CA2418011A1

CA2418011A1 - Method of manufacturing autoclaved, cellular concrete products using bottom ash

Info

Publication number: CA2418011A1
Application number: CA 2418011
Authority: CA
Inventors: Michael P. Mehlhoff
Original assignee: Individual
Current assignee: ENVIROC Inc
Priority date: 2000-08-03
Filing date: 2001-07-31
Publication date: 2002-02-14
Also published as: AU2001283061A1; WO2002011960A1

Abstract

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 to form calcium silicate crystals; (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, temperature, and steam to transform the calcium-silicate crystals into Tobermorite.

Description

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 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 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.

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 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 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.

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.

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 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%

Claims

What is claimed is:

1. A method of manufacturing autoclaved cellular concrete products, comprising the following steps:
a. selecting a suitable quantity of bottom ash;
b. processing said bottom ash to increase its overall surface area and expose silicate compounds located therein;
c. mixing said processed bottom ash with water to form a slurry;
d. mixing cement, lime and aluminum powder with said slurry;
e. pouring said slurry into molds;
f. subjecting said slurry in said molds to a pre-curing at room temperature and atmospheric pressure; and g. subjecting said slurry in said molds to a secondary curing using surface pressure, temperature, and steam sufficient to transform calcium silicate crystals into Tobermorite.

2. The method of manufacturing autoclaved cellular concrete products according to Claim 1, wherein the bottom ash, cement, lime, anhydrite, and aluminum powder are mixed in water in a 71:10:16.40:2.46:0.09 ratio by weight, respectively.

3. The method of manufacturing autoclaved cellular concrete products according to Claim 1, wherein step (g) further comprises subjecting the slurry in the molds to surface pressure of about 250 PSI.

4. The method of manufacturing autoclaved cellular concrete according to Claim 3, wherein step (g) further comprises subjecting the slurry in the molds to a temperature of about 400 degrees F.

5. The method of manufacturing autoclaved cellular concrete products according to Claim l, wherein step (d) further comprises mixing the cement, lime and aluminum powder with about 0.05% siloxane.

6. The method of manufacturing autoclaved cellular concrete products according to Claim 1, wherein step (c) further comprises mixing the processed bottom ash with water, wherein the processed bottom ash has a particle size distribution in the following ranges:
95.6 % - 99 % passing a 200 micrometer sieve, 71.6 % - 88.8 % passing a 90 micrometer sieve, 57.2 % - 76.8 % passing a 63 micrometer sieve, and 42.0 % - 62.8 % passing a 45 micrometer sieve.

7. The method of manufacturing autoclaved cellular concrete products according to Claim 1, wherein step (c) further comprises mixing the processed bottom ash with water, wherein the processed bottom ash has the following particle size distribution:
about 99 % passing a 200 micrometer sieve, about 88.8 % passing a 90 micrometer sieve, about 76.8 % passing a 63 micrometer sieve, and about 62.8 % passing a 45 micrometer sieve.

8. The method of manufacturing autoclaved cellular concrete products according to Claim 1, wherein step (c) further comprises mixing the processed bottom ash with water, wherein the processed bottom ash has the following particle size distribution:

about 95.6 % passing a 200 micrometer sieve, about 71.6 % passing a 90 micrometer sieve, about 57.2 % passing a 63 micrometer sieve, and about 42.0 % passing a 45 micrometer sieve.

9. The method of manufacturing autoclaved cellular concrete products according to Claim 1, wherein the slurry of step (e) comprises the following approximate amounts by weight:
71 % bottom ash, 13.2 % quicklime, 13.2 % Portland cement, 2.51 % anhydrite, and 0.09% aluminum powder.

10. The method of manufacturing autoclaved cellular concrete products according to Claim 1, wherein the slung of step (e) comprises the following approximate amounts by weight:
71 % bottom ash, 14.8 % quicklime, 11.6 % Portland cement, 2.51 % anhydrite, and 0.09% aluminum powder.

11. The method of manufacturing autoclaved cellular concrete products according to Claim l, wherein the slurry of step (e) comprises the following approximate amounts by weight:
71 % bottom ash, 16.4 % quicklime, 10.0 % Portland cement, 2.46 % anhydrite, 0.05 % aluminum powder, and 0.05 % siloxane.

12. The method of manufacturing autoclaved cellular concrete products according to Claim 1, wherein step (b) comprises grinding the bottom ash in a ball mill.