Spray dryer absorber (SDA) material, also known as spray dryer ash, is a byproduct of coal combustion and flue gas scrubbing processes that has self-cementing properties similar to those of class C fly ash. SDA material does not usually meet the existing standards for use as a pozzolan in Portland cement concrete due to its characteristically high sulfur content, and thus unlike fly ash, it is rarely put to beneficial use. This paper presents the results of a study with the objective of developing beneficial uses for SDA material in building materials when combined with tire fiber reinforcement originating from a recycling process. Specifically, spray dryer ash was investigated for use as the primary or even the sole binding component in a mortar or concrete. This study differs from previous research in that it focuses on very high contents of spray dryer ash (80 to 100 percent) in a hardened product. The overarching objective is to divert products that are normally sent to landfills and provide benefit to society in beneficial applications.
Portland cement concretes and mortars are used extensively in construction of buildings, bridges, and other infrastructure ranging from low-strength sidewalks to high-performance airport runways. Despite recent advances in manufacture, Portland cement remains an energy-intensive product that requires mining of raw materials as well as significant energy input and processing. Incorporation of coal fly ash into concrete mixtures is now widely accepted given its capacity to produce an equivalent or even improved hardened concrete product with less Portland cement and, therefore, reduced raw materials extraction and carbon emissions [
Spray dryer ash is produced in far smaller quantities than fly ash in the US. The American Coal Ash Association, ACCA, estimates 1.4 million tons for all dry flue gas desulfurization products, of which spray dryer ash makes up a large portion [
While fly ash use and performance in concrete has been well documented [
The project described here seeks to leverage the self-cementing nature of this material to create a cementitious product with as little Portland cement as possible. Thus, the emphasis of this work is not on achieving the highest possible strength, but in diverting as much waste material as possible into a useful product that maintains adequate properties. In the present study, applications with lower strength requirements (e.g., nonstructural components) are targeted with the cemented SDA combined with fibers collected during the automobile tire recycling process as reinforcement. The results indicate that good compressive and tensile strength is possible. While these results are for a specific SDA material source, they suggest that useful materials may be produced with this presently underutilized industrial byproduct. They also indicate the value of further study of SDA material sources and long-term material performance.
Two different types of materials were studied. The first was a material comprised solely of spray dryer ash and water, with some specimens also including recycled polymer fibers from used automobile tires (described in more detail below). Second, mortar specimens manufactured with spray dryer ash, sand conforming to ASTM C33 [
Table
Bulk chemical composition of rawhide power station SDA.
Compound | Sample 1 Content A (%) | Sample 2 Content B (%) | Sample 3 Content C (%) | ASTM C618 limit for Class C fly ash (2005) | ASTM C 618 limit for class F fly ash (2005) |
---|---|---|---|---|---|
Silicon dioxide, SiO2 | 39.76 | 29.84 | Sum between 50 % and 70 % | Sum greater than 70 % | |
Aluminum oxide, Al2O3 | 14.31 | 14.24 | |||
Iron oxide, Fe2O3 | 5.56 | 5.82 | |||
SiO2 + Al2O3 + Fe2O3 | 59.63 | 49.90 | 60.63 | ||
Calcium oxide, CaO | 23.45 | 26.48 | |||
Sulfur trioxide, SO3 | 6.19 | 10.01 | 3.70 | 5% maximum | 5% maximum |
Magnesium oxide, MgO | 4.06 | 4.93 | |||
Sodium oxide, Na2O | 1.42 | 1.66 | |||
Titanium dioxide, TiO2 | 1.15 | 0.98 | |||
Phosphorous pentoxide, P2O5 | 0.98 | 1.59 | |||
Barium oxide, BaO | 0.61 | 0.68 | |||
Potassium oxide, K2O | 0.53 | 0.48 | |||
Strontium oxide, SrO | 0.33 | 0.42 | |||
Manganese dioxide, MnO2 | <0.01 | 0.02 | |||
Moisture | 1.86 | 1.04 | 1.33 | 3% maximum | 3% maximum |
Loss on ignition | 1.65 | 2.85 | 1.64 | 6% maximum | 6% maximum |
A: sampled 4/3/2007 and tested by SGS North America, Inc., Denver, Colo, USA.
B: sampled 7/26/2007 and tested by Wyoming Analytical Laboratories, Inc., Golden, Colo, USA.
C: reported in Little, 2008 [
The polymer fibers used in this study were obtained from an automobile tire recycling facility and represented a combination of polymers commonly used as tire reinforcement such as nylon, polyester, and aramid with lengths randomly distributed between approximately 2 mm and 30 mm. The steel wires present in most automotive tires were removed magnetically at early stages of the recycling process. The polymer fibers were interspersed with rubber particles varying from fine dust to larger pieces less than 5 mm in dimension as well as raw chopped fibers that were still twisted together in cords. These recycled fibers were chosen for their compatibility with the theme of waste diversion and sustainable construction, which is a prime motivator for developing a cementitious material with industrial byproducts. Because of variability in fiber size and constitution, all fiber fractions are by weight, as without the density it was not possible to determine a volume fraction. However, given the range of specific gravities of the potential reinforcing fibers, a volume fraction very nearly equal to the weight fraction is reasonable.
The mixture proportions of the specimens are shown in Table
Mixture proportions.
Mixture number | Cement type | Cementa | SDAa | Sanda | Watera | Fibersb |
---|---|---|---|---|---|---|
1A | — | 0 | 100 | 0 | 40 | 0 |
1B | — | 0 | 100 | 0 | 35 | 0 |
1C | — | 0 | 100 | 0 | 30 | 0 |
1D | — | 0 | 100 | 0 | 25 | 0 |
1E | — | 0 | 100 | 0 | 25 | 0.5 |
1F | — | 0 | 100 | 0 | 25 | 1.0 |
1G | — | 0 | 100 | 0 | 25 | 2.0 |
2A | — | 0 | 100 | 100 | 40 | 0 |
2B | III | 5 | 95 | 100 | 40 | 0 |
2C | III | 10 | 90 | 100 | 40 | 0 |
2D | III | 15 | 85 | 100 | 40 | 0 |
2E | III | 20 | 80 | 100 | 40 | 0 |
2F | I/II | 5 | 95 | 100 | 40 | 0 |
2G | I/II | 10 | 90 | 100 | 40 | 0 |
2H | I/II | 15 | 84 | 100 | 40 | 0 |
2I | I/II | 20 | 80 | 100 | 40 | 0 |
2J | I/II | 10 | 90 | 100 | 40 | 1.0 |
2K | I/II | 10 | 90 | 100 | 40 | 1.5 |
2L | I/II | 10 | 90 | 100 | 40 | 2.0 |
aThese quantities are expressed as a percent by weight relative to the total weight of SDA and cement in the mixture.
bExpressed as a percent by weight of the total solids (SDA, cement and sand) in the mixture.
The mortar mixtures had a constant water/cementitious materials ratio of 0.40 but had varying amounts of Portland cement and recycled polymer fibers in an effort to improve on the properties observed in the spray dryer ash pastes. Two types of Portland cement were used. Type I/II was used because it is very commonly available. Type III cement was also used because in the case of fly ash, there is usually a reduced rate of strength gain when mixtures including fly ash are compared to mixtures with just Portland cement, and it was anticipated that the same situation might occur with spray dryer ash. Research by Bilodeau and Malhotra [
The compressive strengths of the mixtures in Table
For both pastes and mortars, the constituent materials were mixed approximately according to ASTM C 305 [
Recall that the objective of this study was to divert as much spray dryer ash as possible from landfilling. Thus, the first experiments evaluated the potential of hydrated spray dryer ash alone in manufactured structural and nonstructural construction products. Early strength gain is an important item of consideration for these materials because manufacturers of commercial products require shipment as quickly as possible, often in as little as seven days. Figure
Early compressive strength of hydrated spray dryer ash pastes.
Figure
Early compressive strength of hydrated spray dryer ash with fibers.
Typical cube appearance after compressive testing without (a) and with (b) polymer fibers.
The ultimate compressive strength of hydrated spray dryer ash will depend on the specific chemical composition of the ash and the long-term curing conditions. However, it is of interest to know approximate values for the compressive strength for both neat and fiber-reinforced spray dryer ash. Table
Average 56-day compressive strength and modulus of elasticity measured from cube specimens.
Set | Average 56-day compressive strength (MPa) | Average 56-day modulus of elasticity (MPa) |
---|---|---|
1A | 1.63 | 119.6 |
1B | 7.57 | 408.4 |
1C | 5.40 | 382.8 |
1D | 7.16 | 376.9 |
1E | 12.79 | 565.9 |
1F | 15.34 | 557.2 |
1G | 9.28 | 395.5 |
The highest average strengths observed for the hydrated spray dryer ash with and without fibers were 15.3 MPa and 7.5 MPa, respectively. These strengths were achieved at an age of 56 days and indicate that hydrated spray dryer ash alone is not likely to be suitable for many structural engineering uses. Aesthetically, the finished cubes had limited resistance to scratching or abrasion and for high water ratios especially, seemed to have a chalky finish. The material was also observed to readily absorb water. A cube dipped in water appeared dry in less than one minute because the water had been absorbed into the cube. Based on these results, the mortar mixtures were developed and tested to study potential means of achieving enhanced properties while still utilizing large quantities of spray dryer ash.
Seeking to improve the properties of the spray dryer ash pastes, the researchers considered the addition of sand and small amounts of Portland cement. To maximize spray dryer ash usage, cement quantities of only five, ten, fifteen, and twenty percent were considered. This can be thought of as the inverse of typical fly ash applications, where smaller amounts of fly ash are used as additives to traditional concrete mixtures. As indicated earlier, both Type I/II and Type III cements were tested.
Table
Compressive strength for different types of cement, testing ages and percents of cement added (MPa).
Type I cement (Sets 2F–2I) | Type III cement (Sets 2B–2E) | ||||||||
Days | 0% | 5% | 10% | 15% | 20% | 5% | 10% | 15% | 20% |
7 | 4.09 | 17.62 | 13.03 | 16.19 | 15.88 | 15.18 | 23.00 | 19.85 | 23.04 |
14 | 6.91 | 23.61 | 17.83 | 23.92 | 25.70 | 20.65 | 27.09 | 27.82 | 24.37 |
21 | 8.03 | 24.43 | 24.53 | 32.64 | 32.84 | 23.64 | 29.98 | 27.06 | 31.91 |
28 | 7.99 | 27.37 | 27.15 | 34.00 | 37.18 | 22.87 | 29.26 | 29.23 | 28.79 |
Comparison of compressive strength at 7 days for mixtures with type I/II and type III cement.
Comparison of compressive strength at 28 days for mixtures with type I/II and type III cement.
The modulus of elasticity of the test specimens was computed based on results from the compression testing. These results are shown in Table
Modulus of elasticity for different types of cement, testing ages and percents of cement added (MPa).
Type I cement (Sets 2F–2I) | Type III cement (Sets 2B–2E) | ||||||||
Days | 0% | 5% | 10% | 15% | 20% | 5% | 10% | 15% | 20% |
7 | 686 | 2022 | 1666 | 1965 | 1720 | 1609 | 2354 | 2193 | 2474 |
14 | 1080 | 2681 | 2176 | 2337 | 2099 | 1835 | 2720 | 3095 | 2743 |
21 | 1051 | 2773 | 2612 | 3208 | 3122 | 2513 | 2612 | 2643 | 2938 |
28 | 881 | 2850 | 3172 | 3360 | 2869 | 2428 | 2648 | 2911 | 2819 |
The effect of polymer fibers on the compressive strength was also considered with the addition of cement. Mortars were prepared with ten percent Type I/II cement and varying fiber contents. Figure
Modulus of elasticity for different percentages of fibers for a mortar made with 10% type I/II cement (MPa).
% Fibers | ||||
Days | 0.0% | 1.0% | 1.5% | 2.0% |
7 | 1666 | 2034 | 1724 | 1839 |
14 | 2176 | 2468 | 2804 | 2237 |
21 | 2612 | 2969 | 2596 | 2815 |
28 | 3172 | 2697 | 2739 | 2861 |
Effect of increasing percentages of recycled polymer fiber on the compressive strength of a mortar with ten percent type I/II cement.
Given the mechanics of failure of a brittle material in compression (shear failure along 45-degree planes and splitting along the axis of loading), the addition of reinforcing fibers to a stiff and brittle matrix may not have a significant impact on compressive strength or stiffness. Thus, the strengthening observed in the SDA paste specimens is likely the result of the weaker and less stiff matrix being reinforced by fibers that are relatively stiffer and thus able to reinforce the matrix prior to cracking. Once the matrix itself is stronger and stiffer, as is the case in the mortars with added cement, fibers can have a detrimental effect (due perhaps to a loss in workability) until cracking has occurred, and the fibers deform sufficiently to carry significant loads. These results are consistent with the highly variable results for fiber-reinforced concrete reported by other authors and summarized by Johnston [
Flexural testing was conducted at 14 and 28 days for mortar specimens with the addition of both cement and fibers. Table
Modulus of rupture results for different types of cement, testing ages and percents of cement added (MPa).
Type I cement (Sets 2F–2I) | Type III cement (Sets 2B–2E) | ||||||||
Days | 0% | 5% | 10% | 15% | 20% | 5% | 10% | 15% | 20% |
14 | 1.19 | 3.27 | 2.61 | 3.74 | 5.24 | 2.38 | 3.14 | 3.28 | 3.99 |
28 | 1.54 | 3.05 | 3.11 | 4.85 | 6.44 | 2.58 | 3.74 | 3.34 | 4.21 |
Comparison of MOR at 28 days for mixtures with type I/II and type III cement.
The addition of fibers to concrete typically provides added capacity to the regions of the test specimen in tension, potentially increasing the overall flexural strength of the specimen. Figure
Effect of increasing percentages of recycled polymer fiber on the MOR of a mortar with ten percent type I/II cement.
Overall, the mixtures tested with additions of both cement and tire fiber showed significant promise as a potential engineering material. The strengths and stiffnesses showed significant improvement with as little as 5 percent addition of Portland cement and approximately 1 percent of fibers. Both Type I/II and Type III cements were found to be effective, with Type III cement increasing early strengths, while Type I/II cement produced higher later strengths. In terms of aesthetics and workability, the addition of the cement also eliminated concerns about the chalky finish associated with the SDA-only mixtures. The addition of cement also improved the workability of the pure spray dryer ash mortar, as the spray dryer ash-only mixtures tended to be sticky which made the finishing difficult.
Two phases of testing were used to investigate the use of spray dryer ash as a cementitious material for engineering use. Spray dryer ash alone mixed with water was found to be too weak in compression to offer benefits as a practical engineering material even for moderately structural components such as roof tiles. The addition of recycled polymer fibers increased the strength by a significant amount (10–50 percent), but the resulting strengths were still quite low. Other properties, such as a chalky finish, also indicated that spray dryer ash alone was not suitable as a matrix for most structural engineering applications.
Improvements were found in the properties of sanded mortars through the addition of small amounts of Portland cement to the ash-tire fiber mixture. Compressive strengths at the low end of the range typically considered for conventional concrete (27 MPa) were achieved with only 5% additions of Portland cement. Recycled polymer fibers were shown to benefit the compressive and flexural strengths at additions of around 1 percent by weight, while greater fiber fractions had a limited or detrimental impact on strengths. The fibers were very effective at preventing spalling and loss of material due to fracture and contributed to increased toughness and ductility. This attribute may be desirable for certain applications.
This preliminary testing has been conducted on mortars, and thus, testing of concretes with large aggregate is a necessary next step. If structural applications are to be pursued, these practical applications will also require testing to ensure the durability of the product and its compatibility with reinforcing bar from both a bonding and corrosion perspective. The results of the study presented herein indicate a high potential for useful application of this material and provide justification for further studies focusing on specific applications. Significant waste diversion through beneficial use of spray dryer ash appears to be a viable objective.
The authors gratefully acknowledge the Colorado Commission on Higher Education (CCHE) for support of this research through contract no. 07 GAA 00018. The spray dyer ash and recycled tire fibers were provided for this research by the Platte River Power Authority and Jai Tire, respectively. The authors also acknowledge the assistance of the students who worked on this project including Jeff Eulberg, Stephanie Thomas, Balaji Mahalingam, Fredrick Busch, and Karthik Rechan.