TECHNICAL REPORT
September, 1997 through August, 1998
Project Title: COAL COMBUSTION RESIDUES MANAGEMENT PROJECTS
ICCI Project Number: 97-1/3.4A-2
Principal Investigator: Y. P. Chugh, Southern Illinois University at Carbondale
Other Investigators: S. K. Chaudhuri, H. Wilcox, S. Sengupta, J. Jennings; Southern Illinois University at Carbondale, Illinois; Woodruff Supply Company, Madisonville, Kentucky; Eagle Seal Inc., Benton, Illinois
Project Manager: Wayne Bahr, Office of Coal Development and Marketing, Illinois Department of Commerce and Community Affairs
ABSTRACT
The goals of the project are to: 1) develop, demonstrate and commercialize coal combustion by-products (CCBs)-based ultra-lightweight structural materials (ULSM) in the 30 to 40 pcf range for construction of mine ventilation stoppings in underground mines; 2) develop and demonstrate volume CCBs-based flowable fills; 3) develop and demonstrate large volume CCB based subgrade improvement technique for road construction; 4) maintain Illinois Coal Combustion Residues Sample Bank; and 5) conduct a technology transfer seminar for industries associated with management of CCBs.
The CCBs considered were FBC fly ash from SIUC power plant, and F-ash from Lake of Egypt, Baldwin, and Grand Tower power plants. These CCBs were characterized for physical and chemical properties as part of the project. Two different types of soil were used to develop large volume CCB-based subgrade improvement for road construction.
ULSM in the range of 25 to 40 pcf with compressive strength of 90 to 250 psi (in 4 to 6 days) were developed using at least 77% CCBs, binding agents, and waste fiber. Commercial size blocks (b x 12 x 16 in.) were successfully made which have a maximum 30% by weight water absorption even after several hours. The developed materials do not disintegrate in 3 to 4 wetting/drying cycles.
Mixes of CLSM containing 90% CCBs and 10% cement were developed. The materials had a density between 55 to 65 pcf, strength from 80 to 300 psi, and a flowability of about 18 inches. Corrosivity of the developed material conformed to the EPRI standards.
Subgrade soils treated with SIUC FBC fly has yielded the best results among different CCBs tested. The soils treated with SIUC FBC fly ash developed an immediate bearing value exceeding 25%, California bearing ratio exceeding 20%, and swelling strain of less than 1%. This was an improvement over the untreated soils and met IDOT standard specifications.
EXECUTIVE SUMMARY
The goals of the project are to: 1) develop, demonstrate and commercialize coal combustion by-products (CCBs) based ultra-lightweight structural materials (ULSM) in the 30 to 40 pcf range fro construction of mine ventilation stoppings in underground mines; 2) develop and demonstrate large volume CCB-based flowable fills; 3) develop and demonstrate large volume CCB-based subgrade improvement technique for road construction; 4) maintain Illinois Coal Combustion Residues Sample Bank; and 5) conduct a technology transfer seminar for industries associated with management of CCBs.
The results of the various parts of the project are summarized below.
Characterization of Coal Combustion By-Products
1. FBC fly ash from SIUC and F-fly ash from Lake of Egypt, Baldwin, and Grant Tower power plants were characterized for chemical properties (loss on ignition, oxide composition, pH and leaching characteristics). The bulk chemical composition indicates teh possbility of both sulpho-pozzolanic as well as silico-pozzolanic reactions for FBC fly ash and mainly silico-pozzolanic reactions for F-ash.
2. The phase mineralogy, particle morphology and particle elemental compositions of CCBs were studied by X-ray diffractometer (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray analyzer (EDX), respectively. The presence of anhydrite, lime, and a number of calcium silicates as well as calcium alumino-silicates phases in FBC fly ash suggest its self-cementing properties.
3. FBC fly ash particles are irregular in shape, flaky in nature and most of the particles exist in agglomerated forms. This type of particle morphology of FBC ash in addition to its high LOI value indicate the requirement for a high water to dry powder (W/P) ratio to make a mix. On the other hand, almost all F-ash particles are spherical in nature but numbers of tiny particles of iron compounds are sitting on the surface of each large spherical particle. The tiny iron particles on the surface of F-ash particles hindered the expected pozzolanic activity of F-ash, especially in the presence of foam.
4. The variation in pH with time, temperature and water to dry powder (W/P) ratios were studies for CCBs, cement and their mixes to assess alkali-aggregate reactions. The pH for each case was found to be in 10 to 12, which is desirable for cement hydration and pozzolanic reactions.
Ultra-Lightweight Structural Materials (ULSM)
Characterization of foam
1. The density of foam, liquid held up in foam, and foal collapse rate were studied as a function of foam generating pressure. The required optimum air pressure should be between 40 to 50 psi for making stable and good quality foam of density 0.078 +/- 0.02 g/cc suitable for ULSM.
2. Mearl liquid procured from the Mearl Corporation was found to be an economically attractive and technically viable foaming chemical. Only 1% (V/V) Mearl liquid is enough to make desired quality foam.
Bulk specific gravity of mixes with water to dry powder (W/P) and foam to dry powder (F/P) ratios
1. The bulk specific gravity (BSG) of a mixture depends upon the W/P ratio which directly impacts strength. W/P should be minimized consistent with in-service properties, such as flowability and ability to mold. For each selected dry mis containing at least 77% CCBs (FBC fly ash ash alone or a combination of FBC fly ash and a F-ash) and cement, the BSG was found to increase with increasing W/P ratio to a maximum value and then decrease almost exponentially to a lower limit value. Beyond this value, the added water has no effect on the reduction of mix density. The addition of foam was found to be economical to decrease the mix density for obtaining ULSM of desired quality.
2. For any dry mix of minimum 77% FBC fly ash and other admixtures, the initial W/P ratio should be between 0.7-0.8, before the addition of foam is advantageous to bring down the density of mis to the desired casting density value.
3. The variation in BSG of mix with F/P ratio, for a specific W/P ratio, depends upon the dry mix composition.
Mix development using FBC fly ash and Cement
1. About 45 cubical samples (2 x 2 x 2 in.) were prepared using 80% SIUC FBC fly ash and 20% type-1 OPC for different W/P and F/P ratios in a laboratory scale mixer. The effects of two curing cycles, initial mixing temperature, addition of latex, and MgCl2 were tested.
2. The density, drying shrinkage, compressive strength, and percent strain were tested for both the curing cycles. Initial 48 h room temperature (~82 F and 30% RH) curing before placing in hot air (~150 F and 25% RH) was found better compared to only hot air curing. However, a considerable drying shrinkage occurred in both cases. The outer skin of the products was rough and friable in both cases. The addition of latex and MgCl2 had no effect on improving the quality of products.
3. An exponential relation was found between the density and compressive strength of products. Depending upon the mixing and curing conditions, the compressive strength of products of density 30 to 40 pcf was found to vary between 150 to 400 psi.
4. The formation of thaumacite during hydration of FBC-ash in absence of atmospheric CO2 may be causing the friability of outer skin of products. Thaumacite is a special type of carbonate containing ettringite. It is preferable to add a small amount of sulpho-pozzolanic chemical, especially anhydrite (CaSO4.1/2 H2O), to reduce the CO3/SO4 ratio in the curing process.
5. In presence of foam, the mixing temperature should not be greater than 113 F.
6. A 0.3 cu. yd. grout mixer was used for making commercial size blocks. The required W/P ratio in this mixer was found to be higher than for the laboratory scale mixer.
7. A dry mix containing 80% FBC and 20% binder is not suitable for products of less than 28 pcf.
Mix development using FBC ash, F-ash and OPC
1. Two quick setting blended cements, namely Duracal and Hydrocal, were evaluated as part of the mix to reduce drying shrinkage. The XRD of both the cements and their hydrated products were studied to assess their cementing and setting characteristics.
2. A number of cylindrical samples (3 x 6 in.) were made using dry mixes containing 92% FBC-ash and 8% blended cement in a laboratory scale mixer for initial W/P ratio of 0.7. The best curing cycle was found to be 48 h low temperature steam (~120 F and 95%RH) heat followed by at least 48 hours hot air drying before demolding. The products containing Duracal were found better than those containing Hydrocal. This is probably due to the presence of both sulpho-pozzolanic and silico-pozzolanic ingredients in Duracal. For casting density around 50 pcf, the drying shrinkage was negligible. The density and compressive strength of products vary between 35 to 42 pcf and 170 to 300 psi, respectively. An exponential relation was found between pcf and 170 to 300 psi, respectively. An exponential relation was found between pcf and 170 to 300 psi, respectively. An exponential relation was found between density and compressive strength. However, water absorption for these materials was high (~50%) and so was the failure strain >4%).
3. Dry mixes containing 2% to 3% Duracal, 10% to 20% OPC and the balance FBC-ash were suitable for the development of commercial size (6 x 12 x 16 in.) ULSM blocks of 36 to 40 pcf density for initial W/P ratio of 0.7 to 0.8. The post-failure characteristics of commercial size blocks can be improved using 0.5% (V/V) fibers. Among the different fibers tested, human hair, industrial fibers, and carpet backing fiber having a thin rectangular surface were found to be effective. The developed commercial size blocks had maximum 30% by weight water adsorption characteristics even after several hours, and materials do not disintegrate in 3 to 4 wetting/drying cycles.
Controlled Low Strength Material (CLSM)
1. Mix designs for CLSM having a compressive strength of 80 to 300 psi are being emphasized. Experimental mix designs containing 80% to 95% CCBs, and 5% to 20% cement were attempted. SIUC FBC fly ash, and F-ash from Baldwin, Lake of Egypt, and Grand Tower were the primary CCBs utilized. Mixes were made with and without preformed foam.
2. Materials developed were found to have a density between 55 to 65 pcf and compressive strength ranging from 80 to 300 psi. Flowability values were about 18 inches. Bleed from the mix was found to be typically below 5% based on the amount of water use.d Permeability was on the order of 5x10-5 cm/sec.
3. Corrosivity studies on the developed mixes were done in accordance with the guidelines of Corrosion Potential Evaluation Criteria for Coal Ash (EPRI RP-3176). The parameters determined for evaluation of corrosion potential are (1) Resistivity, (2) pH, (3) Redox Potential, (4) Sulfides, and (5) Moisture. The tests indicated that all the parameters except pH were in the desirable range. The material, however, was found to be well within the corrosion limits.
4. A shake test was conducted to determine the quality of the leachate percolating into the soil. The amount of dissolved solids in the leachate was found to be within the ASTM standards.
5. A spreadsheet was developed for economic evaluation of commercial scale production of CLSM. The evaluation consisted of determination of various project costs, computing the cash flows over the life of the project, and performing a net present value (NPV) analysis to determine the feasibility of the project.
6. Sensitivity analyses were done with the help of the above spreadsheet. Among the various control factors transportation cost was found to be a very critical factor.
7. Field tests of a developed CLSM was performed at the Illinois Coal Development Park facility. The material was found to have the desired strength and did not crack on curing.
Subgrade Stabilization
The overall goal of this project is to develop design, test, and commercialize the use of CCBs for the purpose of subsoil stabilization in road construction projects using guidance and assistance from the Illinois Department of Transportation (IDOT). The CCBs (F-fly ash and FBC fly ash were to be used in conjunction and in lieu of standard subsoil stabilization products such as code "L" lime. Along with using CCBs for the subgrade stabilization, IDOT expressed interest in CCBs as a material for making embankments. Illinois electrical power generating companies and IDOT will benefit from this research. Electric power companies will be able to reduce or eliminate the cost for disposing of the fly ash while finding environmentally safe ways to use the coal fly ash. IDOT will have the potential to lower the costs for road construction using the fly ash from the local power plants. Over the next decade, U.S. infrastructure (highways, bridges) will be refurbished and enhanced. This research is very timely for effective utilization of large volumes of CCBs.
The following results summarize work completed to date.
1) Two types of soil, A-4 and A-6, typically found in southern Illinois were investigated in this study.
2) The testing of A-4 and A-6 solids without stabilization indicated that they do not have adequate IBV. The IBV is reduced ty 2% to 6% upon wetting.
3) When stabilized with a mixture of 8% SIUC FBC fly ash, the A-4 solid showed a higher IBV than A-6 soil. Furthermore, it retained a larger proportion of the strength when the CBR test was performed under soaked wet condition.
4) When A-4 and A-6 soils were mixed with SIUC FBC fly ash, the IBV ranged from 15.55% to 22% which exceeded the IDOT criteria for an IBV of 10%.
5) When A-4 and A-6 soils were mixed with SIUC FBC fly ash or a combination of SIUC FBC fly ash and SIPC F-ash, the optimum moisture content (OMC) was lower than for the untreated soils.
6) When A-4 and A-6 soils were mixed with SIUC FBC fly ash, the swelling strain ranged from 0.55 to 2.2%. Only the addition of 12% SIUC FBC fly ash in A-4 soil resulted in above the 2% swelling strain established by IDOT.
7) When SIUC FBC and SIPC F-ash are mixed together and used as a single admixture, the A-4 and A-6 soils react in a similar manner as compared to the us of SIUC FBC fly ash only.
The results of these studies are being submitted to IDOT for their review. In the near future, it is proposed to develop a demonstration project for secondary road in cooperation with IDOT.