Leachability of trace elements from two stabilized low lime Indian fly ashes

Fly ash is a waste by-product obtained from the burning of coal by thermal power plants for generating electricity. When bulk quantities are involved, in order to arrest the fugitive dust, it is stored wet rather than dry. Fly ash contains trace concentrations of heavy metals and other substances in sufficient quantities to be able to leach out over a period of time. In this study an attempt was made to study the leachabilities of a few selected trace metals: Cd, Cu, Cr, Mn, Pb and Zn from two different types of class F fly ashes. Emphasis is also laid on developing an alternative in order to arrest the relative leachabilities of heavy metals after amending them with suitable additives. A standard laboratory leaching test for combustion residues has been employed to study the leachabilities of these trace elements as a function of liquid to solid ratio and pH. The leachability tests were conducted on powdered fly ash samples before and after amending them suitably with the matrices lime and gypsum; they were compacted to their respective proctor densities and cured for periods of 28 and 180 days. A marked reduction in the relative leachabilities of the trace elements was observed to be present at the end of 28 days. These relative leachability values further reduced marginally when tests were performed at the end of 180 days.     

Swelling behaviour of expansive soil treated with fly ash–GGBS based binder

In this article, the potential of a binder developed by admixing fly ash and ground granulated blast furnace slag (GGBS) to stabilise expansive soils is evaluated. Laboratory tests included determination of free swell index, swell potential and swelling pressure tests of the soil/binder mixtures at different mixing ratio. The test results showed decrease in the swelling behaviour of the soil with increase in binder content. The percent swell–time relationship was observed to fit the hyperbolic curves enabling us to predict the ultimate percent swell from few initial test results. Addition of 1% of lime to the binder showed further improvement in reducing swelling. A good linear relationship is established between percent oedometer swell and modified free swell index (MFSI) for soil/binder mixtures without lime but the same has not been observed in the presence of lime. The compressibility characteristics of the soil/binder mixtures reduced nominally with increase in binder content but in the presence of lime, the compressibility reduced significantly. Binder used in this study has been found to be effective and economic to stabilise expansive soils with lesser amount of chemical additives such as lime.     

Potassium Chloride Treatment to Control Alkali Induced Heave in Black Cotton Soil

The results presented in this paper shows that high concentrations of sodium hydroxide causes abnormal changes on the volume change behaviour of illite–smectite (interstratified mineral) soil due to mineralogical changes. The higher swell that occurs is shown in the form of a new second stage of swelling. Increase in negative charges on soil particles and mineralogical changes after interaction with soil, respectively, are responsible for the swelling in these two stages. However, potassium hydroxide does not induce such high swelling in soils. This is mainly due to the fixation of potassium ions. Hence an attempt has been made to control the swelling induced by sodium hydroxide by making used of potassium chloride as an additive. Potassium fixation which is not substantial at neutral pH is favoured at higher pH Addition of potassium chloride salt solution (as 2 and 5% solution) can reduce only the first stage of swelling by linking the unit layers of mineral by reducing development diffuse double layer near clay surface. Potassium chloride is unable to prevent the formation of mineralogical alteration due to soil alkali interaction and hence the swelling associated with mineralogical changes. X-ray diffraction studies have revealed that mineralogical changes leading to formation of zeolite by soil alkali interaction is not inhibited by potassium ions. Morphological changes studied by scanning electron microscope corroborate these observations. Also the compressibility of soil which is increased in alkali solution is reduced in the presence of potassium salts. This reduction is due to reduction in the first stage of swelling.     

Pozzolanic fly ash as a hydraulic barrier in land fills

The liner plays an important role in controlling migration of contaminants present in the leachate in waste containment systems such as land fills and impoundments. Although questions have been raised about the performance of clay liners, they are increasingly used singly or as double liners in disposal sites. Though the clay liners possess many advantages such as low permeability and large attenuative capacity, they also possess high shrinkage potential and hence can crack under unsaturated conditions causing instability and increase in leakage rates. Further, the permeability of the clay linear can increase due to clay–pollutant interaction. This study examines the potential of pozzolanic fly ash as a hydraulic barrier in land fill. The behaviour of three different types of fly ashes, showing a range of physical properties and chemical composition from three different sources are reported in the study. Geotechnical properties, needed to evaluate the use of fly ashes as barriers, such as shrinkage, compaction, permeability, consolidation and strength characteristics are reported. The results show that fly ashes possess low shrinkage and hence do not crack. Compacted fly ashes undergo very little volume changes. They also show that pozzolanic fly ashes develop good strength properties with time. Pozzolanic fly ashes containing sufficient lime develop strength even without addition of lime. Non-pozzolanic fly ashes do not develop strength even on addition of lime. Fly ashes generally consist of silt size particles and consequently possess high permeability. However, pozzolanic fly ashes with lime exhibit low permeability on curing because of the formation of gelatinous compounds which block the pores. Thus, pozzolanic fly ashes appear to be promising for construction of liners to contain alkaline leachate.     

Technical Note Reactive silica and strength of fly ashes

The use of fly ash in geotechnical engineering depends greatly on its pozzolanic reactivity. Though many factors influence the reactivity of fly ash it is well recognized that reactive silica and lime content play a major role. A new, accurate and reliable method for the determination of reactive silica content of fly ash has been established. The reactive silica content, obtained as acid soluble silica in about 2 to 3 N hydrochloric acid, is found to correlate well with unconfined compressive strength of fly ashes. The reactive silica content of fly ash is also important in the stabilization of soils using fly ash. © Rapid Science Ltd. 1998     

Interactions Between Soils and Laboratory Simulated Electrolyte Solution

To study the impact of salt water intrusion on two types of soils from west coast region of India were investigated in the laboratory. The key characteristics evaluated included Atterberg limits, compaction characteristics, hydraulic conductivity and chemical characteristics of selected soils. The sea at this coast receives effluent from different points and hence the characteristics change with time and locality. Therefore, to maintain uniform composition, 0.5 N sodium chloride solution (NaCl) was prepared in the laboratory and batch tests were used to determine the immediate effect on soils. Soil specimens were prepared by mixing the soils with 0.5 N NaCl in the increments of 0, 5, 10 and 20% by weight to vary the degree of contamination. Experimental results of soils mixed with 0.5 N NaCl showed that the maximum dry density increases and the optimum moisture content (OMC) decreases with increasing sodium chloride concentration. The study also revealed that the hydraulic conductivity of the soils tested increases with increase in sodium chloride concentration. The Atterberg limits of contaminated specimens show a remarkable change when compared with uncontaminated specimens.     

Effect of Pozzolanic Reactivity on Compressibility Characteristics of Stabilised Low Lime Fly Ashes

This paper presents the effects of addition of lime and lime along with gypsum on the compressibility behavior of two class F fly ashes. Since the fly ashes develop strength and exhibit lower compression, consolidation testing with conventional duration of load increment may not be appropriate. Hence, an attempt has been made to assess the minimum duration of load increment necessary to study the compressibility characteristics of such materials. Thus compressibility behavior of fly ashes with additives has been studied using conventional consolidation test with different durations of load increments varying from 30 min to 48 h. The results obtained indicated that 30 min of duration of load increment can be used to assess the compressibility behavior of such materials. The effect of lime which reduces the compression is seen to be maximum from the results obtained with the duration of load increment of 30 min but gradually reduces with higher durations of load increment. It has also been observed that the rate of decrease in the compressibility is maximum up to 2.5% lime and gradual thereafter. The compressibility of lime treated fly ashes further reduces when gypsum is incorporated, the optimum gypsum percentage being 2.5. This reduction in the compressibility of fly ashes enhanced by incorporating lime and gypsum makes them versatile in the construction of embankments and for structural fills, particularly reducing the time required in between laying of each lift.     

Geotechnical properties of stabilised Indian red earth

Locally available soils amended with sufficient bentonite are generally used for construction of liners for water and waste retention facilities. The amount of bentonite required to keep the hydraulic conductivity low varies with the nature of the local soil. Many studies have shown that bentonite content higher than 20% by weight is not usually required. This is also the case with Indian red earth containing predominantly quartz and kaolinite minerals. Incorporating bentonite, though keeps the hydraulic conductivity of soil low, increases the swelling and shrinkage potential; increases the loss of strength due to reduction in cohesion. This paper aims to improve the geotechnical properties of red earth and bentonite mixture with lime or cement. The studies reveal that the geotechnical properties of red earth with 20% by weight bentonite stabilised with 1% by weight of lime or cement are greatly enhanced, particularly after curing for 28 days. it has been shown that the early gain in strength is better with cement whereas its long-term strength gain is better with lime.     

Gypsum Induced Strength Behaviour of Fly Ash-Lime Stabilized Expansive Soil

Physical and engineering properties of soil are improved with various binders and binder combinations. Fly ash and lime are commonly used to improve the properties of expansive soils. An attempt has been made, in this paper, to examine the role of gypsum on the physical and strength behaviour of fly ash-lime stabilized soil. The change in strength behaviour is studied at different curing periods up to 90 days, and the mechanism is elucidated through pH, mineralogical, microstructural and chemical composition study. The strength of soil-fly ash mixture has improved marginally with the addition of lime up to 4 % lime and with curing period for 28 day. Significant increase in strength has been observed with 6 % lime and enhanced significantly after curing for 90 days. The variations in the strength of soil with curing period is due to cation exchange and flocculation initially, and binding of particles with cementitious compounds formed after curing. With addition of 1 % gypsum to soil-fly ash-lime, the strength gain is accelerated as seen at 14 day curing. The accelerated strength early is due to formation of compacted structure with growth of ettringite needles within voids. However, strength at curing for 28 day has been declined due to annoyance of clay matrix with the increase in size of ettringite needle; and again increased after curing for 90 days. The rearrangement of clay matrix and suppression of sulphate effects with formation of cementitious compounds are observed and found to be the main responsible factors for strength recovered.