FINAL TECHNICAL REPORT
September 1, 1997, through September 30, 1999
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. Chaudhary, S. Sengupta, H. Wilcox, 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 large 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-ash from SIUC power plant, 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 types (A-4 & A-6) of soil were used to develop large volume CCB-based subgrade improvement for road construction.
ULSM in the range of 25 to 50 pcf with compressive strength of 90 to 690 psi (in 4 to 6 days) were developed using at least 75% CCBs, binding agents, and waste fiber. Commercial size blocks (6 ´ 12 ´ 16 in. & 8 x 12 x 16 in.) were made using facilities at the Illinois Coal Development Park.
Mixes of CLSM containing a minimum of 73% CCBs and a maximum of 27% cement were developed. The materials had a density between 55 to 110 pcf with strength from 80 to 446 psi, and a flowability of 10 to 18 inches. Corrosivity of the developed material conformed to the EPRI standards.
Subgrade soils treated with SIUC FBC fly ash 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%, a swell less than 1%, which was an improvement over the untreated soils and met IDOT standard specification. A very successful information transfer seminar was developed. About 30 persons from industry and state agencies attended the seminar.
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 for 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 on the various parts of the projects are summarized below.
Characterization of Coal Combustion By-Products
1. FBC fly ash from SIUC and F-fly ash from F-ash from Lake of Egypt, Baldwin, and Grand Tower power plants were characterized for chemical properties (loss on ignition, oxide composition, pH and leaching characteristics). The bulk chemical composition indicates the possibility of both sulpho-pozzolanic as well as silico-pozzolanic reactions for FBC 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 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 of 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 studied for CCBs, cement and their mixes to assess alkali-aggregate reactions. The pH for each case was found to be in a safe zone, 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 foam collapse rate were studied as a function of foam generating pressure. The required optimum air pressure should be between 40 -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 mix containing at least 77% CCBs (FBC ash alone or a combination of FBC-ash and 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-ash and other admixtures, the initial W/P ratio should be between 0.7-0.8, where the addition of foam is advantageous to bring down the density of mix 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 ash and Cement
1. About 45 cubical samples (2 x 2 x 2 in.) were prepared using 80% SIUC FBC-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, 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 (~82oF and 30% RH) curing before placing in hot air (~150oF and 25% RH) was found better compared to initial hot air curing. However, a considerable drying shrinkage occurred in both the cases. The outer skin of the products is rough and friable in both cases. The addition of latex and MgCl2 has no effect on improving the quality of products.
3. An exponential correlation was found between 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-pozzalnic chemical, especially anhydrite (CaSO4.1/2 H2O), to reduce the CO3/SO4 ratio in the curing process.
5. In presence of foam, the initial mixing temperature should not be greater than 113oF.
6. A 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 dry mixes, one containing 60% FBC-ash, 20% F-ash, <20% OPC, and the other containing 70% FBC-ash, 10% F-ash, and 20% OPC were tested for the development of ultra-lightweight materials. It was found that in the presence of foam, the F-ash particles in the mixes were agglomerated and reduced the strength of the products.
2. The agglomeration of F-ash occurred when the casting was less than 35 pcf. The compressive strength of the products of density ranging from 35 to 42 pcf was found to vary in between 240 to 300 psi for both mixes.
Mix development using CCBs and blended cement
1. Two quick setting blended cements, namely Duracal and Hydrocal, were added 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 (~120oF and 95%RH) heating followed by at least 48 hours hot air drying before demolding. The products based on Duracal were found better than 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 correlation 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%-3% Duracal, 10%-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 - 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 backing carpet 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.
4. 250 Commercial size blocks (6 x 12 x 16 in.) using the same formulation mentioned in item #3 was used to make blocks in the 30 to 40 pcf density range. 300 Commercial size blocks (8 x 12 x 16 in.) using the same formulation mentioned in item #3 was used to make blocks in the 40 to 50 pcf density range. The Commercial size blocks (8 x 12 x 16 in.) with the slightly higher density yielded a more consistent and stronger block as compared to the commercial size blocks (6 x 12 x 16 in.). The larger blocks had a flexural strength ( ASTM Test E-72) of 60 psi on one wall when the wet density of the blocks was maintained between 40 and 50 pcf.
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. FBC fly ash SIUC, F-ash from Baldwin, Lake of Egypt, and Grand Towers were the primary CCBs utilized. Mixes were made with and without preformed foam.
2. Materials developed were found to have 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 used. Permeability was on the order of 5´10-5 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 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 the economic evaluation of commercial scale production of CLSM. Economic evaluation consisted of determination of various project costs, computing the cash flows over the life of the project and, performing a NPV analysis to determine the feasibility of the project.
6. A sensitivity analysis was done with the help of the above spreadsheet. Among the various control factors, CCBs transportation cost was found to be a very critical factor.
7. Field test of a developed CLSM was performed at the Illinois Coal Development Park facility.
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) are 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. Currently, millions of tons of coal fly ash are created by electrical power generating companies in this country and most coal fly ash is treated as a waste product and an environmental problem. 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. The testing of A-4 and A-6 soils without stabilization indicated that they do not have adequate immediate bearing value (IBV). The IBV is reduced by 2% to 6% upon wetting.
2. When stabilized with a mixture of 8% SIUC FBC fly ash, the A-4 soil 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 condition.
3. 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%.
4. 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 OMC was lower than the untreated soils.
5. When A-4 and A-6 soils were mixed with SIUC FBC fly ash the swell ranged from .55% to 2.2%. Only the 12% SIUC FBC with A-4 soil was above the 2% mark established by IDOT.
6. 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 use of SIUC FBC fly ash only.
7. Type A-6 soil treated with SIUC FBC fly ash and an asphalt waste dust yielded an IBV of 24.5%, a CBR exceeding 8.5%, and swelling strain of 2.4%. This was an improvement over the untreated soil and the soil treated with SIUC FBC fly ash only.
The results of these studies are being discussed with IDOT and IEPA for their review. In the near future, it is proposed to develop a demonstration project for a parking lot or secondary road in cooperation with IDOT or a private company.
Information Transfer
An information transfer activity was conducted in the College of Engineering Alumni room. Samples of developed materials, commercial blocks, and ULSM were displayed along with poster boards of research results. About thirty (30) professionals from the coal industry, the electric utility industry, and state agencies (Illinois Department of Transportation, Office of Coal Development and Marketing, and Illinois EPA) attended the activity. There was a lot of interest in the commercial blocks, ClSM, and road sub-base development.