Fly Ash as a Portland Cement Raw Material

In 1966, approximately 125 million tons of raw materials, exclusive of fuel, air, water, and power were consumed to produce almost 74 million tons of portland cement in the United States that is about 3,400 pounds of raw material per ton of finished cement produced. To present the figure in another way, a single plant producing 600,000 tons of cement a year would consume about 1,000,000 tons of raw materials.

At a portland cement manufacturing plant, fly ash may be used in the process, principally, at three points. It can be mixed with the finished cement, interground with the cement clinker, or serve as a component in the cement raw batch. In the first two instances it serves as a quick and inexpensive way to increase the capacity of the cement plant if kiln or grinding departments are limiting factors to production, or else to create products with special properties such as portland-pozzolan cement or certain oil-well cements. If it is interground with the cement clinker it also serves the purpose of a grinding aid. As a rule when fly ash is added in these instances, and let me stress that this presently is not a common American practice, it will usually comprise 10 to 30 percent of the mixture. The fly ash, if added to the cement manufacturing process at either of these two points, remains fly ash and although thoroughly blended into the portland cement will perform about the same functions in concrete as if it were added later by being blended with other ingredients during the concrete manufacturing process. As a result, any fly ash consumed in these ways at the cement manufacturing plant will tend to cancel out, at least to some extent, fly ash that might have been added later when making concrete. Many producers feel the market can best be served by the cement customer following the latter course.

The third alternative, fly ash as an ingredient of the raw materials mixture used to manufacture cement, has been relatively neglected in the United States but is of interest for several reasons. Five of the more important are:

  1. Potential consumption would be national in scope.
  2. Demand for such use could become very large the 20 million tons of fly ash generated in the United States in 1965 and the 30 million tons of fly ash estimated to be created by power stations in 1970 is not large when compared to a cement industry that consumed 125 million tons of raw batch ingredients in 1966.
  3. Much of the potential consumption is located in industrialized areas where fly ash is most heavily generated. For example, while the cement industry is national in distribution there are concentrations of cement plants along the Hudson River Valley, in eastern Pennsylvania, near Pittsburgh, near Detroit, along the Mississippi River from St. Louis south to Joppa, Illinois, at the confluence with the Ohio River and on up the Ohio River Valley, and there are other similar geographic concentrations. You will note that many of these cement plant concentrations are along navigable waterways, where real possibility for fly-ash back-haulage in coal or cement carriers exists.
  4. Chemically and physically the increased substitution of fly ash for aluminous and siliceous raw materials in Portland cement manufacture is technically feasible.
  5. For the domestic industry, the use of fly ash as a raw material in cement manufacture is still practically an untapped market.

During the manufacture of the various types of portland cement, one or more raw materials must be blended to achieve proper chemical composition in the kiln feed. The four major chemical components in the kiln feed are lime (CaO), silica (SiO2), alumina (Al2O3), and Iron oxide (Fe2O3). More than 30 different raw materials containing one or more of these compounds may be used in varying blends depending upon availability and the particular type of cement being made. Without going into detail on the relatively minor differences in raw batches for the various types of portland cement, the raw materials blend may be generally described as containing 73-78 percent CaCO3, 12-17 percent SiO2, 1-3 percent Fe2O3, and 2-5 percent Al2O3, with 1-5 percent MgCO3 and less than 1 percent alkalies. The desired composition is usually achieved by blending two or more unlike raw materials. The most important sources for

lime in the raw batch, are as follows:

Cement rock
Limestone
Marl
Shell
Marble
Chalk

For silica:

Sand
Quartz
Quartzite

For Iron oxide:

Iron ore
Pyrite cinders
Blast-furnace flue dust
Mill scale

For alumina:

Clay
Shale
Slag

This list is the longest and most varied and we could go on adding things like schist, bauxite, diaspore, staurolite, kaolin, fly ash, and so forth. A more complete listing appears in Dr. Carl F. Clausen’s “Cement Materials” chapter in A.I.M.E.’s “Industrial Minerals and Rocks.” In addition, many industrial waste products have compositions somewhat resembling the desired composition for a cement raw materials batch. At various times, some have been suggested or actually used as a raw material component. An example of one of these materials would be the by product that results during the winning of magnesium metal from dolomite. Typically such a product may be 56 percent CaO, 1 percent MgO, 5 percent Fe2O2, 3 percent Al2O3, 32 percent SiO2, and 3 percent CaF2. The calcium fluoride content would be lowered by mixing with other ingredients and would be acceptable in kiln feed as a flux.

Now, all of the raw materials also may contain greater or lesser amounts of the other major components, for example the so-called alumina source also may provide all the silica and iron oxide needed, and the desired magnesium carbonate may be derived from almost any of those materials we have listed. Most frequently in actual practice, only two major raw materials needed to be blended to achieve chemical balance with only minor extra amounts of ferruginous or siliceous raw materials added. Plants using a blend of one, three, or four major components are less common. In 1966 the raw materials consumed were as follows:

fly-ash-raw-materials-consumed

The last group, miscellaneous items, is comprised of materials such as fluorspar, pumicite, calcium chloride, soda ash, borax, staurolite, air-entraining agents, grinding aids, and fly ash. Unfortunately, we cannot reveal the exact amount that was fly ash because it would disclose individual company operations, but it was on the order of 100,000 tons. Some of the fly ash reported was ground with the clinker or added to finished cement, although no detail on where it is used in the process is actually reported to the Bureau of Mines. Practically all the fly ash reported was used by three firms in 1966. One of these G & W. H. Corson, Inc., primarily used it as a finished cement additive or ground the fly ash with clinker. At the other two plants: Missouri Portland Cement Co., Joppa, Illinois, and River Cement Co., Festus (Selma) Missouri, all or part of the fly ash was consumed as a raw batch component.

The potential tonnage of raw material that fly ash might substitute for in portland cement is, in essence, the 12.7 million tons of aluminous-siliceous materials, that is, the clay, shale, schist, and blast-furnace slag categories.

Ideally, a kiln feed mixture of cement raw materials should be a single ground up component consisting of a somewhat sandy and muddy limestone — the so-called “cement rock” of the industry.

In reality, an ideal “cement rock” of exactly the right composition is not frequently encountered and in actual practice during 1966, only 22.4 percent of portland cement was made from such a basically single-component kiln feed; for the rest, the principal calcareous ingredient was mixed with clay, shale, or other similar naturally occurring substances in the case of 72.2 percent of the cement; or with granulated iron-blast-furnace slag in the case of 5.4 percent of the cement. (The percentage made by blending limestone with fly ash is, of course, confidential and is concealed in the above breakdown). Silica and iron are generally present in the two calcareous and aluminous components so that additions of iron or silica rich materials are either usually small or even unnecessary.

As a consequence, the 184 portland cement plants reporting in 1966, listed their major raw material ingredients as follows:

fly-ash-raw-materials-ingredients

What these figures mean are that, excluding only the first three categories that used only calcareous ingredients of suitable composition, in 1966 there were 137 portland cement manufacturing plants located in all areas of the United States that represented possible consuming points for large quantities of fly ash.

Composition of Raw Materials

At this point let us compare the chemical composition of fly ash with the normally used siliceous-aluminous ingredients. The ranges of values used are a synthesis of those reported in the literature:

fly-ash-raw-materials slag

As this comparison shows, fly ash compares very favorable as a potential source of alumina and silica for cement manufacture by blending with limestone. It does range somewhat higher in iron oxide, but not seriously so. Some fly ashes could have a poor Fe2O3 to Al2O3 ratio that would require adjusting with other alumina-rich materials to achieve a typical raw batch of 75.0 percent CaCO3 (42.0 percent CaO), 3.5 percent MgCO3 (1.7 percent MgO), 14.0 percent SiO2, 5.0 percent Al2O3, 2 percent Fe2O3, and less than 1 percent alkalies (34.8 percent CO2 equivalent).

For cement manufacture the most important chemical requirement is that the fly ash be of constant composition without appreciable variation in the formula as it is delivered to the cement plant. Only in this way can one avoid frequent costly and time consuming checks and adjustments of the raw mixture to maintain proper kiln feed composition.

Fly Ash Market

Theoretically then, fly ash can substitute for more than 12 million tons of aluminous-siliceous raw materials presently consumed in the United States for portland cement manufacture.

For practical purposes, of course, the actual market for such use would not be that large. Economic factors will dictate whether or not fly ash is substituted for some other material. If a cement operation is stripping clay or shale of proper composition to obtain underlying limestone, it will probably use that clay or shale in the raw batch whether or not fly ash is obtainable nearby. Another factor to be considered is, can the powerplant generating the fly ash make money by selling the product for other use. The fly ash producer certainly will not sell it to a portland cement plant if he can more profitably release it for other use. Convenience would be one advantage in disposing of the fly ash to a nearby cement plant for use as a cement raw material. Little or no processing would be required; and if uniformly of the proper chemical composition the fly ash could be sold and used essentially as is. A subsidiary benefit to using fly ash in place of clay or shale is that we not only dispose usefully of an unsightly byproduct the fly ash, we also enhance aesthetic values by not creating an excavation to obtain clay or shale. The use of fly ash may also actually lower cement manufacturing costs. The fly ash is already fine enough for kiln feed without grinding. It is also generally low in alkalies which are deleterious and bothersome in cement manufacture, and any unburned carbonaceous matter present will simply burn in the kiln contributing a few more Btu’s.

We should also point out that fly ash used as a cement raw ingredient will not conflict quantitatively with fly ash added to finished cement or interground with cement clinker. After passing through the kiln it is no longer fly ash but has combined with other raw materials to form the various component portland cement compounds, the major ones being, — tricalcium silicate (3CaO.SiO2 = C3S), dicalcium silicate (2 CaO.SiO2=C2S), tricalcium aluminate (3CaO.Al2O3=C3A), and a compound approaching tetracalcium alumino-ferrite in composition (4CaO.Al2O3.Fe2O3 = C4AF). Thus, fly ash used as a grinding aid, or used to Increase plant capacity to produce finished cement, or used to moderate the properties of concrete, can be subsequently added without regard to quantities of fly ash already used in the raw batch.

In closing, I would like to point out one interesting possibility for future development involving both cement production and fly ash, similar in a sense, to the way iron and steel complexes may have cement plants nearby due to availability of blast furnace slag. In 1966 the portland cement industry of the United States consumed more than 9.3 million tons of coal in 111 out of the total 184 plants, and the entire portland cement producing industry also consumed 3.9 million barrels of oil, 203.6 billion cubic feet of natural gas, and 9.6 billion kilowatt hours of electrical energy. Now this electricity consumed is roughly 25 kilowatt hours per barrel of cement, and the 59 plants using coal exclusively, used in addition 97.1 pounds of coal per 376-pound barrel of cement produced. To go back to our earlier sample plant producing 600,000 tons of cement per year, that is more than 76 million kilowatt-hours of electrical energy and if entirely coal-fueled, also approximately 150,000 tons of coal as well as perhaps anything up to 200,000 tons of fly ash depending on the composition of the major raw materials to be blended. What I am trying to show here are the intriguing possibilities for contiguous coal-fired power stations and coal-fired cement plants. Both are preferably located near industrialized areas and at the same time with access to low-cost fuel sources and often on navigable water. The factor not in common is the need for limestone in cement manufacture-. Conceivably in the future, availability of fly ash for cement manufacture may be one factor considered in deciding between possible sites for cement plants. In fact, it could well be that some existing powerplant sites, where fly ash is a disposal problem, have all the ingredients for a successful cement plant close at hand for which the economic potential has not been recognized. Based on a comparison with France, where a cement manufacturing industry one-third as large as in the United States consumes almost a million tons of fly ash annually (832,000 tons ground with clinker as an additive or cement supplement and 124,000 tons, in cement kiln feed), the domestic cement manufacturing industry should certainly be a fertile market outlet for about 3 million tons of fly ash each year instead of the approximately 100,000 tons it consumed in 1966.

 

By |2019-08-12T06:56:19-04:00August 9th, 2019|Categories: Non-Metallic & Industrial|Tags: |Comments Off on Fly Ash as a Portland Cement Raw Material

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