British Columbia Mine Reclamation Symposia

Temporal influence of fly ash on soil bulk density and aggregate size distribution Salé, Loretta Y.; Chanasyk, David S.; Naeth, Anne 1996

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Proceedings of the 20th Annual British Columbia Mine Reclamation Symposium  in Kamloops, BC, 1996. The Technical and Research Committee on Reclamation TEMPORAL INFLUENCE OF FLY ASH ON SOIL BULK DENSITY AND AGGREGATE SIZE DISTRIBUTION Loretta Y. Salé1, David S. Chanasyk2 and M. Anne Naeth2 1 Ministry of Employment and Investment,        2 Department of Renewable Resources, General Energy and Minerals Division, Bag 5000, Services Building, University of Alberta, Smithers, BC V0J 2N0 Edmonton, AB T6G 2H1 ABSTRACT Fly ash is a by-product of coal-fired power generation. In 1992, 2.5 million tonnes of fly ash were produced in Alberta of which most was disposed of in lagoons or disposal sites. There is potential to use fly ash as an amendment to improve certain physical characteristics of problem soils. Fly ash is composed predominantly of silt-sized particles and when added to a soil high in clay, the soil texture and other associated physical characteristics, such as bulk density, can be altered to be more desirable for plant growth. The land surrounding the Sundance power generating plant and Highvale coal mine near Edmonton, Alberta is composed predominantly of Luvisolic and Solonetzic soils that have a high clay content. Fly ash from the Sundance power generating plant was added to a clay loam soil in amounts ranging from O to 100% fly ash. The soil:fly ash mixtures were left outdoors at the Highvale coal mine for 16 months. At select times during this period, the mixtures were periodically analyzed for bulk density and aggregate size distribution. Adding 12.5 to 25% fly ash to this soil produced the highest percentage of soil aggregates within the ideal range (1-5 mm) for seed-soil contact. Soil bulk density increased with fly ash additions of up to 25%, then decreased. Fly ash alone had the lowest bulk density. Hence, fly ash may be a suitable soil amendment, although other environmental issues related to its use must also be addressed. INTRODUCTION Fly ash is a by-product of coal-fired power generation plants that has little current utility (10% of the amount produced is used in the cement industry). Fly ash is the portion of ash produced in coal combustion that has a sufficiently small particle size to be carried away from the boiler in the flue gas (El-Mogazi et al. 1988). Bottom ash is composed of fine- and coarse-grained particles (ash and slag) and remains in the boiler (Carlson and Adriano 1993). Of all the ash produced in Alberta, about 70% is fly ash with the 184 Proceedings of the 20th Annual British Columbia Mine Reclamation Symposium  in Kamloops, BC, 1996. The Technical and Research Committee on Reclamation remainder being bottom ash. Fly ash that is not sold for commercial use is buried in disposal sites or lagooned. The physical, chemical and mineralogical characteristics of fly ash depend on the parent coal source, the method of combustion and the efficiency and type of emission control device (Adriano et al. 1980; Carlson and Adriano 1993). In general, fly ash is composed of predominantly silt-sized, spherical, amorphous ferro-aluminosilicate minerals (Adriano et al. 1980; Carlson and Adriano 1993). Fly ash is generally characterized as having low permeability, low bulk density and high specific surface area (Page et al. 1979). Soils with a high clay content are generally considered difficult to cultivate, and under dry soil conditions, large, dense and hard clods form (Hadas and Wolf 1983; Shiel et al. 1988). The clods can lead to poor seed-soil contact and reduced germination and pose a mechanical impedance to emerging seedlings (Malik et al. 1985; Wild 1988). Alberta's agricultural land is a mosaic of soils, including several problem soils with limiting soil physical properties. There are approximately 6.7 million hectares of fine textured soils, 6 million hectares of coarse textured soils and 20 million hectares of Luvisolic soils in Alberta to name a few (Brierley et al. 1992). There is potential to manipulate a problem soil's physical characteristics by the addition of fly ash because of its silt-sized particles and component calcium. Watson (1994) found that the textures of a silty clay and a sandy loam soil could be changed to loam by addition of fly ash. Chang and co-workers (1977) significantly reduced bulk density of California soils of various textures by the addition of as little as 2.5% v/v fly ash. Studies on the potential use of fly ash as an amendment for soil physical properties are limited. Within Alberta, the research is even more scarce. Watson (1994) reported from laboratory experiments strength and water retention characteristics of two types of Alberta fly ashes mixed with two types of soil. There is a need to quantify the influence of fly ash addition on soil physical properties under field conditions. Information is limited not only on how soil physical properties change after fly ash addition to soil but also on its dynamics. The purpose of this study was to determine the temporal influence of adding fly ash to a problem soil on select soil physical parameters. Fly ash:soil mixtures were analyzed at four time intervals: 185 Proceedings of the 20th Annual British Columbia Mine Reclamation Symposium  in Kamloops, BC, 1996. The Technical and Research Committee on Reclamation 1) upon mixing, 2) after one summer, 3) after one summer and a winter, 4) after the second summer following a summer and a winter. Soil parameters used to evaluate a soil's potential to form clods were bulk density and dry aggregate size distribution. METHODS Fly Ash and Soil Fresh fly ash was collected directly from the electrostatic precipitators (emission control devices that remove particulates discharged to the atmosphere) at the Sundance coal-fired power plant located approximately 100 km west of Edmonton, Alberta. The problem soil used was the surface horizon (15-cm depth) from a reconstructed soil on the Highvale coal mine. The soil was the A horizon of a Dark Gray Luvisol with a minor contribution from a Gray Solonetz with poor tilth prior to mining. The soil was a clay loam with a sodium adsorption ratio of 6.4 and organic carbon content of 1.6%. The fly ash was silt loam in texture. The physical and chemical properties of the soil and fly ash are summarized in Tables 1 and 2. Fly Ash:Soil Mixtures The soil sample was air dried, then crumbled to pass a 9.5-mm sieve. Fresh fly ash was mixed with the clay loam soil in rates of 0, 6.25, 12.5, 25, 50, 75 and 100% fly ash on a volume basis. The mixtures were gently poured into plastic pots (ID = 20 cm, volume 553 cm3) to a depth equal to that of cultivation in the field (15 cm). The bulk densities of the fly ash:soil mixtures were approximately 1.09 Mg m"3. Holes were punched into the bottom of the pots to allow drainage. The pots were buried on the mine on July 8, 1993 at the same site where the soil was collected so that the mixture within the pot was level with the surrounding soil. The 63 pots were spaced at least 30 cm apart and were arranged in a completely randomized design with seven rows and nine columns. Three replicates of each mixture (3x7) were removed from the ground at different sampling times (October 20, 1993; April 15, 1994; and October 20, 1994), covered with lids, then stored at room temperature for subsequent analysis. Any weeds that grew in the pots were removed manually during the course of the experiment. 186 Proceedings of the 20th Annual British Columbia Mine Reclamation Symposium  in Kamloops, BC, 1996. The Technical and Research Committee on Reclamation MEASUREMENTS Bulk Density A Uhland core sampler (7.6 cm in diameter x 7.6 cm high) was inserted into the center of each pot until the top of the sleeve was level with the soil surface. Bulk densities were determined by using the oven-dried mass of soil collected in the core and the volume (344.8 cm3) of the core. The bulk densities that were determined for the 0-7.6 cm depth increment were assumed to be the same for the 7.6-15 cm depth. The diameter of the pot restricted the removal of a sample from the lower depth. The samples were saved and later used in the dry sieving analysis. Dry Sieving The soil samples saved from the bulk density determinations were air dried, then placed on a nest of sieves and gently shaken by hand to allow the aggregates to pass through the sieves. The nest was composed of the following sieve sizes: 19.0, 12.5, 9.5, 8.0, 6.3, 4.0, 2.0, 0.500, 0.250, 0.125 mm, and a pan to catch aggregates <0.125 mm. The aggregates that remained on each sieve were weighed. Aggregate size distribution (% of sample) per sieve size and mean weight diameters (MWD) were then calculated as (Kemper and Rosenau 1986):  where n = number of sieves, xi = mean diameter and wi = proportion of the total sample weight. The aggregates collected on sieves were combined into three groups and their weights (% of sample) added together: Group A for aggregates <0.250 mm, Group B for aggregates 0.500 to 4.0 mm, inclusive, and Group C for aggregates 6.3 to 19.0 mm, inclusive. Statistical Analyses Soil water content might change with storing the pots for different lengths of time after field removal. Therefore, since bulk density is a function of soil water, this parameter was compared among rates within each sampling time separately using t-Tests (p<0.05) (SAS Institute Inc. 1992). Mean weight diameter and grouped dry sieving data are assumed to not be influenced by storage time and were analyzed using t-Tests within each sampling time (p<0.05) (SAS Institute Inc. 1992). 187 Proceedings of the 20th Annual British Columbia Mine Reclamation Symposium  in Kamloops, BC, 1996. The Technical and Research Committee on Reclamation RESULTS AND DISCUSSION Climatic Conditions From July (date of pot burial) to October 31, 1993 there was 171 mm of rain, with one major single day rainfall event on July 21, 1993 (74 mm). Over the winter, 42.7 cm of snow accumulated in the field (measured March 3, 1994). During summer 1994 (April 1 to October 31), at least 290 mm of rain fell on the site with the largest single precipitation amount on August 17 (31 mm); all other events were < 20 mm. Due to a mechanical problem with the meteorological station used, not all precipitation was measured. Bulk Density Bulk density generally increased as percent fly ash increased to 12.5 or 25%, then decreased with additional fly ash (Table 3) with the most dramatic increases occurring with as little as 6.25% fly ash (except at Time 3 and 4). Bulk density generally decreased with time. Bulk densities measured at Time 4 were substantially lower than those at Time 2 (both fall measurements) and these effects were consistent across treatments. Over-winter (Time 2 to Time 3) changes in bulk density were greatest for 6.25, 12.5 and 100% fly ash (-0.18, -0.15 and -0.10 Mg m-3, respectively). Other mixtures had little or no change in bulk density over this time period. In addition, growing season changes in bulk density were much greater in 1994 compared to 1993 although the elapsed period was almost twice as long in 1994. Mean Weight Diameter (MWD) On a given sampling date MWD generally declined as percent fly ash increased (except 100% fly ash; Figure 1). For the 0, 6.25, 12.5 and 25% fly ash treatments, MWD increased over time. In the 50 and 75% fly ash treatments, MWDs increased by Time 3 then declined by Time 4. Adding fly ash to the soil did not change MWD for the O to 25% fly ash treatments until Time 2. Interesting changes in MWD occurred once the mixtures were exposed to the environment.  After the first summer, large clods formed in the 100% fly ash treatment, but treatments with soil had dramatically reduced clod formation.  The MWDs for the 0 to 12.5% fly ash treatments, however, were still higher at Time 2 than they were at Time 1. The pattern of increasing MWD over time was consistent for the 6.25, 12.5 and 25% fly ash treatments (Figure 1). The average increase in MWD for these three rates of fly ash addition between Time 1 and 2 was 174%, Time 2 and 3 was 139% and Time 3 and 4 was 151%, thus illustrating the influence of freeze- 188 Proceedings of the 20th Annual British Columbia Mine Reclamation Symposium  in Kamloops, BC, 1996. The Technical and Research Committee on Reclamation thaw cycles (Time 2 to 3) on reducing aggregate size distribution. Over-winter increases in MWD were consistent for all treatments except 100% fly ash, averaging 187% across the six treatments. Increases in MWD during the summer periods were similar in 1993 and 1994 for the 6.25 and 12.5% fly ash treatments. In contrast, the MWD of the 50% fly ash mixture did not change during these periods, but changed most during the over-winter period. The low MWD of 100% fly ash at Time 1 wais undoubtedly due to fly ash's powdery nature when dry. Its consistent decrease in MWD over Times 2, 3 and 4 was not evidenced in any other treatment (Figure 1). The consistent decreases in bulk density for all treatments between Times 2 and 4 were matched with consistent increases in MWD for the same period, with the exception of the 100% fly ash treatment. Decreases in both bulk density (Table 3) and MWD (Figure 1) between Times 2 and 4 occurred in this latter treatment. The soil's cloddy nature was evidenced by having the highest MWDs on all but one of the sampling dates (Figure 1). The lowest MWD was measured in the 75% fly ash treatment on the last three sampling times. The ideal aggregate size distribution to optimize seed-soil contact is 0.5-1 to 5-6 mm (Russell 1961). The 0 and 6.25% fly ash treatments had mean weight diameters that exceeded the ideal range for samples collected at Time 3 and 4. The 100% fly ash treatment was the only other treatment that formed aggregates greater than 6 mm (Time 2). If > 12.5% fly ash is added to this soil, mean weight diameter of aggregates would be reduced to within the optimal range by 9 months after being exposed to the environment (Time 3) and would remain so by 16 months (Time 4). Grouped Dry Sieving After the soil was crushed to pass a 9.5-mm sieve, it had the highest percentage of aggregates within the ideal range (Group B: 0.5 to 4 mm) for seedbed preparation (Russell 1961) as well as in Group C (6.3 to 19 mm) (Table 4). However, once the soil had been exposed to the environment, its percentage within Group B decreased and within Group C increased (Group C contains the large clods that lead to poor seed-soil contact). This trend of decreasing percent in Group B over time was generally characteristic of all fly ash:soil mixtures. The 12.5% fly ash treatment maintained the highest percentage of aggregates within Group B after being subjected to the environment although the differences among the 0 to 50% fly ash treatments were not significantly different. In general, as percent fly ash in the mixture increased, the percent of aggregates within Group C decreased (except 100% fly ash which formed large clods over time 189 Proceedings of the 20th Annual British Columbia Mine Reclamation Symposium  in Kamloops, BC, 1996. The Technical and Research Committee on Reclamation likely due to cementing properties; note the dramatic increase in Group C percent for 100% fly ash between Times 1 and 2). The Group C percent also increased markedly for the 0% fly ash treatment over time, a reflection of the cloddy nature of the soil. At least 25% fly ash had to be added before the increase in Group C percent over time was small. The 50% fly ash treatment was the most stable (unchanging proportions for a given group) over time. CONCLUSIONS Adding up to 25% fly ash increased bulk density; adding more decreased it. In general, bulk densities declined over time for most of the treatments to levels below 1.0 Mg m-3. Adding 12.5 or 25% fly ash to this soil produced the highest percentage of aggregates within the ideal range for seed-soil contact. MWD for the 6.25, 12.5 and 25% fly ash treatments decreased over-winter, demonstrating the influence of freeze-thaw cycles on these parameters, then these parameters increased slightly by the following autumn. When considering the physical changes that occurred in the fly ash:soil mixtures, adding 25% fly ash maintained a desirable level of aggregation, thus reducing this soil's cloddy nature. 190 Proceedings of the 20th Annual British Columbia Mine Reclamation Symposium  in Kamloops, BC, 1996. The Technical and Research Committee on Reclamation Table 1. Physical and chemical characteristics of the fly ash and soil prior to the experiment.  191Proceedings of the 20th Annual British Columbia Mine Reclamation Symposium  in Kamloops, BC, 1996. The Technical and Research Committee on Reclamation   192 Proceedings of the 20th Annual British Columbia Mine Reclamation Symposium  in Kamloops, BC, 1996. The Technical and Research Committee on Reclamation  193 Proceedings of the 20th Annual British Columbia Mine Reclamation Symposium  in Kamloops, BC, 1996. The Technical and Research Committee on Reclamation REFERENCES Adriano, D.C., A.L. Page, A.A. Elseewi, A.C. Chang and I. Straughan. 1980. Utilization and disposal of fly ash and other coal residues in terrestrial ecosystems: A review. Journal of Environmental Quality 9:333-344.  Agriculture Canada Expert Committee on Soil Survey (ACESS). 1987. The Canadian system of soil classification. 2nd ed. Agriculture Canada Publication 1646.  Brierley, J.A., G.M. Coen and B.D. Walker.  1992. The scope of problem soils in Alberta. Pp. 1-16 In 29th Annual Alberta Soil Science Workshop Proc., Lethbridge, AB. 19-20 Feb. 1992.  Carlson, C.L. and D.C. Adriano. 1993. Environmental impacts of coal combustion residues. Journal of Environmental Quality 22:227-247. Chang, A.C., LJ. Lund, A.L. Page and J.E. Warneke. 1977. Physical properties of fly ash-amended soils. Journal of Environmental Quality 6:267-270.  El-Mogazi, D., DJ. Lisk and L.H. Weinstein. 1988. A review of physical, chemical, and biological properties of fly ash and effects on agricultural ecosystems. Science of the Total Environment 74:1-37.  Hadas, A. and D. Wolf. 1983. Energy efficiency in tilling dry clod-forming soils. Soil Tillage Research 3:47-59.  Kemper, W.D. and R.C. Rosenau.  1986. Aggregate stability and size distribution. Pp. 425-442. In A. Klute (ed.) Methods of soil analysis. Part 1 Agronomy. 2nd ed. American Society of Agronomy and Soil Science Society of America, Madison, WI.  Malik, R.S., B.S. Jhorar and I.S. Dahiya. 1985. Influence of seedbed tilth on emergence and root and shoot growth of seedlings of some crops. Experimental Agriculture 21:59-65.  Page, A.L., A.A. Elseewi and LR. Straughan. 1979. Physical and chemical properties of fly ash from coal-fired power plants with reference to environmental impacts. Residue Review 71:83-120. Russell, E.W. 1961. Soil conditions and plant growth. 9th ed. Longman, NY.  SAS Institute Inc. 1992. SAS/STATS user's guide. 6.08th ed. SAS Institute Inc., Cary, NC.  Shiel, R.S., M.A. Adey and M. Lodder.  1988. The effect of successive wet/dry cycles on aggregate size distribution in a clay texture soil. Journal of Soil Science 39:71-80.  Watson, L.D. 1994. Effects of fly ash-induced textural changes on soil water retention and soil strength. M.Sc. Thesis. University of Alberta.  Wild, A. 1988. Russell's soil conditions and plant growth. 1 lth ed. Longman Group, London, England. 194 Proceedings of the 20th Annual British Columbia Mine Reclamation Symposium  in Kamloops, BC, 1996. The Technical and Research Committee on Reclamation   Figure 1 Mean weight diameter (mm) of aggregates among fly ash:soil mixtures sampled at four times. Means with the same letters within each sampling time are not significantly different (t-Test, p<0.05). Time 1 = July 8,1993; Date of mixing Time 2 = October 20,1993; 3.5 months after mixing Time 3 = April 15, 1994; 9 months after mixing Time 4 = October 20,1994; 15.5 months after mixing 195 


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