Time Dependent Strength and Stiffness of PCC Bottom Ash-Bentonite Mixtures

Utilization of bottom ash from burning of pulverized coal in construction-related applications has received some attention within the last decade. Its use in geotechnical engineering applications is still very limited, however. Within the last few years several studies have been completed to evaluate strength, stiffness, and durability properties of pulverized coal combustion (PCC) bottom ash mixed with various admixtures. Studies have shown that the physical properties of bottom ash obtained from burning of pulverized coal are similar to that of natural sand with particle sizes ranging from fine gravel to fine sand and low percentages of silt and clay sized particles. However, unlike sand, chemical composition of bottom ash results in change of strength and stiffness characteristics of the bottom ash-admixture mixtures with time. In this study, change in strength and stiffness characteristics of Illinois PCC bottom ash and bentonite mixtures with time are evaluated. A series of unconfined compression tests on bottom ash-bentonite mixtures at various curing ages was performed in the laboratory. Results presented show that strength and stiffness of bottom ash-bentonite mixtures changed significantly with time.     

Utilization of Illinois PCC Dry Bottom Ash for Compacted Landfill Barriers

Compacted soil barriers are one of the most important components of municipal waste landfills. The material used to construct a landfill liner and/or cap must prevent the flow of fluids through them. Soils with low values of permeability (such as compacted clays) are often used to construct landfill barriers. Natural sands and other cohesionless materials are used to construct hydraulic barriers by adding admixtures to modify their properties. Several studies have been conducted that dealt with determining geotechnical engineering properties of sand-bentonite mixtures. Pulverized coal combustion (PCC) dry bottom ash is a coal combustion by-product of burning coal to produce electricity. Because of the increasing costs associated with the disposal of bottom ash and the environmental regulations in place, there is a need to develop alternate methods for profitable and environmentally safe uses of this waste material. Most scientists and researchers have concluded that bottom ash has geotechnical characteristics similar to those of sands. However, information on the use of bottom ash, with or without admixtures, in the construction of landfill barriers is limited. Most of the available literature on the engineering properties of bottom ash deals with its use as a fill material. The physical and chemical characteristics of bottom ash depend on several factors including type of coal used and type of boiler and collection system. This paper presents the results of an experimental study conducted to determine the possible use of Illinois PCC dry bottom ash amended with bentonite to construct landfill barriers. Test results presented show that the average value of hydraulic conductivity of Illinois PCC dry bottom ash with 15% bentonite content is close to the acceptable value required for its use as hydraulic barrier. Therefore, it was concluded that Illinois PCC dry bottom ash, modified with 15% or higher bentonite content, is likely to provide adequate hydraulic conductivity for its use to construct landfill barriers.     

Effect of Bentonite on Compacted Clay Landfill Barriers

Compacted clay barriers (liner and cap) are one of the most important components of municipal waste landfills. On-site soils are generally used to construct the clay barriers as long as they can be compacted to standard specifications, including hydraulic conductivity. Wherever the available on-site soils are not suitable to be used for constructing clay barriers, soils amended with bentonite are commonly used. This article presents the results of an experimental study conducted on compacted clay-bentonite mixtures to develop data on the effects of bentonite on engineering properties of compacted clay-bentonite mixtures. Clay-bentonite mixtures with bentonite contents of up to 7% were tested to determine consistency using Atterberg limits, moisture-density relationships using standard Proctor compaction tests, shear strength using unconfined compression tests, hydraulic conductivity using triaxial flexible-wall hydraulic conductivity tests, and consolidation properties using one-dimensional consolidation tests. Unconfined compression tests were also performed with 11% bentonite content. The laboratory test results indicated that liquid limit, plastic limit, and plasticity index increased linearly with increased amount of bentonite. The addition of bentonite resulted in decreased maximum dry unit weight but the optimum moisture content increased slightly. Unconfined compression strength of compacted clay-bentonite mixtures increased linearly with an increase in the amount of bentonite. Hydraulic conductivity of compacted clay-bentonite mixtures decreased nonlinearly with increased amount of bentonite, but a linear relationship was observed between logarithm of hydraulic conductivity and bentonite content. The compression index increased slightly from 0 to 3% bentonite content but increased nonlinearly beyond 3% bentonite content, whereas the swelling index of clay-bentonite mixtures has been observed to increase approximately linearly with increase in the amount of bentonite.     

粉煤灰改良砂姜黑土对麦田生态因子及重金属残留的影响

采用盆栽试验,研究了粉煤灰改良砂姜黑土对麦田生态因子及Cd、Cr、Pb、Hg和As残留的影响.结果表明,粉煤灰改良砂姜黑土可以降低土壤容重和土壤比重,减少土壤粘粒含量,增加土壤孔隙度和土壤渗透系数,提高耕层地温,促进土壤微生物活动和养分转化;土壤含水量高时具有散湿作用,土壤含水量低于10%后,具有保墒作用;在粉煤灰用量为6×10⁴～18×10⁴kg·hm^-2的条件下,土壤及小麦子粒中重金属元素的积累量远低于国际污染标准值,即在此范围内,施用粉煤灰改良砂姜黑土是安全可靠的.     

A determination of protective microhabitats for bacteria introduced into soil

Abstract Survival studies with rhizobia introduced into loamy sand showed that a kaolinite amendment of the soil improved the survival of Rhizobium , and that bentonite had a very strong positive effect on rhizobial survival. The survival level was significantly higher in soil amended with 10% than with 5% bentonite. The amount of water present in the bentonite amended soil had a significant influence on rhizobial survival; in drier soil, survival levels were highest. For the loamy sand, the loamy sand amended with 5 and 10% bentonite or with 10% kaolinite, the number of rhizobial cells surviving on day 57 after introducing 2.5–5.0×10⁷ cells g⁻¹ dry soil could be described using the distribution of pores from three size classes in a mathematical relationship. Pores with necks < 3 μm and between 3 and 6 μm positively affected the survival of introduced rhizobia whereas pores with necks > 6 μm had a negative effect.     

Chemical Compatibility of Lime Stabilized Indian Red Earth as Liner Material

Geotechnical liners are widely used to contain leachate generated within landfills and minimize the risk of sub-surface and underground water contamination. In this study, an attempt has been made to utilize locally available soil red earth as liner material. The collected red earth contains mostly quartz and kaolinitic minerals. Studies have shown that bentonite content higher than 20% by weight is not usually required. This study aims to assess the red earth with 20% by weight of bentonite as liner material. Further, the studies are being carried out to improve the amended material by stabilizing the mixture with 1% by weight of lime. The relative merits of these materials under different physico-chemical environments are studied. The assessment of the liner material is based on their basic and geotechnical properties. The studies reveal that the geotechnical properties of red earth with 20% by weight bentonite stabilized with 1% by weight of lime enhanced, particularly after curing for sufficient period. The pore fluids such as HCl and CCl ₄ increased volume change. The hydraulic conductivity of soils, which increased on treating with lime initially, decreased with curing. However, the hydraulic conductivity of stabilized soil increased in the presence of HCl and CCl ₄ . The strength of stabilized soil is affected with the fluids NaCl and HCl solutions.     

Properties of Fly Ash as Hydraulic Barrier

This study examines the suitability of pozzolanic fly ash as a hydraulic barrier and the use of bentonite to enhance geotechnical properties of fly ash. The behavior of fly ash is studied not only with water but also with different pore fluids, such as acid, alkali, salts, and neutral organic fluid to assess its chemical compatibility. While some geotechnical properties of fly ash meet the requirements of liner material, the disadvantage of using of fly ash alone is that it has a low cation exchange capacity and high hydraulic conductivity. The compressibility of fly ash reduces with alkaline solution but increases with acidic solutions. While alkaline or neutral inorganic solutions do not affect the hydraulic conductivity of fly ash, the addition of dilute acid increases the hydraulic conductivity. Addition of bentonite improves the geotechnical properties of fly ash such as cation exchange capacity, shrinkage and volume change behavior, etc. Fly ash-bentonite mixtures possess low shrinkage and hence do not crack. Compacted fly ash-bentonite mixtures undergo very little volume changes under various stress conditions. The hydraulic conductivity of fly ash is reduced after amendment with bentonite. Though the unconfined compressive strength of the mixture is lower than that of fly ash alone, the fly ash-bentonite mixture still possesses good strength. The compressibility of fly ash bentonite mixtures are lower with different pore fluids studied than with water. The hydraulic conductivity of fly ash-bentonite mixtures are slightly higher in different pore fluids studied than with water.     

The Influence of Hydraulic Gradient and Rate of Erosion on Hydraulic Conductivity of Sand-Bentonite Mixtures

The use of sand-bentonite mixtures as liner materials for waste disposal is very common. In the laboratory, this study investigated hydraulic conductivities of such mixtures at different hydraulic pressure (hydraulic gradient), dry unit weights, and bentonite contents. The bentonite content and the dry unit weight of the samples were both important factors, significantly affecting the hydraulic conductivity of the liner material. A bentonite content of 5% was found to be sufficient in reaching a hydraulic conductivity under 10^?9 m/s, when the liner material was compacted under near optimum moisture content. Nevertheless, hydraulic conductivity was found to increase with hydraulic pressures, especially for the 5% bentonite mixtures subjected to pressure above 40 kPa, suggesting some degree of internal erosion (washing out of particles).

Therefore, this paper discuses the influence of internal erosion of the mixtures under a given hydraulic gradient, on the final value of k. The internal erosion of the tested mixtures was found to be influenced mainly by porosity, which can be reduced by properly selecting the sand particle size distribution and the bentonite percentage. Furthermore, this study proposed an empirical expression to predict the risk of internal erosion in the sand-bentonite mixtures, and therefore of k being higher than planned. This expression can be used for designing bentonite content and compaction to achieve very low permeability.     

Use of bottom ash of waste coal as an effective microbial carrier

An experiment was done to determine the efficacy of waste bottom ash as an effective microbial carrier. Bottom ash found to be a suitable microbial carrier. The average of viable cells of Paenibacillus polymyxa GS01 (as a test biocontrol agent) in bottom ash samples was about 10(8) cfu/10 ± 2 mg. The surface of bottom ash coated with 5% PVA w/v was most effective for improvement of cell viability. TSB medium containing 50 mg/L of MnSO(4) · H(2)O was the best for spore production of P. polymyxa GS01. Thus waste bottom ash coating with 5% PVA is likely to be suitable for use as a microbial carrier.     

Organic emissions from petroleum‐contaminated soil fixed in concrete

This study investigated a solidification treatment process for soils that are contaminated with hydrocarbons at levels of 0.5 and 3.0% by weight of benzene. The contaminated soils were bound in a concrete matrix and the migration of organics from the concrete to air was evaluated. If the hydrocarbon emissions are sufficiently attenuated, the concrete containing such contaminated soil can be used for exterior construction applications.

The experimental specimens consisted of concrete mixtures in which 40% of the sand was replaced with the contaminated soil. The mixtures’ ratio of cement, aggregate, sand, and water is 1:1.5:1.5:0.5 by weight. The study included specimens with and without class C fly ash replacing 10% of the cement. In addition, two unfixed control specimens were prepared for each contamination level. The concrete‐soil mixtures were placed in sealed jars and air was passed through the head space of the jars and then through carbon adsorption tubes for measurement of contaminant flux from the surface of the specimens. Measurements were made during and after concrete curing. The results were fitted to a Fickian diffusion model to estimate effective diffusivity in the concrete‐soil specimens.

The test results showed that the effective diffusivity of the contaminant within the concrete was reduced by three to five orders of magnitude over the molecular diffusivities in unfixed contaminated soil used as control. It was observed that the presence of fly ash in the concrete affects the hydrocarbon release and causes an additional decrease in effective diffusivity of about one order of magnitude. Contaminant emissions during the curing phase were found to exceed rates predicted by the Fickian model. This is apparently due to the water used in the concrete. Total emissions, however, never exceeded values emitted from the unfixed controls. This study indicates that fixation of low hydrocarbon levels within concrete is a technically viable and safe technology for recycling petroleum‐contaminated soil.     

Fly ash effect on improving soil properties and rice productivity in Korean paddy soils

Paddy soils in Korea generally require the addition of Si to enhance rice productivity. Coal combustion fly ash, which has a high available Si content and alkaline pH, was selected as a potential source of Si in this study. Two field experiments were carried out to evaluate rice (Oryza sativa) productivity in silt loam and loamy sand soils to which 0, 40, 80, and 120 Mg ha(-1) of fly ash were added with 2 Mg ha(-1) Si as a control. Fly ash increased the soil pH and available Si and P contents of both soils. The amount of available B increased to a maximum of 2.57 mg kg(-1), and the B content of the rice plants increased to a maximum of 52-53 mg kg(-1) following the addition of 120 Mg ha(-1) fly ash. The rice plants did not show toxicity effects. The highest rice yields were achieved following the addition of around 90 Mg ha(-1) fly ash. The application of fly ash increased Si, P and K uptake by the rice plants, but did not result in an excessive uptake of heavy metals in the submerged paddy soil. In conclusion, fly ash could be a good supplement to other inorganic soil amendments to improve the nutrient balance in paddy soils.     

Fly ash as a soil ameliorant for improving crop production--a review

Fly ash, a resultant of combustion of coal at high temperature, has been regarded as a problematic solid waste all over the world. Many possible beneficial applications of fly ash are being evaluated to minimize waste, decrease cost of disposal and provide value-added products. The conventional disposal methods for fly ash lead to degradation of arable land and contamination of the ground water. However fly ash is a useful ameliorant that may improve the physical, chemical and biological properties of problem soils and is a source of readily available plant macro and micronutrients. In conjunction with organic manure and microbial inoculants, fly ash can enhance plant biomass production from degraded soils. Detailed studies on the nature and composition of fly ash, conducted during the latter half of the 20th century have helped in repeatedly confirming the various useful applications of this hitherto neglected industrial waste. The purpose of this paper is to review the available information on various attributes of fly ash and explore the possibility of exploiting them for agronomic advantage.     

Use of Bottom Ash of Waste Coal as an Effective Microbial Carrier

An experiment was done to determine the efficacy of waste bottom ash as an effective microbial carrier. Bottom ash found to be a suitable microbial carrier. The average of viable cells of Paenibacillus polymyxa GS01 (as a test biocontrol agent) in bottom ash samples was about 10⁸ cfu/10±2 mg. The surface of bottom ash coated with 5% PVA w/v was most effective for improvement of cell viability. TSB medium containing 50 mg/L of MnSO₄·H₂O was the best for spore production of P. polymyxa GS01. Thus waste bottom ash coating with 5% PVA is likely to be suitable for use as a microbial carrier.     

Reuse options for coal fired power plant bottom ash and fly ash

Reuse options for coal fly ash and coal bottom ash are reviewed in this paper. Although, significant quantities of coal fly ash and coal bottom ash are produced worldwide every year, less than 30 % of coal ash produced is reused. Coal ash is mainly reused in civil engineering applications such as road construction, embankments, construction materials, geo-polymer applications and in cement production. Other potential reuse options for coal ash include applications such as glass ceramics, water and wastewater treatment, agriculture as well as for making high value products (e.g. telescope mirrors, break-liners, fire proof products etc.). Considering that only a small fraction of coal ash is reused, other reuse options for commercial applications need to be explored.     

Encapsulation of R. planticola Rs-2 from alginate-starch-bentonite and its controlled release and swelling behavior under simulated soil conditions

The plant growth-promoting bacteria (PGPR) Raoultella planticola Rs-2 was encapsulated with the various blends of alginate, starch, and bentonite for development of controlled-release formulations. The stability and release characteristics of these different capsule formulations were evaluated. The entrapment efficiency of Rs-2 in the beads (capsules) was more than 99%. The diameter of dry beads ranged from 0.98 to 1.41 mm. The bacteria release efficiency, swelling ratio, and biodegradability of the different bead formulations were enhanced by increasing the starch or alginate contents, but were impeded by higher bentonite content. The release kinetics of viable cells from capsules and the swelling ratio of capsules were studied in simulated soil media of varying temperature, moisture, pH, and salt content. The release of loaded Rs-2 cells and swelling of capsules are greatly affected by moisture, temperature, pH and salt content of the release medium. The release of viable Rs-2 cells from capsules was positively associated with the swelling properties of the capsules. The release of Rs-2 cells occurred through a Case II diffusion mechanism. In summary, this work indicates that alginate-starch-bentonite blends are a viable option for the development of efficient controlled-release formulations of Rs-2 biofertilizer, and which could have a promising application in natural field conditions.     

Co-firing coal with rice husk and bamboo and the impact on particulate matters and associated polycyclic aromatic hydrocarbon emissions

The potential of co-firing rice husk and bamboo with coal was studied in a bench-scale pulverized fuel combustion reactor. Experimental parameters including biomass blending ratio in the fuel mixture, biomass grinding size, excess air ratio and relative moisture content in the biomass were investigated. Particulate Matters in the forms of PM(10), PM(2.1), ultra fine particles as well as the associated Polycyclic Aromatic Hydrocarbons (PAHs) emissions were evaluated. An operation range between 10% and 30% of biomass to coal ratio was found to be the optimum range in terms of minimum pollutant emissions per unit energy output. Co-combustion of coal with biomass seemed to have the effect of moving the fly-ash in PM(2.1) to a larger size range, but increasing the number counts of the ultra fine particles. It was noted that the much higher volatile matter content in the biomass fuels has played a key role in improving the combustion performance in the system. However, slagging, fouling and formation of clinker could be the issues requiring attention when using biomass co-combustion in conventional boilers.     

The uptake of cadmium,lead and selenium by barley and cabbage grown on soils amended with refuse incinerator fly ash

Summary Barley and cabbage plants grown in the greenhouse on soils amended with refuse incinerator fly ash contained significantly elevated levels of Cd, Pb and Se, with Cd uptake being greatest in both plant species. Cabbage grown on 20% ash amended soil contained 146 times more Cd than controls. Cadmium and Se appeared to be less available in a successive barley crop after overwintering the pots of soil outside, but elemental concentrations still remained elevated. Comparisons with data from other studies indicated that Cd availability was greater from refuse fly ash than from sewage sludge.     

Effects of different organic waste amendments on soil microaggregates stability and molecular sizes of humic substances

Three soils which had been amended for several years with pig slurry, cattle slurry, and sewage sludge were dry-sieved to obtain microaggregates in the size range of 250–125, 125–50, and <50 μm. With amendments, aggregate size distribution of whole soils was shifted to larger sizes, especially for the most fragile soil, whereas percent content of microaggregates decreased except for the lower size aggregates of the fragile soil. Particle size distribution of microaggregates revealed an increase in percent sand and a reduction of percent silt and clay in the <50 μg size fraction for all soils. These results showed the aggregation effect induced by the organic waste additions. Aggregate stability of microaggregates revealed significant correlation with humic substances content (humic acids alone and humic plus fulvic acids) and non significant with total organic matter substantiating the belief that humic substances are the predominant binding agents in this aggregation range. Molecular weight distribution of humic acids extracted from microaggregates of unamended soils demonstrated that the lower the soil aggregate size distribution, the larger the contribution of the high molecular weight fraction. All microaggregates from amended soils showed a progressive increase of the high molecular weight humic acids with decreasing size, reaching a maximum in the <50 μm fraction. In this aggregate size a parallel enhancement of the aggregate stability was also evident. It is concluded that a close relationship exists between aggregate stability and high molecular weight humic substances. Additions to soils of organic material containing high molecular weight constituents would represent a useful management practice to improve aggregate stability.     

Phytotoxic doses of boron in contrasting soils depend on soil water content

Boron (B) affects plant growth in soil at B doses (mg added B kg^-1 soil) that appear in the range of natural background B concentrations. A study was set up to determine B bioavailability by testing B toxicity to plant as affected by soil properties and ageing after soil dosing. Nineteen soils (pH 4.4?C7.8) and 3 synthetic soils (sand-peat mixtures) were amended with 7 doses of H₃BO₃. Barley root elongation was determined immediately after B amendment and after 1 and 5 months ageing. Soil solution B concentrations increased linearly with added B concentrations with almost no detectable adsorption. In contrast, the ratio of aqua regia soluble B/soil solution B in unamended soils (no B added) was 10?C25 times higher than in B amended soils at similar aqua regia soluble B concentrations illustrating a much lower B availability in unamended soils. Soil solution B concentrations did not decrease by ageing. The toxic B doses or soil B concentrations that decreased barley root growth by 10% (EC10 values) varied about tenfold (respectively 3?C27 mg added B kg^-1 and 5?C52 mg B kg^-1) among soils. Corresponding thresholds in soil solution varied less than fourfold (16?C59 mg B l^-1). Soil ageing for 5 months did not significantly change EC10 and EC50 values, expressed either as total soil B or as soil solution B, unless in 1 soil. Variability in EC10 and EC50 values was explained by various soil properties (soil moisture content, background B, %clay, cation exchange capacity), but covariance of these properties with the soil moisture content suggest that B dilution is the critical factor explaining B toxicity. It is concluded that effects of B amendments do not decrease by ageing and that soil solution B or B doses corrected for soil moisture content may be used as an index for B toxicity across different soils.     

Sedimentary Residual Soil as a Waste Containment Barrier Material

Compacted soil liners are widely used as a waste containment barrier to control or restrict the migration of contaminant/leachate from the landfill into the environment because of their low hydraulic conductivity, attenuation capacity, resistance to damage or puncture, and cost effectiveness. Compacted soil liners are usually composed of natural inorganic clays or clayey soils. If natural clayey soils are not available, kaolinite or commercially available high swelling clay (bentonite) can be mixed with local soils or sand. This study examines the potential of a sedimentary residual soil as a waste containment barrier in landfills. The laboratory experiments conducted were: grain size distribution, Atterberg limits, swelling tests, compaction, volumetric shrinkage strain, unconfined compression, hydraulic conductivity and cation exchange capacity. The experimental results were compared with those recommended by various researchers for evaluation of its suitability. Test results showed that the soil compacted with modified Proctor compaction effort possesses low hydraulic conductivity (≤1 × 10^?7 cm/s) and adequate strength. In addition, compacted sedimentary residual soil exhibited little volumetric shrinkage strain of below 4% at this compaction effort. Thus, the sedimentary residual soil could be effectively used for the construction of a waste containment barrier in landfills.