We've many times documented that the solid residua resulting from our essential use of Coal in the generation pf genuinely economical electric power is a valuable raw material mineral resource; one which can be productively utilized and consumed in multiple ways in the making of Portland-type Cement, PC, and Portland Cement Concrete, PCC.
Such use of Coal Combustion Products, CCP's, can actually enhance and improve the physical properties of the cured concrete, while at the same time reducing the need to extract virgin resources, thus saving money and helping to preserve some segments of the natural environment.
Although, as we've documented, for one instance, in:
West Virginia Coal Association | Pittsburgh Converts Coal Ash and Flue Gas into Cement | Research & Development; concerning:
"United States Patent 5,766,339 - Producing Cement from a Flue Gas Desulfurization Waste; 1998; Assignee: Dravo Lime Company, Pittsburgh; Abstract: Cement is produced (from) a moist mixture of a flue gas desulfurization process waste product (and) aluminum, iron, silica and carbon (and) wherein said source of aluminum and iron comprises fly ash";
that PC itself can be made from CCP's, in a process that not only conserves natural resources but reduces the amount of Carbon Dioxide emitted from the cement-making process, it has been known for a longer period of time that CCP's can be used as property-enhancing fine aggregates for PCC, as replacements for the conventional sand.
That, as we've documented, again for just one example, in:
West Virginia Coal Association | Coal Ash Concrete More Durable, Resists Chemical Attack | Research & Development; concerning:
"United States Patent 5,772,752 - Sulfate and Acid Resistant Concrete and Mortar; 1998; Assignee: New Jersey Institute of Technology; Abstract: The present invention relates to concrete, mortar and other hardenable mixtures comprising cement and fly ash for use in construction and other applications, which hardenable mixtures demonstrate significant levels of acid and sulfate resistance while maintaining acceptable compressive strength properties."
And, as we've documented, again for just one example, in:
West Virginia Coal Association | Wyoming Converts Coal Ash to Construction Aggregates | Research & Development; concerning:
"United States Patent 6,334,895 - Producing Manufactured Materials from Coal Combustion Ash; 2002; Assignee: The University of Wyoming Research Corporation; Abstract: This invention discloses a system for cold bond processing of combustion ash which enhances various characteristics of the resulting cured consolidated combustion ash materials. Specifically, the invention relates to processing techniques which enhances both density and strength of the of the consolidated combustion ash materials. The invention also relates to processing techniques which control various chemical reactions which assure that certain types of minerals are formed in the proper amounts which results in a cured consolidated combustion ash material which has greater dimensional stability and enhanced resistance to degradation. Embodiments for both normal weight and light weight combustion ash aggregates are disclosed which meet various ASTM and AASHTO (American Association of State Highway and Transportation Officials) specifications";
technical processes exist which enable the production of coarse aggregates, as a replacement for gravel and crushed stone, for use in PCC, as well.
There is, however, in addition, another CCP which can, even without being processed, as via the technique disclosed in the above "United States Patent 6,334,895 - Producing Manufactured Materials from Coal Combustion Ash", or others related, be utilized as coarse aggregate in PCC: "cinders", or, more technically correct: Boiler Slag.
As can be learned from the much-vilified US EPA:
Boiler Slag | Coal Combustion Products Partnership | US EPA; "Boiler slag is the molten bottom ash collected at the base of slag tap and cyclone type furnaces that is quenched with water. When the molten slag comes in contact with the quenching water, it fractures, crystallizes, and forms pellets.
This boiler slag material is made up of hard, black, angular particles that have a smooth, glassy appearance. Boiler slag is generally a black granular material. Boiler slag particles are uniform in size, hard, and durable with a resistance to surface wear. In addition, the permanent black color of this material is desirable for asphalt applications and aids in the melting of snow. Boiler slag is in high demand for beneficial use applications; however, supplies are decreasing because of the removal from service of aging power plants that produce boiler slag. Boiler slag applications include its use as a: Component of blasting grit and roofing granules; Mineral filler in asphalt; Fill material for structural applications and embankments; Raw material in concrete products; Snow and ice traction control material."
And, even though "supplies are decreasing", we wanted herein to document that, if we do have any surplus Boiler Slag available, it can be utilized, along with Fly Ash, in the making of PCC that has desirable qualities; one of those qualities being that rivers don't have to be dredged, and energy doesn't have to be expended crushing rock at stone quarries, in order to provide coarse aggregate for PCC.
Comment follows excerpts from the initial link in this dispatch to:
"United States Patent 2,250,107 - Concrete
Date: July, 1941
Inventor: John S. Mills, Detroit
Assignee: Detroit Edison Company, Michigan, a corporation of New York
This invention relates to concretes and more particularly to Portland cement concretes designed and intended for use in the erection of reinforced concrete structures. Concretes suitable for such use comprise mixtures of cement, fine aggregate and coarse aggregate ... and, at the time of mixing and placing in forms, those quantities of water which are deemed necessary to obtain desired consistencies when wet, and compressive strengths when dry.
The coarse aggregate employed may be pebbles, crushed rock of various types, and products of various types of combustion such as slag and cinders. The function of the coarse aggregate in the concrete is to lend bulk and strength at an expense less than that of the cement and fine aggregates, and also to reduce the overall shrinkage induced by the shrinkage of the cement upon setting.
The function of the fine aggregate, generally speaking, is to form, with the cement and water, a mortar or matrix which will support the coarse aggregate in suspension after the mixing operation. It will be understood that, immediately after the concrete has been poured, the denser particles of the coarse aggregate tend to settle towards the bottom of the matrix and it frequently happens that the lighter bonding matrix fails to follow the coarse particles downwardly. This results in an undue concentration of the coarse aggregate near the bottom of the forms in which the concrete has been poured and likewise often causes the bonding matrix to fail to make adequate bond with all surfaces of the coarse aggregate particles so that the actual strength of the concrete mixture is less than the designed strength. It is important therefore to have a matrix of sufficient fluidity to facilitate pouring but which also is sufficiently viscous so that the segregation of the coarse aggregate is prevented and a satisfactory bond is made with all sides of the particles of the coarse aggregate.
Sand has been the material most commonly used as a fine aggregate.
I have discovered that a material, heretofore never made use of as a fine aggregate for concrete, or suggested as useful for that purpose, has properties which render it especially valuable as a fine aggregate.
It may be used with all kinds of coarse aggregates and has been found to be especially suitable for use with cinders.
This material, called fly ash, is the ash produced when pulverized coals, usually bituminous in nature, are burned in suspension.
Fly ash as a fine aggregate in concrete may be used in combination with sand, but in all cases where it is used, it should be considered a substitute for all or part of the sand or other fine aggregate which would otherwise have to be used.
(Which "other fine aggregate" would otherwise entail the expenditure of energy, with concomitant emissions of Carbon Dioxide, and the disruption of the environment to be obtained.)
The resulting concrete is, while wet, more easily worked than a sand concrete and, when dry lighter, stronger, more uniform and more elastic than a concrete of the same approximate composition containing sand as the sole fine aggregate.
(That is, Fly Ash concrete is, simply, better.)
The proportion of fly ash used in any batch of concrete may be expressed as the amount of cement used. In accordance with my invention, the ratio of fly ash to cement may vary between 1 and 5.4 or more, the amounts or cement and fly ash being measured in cubic feet. Preferably this ration varies between 1.5 and 2.5 times the amount of cement.
(We use a lot, really a lot, of Cement for the manufacture of Concrete in this country each year as it is. The above specifications, if implemented, would lead to us using a lot, really a lot, of Coal Ash.)
In mixing fly ash concretes in which cinders are the coarse aggregate, I have found that fly ash is extremely beneficial because its specific gravity more nearly approaches that of the cinders. This results in an intimate mixture of fly ash and cinders, with the fly ash particles serving to fill or plug any of the apertures in the surface of the larger cinder particles. Heretofore these apertures have been plugged by the cement alone, inasmuch as it is the only ingredient of the concrete which is fine enough to serve this purpose.
With fly ash in the concrete, it's particles which are as fine as those of the cement, tend to take over this plugging function leaving additional cement available for extra strength.
In general, fly ash concretes have a finer texture and are, therefore, more pleasing to the eye.
They may be mixed quite readily and are usually lower in cost than sand concretes inasmuch as fly ash is essentially a waste product.
For example, in one case it was found that fly ash concrete could be mixed for about ten per cent less than a standard sand-pebble concrete.
(Note, though, the additional potential energy and cost savings because the Fly Ash concretes can "be mixed quite readily"; for an implied cost "ten percent less than" sand concretes. In sum, the Fly Ash concretes are cheaper and prettier, i.e., "more pleasing to the eye".)
Claims: Concrete formed from an easily workable homogenous plastic mass comprising cement and aggregate, said mass being characterized by the presence of fine aggregate which consists of fly ash and sand ... the amount of fly ash in the fine aggregate being, by volume, from approximately 1.5 to 2.5 times the amount of cement, and exceeding the amount of sand, the concrete being strong in compression, lighter in weight than similar concrete in which the fine aggregate is wholly sand.
A lightweight high-strength concrete mixture comprising cement, a coarse aggregate of cinders, and a fine aggregate of fly ash, the fly ash being present in a volume greater that the volume of cement present and having substantially the same specific gravity as the cinders whereby the fly ash particles mix thoroughly with the cinders without segregation ... ."
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We'll close our excerpts there so that we can emphasize yet another benefit accruing to the use of Coal Ash and Coal Boiler Slag, "cinders", combined as the, respectively, fine and coarse aggregates in PCC.
Note in the closing paragraph that using both of them in a concrete mix, because of their nearly-equal specific gravities, prevents "segregation".
What Detroit Edison is referring to, using a more generic label, is what is more commonly known as "settling"; that is, the coarser and heavier aggregates will tend to settle downwardly in a concrete mass after it has been poured and before it sets.
In a large concrete structure, than can result in variations in density and strength from one portion of the structure to another, and voids and what are sometimes known in the trade as "sand trails", zones of weakness and greater porosity, within the concrete.
More about that phenomenon, and other things related to concrete, can be learned via:
"A Guide to Determining the Optimal Gradation of Concrete Aggregates; Strategic Highway Research Program; National Research Council; Washington, DC; 1993; PUBL. NO. SHRP-C-334; This guide provides a means to determine the optimal gradation of sand and coarse aggregate materials for highway concrete pavements. The proportioning of sand and coarse aggregate components has an important effect on the properties of both the fresh and hardened concrete.
Unsatisfactory gradation may lead to: 1. Segregation of the mortar from the coarse aggregates. 2. Bleeding of water below and around larger aggregates and on the surface of the concrete. 3. Settling of aggregates,
Effective particle packing seeks to select proper sizes and proportions of small particle shaped materials to fill larger voids. Depending on the workability and the consolidation during placement of the concrete, the packing of the aggregates may vary. In some cases aggregates may settle, causing internal bleeding leading to water-filled gaps surrounding coarse aggregates. The physics of packing can control these phenomena in the fresh concrete and thereby improve the durability of the hardened concrete."
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And, apparently, the combined use of Coal Fly Ash and Coal Boiler Slag, "cinders", because of their nearly equivalent specific gravities, as substitutes for both sand and gravel aggregates in Portland Cement Concrete, serves to help prevent "segregation", or "settling", with the end result being a more consistent, stronger and more durable, Portland Cement Concrete that is "lower in cost than sand concretes".