Note: Descriptions are shown in the official language in which they were submitted.
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TITLE: METHOD OF MANUFACTURING AUTOCLAVED, CELLULAR CONCRETE
PRODUCTS USING BOTTOM ASH
FIELD OF THE INVENTION
This invention pertains to methods of manufacturing cement or concrete
building
materials. More particularly, the invention relates to methods of
manufacturing autoclaved
cellular concrete products utilizing waste or by-product from the burning of
coal. Thus, the
invention relates to both a method for manufacturing building materials and a
method for
utilizing an environmentally undesirable waste product to produce useful
materials.
BACKGROUND OF THE INVENTION
Bottom ash is a by-product of the burning of coal. Coal is a solid, dark-
colored fossil fuel
found in deposits of sedimentary rocks, that is formed from once-living plant
and animal matter.
Coal is commonly burned in many locations around the world to produce energy
and to
manufacture steel. It is also a source of chemicals used to maxmfacture
pharmaceuticals,
fertilizers, pesticides, and other products.
Coal is classified according to its fixed carbon content or the amount of
carbon produced
when the coal is heated under controlled conditions. Higher grades of coal
have higher fixed
carbon content, less water content, and fewer inorganic impurities. Upon
combustion, the
inorganic impurities in coal form ash, referred to as fuel ash. Typically,
fuel ash includes
minerals such as pyrite and marcasite that are formed from metals that
accumulate in living
plants, and quartz and clay and other minerals that have been deposited in the
coal by wind and
groundwater. There are generally two types of fuel ash, namely fly ash and
bottom ash.
When coal is burned, airborne by-products, such as carbon dioxide, sulfur
dioxide gas and
airborne ash, known as fly ash, are produced. In the United States, statutes,
such as the U.S.
Clean Air Act, have been enacted to reduce the release of these by-products
into the
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environment.
In addition to airborne by products, non-airborne by-products are produced in
large
quantities, also raising environmental concerns. Such by-products, along with
unburned
combustible by-products, are collectively known as bottom ash and accumulate
in the coal
furnaces.
Heretofore, little use has been found for bottom ash and it is usually
discarded in landfills.
These significant quantities of bottom ash take up valuable space in
landfills. Generally
speaking, in recent years in the United States and in many other countries
throughout the world,
there has been an increased effort to reduce the amount of waste discarded in
landfills, by either
reducing the amount of waste products produced or by converting waste products
into useful
materials, or by doing both.
The presently claimed invention seeks to reduce the amount of bottom ash
stored in
landfills, and to convert what would otherwise be useless material into useful
and valuable
products. One useful product into which bottom ash may be incorporated is
autoclaved, aerated
concrete.
Autoclaved, aerated concrete (known as and referred to herein as "AAC") is a
lightweight
material used in place of concrete to manufacture various building materials,
including blocks,
panels, slabs and the like. AAC is a mixture of cement, lime, and fine silica
ash, foamed or
expanded with an aluminum powder, then autoclaved to produce a lightweight
building material.
With a weight about the same as wood, AAC blocks and panels provide a builder
with a versatile
and durable building product that is easy to modify and use in the field at an
economical cost. It
is also an energy efficient system that provides superior fire protection,
sound attenuation, and
insulating properties. Walls, floors, and roofs of a building can be
constructed with this product
with significant savings of time. AAC buildings may be erected during adverse
weather
throughout the year.
AAC products were first developed in Europe in the early 1920's as an
alternative
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building material to lumber. A Swedish architect, Axel Johanson, introduced
the product to
Europe. Since that time, AAC has become widely used in building construction
around the
world. Autoclaved cellular concrete products are discussed in the Comite Euro-
International du
Beton "Manual of Design and Technology," which is herein incorporated by
reference.
AAC products are made of 10% to 15% by weight of calcium-silicate penta-
hydrate
crystals, known as Tobermorite, in which the atoms are approximately.llA
apart.
The manufacture of AAC products is analogous to baking bread: yeast causes the
other
ingredients to expand or fill with air, and then it is baked to form bread.
Like the ingredients
used to make bread, the types of ingredients, the size of the particles, the
order of mixing, and the
reaction times are all important aspects that must be taken into account to
manufacture high
standaxd AAC products.
Heretofore, the main source of silica used in AAC products was sand, which is
ground
into fine particles to increase its overall surface area. In order to produce
finished AAC products
including the mineral Tobermorite, which provides excellent properties such as
strength and
resistance to shrinkage upon curing, sand containing about 75% to 95% silica
is required.
However, the price of sand is a factor which in part determines the price of
AAC products.
Ideally, in some locations where bottom ash is abundant, it would be desirable
to use
bottom ash to manufacture AAC products. Unfortunately, bottom ash has
relatively low silica
content (about 50 % to 60 %) and does not produce a sufficient amount of
Tobermorite.
One aspect of this invention relates to a method for obtaining the desirable
formation of
Tobermorite to form AAC materials, using bottom ash.
The present inventor has determined that it would be highly desirable to
manufacture
AAC products using ingredients less expensive than sand, and has created a
method for
converting bottom ash, an otherwise useless and environmentally unfriendly by-
product, into a
substance that is useful in manufacturing AAC products, generally at a
significantly lower cost
than prior art methods using silica sand as the source of Tobermorite.
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SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method of manufacturing
autoclaved,
cellular concrete products.
It is another object of the present invention to provide a method of
manufacturing
autoclaved, cellular concrete products using bottom ash.
It is yet a further object of the invention to provide a method for converting
bottom ash
into a useful component of autoclaved, cellular concrete products.
These and other objects of the invention which will become apparent are met by
a method
of manufacturing autoclaved cellular concrete products using bottom ash, that
uses the following
steps: (1) selecting a suitable quantity of bottom ash; (2) processing the
bottom ash into fine
pellets to increase its overall surface area and expose silicate compounds
located therein; (3)
mixing the processed bottom ash with water to form a slurry; (4) mixing
cement, lime and
aluminum powder into the slurry; (5) pouring the slurry into molds to form
various construction
components; (6) initially curing the slurry in the molds at room temperature
and at atmospheric
pressure; and (7) curing the slurry in the molds using pressure, raised
temperature and steam.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic showing the steps used in the method disclosed herein.
Figure 2 is a perspective view of an autoclaved, aerated concrete plant.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to the accompanying Figures, there is shown and described a method
of
manufacturing autoclaved aerated concrete (AAC) products using bottom ash,
that uses the
following steps: (1) selecting a suitable quantity of bottom ash; (2)
processing the bottom ash
into fine pellets to increase its overall surface area and expose silicate
compounds located
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therein; (3) mixing the processed bottom ash with water to form a slurry; (4)
mixing cement, lime
and aluminum powder into the slurry; (5) pouring the slurry into molds to form
various
construction components; (6) initially curing the slurry in the molds at room
temperature and
atmospheric pressure; and (7) curing the slurry in the molds using surface
pressure, raised
temperature, and steam.
Production of AAC products is carried out in a large processing plant that
first processes
and stores the necessary ingredients, then follows very specific, prescribed
steps to manufacture
the finished products. The main ingredient is finely ground bottom ash (about
70% by weight)
which is mixed with water, lime (about 16% by weight) and cement (about 13% by
weight), and
about 4 % by weight of other ingredients which act as binders and reaction
carriers, such as
aluminum power, siloxane and anhydrite (calcium sulfate). Aluminum metal
powder (less than
about 0.01 % by volume) is added to the mixture. The aluminum reacts with
water in the slurry
mixture to produce small volumes of gas that dissipate and leave open and
closed air pockets in
the product. These bubbles contribute to the lightweight characteristics of
the final product.
Sufficient water is included in the slurry so as to create a slurry that may
be mixed and
handled as desirable and as known to one of ordinary skill in the art. The
slurry is then poured
into molds and cured at room temperature and atmospheric pressure, and
thereafter subjected to
further curing under pressure and high temperatures. More specifically, the
process involves
subjecting the molds containing slurry to autoclaving with pressurized steam
and high
temperature to form AAC products containing about 10 % to about 15% of the
calcium silicate
converted to Tobermorite. Preferably, the pressurized steam is at about 250
PSI and the slurry in
the molds is subjected to a temperature of about 400 degrees F.
Refernng to Figure 2, which depicts the processing plant 10, the bottom ash is
collected
from local coal burning, steam-energy power plants and stored in silos. The
bottom ash is then
fed via a conveyor belt to a ball mill that finely grinds the bottom ash into
small, sand-like
pellets. During the grinding process, nodes on the bottom ash particles are
broken off and
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smashed into finer particles, similar to the pieces formed when smashing
eggshells. A sufficient
amount of water is then added to the small pellets to form a slurry that is
pumped to a slurry tank.
An agitator in the slurry tank is used to keep the particles in suspension.
The cement and lime binders are dry powders that are stored in large storage
tanks
located in the plant. The aluminum powder is stored in a separate location in
the plant for
additional safety. Preferably, prior to mixing, the aluminum powder is
separately weighed for
each casting and dispersed in a water suspension. Thus, the aluminum may be
introduced in
powder form, or in a suspension. The suspension may be in the form of a paste.
The slurry of bottom ash, cement, lime binders, anyhydrite (calcium sulfate)
and
aluminum powder are then mixed together to form a mixture that can be
discharged into molds.
As an optional step, siloxane (0.05% by volume) may be added to the slurry
mixture.
Addition of siloxane creates an outer water repelling surface on the products,
thereby reducing
the absorption of moisture through the outer surface of the product. Siloxane
is commercially
available from various sources, including Horscht Chemical Co. An example of a
commercially
available siloxane that may be used in the process of this invention is
blacker Chemie VP1307.
Mixing may be accomplished by any suitable means, such as using a combination
mixer/balance machine. The combination mixer/balance machine thoroughly mixes
the
ingredients to obtain a homogeneous mixture that can be discharged into molds.
During the
mixing process, calcium in the cement binds with the silica on and in the
pieces of the bottom ash
to form calcium silicate. The cement may be portland cement and the lime may
be quicklime.
The filled molds, also called castings, are moved to a curing area.
In the pre-curing area, the mixture expands in volume in the molds as the
cement and
lime react to remove the excess water and stiffen the mixture into a gel. The
aluminum power in
the mixture reacts with water to form gas bubbles within the casting. When the
gel has attained
sufficient strength, the mold is removed and the casting is cut into product
sizes. Typically, the
curing period lasts approximately 2 to 2-1l2 hours.
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The castings are cut to size by any suitable means. Thin, vibrating wires are
an example
of a suitable means for cutting the cured castings. For example, the castings
may be cut with thin
vibrating wires to make product sizes useable in building construction, for
example, bloclcs 4 to
12 inches in height, 8 to 24 inches in width, and 1 to 4 feet in length, and
panels, with or without
steel reinforcing rods, 4 to 12 inches in height, 24 inches in width, and 4 to
20 feet in length.
Thereafter, the cut castings are cured again, but now under steam pressure for
a time
sufficient to transform the planar calcium silicate crystals into the lattice
crystal "Tobermorite",
which gives the product its dimensional stability and strength. Typically, the
cut castings are
subjected to curing under steam pressure for about eight (8) to twelve (12)
hours.
After being cured under steam pressure, each cut casting is cut to close
tolerance,
inspected and stacked into units. The units are then wrapped with protective
wrapping material
to prevent moisture from entering or escaping from each casting. Products are
generally
considered saleable when the density is reduced from about 50 lbs. per cubic
foot to about 40 lbs.
per cubic foot. After the castings have cured, the castings contain
approximately 4.99 % water
by weight.
Bottom ash used in the invention can be obtained from any coal-fired boiler.
The primary
source of bottom ash will be, in most instances, local coal burning, steam-
electric power plants.
The bottom ash is usually widely available from such plants and can be
obtained at little or no
expense.
The bottom ash is ground in such a way as to increase the propensity of the
bottom ash to
react with the lime and water to form the mineral Tobermorite under the heat
and pressure of an
autoclave. Tobermorite is formed of calcium-silicate penta-hydrate crystals.
Thus, an aspect of
the invention relates to the discovery that, by grinding bottom ash, the
interior of the bottom ash
particles is revealed, thus exposing the silica crystals therein, which
permits the reaction with the
calcium (which forms Tobermorite, the calcium-silicate-hydrate crystal) in the
mixture to take
place more efficiently and more completely than with the use of fly ash.
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The desirability of Tobermorite stems from its ability to produce cement
wherein the
particles are large enough that once the product dries out to equilibrium with
normal air, it will
not shrink so much that it cracks significantly. Tobermorite has a lattice
distance of about 11
Angstrom, and is composed of plate-shaped crystals which combine to form a
rigid lattice. The
relatively larger crystals of Tobermorite prevent significant shrinking and
cracking upon drying.
Bottom ash, which has a relatively lower silica content than the sand
traditionally used in
the production of AAC products, can now be used to produce AAC products having
physical and
chemical characteristics at least as good, and usually better, than the prior
art's use of sand or fly
ash.
Fly ash, which is another by-product of the coal burning process, may be used
to form
AAC products. However, bottom ash has unexpected advantages over fly ash in
the production
of AAC products. In particular, bottom ash is composed of particles that tend
to be too large to
become airborne, and therefore the transport of bottom ash is much easier and
less hazardous
than the much smaller fly ash. With bottom ash, there is little danger of the
particles becoming
airborne, and thus breathed in by workers at the coal-burning plant or at the
AAC plant, or during
transport of the material therebetween.
The following are three examples of compositions for use in preferred
embodiments of
the method of the invention (amounts shown in percentage (%) by weight):
Com onent Exam le Exam le Exam le 3
1 2
Bottom Ash 71.00% 71.00% 71.00%
uicklime 13.20% 14.80% 16.40%
Portland Cement 13.20% 11.60% 10.00%
Anh drite Calcium sulfate 2.51 % 2.51 % 2.46
Siloxane p 0 0.05
Aluminum Powder 0.09 % 0.09 % 0.09
Com ressive Stren h 2.51 N/mm 3.18 N/mm 3.95 N/mrn
Bottom Ash Particle Size Distribution
Passin 200 micrometer sieve 99 % 98.7 % 95.6
Passin 90 micrometer sieve 88.8 % 83.2 % 71.6
Passin 63 micrometer sieve 76.8 % 68.4 % 57.2
Passing 45 micrometer sieve 62.8% 54.8% 42.0%
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