Semisequicentennial Transportation Conference Proceedings
May 1996, Iowa State University, Ames, Iowa
Assessment of Cementitious Coal Combustion Byproducts for Highway Base Materials
N. Ghafoori, L. Wang, and S. Kassel
Department of Civil Engineering,
Southern Illinois University at Carbondale,
Carbondale, Illinois 62901.
In summer 1994 the Illinois Clean Coal Institute and Southern Illinois University at Carbondale constructed and monitored an experimental road to examine the potential of fluidized bed combustion (FBC) residues for highway base materials. Slab sections containing three different proportions of prehydrated FBC spent bed, as a fine aggregate; pulverized coal combustion (PCC) fly ash, as a cementition binder; and crushed limestone, as a coarse aggregate, were constructed using the construction technology known as roller compacted concrete. The zero-slump fresh matrices were prepared at their optimum moisture content and consolidated into 152-millimeter (six inch) thick slabs using a steel vibratory roller. Once sufficient strength was attained, the base sections were covered by a 50-millimeter (two inch) layer of asphalt concrete. Appropriate control joints were also provided. The test results conclude that, with proper combinations of industrial wastes and construction technologies, high performance pavement base courses can be built without making a project less feasible, less durable, and more expensive. As an added bonus, the judicial use of byproduct residues should alleviate the environmental concerns associated with coal waste disposal. Key words: coal, wastes, base, pavement, strength.
Illinois is the largest high-sulfur coal producing state in the United States. In an attempt to control Sox emissions and conform with the new environmental clean air regulations, several power producing and cogeneration plants throughout the state use a technology called fluidized bed combustion (FBC). In addition to its ability to reduce the emission of nitrogen and sulfur dioxides, FBC offers cycle efficiency improvements and lower costs of electricity (1).
The FBC process generates large quantities of byproduct residues that are vastly different from those produced by conventional pulverized coal combustion (PCC) units. To date, three FBC solid waste streams have been identified: spent bed obtained from the bottom of boiler, fly ash collected by either fabric filters or electrostatic precipitators, and char which is a coarser flue gas residue collected by particulate collection devices (2). Unlike PCC combustion residues, FBC wastes contain small amounts of pozzolan oxides (SiO2 + Al2O3 + Fe2O3), moderate quantities of anhydrate calcium sulfate (CaSo4 H H2O), and large amounts of calcium oxides (CaO, mostly in free form) (3). When FBC residue is properly conditioned, it can be used as a catalyst to boost pozzolanic/cementitious reactivities of PCC fly ash (3,4).
As part of a broad-based research program aimed at evaluating FBC-based materials and their potential applications for the construction industry, a field demonstration project was conducted to ascertain the suitability of FBC/PCC residues in road base construction. This paper presents field data on short- and long-term engineering properties of three pavement base sections that were constructed and monitored under different traffic and climatic conditions, and ages.
The raw materials used for the construction of the base sections consisted of FBC spent bed, PCC fly ash, crushed limestone coarse aggregate, and tap water. The chemical properties of FBC spent bed, obtained from a plant burning high-sulfur Illinois coal, are shown in Table 1. Its saturated surface dry specific gravity and absorption were calculated at 2.19 and 14.61 percent, respectively. The fineness modules of the FBC spent bed, obtained via a sieve analysis, indicated a value of 1.8. The PCC class F fly ash selected for this project complied with the requirement of ASTM C 618 (5). Its chemical compositions are also shown in Table 1. The crushed limestone coarse aggregate had a maximum nominal size of 19 millimeters (I inch) and saturated surface dry specific gravity of 2.67. Initial mixture design testings were conducted in the laboratory to ascertain appropriate FBC spent bed to PCC fly ash ratios and mixture moisture contents (6). The dry quantities of the matrix constituents, expressed in terms of percent by mass of total dry solids, are shown in Table 2, along with the optimum nominal moisture content and air-dry density of the compacted base sections. Core samples (76 x 152 millimeters (3 x 6 inch)) taken at different ages were tested for compression (ASTM C 39), splitting-tension (ASTM C 496), modulus of elasticity (ASTM C 469), length change (ASTM C 157), and resistance to freezing and thawing (ASTM D 560, fully submerged).
|TABLE 1 Chemical Compositions of FBC Spent Bed and PCC Fly Ash|
|Chemical Composition||FBC Spent Bed||PCC Fly Ash||Fly Ash Specifications (ASTM C 618)|
|Silicon Oxide (SiO2)||9.70||49.10|||
|Aluminum Oxide (A12O3)||3.69||25.50|||
|Iron Oxide (Fe2O3)||2.16||16.60|||
|Total (SiO2+A12O3+Fe2O3)||15.55||91.20||50.0 Minimum, Class C
70.0 Minimum, Class F
|Sulfur Trioxide (SO3)||24.42||0.50||50.0 Maximum|
|Calcium Oxide (CaO)||53.10||1.56||Less than 10%, Class F
More than 10%, Class C
|Magnesium Oxide (MgO)||0.88||0.89|||
|Loss on Ignition||0.80||0.38||6.0 Maximum|
|Free Moisture||0.0||0.16||3.0 Maximum|
|Water of Hydration||2.65||0.0|||
|Available Alkalies as Na2O||N/A||0.08||1.5 Maximum|
|TABLE 2 Mixture Proportion Details|
|Mix. No.||FBC Spent Bed (%)||PCC Fly Ash (%)||Limestone Coarse Aggregate (%)||Nominal Moisture Content (%)||Air-Dry Density (Kg/m3)|
Note: All percentages are by mass of total dry solids 1 pfc=16.02 Kg/m3
RESULTS AND DISCUSSION
When pozzolan silicates and aluminates of PCC fly ash combine with hydrated lime supply by FBC spent bed, cementitious products such as calcium silicate and calcium aluminate hydrates are formed. These reactions are combined with calcium sulfate of FBC, in the presence of sufficient water and temperature, to form expansive reactions called ettringite. Thus, combination of FBC spent bed and PCC fly ash poses the question of internal sulfate attack which may be accompanied by severe expansion and loss of strength.
The linear expansion of the base course FBC/PCC roller compacted concretes is depicted in Figure 1. The expansion strains were minimal and stabilized after six weeks from the date of construction and, since then, remained unchanged. The increase in fly ash content resulted in a lower expansion due to (1) reduced availability of gypsum and mixing water and (2) increased paste quality which, in turn, improved the ability of mixtures to resist sulfate attack.
The impact of mixture proportion, concrete age, and testing condition on compressive and splitting-tensile strengths, and modulus of elasticity of FBC/PCC slabs are reported in Table 3. Both strength and stiffness greatly depended on mixture proportion, i.e., fly ash content, and age of roller compacted concrete slabs. The air dry compressive strength increased by approximately 60 percent when fly ash content was elevated from eight percent to 13.3 percent (by mass of total dry solids). A 10 percent increase in air-dry compressive strength was observed from 13.33 percent to 20 percent fly ash content. For the same mixture proportions, the improvements in wet compressive strength were significantly higher, an increase of 117 percent and 15 percent, respectively.
|TABLE 3 Compressive Strength, Splitting-Tensile Resistance, and Static Modulus of Elasticity of Base Course FBC/PCC Roller Compacted Concrete Mixtures|
|Mix.No.||Air Dry Compressive Strength (MPa)||Wet Compressive Strength (MPa)||Splitting-Tensile Strength (MPa)||Modulus of Elasticity (103 MPa)|
|Concrete Age (Days)||Concrete Age (Days)||Concrete Age (Days)||Concrete Age (Days)|
Not Available 1 Ksi = 6.895 MPa
Test results also indicate that sensitivity of FBC/PCC mixtures to moisture reduces as the percentage of the cementitious binder increases. For the slabs containing eight percent, 13.33 percent, and 20 percent fly ash, the average increase in compressive strength under air-dry conditions were 68 percent, 24 percent, and 19 percent, respectively, higher than those obtained under saturated conditions.
The splitting-tensile strength of the experimental base slabs at various ages were investigated and the results are presented in Table 3. Similar to the compressive strength, the splitting-tensile strength was a function of the matrix proportion and the age of concrete. For the same mixture proportion, the magnitude of splitting-
tensile resistance averaged 11.5 percent of the compressive strength, a pattern similar to that of cement-based mixtures.
The static modulus of elasticity, as suggested by American Concrete Institute (ACI), of the FBC/PCC roller compacted concrete base slabs are shown in Table 3. The elastic modulus increased with age and remained at a value comparable to that of conventional concrete of the same strength level.
Table 4 documents the freezing and thawing performance under an accelerated laboratory test devised for base course mixtures. The core samples were fully submerged in water throughout freezing and thawing cycles. With the exception of the mixture group C1, the remaining test samples performed well under the severe conditions of the laboratory tests. To date, inspection of the core samples, taken 18 months after construction of the base slabs, indicates no deterioration due to freezing and thawing cycles of the winter climate.
|TABLE 4 Freezing and Thawing Resistance of Base Course FBC/PCC Roller Compacted Concrete Mixtures|
|Number of Freezing and Thawing Cycles|
|Mass Loss (%)|
Cycle in which testing was terminated
Suitable mixtures for pavement base courses containing FBC spent bed, as a fine aggregate, and PCC fly ash, as a cementitious binder, have been developed. Mechanical properties and long-term durability were greatly affected by mixture proportion and concrete age. The trial base course slabs offered compressive strength of 8.72 MPa to 39.67 MPa (1264 Psi to 5754 Psi) under wet and air-dry conditions. The splitting-tensile strengths were nearly 11.5 percent of the compressive strengths. The static modulus of elasticity varied from 24400 MPa (3.54 x 106 Psi) to 28740 MPa (4.17 x 106 Psi), indicating good resistance to deformation. The presence of gypsum (calcium sulfate) in FBC spent bed raises the question of internal sulfate attack which may affect durability. Based on one-year observations, expansion strains were minimal and stabilized within six weeks of the date of construction. Examination of the cores taken 18 months after construction revealed no damage due to freezing and thawing cycles of the winter climate.
- L. Yverbaum. Fluidized Bed Combustion of Coal and Waste Materials. Noyes Data Corporation, 1987.
- Electric Power Research Institute. Atmospheric Fluidized Bed Combustion Waste Management Design and Guidelines. Report No. CS-6053. Prepared by Baker/TSA, Inc. and ICF Technology Incorporated, 1988.
- N. Ghafoori. Evaluation and Utilization of Illinois FBC Residues for