We enclose herein two United States Patents, issued to the University of Texas, which confirm, and help to emphasize, some earlier reports we've made concerning the rather vast potentials we have available to us for profitably utilizing the Coal Ash byproduct of our economical Coal-based power generation, in the making of Portland-type cement and concrete.
Some preamble is necessary so that the true significance of what is described herein can be made clear.
First, as we've alluded to and superficially described in previous reports, conventional Portland-type cement is typically made from, basically, limestone, which is calcined at high temperatures in a cement kiln, with the consequent emission of copious amounts of Carbon Dioxide.
The product of a cement kiln isn't cement itself, but what is known as cement "clinker", which is subsequently ground fine, in a mill, with the admixture of some other ingredients, to form the powdered product we typically think of as dry cement.
The dry cement is then blended and mixed with water and different forms of aggregate, and then sets up, or cures, into concrete.
As we've documented, for one instance, in:
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 by forming a moist mixture of a flue gas desulfurization process waste product (by) adding a source of aluminum, iron, carbon, and a siliceous material to said flue gas desulfurization process waste product to form a moist mixture thereof (and) calcining said dry agglomerated kiln feedstock in a rotary kiln to produce a cement clinker; and ... wherein said source of aluminum and iron comprises fly ash";
such cement clinker can be produced from, essentially, nothing but the wastes from Coal combustion.
Moreover, once the clinker is formed by calcination in the kiln, while it's still hot, as seen in:
West Virginia Coal Association | Canada Coal Ash Cement | Research & Development; concerning: "United States Patent 5,837,052 - Process for Producing Cement Clinker Containing Coal Ash; 1998; Assignee: Lafarge Canada, Inc.; Abstract: Contaminated coal ash, for example flyash contaminated with carbon is introduced to hot clinker in a cooler downstream of a cement kiln; the carbon is combusted in the cooler to provide a cement clinker having an effective content of flyash free of carbon; this permits use of flyash contaminated with carbon without the need for separate special steps for carbon removal; volatile contaminants or contaminants having a volatile component, for example adsorbed ammonia are similarly removed in the cooler by volatilization. A process for producing a cement clinker having a content of pozzolanic coal ash comprising: a) producing hot cement clinker from cement clinker raw ingredients in a cement kiln; b) feeding hot cement clinker from step a) into a cooler; c) feeding a pozzolanic coal ash, having a content of a contaminant, into contact with said hot cement clinker in said cooler and liberating said contaminant from said coal ash with heat evolved from said hot cement clinker as said hot cement clinker cools in said cooler; and d) recovering a cooled cement clinker containing said coal ash free of said contaminant";
more raw Coal Ash can be blended into the hot clinker, which burns out any residual carbon in that raw Ash and incorporates more Ash into the clinker, which could, as in the above-cited process of Dravo Lime Company's "United States Patent 5,766,339", have been made from nothing but Coal combustion wastes to begin with.
Again, once the cement clinker is finally formed and cooled - there is such a thing as a "clinker cooler" in cement plants - the clinker, having the appearance of rough stones, or slag, is ground or milled, often with the admixture of other minerals, to form the powdery substance we think of as dry cement.
And, herein, via the initial and one following link in this dispatch, we see that the University of Texas has devised a way in which even more Coal Ash can be consumed, as one of the additive minerals with which cement clinker, once it is finally formed, is ground or milled to form powdered dry cement and, ultimately, a concrete that exhibits higher performance than conventional Portland cement concrete in hostile chemical environments.
However, there is one caveat we must make note of. The processes described herein work because they utilize "Class C" Fly Ash, which is what you get by combusting sub-bituminous, lignite-type Coal, as is more commonly mined in the western US, including Texas.
We don't want, due to our disabilities and various other limitations, especially absent now some of our former technical advisors who gave up in frustration and weariness, to speculate too much, but:
The primary difference between Class C Fly Ash and Class F Fly Ash, which is what we get by combusting our high-quality West Virginia and Pennsylvania bituminous Coal, seems to be the relative amounts of lime, or Calcium Oxide, CaO, remaining in the Ash; which is, of course, the result primarily of Calcium Carbonate, limestone, that was present in the as-mined Coal.
In lignite, the Calcium Carbonate, with other related minerals, is present most often as finely-divided particulates that can't be separated by Coal-cleaning processes, whereas any limestone contaminating our as-mined eastern bituminous Coal is usually present in larger masses that come from the "top" or "bottom", due to falls or deliberate mining for construction purposes, and/or as intermittent thin seams within the main seam of the Coal itself.
Some differences in how and why the two different types of Coal Ash behave the way they do, based mostly on their relative Calcium contents, but with some other factors as well involved, can be learned via a dissertation on the subject provided by the US Federal Highways Administration, about which we've previously reported, via:
West Virginia Coal Association | Federal Highway Administration Recommends Fly Ash Concrete | Research & Development; wherein we're told, in part:
"Fly ashes are finely divided residue resulting from the combustion of ground or powdered coal. They are generally finer than cement and consist mainly of glassy-spherical particles as well as residues of hematite and magnetite, char, and some crystalline phases formed during cooling. Use of fly ash in concrete started in the United States in the early 1930's. The major breakthrough in using fly ash in concrete was the construction of Hungry Horse Dam in 1948, utilizing 120,000 metric tons of fly ash. This decision by the U.S. Bureau of Reclamation paved the way for using fly ash in concrete constructions.
In addition to economic and ecological benefits, the use of fly ash in concrete improves its workability, reduces segregation, bleeding, heat evolution and permeability, inhibits alkali-aggregate reaction, and enhances sulfate resistance.
(The above-noted enhancement in "sulfate resistance" is a key point of our current presentation.)
One of the most important fields of application for fly ash is Portland Cement Concrete pavement, where a large quantity of concrete is used and economy is an important factor in concrete pavement construction. FHWA has been encouraging the use of fly ash in concrete. When the price of fly ash concrete is equal to, or less than, the price of mixes with only portland cement, fly ash concretes are given preference if technically appropriate under FHWA guidelines.
Two major classes of fly ash are specified in ASTM C 618 on the basis of their chemical composition resulting from the type of coal burned; these are designated Class F and Class C. Class F is fly ash normally produced from burning anthracite or bituminous coal, and Class C is normally produced from the burning of subbituminous coal and lignite (as are found in some of the western states of the United States).
Class C ash usually has cementitious properties in addition to pozzolanic properties due to free lime, whereas Class F is rarely cementitious when mixed with water alone. All fly ashes used in the United States before 1975 were Class F."
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With some apology for such a lengthy excerpt from a prior dispatch, we do suggest that the full body of FHWA literature concerning the use of Coal Ash in highway construction is worthy of study by anyone genuinely interested in Coal, and in the health and vitality of the Coal industry and US Coal Country.
But, the key point to note is that the pertintent difference, for our purposes of discussion herein, between Lignite Ash and Bituminous Ash is that the Lignite Ash inherently contains more "free lime"; which is, in fact, just Calcium Oxide, CaO, with, perhaps, some Magnesium Oxide, MgO, which is nothing more than calcined limestone, primarily CaCO3, which often contains some variable amount of MgCO3; and, which Oxides are what we get from a cement kiln in any case.
So, conjecturally, all we might need to do to make Class F Ash more like Class C Ash is add a little calcined limestone to it. Which, given the number of cement kilns in the US, and the rather vast beds of limestone we have in some parts of US Coal Country, shouldn't be seen as all that much of a problem.
That said, following, as excerpted from the initial link in this dispatch, with an additional link and excerpts appended, we see that the University of Texas has established that Coal Ash Concretes should be the material of choice for use in construction in harsh chemical environments:
"US Patent 5,578,122 - Concretes Containing Class C Fly Ash that are Stable in Sulphate Environments
Date: November, 1996
Inventor: Ramon Carrasquillo, Texas
Assignee: The University of Texas System, Austin
Abstract: Methods for combining Class C fly ash into cementitious mixtures for producing concretes that are resistant to sulfate-containing environments. In one method, Class C fly ash is intergound with portland cement clinker and gypsum to produce a cementitions mixture, which, when combined with water and an aggregate produces a hardened concrete that is resistant to sulfate environments. Alternatively, portland cement clinker and gypsum are first interground and the resultant mixture is admixed with Class C fly ash to produce a cementitious mixture. Thie cementitious mixture, in combination with water and an aggregage, produces a hardened concrete that has improved resistance to sulfate environments. In other aspects, a concrete that is stable in sulfate environments is produced by admixing portland cement, Class C fly ash and water containing a source of ions selected from the group consisting of sulfate and hydroxyl anions. The resultant concrete is capable of hardening in sulfate environments without the formation of such quantities of volume-expanding compositions in the hardened concrete as would cause the hardened concrete to undergo stress failure.
(Note, in the above, that the cement clinker, as perhaps made via the above-cited processes of "United States Patent 5,766,339 - Producing Cement from a Flue Gas Desulfurization Waste" and "United States Patent 5,837,052 - Process for Producing Cement Clinker Containing Coal Ash", is to be "interground" in the cement mill with "gypsum" before being "admixed with Class C fly ash"; and, we remind you of our report:
Pittsburgh Makes Coal Flue Gas Gypsum for Fly Ash Cement | Research & Development; concerning:
"United States Patent 5,312,609 - Sulfur Dioxide Removal from Gaseous Streams with Gypsum Product Formation; 1994; Assignee: Dravo Lime Company, Pittsburgh; Abstract: A method is provided for removing sulfur dioxide from a hot gaseous stream while directly producing .alpha.-hemihydrate gypsum from a scrubber effluent";
wherein it's further explained that synthetic gypsum can be made from flue gas desulfurization sludge.)
Claims: A method for producing a cementitious mixture for use in producing a sulfate resistant hardened concrete.
A method for producing a cementitious mixture for use in producing a sulfate resistant hardened concrete substantially free of any calcium aluminate, calcium hydroxide, and iron oxide compounds that react with sulfate ions from sulfate containing environments, the method comprising the step of:
dry milling from 25% to 75% Class C fly ash, said Class C fly ash containing compounds selected from the group consisting of calcium aluminate, calcium hydroxide and iron oxides, with from 25% to 75% portland cement clinker and from 5% to 10% gypsum, such that at least an amount of sulfate is provided to convert the calcium aluminate to ettringite;
(and) wherein the cementitious mixture when admixed with an aggregate and water, forms a hardened concrete substantially free of any calcium aluminate, calcium hydroxide and iron oxide compounds that react with sulfate ions from sulfate containing environments to form volume expanding compositions that lead to stress failure of the hardened concrete.
(Concerning the above overly-technical jargon "ettringite", see:
Ettringite - Wikipedia, the free encyclopedia; "Ettringite is a hydrous calcium aluminum sulfate mineral with formula: Ca6Al2(SO4)3(OH)12ยท26H2O. Ettringite was first described in 1874 for an occurrence near the Ettringer Bellerberg volcano, Germany".
And, what the heck, since "the cementitious mixture" is to be "admixed with an aggregate", as seen 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 specifications";
we can make that "aggregate", too, out of Coal Ash.)
Fly ash is a combustion product produced when coal is burned in power plants. All fly ash, however, is not the same. The chemistry of the fly ash is dependent upon the nature of the coal from which it is obtained. Thus, ASTM C618 (incorporated by reference) defines two classes of fly ash: Classes C and F. Class F is obtained by burning anthracite or bituminous coal while Class C is the combustion product of sub-bituminous coal or lignite. Class C fly ash often contains significant amounts of calcium mineral matter, while Class F rarely does. Thus, Class C fly ash has cementitious and pozzolanic properties while Class F is rarely cementitious when mixed with water alone.
(Concerning just what "pozzolanic properties" might be, see, for one instance:
West Virginia Coal Association | University of Kentucky Prepares Coal Ash for Market | Research & Development; which centers on exposition and explanation of: "United States Patent 6,533,848 - High Quality Polymer Filler and Super-Pozzolan from Fly Ash; 2003; Assignee: The University of Kentucky; Abstract: A novel method for producing fly ash material ... suitable for use as filler material in the plastics industry and super pozzolan for the concrete industry".)
Summary: The invention provides methods of treating Class C fly ash and methods of blending the Class C fly ash into portland cement to produce concretes that do not undergo stress cracking when exposed to sulfate-containing environments.
In one aspect of the invention, the Class C fly ash is interground with other compositions. Thus, in a preferred method, Class C fly ash is ground with portland cement clinker, and gypsum in certain proportions in a mill to provide a sulfate-resistant cement blend, like the Type IP (normal sulfate-resistant) and Type IIP (medium sulfate resistant) cements. In another preferred embodiment, the invention also provides a method of using Class C fly ash by first inter-grinding a mixture of portland cement clinker and gypsum. The ground mixture is then mixed with fly ash, in certain proportions, to produce a cement product that can be used for making concrete that is sulfate resistant.
In another aspect of the invention, treatment of the Class C fly ash takes place during mixing with cement to make concrete. In one preferred method, sulfate ions added to mixing water is combined with a cement-fly ash mixture and suitable aggregate to make a concrete that is stable when exposed to an environment containing sulfate ions. In another preferred method, an alkaline-based additive is added to mixing water for the concrete containing the fly ash-cement mixture. As a result of adding the alkaline-based additive, it is theorized without being bound, that the reactivity of fly ash is increased at the early stages of a concrete hardening, so that a sulfate resistant concrete is produced."
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In the above summary is indirectly revealed a key fact: Though we might have to add some Calcium Oxide to our Class F bituminous Coal Ash, in order to add some cementitious properties to it's pozzolanic properties, we wouldn't, in most cases, have to apply the process of our subject herein, "US Patent 5,578,122 - Concretes Containing Class C Fly Ash that are Stable in Sulphate Environments", to make our eastern Class F Ash suitable for use in "Concretes" that would be "Stable in Sulphate Environments".
And, this is really all about making any Coal Ash that might contain "calcium aluminate, calcium hydroxide, and iron oxide compounds", which are most often contained in the Ash from combusting lignite, more suitable for use in Concretes that might be utilized in harsh chemical environments, especially where Sulfur compounds are present.
That might, or might not, apply to some eastern US bituminous Coal Ash, depending upon the chemistry and composition of specific Coal seams.
But, it also indicates, that, with the addition of a little Calcium Oxide, eastern Coal Ash might be suitable for use in "sulfate resistant concrete" without need of the processes disclosed in the above "US Patent 5,578,122 - Concretes Containing Class C Fly Ash that are Stable in Sulphate Environments" and the similar process disclosed in its companion:
US Patent 5,573,588 - Concretes Containing Class C Ash that are Stable in Sulphate Environments
Date: November, 1996
Inventor: Ramon Carrasquillo, Texas
Assignee: The University of Texas System, Austin
Abstract: Class C fly ash containing cementitious mixtures for producing concretes that are resistant to sulfate-containing environments. Class C fly ash is intergound with portland cement clinker and gypsum to produce a cementitions mixture, which, when combined with water and an aggregate produces a hardened concrete that is resistant to sulfate environments. Alternatively, portland cement clinker and gypsum are first interground and the resultant mixture is admixed with Class C fly ash to produce a cementitious mixture. This cementitious mixture, in combination with water and an aggregate, produces a hardened concrete that has improved resistance to sulfate environments. In other aspects, a concrete that is stable in sulfate environments is produced by admixing portland cement, Class C fly ash and water containing a source of ions selected from the group consisting of sulfate and hydroxyl anions. The resultant concrete is capable of hardening in sulfate environments without the formation of such quantities of volume-expanding compositions in the hardened concrete as would cause the hardened concrete to undergo stress failure.
Claims: A Class C fly ash containing cementitious mixture for use in producing a sulfate resistant hardened concrete, the cementitious mixture comprising: 25 wt % to 70 wt % Class C fly ash, said fly ash containing compounds selected from the group consisting of calcium aluminate, calcium hydroxide and iron oxides,
(and) 70 wt % to 25 wt % portland cement clinker, and
(and) 5 wt % to 10 wt % of gypsum such that at least an amount of sulfate is present to convert the calcium aluminate to ettringite;
wherein the cementitious mixture is produced by a process comprising the step of dry milling together said Class C fly ash, portland cement clinker and gypsum; and
wherein the cementitious mixture, when admixed with an aggregate and water, forms a sulfate resistant hardened concrete substantially free of any calcium aluminate, calcium hydroxide and iron oxide compounds that react with sulfate ions from sulfate containing environments to form volume expanding compositions that lead to stress failure of the hardened concrete.
Background and Field: The invention relates to the use of a coal combustion product, fly ash, as an additive in concretes. More particularly, the invention provides methods of treating Class C fly ash, and concrete mixtures containing Class C fly ash, to prevent the formation, in sulfate ion-containing environments, of volume-expanding compositions within the hardened concrete that cause stress failure of the concrete.
Summary: The invention provides methods of treating Class C fly ash and methods of blending the Class C fly ash into portland cement to produce concretes that do not undergo stress cracking when exposed to sulfate-containing environments."
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We'll close our excerpts there so that we can emphasize what we think to be an important point.
In our introductory comments, we provided discussion about whether or not Class F Fly Ash from our eastern US bituminous Coals would work in the processes of our subjects herein.
But, both "US Patent 5,573,588" and "US Patent 5,578,122" present the option of adding Class C Ash to the Portland cement clinker after the clinker has been made, in order to form a sulphate-resistant concrete, as in: the "cementitious mixture is produced by a process comprising the step of dry milling together said Class C fly ash, portland cement clinker and gypsum".
And, again, with apologies for the repetition, as we cited in our introductory comments:
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; Cement is produced by forming a moist mixture of a flue gas desulfurization process waste product containing 80-95 percent by weight calcium sulfite hemihydrate and 5-20 percent by weight calcium sulfate hemihydrate, aluminum, iron, silica and carbon, agglomerating the moist mixture while drying the same to form a feedstock, and calcining the dry agglomerated feedstock in a rotary kiln. Sulfur dioxide released from the calcium sulfite hemihydrate and calcium sulfate hemihydrate during calcination may be used to produce sulfuric acid, while heat recovered in the process is used to dry the agglomerating feedstock. Claims: A process for producing cement from a flue gas desulfurization process waste product, comprising: providing a moist flue gas desulfurization process waste product containing 80-95 percent by weight of solids of calcium sulfite hemihydrate and 5-20 percent by weight of solids of calcium sulfate hemihydrate (and) adding a source of aluminum, iron, carbon, and a siliceous material to said flue gas desulfurization process waste product to form a moist mixture thereof; agglomerating said moist mixture while removing water therefrom, by contact with hot air, to provide a dry agglomerated kiln feedstock (and) calcining said dry agglomerated kiln feedstock in a rotary kiln to produce a cement clinker (and) wherein said source of aluminum and iron comprises fly ash"; and:
Pittsburgh Makes Coal Flue Gas Gypsum for Fly Ash Cement | Research & Development; concerning:
"United States Patent 5,312,609 - Sulfur Dioxide Removal from Gaseous Streams with Gypsum Product Formation";
since we can definitely make the Portland cement-type clinker itself out of good old Pittsburgh-area Class F bituminous Coal Fly Ash, and other Coal combustion byproducts, and, yet again, since the processes of our subjects herein concern themselves primarily just with "blending the Class C fly ash into portland cement to produce concretes that do not undergo stress cracking when exposed to sulfate-containing environments", there seems no reason both classes of Coal Ash couldn't be combined in a total process of cement-making and subsequent concrete-making that would consume a maximized amount of Coal Ash from across the country, in the production of high-performance, more chemically-resistant and far more durable, concrete.