Testing Framework for Analysis of Time-Dependent Behavior of Coal Combustion Products

Plaks, Nicholas T.; Palomino, Angelica M.; Scheetz, Barry E.; Braun, Gregory D.

doi:10.1061/(ASCE)MT.1943-5533.0001326

Open access

Technical Notes

May 18, 2015

Testing Framework for Analysis of Time-Dependent Behavior of Coal Combustion Products

Authors: Nicholas T. Plaks, Angelica M. Palomino, A.M.ASCE [email protected], Barry E. Scheetz, and Gregory D. Braun, A.M.ASCEAuthor Affiliations

Publication: Journal of Materials in Civil Engineering

Volume 28, Issue 1

https://doi.org/10.1061/(ASCE)MT.1943-5533.0001326

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Abstract

Coal combustion products (CCPs) are by-products created when coal is burned for energy production. These products include predominantly fly ash, bottom ash, and flue gas desulfurization (FGD) by-products. These materials can be a viable alternative to natural resources for the construction of engineered fills. Because of the variability in the chemical composition, CCPs require mineralogical, chemical, and mechanical characterization to ensure its applicability as a construction material. Furthermore, CCP characterization should also address the material’s changing properties with time. To date, only limited standards have been proposed to characterize these materials to determine the viability of their use as structural fill. The purpose of this study is to apply a minimal set of practical mechanical and chemical tests that will characterize and predict CCP time-based performance intended for large-volume civil engineering applications such as embankments and mine reclamation. Unconfined compressive strength tests, hydraulic conductivity measurements, and X-ray diffraction (XRD) analysis were performed at curing times from 1 to 180 days on three materials: fluidized bed combustion (FBC) ash, flue gas desulfurization (FGD) ash, and pulverized coal (PC) class F fly ash. Results show that the unconfined compressive strength varied between each of the materials and over time for both the FBC and PC fly ashes, whereas the strength of the FGD ash did not appear to change over time. The FBC ash experienced a significant strength gain with increased curing time. The XRD characterization shows that the strength gain in the FBC ash is likely caused by the formation of ettringite. The FBC ash also showed significant changes in hydraulic conductivity with curing time.

Introduction

In 2012 alone, the United States produced more than 109 million tons of coal combustion products (CCPs), by-products of coal fired energy production. These CCPs most abundantly include fly ash, bottom ash, and flue gas desulfurization (FGD) by-products. Of the 109 million tons produced, approximately 47% of these CCPs were beneficially used. The beneficial use of CCPs includes practices such as using fly ash as a supplemental cementitious material in portland cement concrete and as a fill material in large-volume civil engineering applications. The remaining 53% of CCPs produced were disposed of in landfills. Fig. 1 outlines the beneficial use of CCPs versus production from 1966 to 2012 [American Coal Ash Association (ACAA) 2014]. This figure clearly illustrates the gap between the amounts of CCPs produced and recycled.

Fig. 1. CCP production and beneficial use (data from ACAA 2014)

There are significant advantages to using CCPs for large-volume civil engineering applications, such as mine land reclamation, in which the CCPs could be used as a fill material to reinstate the original grade of the land. The repurposing of CCPs as opposed to disposal would reserve landfills to be used for residential waste, which currently has no other viable disposal methods. Other advantages include the potential cementitious nature of CCPs (strength gain with time), low unit weight, and the immediate availability of large volumes of material (Butalia and Wolfe 2001; ACAA 2014).

Large-volume CCP applications have approximately a 20-year history in Pennsylvania through mine land reclamation projects. The Pennsylvania Department of Environmental Protection (PA DEP) has over the course of these two decades developed strict regulation standards that culminated in 2010 with the adoption of Chapter 290: Beneficial Use of Coal Ash into Title 25 of the Pennsylvania Code (Commonwealth of Pennsylvania 2010). Chapter 290 provides a required chemical and material testing framework for CCP beneficial use. The framework focuses on environmental testing of CCPs, requiring bulk chemical analysis and the synthetic precipitation leaching procedure (SPLP) to be performed every quarter to maintain statewide beneficial-use certification. Any CCPs that do not meet the leachate standards prescribed by Chapter 290 are not permitted for beneficial-use applications, specifically mine land reclamation. These measures ensure that a potentially hazardous CCP will not degrade groundwater sources and streams. However, Chapter 290 neglects the performance of CCPs as a construction material, in particular its performance over time.

The material property most typically considered when using a CCP for large-volume uses is maximum dry density. A material that compacts to a lower maximum dry density than another material is better suited as a bulk fill material, in which settlement of the existing ground surface is a concern. However, what is not typically considered is how these materials may evolve over time, thereby changing the material properties such as strength and hydraulic conductivity.

The purpose of this study is to develop a minimal set of practical mechanical and chemical tests that will characterize and predict CCP time-based performance in large-volume civil engineering applications. These parameters should provide the appropriate data needed to allow the high-volume use of CCPs without negative structural or environmental impacts. The long-term goal is to implement a testing procedure that will provide for the minimal initial material characterization and minimal mechanical properties over a set period. This procedure was used to characterize three CCP sources for beneficial use. These sources include a fluidized bed combustion (FBC) ash, a FGD material, and a pulverized coal (PC) fly ash.

Materials

Fluidized Bed Combustion Ash

Fluidized bed combustion ash is produced at power plants that typically burn coal mine refuse. Coal mine refuse, or waste coal, is a low–British thermal unit (BTU) material typically discarded by the mining industry. Coal mine refuse from bituminous and anthracite mining is called gob and culm, respectively (Dalberto et al. 2004). Fluidized bed combustion ash has been observed to gain strength as a function of time (Deschamps 1998). Fluidized bed combustion bottom ash consists of heavier, coarser particles that collect on the bottom of the combustion chamber and are removed with a conveyer system. Fluidized bed combustion fly ash particles are much smaller and are collected with an electrostatic precipitator. The FBC ash used for this study was collected from a Western Pennsylvania FBC power plant. The FBC ash used in this study is a blend of 45% bottom ash and 55% fly ash by mass. This blend was adopted to match the blend used by the FBC power plant for beneficial-use projects.

Flue Gas Desulfurization Material

Burning coal for electricity produces pollutants that are potentially harmful to the environment and to human health, and regulations are in place to minimize the release of these pollutants into the environment. The EPA limits the emissions of sulfur dioxide into the atmosphere through the Clean Air Act Amendments of 1990 and the Clean Air Interstate Rule (CAIR). Coal-fired power plants comply with this regulation by implementing a lime or limestone reagent in combination with a forced oxidation system to act as a “scrubber.” In this process, limestone slurry is used to dissolve sulfur dioxide from flue gas. The slurry and dissolved sulfur oxide forms calcium sulfite hemihydrate. Forced oxidation refers to the process through which limestone slurry is aerated. The oxygen bubbled through the system causes calcium sulfite hemihydrate to form gypsum. Forced oxidation has been found to produce more granular particles compared with other oxidation practices (Gaikwad 2003). This process is known as flue gas desulfurization. In 2012, more than 31 million tons of FGD materials were produced in the United States with only 41% beneficially used, mostly in the manufacture of wall board for the housing and building industry (ACAA 2014). Flue gas desulfurization material was selected for this study because of the decline of its use in the current housing market. The FGD material used in this study was collected from the same Eastern Pennsylvania pulverized coal power plant that produced the PC fly ash investigated in this study.

Pulverized Coal Fly Ash

Pulverized coal fly ash is produced at conventional coal-fired power plants and is removed from the flue gas by using electrostatic precipitators. The PC fly ash, classified as class F, is very well studied and has various standards associated with its use. Therefore, class F fly ash was chosen for this study as a point of comparison with the FGD material and FBC ash. The PC fly ash in this study was collected from an Eastern Pennsylvania pulverized coal-fired power plant. The fly ash was produced through the burning of bituminous coal mined from the Pittsburgh Coal Seam in Western Pennsylvania.

Material Characterization

Several tests were performed to characterize the chemical and physical properties of the three selected CCPs. Chemical analysis of the CCPs was performed to identify any potential negative environmental impacts that could occur from these materials. The baseline chemical analysis for the CCP materials used in this study is presented in Table 1. On the basis of this analysis, the CCPs were determined to pose no environmental concerns in their initial state. The PA DEP currently certifies both the PC fly ash and the FBC ash for statewide beneficial use.

Table 1. Baseline Chemical Analysis of CCPs Used in This Study

Component type	Analyte	Amount detected (%)
Component type	Analyte	FGD	FBC	Class F fly ash
Major components^a	${SiO}_{2}$	0.71	40.36	43.37
	${Al}_{2} O_{3}$	0.21	16.98	22.27
	${Fe}_{2} O_{3}$	0.17	7.01	15.37
	MnO	0.002	0.027	0.0.27
	MgO	0.05	1.33	0.81
	CaO	32.26	14.42	3.94
	${Na}_{2} O$	0.03	0.2	1.67
	$K_{2} O$	0.04	2.09	1.59
	${TiO}_{2}$	0.006	0.919	1.156
Anions^b	Cl	$< 0.01$	0.06	0.06
	F	0.13	0.02	$< 0.01$
	${SO}_{3}$	$< 0.3$	$< 0.3$	1.1
	${SO}_{4}$	57.1	12.2	2.4
	$P_{2} O_{5}$	0.02	0.12	0.41
	S	6.46	3.76	1.1
Loss on ignition (LOI)^a		20.81	5.08	7.98
Minor/trace^b (ppm)	As	$< 0.5$	91.9	140
	Ba	6	561	817
	Cd	$< 0.5$	$< 0.5$	1.1
	Pb	$< 5$	38	54
	Se	$< 3$	15	$< 3$

Note: Chemical analysis was conducted by Actlabs, Ontario, Canada.

a

Analysis method: fusion-inductively coupled plasma optical emission spectrometry (FUS-ICP).

b

Analysis methods: Cl, As, Se = instrumental neutron activation analysis (INAA); F = fusion-ion selective electrode analysis (FUS-ISE);

{SO}_{3} {SO}_{4}

= combustion infrared detection (IR);

P_{2} O_{5}

, Ba = FUS-ICP; S, Cd, Pb = total digestion inductively coupled plasma optical emission spectrometry (TD-ICP).

The material characterization includes particle-size distribution (PSD), scanning electron microscopy, and specific-surface and specific-gravity measurements. The particle-size distribution, shown in Fig. 2, was determined by using monochromatic laser light diffraction (Malvern Mastersizer, Westborough, Massachusetts). Specific surface (

S_{a}

) was determined by using the BET method (Micromeritics Gemini Unit, Norcross, Georgia), and specific gravity was determined by following ASTM D854 (ASTM 2007c). These parameters, along with the

d_{50}

for each material, are listed in Table 2. The FGD material is more uniformly graded compared with the FBC and PC fly ash particles because the FGD is predominantly composed of gypsum crystals forming under the same conditions (the composition of this material is discussed further in the “XRD Analysis” section). Also, the FGD material exhibits the highest specific surface (Table 2) of the three CCPs tested in this study, likely because of the crystal imperfections found at the edges of the particles. Scanning electron microscopy (SEM; Hitachi S-3000H, Clarksburg, Maryland) was used to image the FGD material [Fig. 3(a)], FBC ash [Fig. 3(b)], and PC fly ash [Fig. 3(c)]. The micrographs clearly show that the particles of the three CCPs vary in both shape and size. The FGD particles are crystalline and elongated compared with the angular FBC ash particles and the spherical PC fly ash particles.

Fig. 2. Particle-size distribution of CCPs tested in this study

Table 2. Material Properties of CCPs Used in This Study

CCP	$G_{s}$	$S_{a}$ ( $m^{2} / g$ )	$d_{50}$ (μm)	Optimum moisture content (%)	Maximum dry density ( $kg / m^{3}$ )
FGD material	2.57	9.14	40.2	17	1,470
FBC ash	3.09	7.2	18.2	26	1,415
PC fly ash	2.62	2.69	15.3	19	1,582

Fig. 3. SEM micrographs: (a) FGD material; (b) FBC ash; (c) PC fly ash

Experimental Methods

The framework proposed by Plaks (2010) provides for a minimal set of testing that quantifies the mechanical behavior of CCPs over a designated time frame. Testing methods were selected on the basis of the most critical material properties needed for using the material as a large-volume fill. The mechanical testing framework addresses three fundamental particulate material properties: shear strength, hydraulic conductivity, and compressibility (consolidation). In particular, the hydraulic conductivity of these materials provides insight into how the CCP placed in bulk will affect the surrounding hydrology and whether the material is permeable enough for leaching to be a concern. The full mechanical testing framework is illustrated in Fig. 4. In this study, the CCPs were subjected to unconfined compression strength tests and hydraulic conductivity measurements over the course of a set curing time of up to 180 days. Samples of the strength test specimens were analyzed by using X-ray diffraction (XRD).

Fig. 4. Proposed mechanical testing framework

Unconfined compression (UC) strength tests were performed to obtain minimum strength characteristics for the FGD material, FBC ash, and class F fly ash. The UC tests were completed for each material at various curing durations to monitor any time-dependent strength behavior. The materials were cured for 1, 3, 7, 14, 28, 56, 90, and 180 days. These curing durations were selected according to the typical timetable used for monitoring concrete strength with curing time. Three samples of each material were prepared for each curing period to ensure repeatable results. The samples were prepared at optimum moisture content and compacted to the maximum dry density as determined by ASTM D698 (ASTM 2007b). The optimum moisture content and maximum dry density of each CCP tested in this study are listed in Table 2.

The UC tests were carried out in accordance to ASTM D2166-00 (ASTM 2007a) with the following exceptions: The samples were prepared at optimum water content and maximum dry density using a standard proctor mold. The samples were compacted in the Proctor mold in three equal lifts using a standard Proctor hammer at 25 blows per lift. The samples were wrapped in plastic wrap and cured at typical room conditions in plastic air-tight bags (double-bagged) to prevent moisture loss. The FGD and class F fly ash samples were tested by using a Geocomp Load Trac II frame (Acton, Massachusetts), using a constant strain rate of 0.5% per minute.

The FBC samples could not be tested in the Geocomp Load Trac II frame because the strength of the material exceeded the capacity of the available load cell. Therefore, the FBC samples were tested on a Boart Longyear model CM-625 (South Jordan, Utah) concrete compression testing machine. Because of the limitations of the device, a constant strain rate could not be applied. Alternatively, a constant loading rate within the range of

6.9 - 20.7 kPa / s

(

1 - 3 psi / s

) was applied until failure. This was the lowest range of loading rate achievable with this device. Representative samples of each CCP type at each curing duration were analyzed by using X-ray diffraction.

Hydraulic conductivity was determined by using a specially designed pressurized permeability cell shown in Fig. 5, following the procedures for a constant head test. The permeability cell consisted of a hollow metal cylinder that holds the sample. The sample was compacted into a Tygon tube (Saint-Gobain, Valley Forge, Pennsylvania) segment with an inner diameter of 2.54 cm (Fig. 6), which acted as both the compaction mold and the flexible membrane. High-pressure water lines were connected to the cell to provide confining and driving pressures independently. The test was performed by using a gas to pressurize distilled water. The distilled water was supplied through an external reservoir with an inner piston. Pressurized gas was used to drive the piston that in turn pressurized the water in the reservoir, thus preventing gas-water interaction. Confining pressure was applied to the sample before the driving pressure. The sample was allowed to saturate and reach a steady-state flow rate at the sample exit.

Fig. 5. Pressurized hydraulic conductivity cell (image by Nicholas Plaks)

Fig. 6. Hydraulic conductivity sample preparation using Tygon tubing as the sample mold and flexible membrane (image by Nicholas Plaks)

Two specimens per curing duration were prepared at the respective optimum moisture content and compacted in three equal lifts by using a tamper. The specimens were compacted to the maximum dry density to maintain consistent void ratios between specimens. The specimens were tested at curing durations of 1, 3, 7, 14, 28, 56, 90, and 180 days. These curing durations were selected to match those of the strength testing. Table 3 summarizes the confining and driving pressures for each material. A lower driving pressure was used for the FGD material to prevent dissolution during measurement.

Table 3. Hydraulic Conductivity Test Parameters for the CCPs Used in This Study

Material	Driving pressure [kPa (psi)]	Confining pressure [kPa (psi)]	Outlet pressure [kPa (psi)]
FGD material	345 (50)	689 (100)	101 (14.7)
FBC ash	1,034 (150)	1,379 (200)	101 (14.7)
PC fly ash	1,034 (150)	1,379 (200)	101 (14.7)

Using this pressurized hydraulic conductivity cell has the following advantages:

•

The hydraulic conductivity of fine-grained materials can be measured in a timely manner compared with conventional falling head tests;

•

The system is applicable to cementitious materials (Roy et al. 1993);

•

Confining pressure minimizes the development of preferential flow paths at the material/cell wall (Tygon tubing) interface; and

•

The samples can be prepared and cured in the Tygon tubing for indefinite time intervals as long as they are well sealed to prevent moisture loss. The Tygon tubing doubles as the flexible membrane during the hydraulic conductivity test so that the sample does not have to be removed from the tubing (preparation mold) before testing.

The effluent from the hydraulic conductivity tests (approximately the first pore volume of fluid, which is equal to the volume of voids in the sample) was collected from each sample at each curing duration for chemical analysis. The chemical composition of the effluent is important in examining changes in the leaching behavior of the CCP to identify any potentially harmful constituents. The pore fluid chemical analysis at day 1 and day 90 for each CCP type is summarized in Table 4. Day 90 was selected instead of day 180 because the pore fluid at this curing time could not be collected for the PC fly ash material. The chemical analysis was conducted by Activation Laboratories (Ontario, Canada), and the techniques used are listed in Table 4.

Table 4. CCP Effluent Chemical Analysis of Selected Analytes

Analyte	Leaching limit^a ( $mg / L$ )	Concentration ( $mg / L$ )
Analyte	Leaching limit^a ( $mg / L$ )	FGD day 1	FGD day 90	FBC day 1	FBC day 90	Class F fly ash day 1	Class F fly ash day 90
Hg^b	0.05	$< 0.002$	$< 0.0002$	0.002	$< 0.0002$	$< 0.002$	$< 0.0002$
Pb^b	0.375	0.00036	0.00008	0.001	0.00027	$< 0.0001$	0.00004
Se^b	0.5	0.0257	0.0203	0.0171	0.0021	0.0974	0.0176
As^b	0.25	0.00043	0.00085	0.00046	0.00157	0.00162	0.036
Ba^b	50	0.0034	0.003	0.12	0.0202	0.0875	0.0456
Cd^b	0.125	0.00047	0.00056	$< 0.00001$	0.00002	0.0001	0.00001
Al^b	5	0.028	0.013	0.02	1.780	2.7	$> 2$
${SO}_{4}$ ^c	2,500	1,440	1,460	1,070	481	867	70.1
Ca^b	No limit	$> 200$	$> 20$	$> 200$	$> 20$	$> 200$	$> 20$

Note: Chemical analysis was conducted by Actlabs (Ontario, Canada).

a

PA DEP maximum leaching limits.

b

Analysis method: inductively coupled plasma mass spectroscopy (ICP-MS).

c

Analysis method: combustion infrared detection.

Results and Discussion

Unconfined Compressive Strength

Unconfined compressive strength tests were performed on the FBC ash, FGD ash, and PC fly ash at various curing durations to obtain minimum strength characteristics. Fig. 7 illustrates the peak strength values for all three materials as a function of curing time. The peak strength values represent the average of three test samples for each material and curing duration.

Fig. 7. Peak strength versus curing time for FGD material, FBC ash, and PC fly ash

The FBC ash experiences significant strength gain characteristics as a function of curing time. The unconfined compressive strength ranges from 393 kPa (57 psi) for the 1-day strength to 15,380 kPa (2,232 psi) for the 180-day strength. This increase in unconfined compressive strength is in part attributable to the coal-burning process. Fluidized bed combustion ash is produced from power plants that were created to burn waste coal, either gob (bituminous coal waste) or culm (anthracite coal waste). Waste coal is characteristically high in sulfur content (Dalberto et al. 2004; Deshamps 1998). Crushed limestone is added to the burning process, which acts as a sorbent to capture the sulfur oxides released from the waste coal (Behr-Andres and Hutzler 1994). The addition of crushed limestone results in the production of a coal ash with a lime content much higher than a traditional class F fly ash produced in a pulverized coal-fired power plant. The difference in lime content can be observed in Table 1. It is this lime content in the FBC ash that causes the coal ash to gain strength through the formation of ettringite. After day 1, the FBC ash shows the greatest strength of all three materials tested here for the remaining curing durations. The typical failure mode observed for this material, including near-vertical fracturing and spalling at the base, is shown in Fig. 8.

Fig. 8. Vertical cracking and spalling distress in FBC ash (image by Nicholas Plaks)

The peak strength of the FGD material remains relatively constant with respect to curing time. The UC strength ranges from 63 kPa (9.2 psi) at day 1 to 95 kPa (14 psi) at day 180. No significant strength gain was observed over the tested curing durations. The FGD material typically failed as shown in Fig. 9.

Fig. 9. FGD material failed UC strength specimen (image by Nicholas Plaks)

The unconfined compressive strength of the PC fly ash did not vary much for the first 28 days, ranging from 297 kPa (43 psi) to 349 kPa (51 psi). At 56 days, a significant drop in UC strength, from 349 to 105 kPa, was observed. The 56-day specimens were extremely fractured and cracked as shown in Fig. 10. The 90- and 180-day specimens had even more extensive fracturing and cracking, indicating that no significant particle bonding occurred. These specimens failed under self-weight and so could not be tested.

Fig. 10. PC fly ash failed UC strength specimen (image by Nicholas Plaks)

XRD Analysis

One potential cause of the observed strength increase of the FBC ash is the evolution of the mineralogical constituents over time. Fig. 11 shows the XRD signatures for the FBC ash at different curing periods ranging from 1 to 180 days. The peak associated with the presence of ettringite grows with each signature as the curing time increases, indicating an increase in the percentage of ettringite. Ettringite is a known cementicious hydration product, and the increasing presence of this mineral over time may explain the early strength gain of the material. The ettringite peak at 7 days of curing is much higher compared with the 3-day peak, which corresponds to the significant strength gain at 7 days. The chemical reaction for ettringite formation is as follows (Damidot et al. 2011):

2 {Ca}^{2 +} + 2 Al {(OH)}_{4}^{-} + 4 {OH}^{-} + 2 {SO}_{4}^{2 -} + 26 H_{2} O \to {Ca}_{6} {[Al {(OH)}_{6}]}_{2} \cdot {({SO}_{4})}_{3} \cdot 26 H_{2} O

Fig. 11. XRD patterns for FBC ash initially mixed at optimum water content; Samples cured for 1–180 days; Signatures are stacked by staggering the counts in increments of 10,000

The XRD analysis with time for the FBC material is consistent with this reaction.

Fig. 12 shows the XRD patterns for the FGD material over a curing period of 180 days. The only mineralogical component in the FGD material is gypsum, and the intensities of the peaks corresponding to gypsum remain fairly constant. The FGD material is composed almost entirely of gypsum, which by itself will not react to form any cementicious hydration products. This consistent mineralogical content over time parallels the relatively constant UC strength.

Fig. 12. XRD patterns for FGD material initially mixed at optimum water content; samples cured for 1–180 days; signatures are stacked by staggering the counts in increments of 10,000

The XRD signatures for the PC fly ash at the specified curing times of up to 90 days are shown in Fig. 13. No XRD signature was collected for the 180-day curing time because of an error in the sample collection process. No significant changes in the PC fly ash mineralogy were observed through the 90 days of curing. In particular, no cementicious hydration products were observed. The PC fly ash does not possess intrinsic cementation characteristics, which could explain the late-term fracturing.

Fig. 13. XRD patterns for PC fly ash initially mixed at optimum water content; samples cured for 1–90 days; signatures are stacked by staggering the counts in increments of 10,000

Hydraulic Conductivity

Permeability analysis was performed to examine how the permeability characteristics of FGD material, FBC ash, and PC fly ash vary with respect to curing time. The hydraulic conductivity of the CCPs as a function of curing time is shown in Fig. 14. The hydraulic conductivity of the FGD material remains relatively constant as a function of time. Values range from

1.25 \times 10^{- 4}

to

1.59 \times 10^{- 4} cm / s

. The hydraulic conductivity of the PC fly ash also remains relatively constant with curing time, with values ranging from

1.87 \times 10^{- 5}

to

3.90 \times 10^{- 5} cm / s

. However, for both the FGD and PC fly ash materials, the hydraulic conductivity was not obtainable at the 180-day curing time. The prepared specimens became extensively fractured during curing and could not sustain placement in the hydraulic conductivity apparatus. The mineralogical content, as observed through XRD analysis, did not significantly change for either material over the same period (Figs. 12 and 13).

Fig. 14. Hydraulic conductivity of FBC ash, PC fly ash, and FGD material with curing time

Unlike the other two CCPs tested in this study, the FBC ash exhibits a wide range of permeability characteristics. Initially, the hydraulic conductivity decreases over the first 28 days to

4.06 \times 10^{- 7} cm / s

. The hydraulic conductivity then increases over 56 and 90 days to

3.04 \times 10^{- 5} cm / s

.

The reasons for the observed fluctuations in the hydraulic conductivity values of the FBC ash may be attributable to the mineralogical composition. The XRD analysis shows that the mineralogy of this CCP changes over time. As previously stated, ettringite forms as the material cures over time. Although this mineral may be the cause of the observed increase in strength of the FBC ash, its formation may also influence the pore-size distribution and pore connectivity over time. It has been observed that ettringite forms into needlelike crystals (Yoon et al. 2007; Deschamps 1998). It is possible that as ettringite formed in the compacted CCP, the needlelike crystals partially blocked some of the preferential flow paths in the CCP matrix, constricting water flow in the earlier curing times. Yet, a significant increase in hydraulic conductivity is seen after 28 days—up to

4.56 \times 10^{- 5} cm / s

at 180 days. In other words, this material gains strength over time while either decreasing or increasing in hydraulic conductivity. Because leaching is a major concern with these materials, further investigation is needed to understand exactly how ettringite formation affects CCP hydraulic conductivity.

Conclusions

This study investigates the variability in behavior over time of three types of coal combustion products: a FBC ash, a FGD ash, and a PC class F fly ash. The long-term goal is to implement a testing procedure that will provide for the minimal initial material characterization and minimal mechanical properties over a set time, in this case 180 days, to capture the influence of changes in the material constituents. This particular framework characterizes the differences in mechanical behavior of varying types of CCPs. The three types of CCPs presented in this study behave very differently in their abilities to resist loads and hydraulic flow. The FBC ash has the most desirable behavior with regard to increasing compressive strength but has a variable, ultimately increasing, hydraulic conductivity with time.

The following conclusions can be drawn from this study:

The CCP type and curing time have an impact on the strength of the material. The FBC ash significantly increased in strength over the observed 180-day curing period from 393 to 15,380 kPa. The FGD ash showed little change in strength over the same period, whereas the PC class F fly ash lost a significant amount of strength after 28 days. The FBC ash had the highest strength, followed by the PC fly ash and then the FGD material over the first 28 days.

The CCP type and curing time have an impact on the hydraulic conductivity of the material. The FGD and PC class F ash materials both had relatively consistent hydraulic conductivities, with the FGD having a higher value than the PC ash. However, both of these materials became unsuitable for measurement between 90 and 180 days. The hydraulic conductivity of the FBC ash varied significantly over the 180-day period, first decreasing over an order of magnitude, then increasing to

4.56 \times 10^{- 5} cm / s

, just above its value at 1 day.

The strength gain and variability of the hydraulic conductivity of the FBC ash are likely attributable to the formation of ettringite. The evolution in chemical composition was observed by using XRD analysis.

On the basis of these findings, careful and consistent attention should be paid to CCP products when being considered for use in engineering applications. Further studies should be conducted to evaluate the long-term impact of ettringite on strength and hydraulic conductivity.

This testing framework shows the differences in material properties and mechanical behavior of CCP types, such as FBC ash, PC fly ash, and FGD material. Strength gain behavior is ideal in a material used as bulk fill for embankment construction and mine land reclamation in which materials with high shear strengths are needed to ensure stability. Fill materials with low hydraulic conductivities are ideal for mine land reclamation practices in which reducing infiltration of water is important for preventing or stopping acid mine drainage. These CCPs can be properly used for specific beneficial-use applications by understanding these differences in material properties, especially over time.

Acknowledgments

The authors would like to thank the Pennsylvania Coal Ash Research Group for funding this study and Dan Fura for his technical expertise and laboratory support.

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Information & Authors

Information

Published In

Journal of Materials in Civil Engineering

Volume 28 • Issue 1 • January 2016

Copyright

This work is made available under the terms of the Creative Commons Attribution 4.0 International license, http://creativecommons.org/licenses/by/4.0/.

History

Received: Aug 14, 2014

Accepted: Mar 6, 2015

Published online: May 18, 2015

Discussion open until: Oct 18, 2015

Published in print: Jan 1, 2016

Authors

Affiliations

Nicholas T. Plaks

Project Engineer II, Mortenson Construction Company, Denver, CO 80202.

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Angelica M. Palomino, A.M.ASCE [email protected]

Assistant Professor, Dept. of Civil and Environmental Engineering, 423 J.D. Tickle, Univ. of Tennessee, Knoxville, TN 37996 (corresponding author). E-mail: [email protected]

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Barry E. Scheetz

Professor, Dept. of Civil and Environmental Engineering, Pennsylvania State Univ., 201 Transportation Research Building, University Park, PA 16802.

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Gregory D. Braun, A.M.ASCE

Geotechnical Designer, Gannett Fleming, Inc., 207 Senate Ave., Camp Hill, PA 17043.

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Testing Framework for Analysis of Time-Dependent Behavior of Coal Combustion Products

Abstract

Introduction