Micro-Fabric Transformations of Ball Clay in Alkaline Environment

Based on the strong evidence of case histories, this study focused on mineralogical and morphological changes of an artificial kaolinitic soil -Ball clay, when exposed to different concentrations of sodium hydroxide (0.1N, 1N, 4N, and 8N) under different curing periods (7, 28 and 100 days). Sediment volume tests are conducted on Ball clay with all combinations and results are analyzed with the help of analytical techniques. XRD and SEM studies are analyzed to understand the micro-level changes of alkali contaminated Ball clay. Mineralogical and morphological transmutations of Ball clay are investigated for 7, 28, and 100 days curing period. Results revealed new mineral formations like Sodalite under 4N and 8N concentrations of NaOH with 100 days interactions are well observed. The morphological transformation from needle shape to pellet shape is clear evidence of the rate of dissolution and precipitation of minerals under 100 days curing periods.


INTRODUCTION
Soil pollution occurs from a broad range of sources which includes over usage of chemicals and accidental spillovers (Ibrahim 2004). All types of soil have direct or indirect, short-term or long-term effects on the soil behavior, which leads to partial or complete transport of contaminants into the ground. The Scientific-Research Institute of Foundations and Underground Structures has investigated the physical and chemical reactions which take place between clayey minerals and acids and alkalis (Sokolovich 1995). In recent years, the detrimental failures of industrial structures are mostly associated with contamination of foundation soils with either acids or alkalis, and alkali contamination has a significant effect on the soil (Sokolovich & Troitskii 1976, Sibley & Vadgama 1986, Rao & Rao 1994, Assa'ad 1998. Sokolovich & Troitskii (1976) reported the heaving of sand due to leakage of NaOH, NH4Cl, and soda solutions into the subsoil for five year period of the Krasnopresensk sugar refinery in Moscow. Chunikhin et al. (1988) observed swelling of alkali-saturated soil depends on the gradation, mineral composition, and the amount of clayey fraction contained in the ground. Sivapullaiah et al. (2004) presented the structural distortion of buildings in an alumina extraction plant, Karnataka, India, caused by prolonged contamination with alkali, which led to the heaving of subsoil.
Interaction of clay minerals under highly alkaline conditions results in dissolution and precipitation (Elert et al. 2005, Taubald et al. 2000, Bauer & Velde 1999, Bauer & Berger 1998, Cuadros & Linares 1996. From the existing literature, it is well-known that transformations of clay minerals depend on the type of the reacting mineral, its chemical composition, and the concentration of the alkaline solutions. Based on the literature on mineral transformations, it was concluded that under high alkaline conditions, dissolution of primary and secondary minerals results in the formation of new minerals which belongs to the family of zeolites Studies revealed, natural expansive and non-expansive soils under different alkali contaminations significantly affect the engineering properties of soil. Hence in this study, artificially available Ball clay with the same mineral composition with different proportions has been considered. Different concentrations (0.1N, 1N, 4N & 8N) of sodium hydroxide for alternate curing periods (7,28 & 100days).

Ball Clay
Ball clay known as 'plastic clay' originated from sedimentary rocks becomes highly plastic under moist conditions. Ball clay exhibits highly variable compositions (Table 1) with kaolinite as a predominant mineral along with quartz and mica. A typical Ball clay powder is light grey or cream color in appearance and is extensively used in the field of ceramics. Commercially available Ball clay has been purchased from Godavari Mines and Minerals, Visakhapatnam, Andhra Vol. 20, No. 4, 2021 • Nature Environment and Pollution Technology Pradesh, India. Lumps forms of Ball clay are oven-dried for moisture removal and pulverized in the laboratory. Finally, the soil was allowed to pass through no. 40 (425 μ) sieve before making it useful.

Preparation of Sodium Hydroxide (NaOH) Solutions
Sodium hydroxide, also known as Lye and caustic soda and is commercially available in pellets form, is purchased from Fisher scientific and the required concentration of solutions using distilled water is prepared. Being deliquescent, NaOH readily absorbs moisture from the air, and hence it is stored in an airtight container. The author prepared Sodium hydroxide solutions of 0.1N, 1N, 4N, and 8N by dissolving 4 g, 40 g, 160 g, and 320 g of analytical grade sodium hydroxide pellets with distilled water in glass bottles. As the solution liberates heat, it is placed in a water bath to maintain an average room temperature. The bottles are closed to avoid evaporation. Later the solution is transferred into a volumetric flask and makes up to 1 L with distilled water (ASTM E 200-08 2008).

Sediment Volume Test
The author followed the procedure established by Rao & Sridharan (1985) for determining the sediment volume test (Fig. 1) by considering the limitations of the test method proposed by Holtz and Gibbs. The soil samples which are more vulnerable to alkali attack with a more specific surface area were sieved through 75 μm. 10 g of Ball clay with 40 mL of distilled water with the required alkali concentration solution in 100mL graduated cylinders is mixed initially. The suspension is then thoroughly and repeatedly stirred. The sample is allowed to equilibrate for 24 hours to ensure thorough wetting. Suspensions are made up to 100mL with distilled water and closed with caps. The graduated cylinders are left undisturbed for 24 hours duration for significant equilibrium and volume in terms of cc.gm -1 is noted respectively. The supernatant from every cylinder is drained after the completion of sufficient curing periods (7, 28, and 100 days). All wet samples are collected in containers and allowed to dry until the moisture gets vanished. The soil samples were ground into a fine powder, moisture-free to make them fit for X-Ray diffraction (XRD) and Scanning Electron Microscopy (SEM) analysis.
Samples are analyzed from 6 o 2θ to 70 o 2θ/, at a step size of 0.017, using copper k alpha radiation, X-Ray Diffractometer, PANalytica, and Netherland. X-Ray diffraction patterns are obtained for the chemical mixed soil

Sediment volume test
The author followed the procedure established by Rao & Sridharan (1985) for determining the sediment volume test (Fig.1) by considering the limitations of the test method proposed by Holtz and Gibbs. The soil samples which are more vulnerable to alkali attack with a more specific surface area were sieved through 75 μm. 10 g of Ball clay with 40 mL of distilled water with the required alkali concentration solution in 100mL graduated cylinders is mixed initially.
The suspension is then thoroughly and repeatedly stirred. The sample is allowed to equilibrate for 24 hours to ensure thorough wetting. Suspensions are made up to 100mL with distilled water and closed with caps. The graduated cylinders are left undisturbed for 24 hours duration for significant equilibrium and volume in terms of cc.gm -1 is noted respectively. The supernatant from every cylinder is drained after the completion of sufficient curing periods (7, 28, and 100 days). All wet samples are collected in containers and allowed to dry until the moisture gets vanished. The soil samples were ground into a fine powder, moisture-free to make them fit for X-Ray diffraction (XRD) and Scanning Electron Microscopy (SEM) analysis. specimens to detect the formation of any new compounds due to the chemical reaction with the soil mineral. SEM is employed for characterizing and examining the morphology of soil samples. SEM-TESCAN was the model used for the present study. A small oven-dried sample is glued to the SEM stub along with the gold-coated sputter before scanning. SEM images under 2 μm, 5 μm, and 10 μm are considered for the identification of morphological changes.

X-Ray Fluorescence Spectrometer (XRF) Test
The chemical composition of clay was evaluated with the help of X-Ray Fluorescence (XRF) spectroscopy technique. Phillips PW 2404 X-Ray fluorescence spectrometer was used for XRF analysis. About 5 g of dried soil sample finer than 20 μ is placed on glass holders and scanned to obtain the chemical composition (Table 2).

RESULTS AND DISCUSSION
Quantification of sediment volume test in Table 3

Morphological Changes of Ball Clay (BC) under 7 Days Interaction period
Stratified needle form morphology of Ball clay remained unchanged under 7 days of interaction period though the concentration of NaOH was increased to 8N (Fig. 3).

Mineralogical Changes of Ball Clay under 28 Days Interactions Period
Ball clay with its mineralogical transformations is clearly highlighted in Fig.4  changes are observed in the XRD patterns of Ball clay under all concentrations of solutions as it is not inclined towards the dissolution process of kaolinite minerals.

Morphological Changes of Ball Clay (BC) under 7 Days Interaction period
Stratified needle form morphology of Ball clay remained unchanged under 7 days of inte period though the concentration of NaOH was increased to 8N (Fig. 3).  changes are observed in the XRD patterns of Ball clay under all concentrat solutions as it is not inclined towards the dissolution process of kaolinite minera

Morphological Changes of Ball Clay (BC) under 7 Days Interaction period
Stratified needle form morphology of Ball clay remained unchanged under 7 day period though the concentration of NaOH was increased to 8N (Fig. 3).  No changes are observed in the XRD pattern of soil when it interacts with 0.1N and 1N NaOH solutions for 28 days. The reaction of Ball clay with 4N NaOH leads to the formation of new peaks of sodalites (6.34, 3.66, and 2.82 [Å]), which belong to the zeolite family. Formations of natrite (sodium carbonate) peaks at 3.23, 2.96, 2.71, and 2.62[Å] are also identified. Further increase in the concentration of NaOH to 8N, triggered the formations of sodalites and natrites with more intensities. Complete dissolution of kaolinite minerals is well observed from the XRD studies and confirms the transformation of kaolinite to zeolitic minerals under 4N and 8N NaOH solutions.

Morphological Changes of Ball Clay under 28 Days Interaction Period
No textural changes were observed in Ball clay when it interacts with 0.1N and 1N NaOH concentrations. However, morphological changes were observed with disintegrated structures when exposed under 4N and 8N NaOH concentrations. A cubic crystalline formation with a weathered texture was identified under 4N NaOH conditions. Compacted pellets with cotton Ball-type morphology are observed when Ball clay interacts with 8N NaOH, which is a typical morphology exhibited by sodalite (Barnes et al. 1999). The open structural formations further confirm the new mineral, which belongs to the zeolite family (Fig. 5).  1N, 1N, 4N, and 8N NaOH solution for 100 days in comparison with uncontaminated soil. No changes are observed in the XRD patterns of Ball clay when it interacts with 0.1N NaOH solutions for100 days. Subsequently, with 1N NaOH, the intensity of kaolinite peaks is reduced due to its dissolution. The silica and alumina released from the dissolution process react with sodium ions under high alkali conditions leading to the formation of new lay with its mineralogical transformations is clearly highlighted in Fig.4. Quartz (Peaks 5,3.34,1.82,and 1.37 [Å]) along with kaolinite (Peaks at 7.14,4.45,3.57,2.49,and ]) are identified as their major minerals.  morphology exhibited by sodalite (Barnes et al. 1999). The open structural fo confirm the new mineral, which belongs to the zeolite family (Fig.5).

Morphological Changes of Ball Clay under 100s day Interaction period
From SEM images of Fig. 7, it is clear that there are no specific morphologic Ball clay interacts with 0.1N NaOH for 100 days. A subtle change in texture i the same Ball clay interacts with 1N NaOH with initial formations of sodalite  peaks pertaining to sodalite (6.34, 3.66, and 2.82 [Å]) -a sodium alumina silicate hydroxide hydrate. However, there is a complete dissolution of kaolinite minerals under 4N NaOH concentrations with strong peaks of sodalite and satrite (3.23, 2.96, 2.71, and 2.62 [Å]). Ball clay when it interacts with 8N NaOH concentration enhanced the formation of sodalite peaks with high intensities along with cancrinite (6.32, 2.42, 1.42 [Å]) and nitratine (sodium nitrate). Cancrinite which is identified in 8N NaOH also belongs to the zeolite group. Further, the XRD pattern of 8N NaOH highlights the maximum dissolution of strong peaks pertaining to quartz.

Morphological Changes of Ball Clay under 100s Day Interaction period
From SEM images of Fig. 7, it is clear that there are no specific morphological changes when Ball clay interacts with 0.1N NaOH for 100 days. A subtle change in texture is observed when the same Ball clay interacts with 1N NaOH with initial formations of sodalites. However, Ball clay exhibited drastic changes in its morphology when it interacts with 4N and 8N NaOH solutions. Typical compacted pellet type morphology clearly reflects under 4N concentration of NaOH indicating the complete dissolution of kaolinite minerals which triggers the formation of sodalite minerals. The micropores with specific sizes on the surface of the texture, confirm the cubic crystalline zeolitic formations. Moreover, cancrinite of zeolite group further made the morphology in 8N NaOH look more compact and denser with pellet type formations. The summary of transmutations of alkali contaminated Ball clay is highlighted in the conclusion under Table 3.

CONCLUSION
Under 0.1N and 1N concentrations of NaOH with 7 days of interactions, Ball clay exhibited low dissolution rates of alumina silicates. At a higher concentration (4N and 8N NaOH) with short-term (7 days) interaction, Ball clay has not shown any significant changes in mineralogy and morphology. However, a higher rate of dissolution and precipitation of Ball clay are observed under 4N and 8N concentrations of NaOH with 100 days of interaction. Transformation of basic needle shape morphology into compacted pellets with open-type textural voids confirms the formations of sodium-based zeolite.

CONCLUSION
Under 0.1N and 1N concentrations of NaOH with 7 days of interactions, Ball clay exhibited low dissolution rates of alumina silicates. At a higher concentration (4N and 8N NaOH) with short-term (7 days) interaction, Ball clay has not shown any significant changes in mineralogy and morphology. However, a higher rate of dissolution and precipitation of Ball clay are observed under 4N and 8N concentrations of NaOH with 100 days of interaction.
Transformation of basic needle shape morphology into compacted pellets with open-type textural voids confirms the formations of sodium-based zeolite.