendency of 22%
(see
Figures 7-10).
This information is summarized in Table 1 below:
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Table 1
2.22%
2% water (10% H2SO4
Control QL added added
Normal Slaking (1)
T 60 (minutes) 0.9 1.6 2.6
10 second heat 20 C 5 C 5 C
rise C
Agglomeration 3% 1 % 2%
Gypsum Slaking (2)
T 60 (minutes) 3.2 5 5.5
10 second heat 18 C 5 C 4 C
rise C
Agglomeration 80% 38% 22%
(1) Normal slaking 150 grams of pulverized quicklime + 600 grams of water.
(2) 150 grams pulverized quicklime + 19 grams gypsum + 600 grams water.
From Table 1, it can be seen that the use of chemical additives such as
water or any other material that slows the initial 10-30 seconds of reactivity
of
quicklime will result in a quicklime that when tested with gypsum present and
used
in aerated autoclaved concrete mixes will produce a quicklime that has
significantly
lower agglomeration tendencies than normal soft burned high reactive
quicklime.
The addition a dilute solution of sulfuric acid, or other chemicals that
decrease the reactivity of quicklime, will also significantly decrease the
agglomeration
tendency of that quicklime if the additive is added in such a manner as to
significantly
lower the temperature rise of the initial 10-30 seconds of the slaking heat
rise.
Figure 11 illustrates, in simplified fashion, the operation of an aerated
autoclaved concrete plant. A typical plant of this type includes a casting
station 1
and a steam curing station which, in this case, is comprised of two parallel
autoclaves 2, 3. The plant is contained within a suitable housing 4. The
casting
station 1 is centrally located between two waiting positions 5 and 6,
respectively.
These waiting positions are connected with the autoclaves 2, 3 by means of
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transport tracks 7, 8 whereby green bodies of stiffened porous material can be
introduced into the autoclaves in the direction indicated by the arrows.
Each autoclave has two doors 9 and 10 at the entrance and exits thereof,
respectively. At each exit there are further transport tracks 11, 12,
respectively,
which are connected by a transverse conveyor 13 which is indicated in the
drawing
in simplified fashion by dotted lines.
A number of trucks 14 are used in the plant and include suitable bottoms and
sides to constitute the necessary mold elements. In the plant design shown in
Figure 11, a
return track 15 for the trucks 14 runs from the transverse transporter 13 to
the
casting station 1. The plant design illustrated is a typical design of the
type
described, for example, in United States Patent No. 4,613,472 to Svanholm,
issued
September 23, 1986, and is merely intended to be representative of the prior
art in
illustrating aerating autoclaved concrete plant layouts of the type suitable
for use in
practicing the present invention.
The method of the present invention, and in particular, the introduction
of chemical additives to the conventional aerated autoclaved concrete mix,
allows for
the production of aerated autoclaved concrete of various desired properties,
without
modification to conventional quicklime calcination processes and independent
of the
variability in quality of the quicklime. Significant cost savings are achieved
by using
chemical additives to alter the reactivity of the quicklime instead of
modifying the
catcination process.
It should be apparent from the foregoing that an invention having
significant advantages has been provided. While the invention is shown in only
one
of its forms, it is not just limited but is susceptible to various changes and
modifications without departing from the spirit thereof.
