Morphological and Strength Properties of Tanjung Bin Coal Ash Mixtures for applied in Geotechnical Engineering Work

In Malaysia, coal has been used as a raw material to generate electricity since 1988. In the past, most of the wastage of coal burning especially the bottom ash was not managed properly as it was dumped in the waste pond and accumulated drastically.This paper focuses on some properties of coal ash mixtures (fly ash and bottom ash mixtures) from Tanjung Bin power plant. The characteristics studied were morphological properties, compaction behaviour and strength properties. Strength properties of coal ash mixtures are carried out by conducting direct shear test and unconfined compression test. Besides, morphology and mineralogy of coal ash mixtures are studied using scanning electron microscope (SEM) and x-ray diffraction (XRD). The coal ash mixtures were compacted at 95% of maximum dry density, sealed and cured for 0, 14, and 28 days before they were analysed for shear strength, morphological and mineralogical analyses. The shear strength of coal ash mixtures varied depending on the fly ash compositions. The maximum shear strength was obtained at mixture with 50%FA: 50%BA and the value increased with curing periods. The friction angle obtained ranged from 27 to 37. Morphological analysis showed that the number of irregular shaped particles increased confirming change in material type with curing period. From mineralogical analysis, the crystalline compounds present in Tanjung Bin coal ash were Mullite, Quartz, Calcium Phosphide, Calcite, Cristobalite and Hematite. It can be concluded that the coal ash mixtures can advantageously be applied in the construction of embankments, roads, reclamation and fill behind retaining structures. Keywords— Coal ash mixtures; Fly ash; Bottom ash; Morphological; Shear strength


I. INTRODUCTION
Every year, the coal fired power plant produced large volume of coal ash which are fly ash and bottom ash over the world. Malaysia also is not excepting as a producer of large volume of fly ash and bottom ash. Even though there is no report about the producing of coal ash annually in Malaysia, but basically, there is about 10% of total weight of the coal burned produces ash 4. Use of coal ash in construction projects requiring large material volumes, such as highway embankment construction, back fill, soil improvement etc. may offer an attractive alternative to recycle this materials. Besides that, using large volume of the coal ash in geotechnical application is a highly promising solution to the disposal problem.
However, from literature studies, it is observed that there is limited investigation on coal ash in Malaysia. Although there are a lots of studied related to the properties of coal ash, but the investigation about the local coal ash is very limited. Therefore, it is necessary to provide the information based on the laboratory evaluation of the locally available coal ash especially on the coal ash mixtures between fly ash and bottom ash. Then, the findings of this study will help the coal fired power plant companies to overcome the disposal problem and will be useful to introduce the coal ash mixture as new material in geotechnical engineering work.
The main purpose of this study is to find out whether the coal ash mixtures can be used as a new material in geotechnical engineering work. Thus, the objectives of this study were to determine the mineralogical properties of coal ash mixtures and to determine the compaction behavior of different proportion of coal ash mixtures. Also, to determine the morphological and shear strength properties of 95% compacted coal ash mixtures with time.

II. LITERATURE REVIEW
Numerous studies have been completed in which the engineering properties and the physical and morphological characteristic of fly ash, bottom ash and coal ash mixtures were determined in the laboratory. 9 investigated the specific gravity values of fly ash and bottom ash from Wabash River Plant is 2.30 and 2.32 respectively and from A. B. Brown plant is 2.81 and 2.62 respectively. However, 10 investigated that the specific gravity values of fly ash and bottom ash from Tanjung Bin power plant is 2.30 and 1.99 respectively. 6 stated that the specific value is depending on the chemical composition especially the amount of iron oxide and the particle structure.
10 investigated majority of the sizes fly ash is occurred in a range from 0.03 mm to 0.001 mm this can graded as mostly fine silt to fine sand sizes. And for bottom ash, their sizes ranging from fine gravel to fine sands sizes and the majority sizes occurred in a range between 10 mm and 0.075 mm. 10 provided that the shapes of the gradation curves of ash mixtures became well graded with increasing bottom ash content in the ash mixtures.
The study made by 9 on the various mixtures of coal ash show that, as the fly ash content increased from 50% to 100%, the maximum dry unit weight decreased, whereas the optimum water content increased. Overall the values of maximum dry unit weight of ash were found to be lower than those of typical compacted soils.
9 investigated the fly ash particles were rounded, spherical in shape and their surfaces appeared to be smooth. Different to the bottom ash particles because it was angular and irregular in shape and had rough, gritty surface textures. Both of fly ash and bottom ash has particle agglomerations which ranged from lightly cemented to strongly bond. According to 10, after curing period, the number of irregular shaped particles is increased and particle size also increased for fly ash. This is due to the pozzolanic reaction has occurred. For bottom ash, compacted specimen is tested and after curing time fine fractions of shattered and irregular bottom ash particles are predominant.
7 investigated the hematite, quartz, lime and anhydrite are the major crystalline found in coal fly ash from XRD analysis. He also stated that the gehlenite and melilite are present in coal fly ash. According to 5 , the main components of the fly ash are the glass aluminium-silicate matrix, mullite, quartz and magnetite. A similar composition was obtained for bottom ash, with a higher content of magnetite.
The difference of shear strength depends on granular size because smaller size of soil particle will easily overcome the interlocking between particles while it is in friction rather than coarser particle of soil. Other than that, the shape of granular soil will also affect the total of shear strength of the soil. Angular textures of the particles will produce great interlocking features between the soil particle thus result in higher shear strength rather than rounded particles of granular. For all ash mixtures tested have similarity peak friction angel with typically sand ranging from 29 o to 37 o 8.

A. Preparation of Sample
Coal ash sample used in this study were extracted from Tanjung Bin power plant, Pontian, Johor ( fig. 1). Firstly, the fly ash and bottom ash sample obtained is oven dry ( fig.2). After oven dried, the samples are sieve with 2.00 mm sieve size. Then, the fly ash and bottom ash are mix together with mixture proportion are 0%, 30%, 50%, 70%, 90% and 100% of fly ash and measured by weight of total mixtures. The coal ash mixtures samples were stored inside plastic bag to maintain their dry condition. The specimens used for direct shear test, unconfined compression test (UCT), morphology and mineralogy testing will be based on the compacted sample with moisture content at 95% maximum dry density and the specimens were cured inside plastic bag to maintain their moisture content condition for 14 and 28 days. All curing samples were stored into the curing tank.

B. Laboratory Works
Laboratory works consist of test on compaction, morphology and strength properties of coal ash mixtures ( Table 1). The compaction test of coal ash mixtures are carried out to determine the compaction behavior. After that, the morphological and mineralogical properties of coal ash mixtures are carried out consisting of microscopic examination (SEM) and X-ray diffraction test (XRD). Last, the strength properties of coal ash mixtures is carried out which are direct shear test and unconfined compression test (UCT).

C. Data Analysis
Results gained from each test were recorded in form of table and graph to identify the trend of the collected data compared to the previous study related on fly ash and bottom ash mixtures. After that, the recorded data were analysed to determine the morphological properties and strength properties of the different mixtures. Table 2 shows the maximum dry density and optimum moisture content of compacted coal ash mixtures. The results show that, as the fly ash composition increased from 50% to 100%, the maximum dry density decreased, while the optimum moisture content increased. However, this results show reversed pattern as the fly ash composition increased from 0% to 50%. This is because at a certain level of fly ash content, the bottom ash particles may be completely separated, floating in a fly ash matrix. Besides that, the gradations of the ash mixtures varying with different mixture ratios also explain the change in dry density. Increasing bottom ash content in coal ash mixtures leads to the increasingly more well graded size distribution, which allows the fly and bottom ash particles to pack more closely and hence, increased the dry density.

B. Microscopic Examination (SEM)
From SEM analysis, the SEM micrograph of the coal ash mixtures particles from Tanjung Bin power plant shown in Figure 3, Figure 4 and Figure 5 at 0 day, 14 days and 28 days curing periods, respectively. The results show that the 0%FA mixture, the particles were angular and irregular in shape and had rough, gritty surface textures. The surfaces of the particles also free of dust, clean and shiny. However, as a fly ash composition increased from 0% to 100%, the angular and irregular shapes were decreased and replaced by rounded and spherical in particles shape. This is because, the fly ash is well rounded, spherical in shapes, round glassy spheres, number of irregular shaped particles and their surface appear to be very smooth. As the curing age of specimen increases, the particle size also increases due to the pozzolanic reaction hence, the number of irregular shaped particles increases. Besides that, there are also some agglomerates bonded particles formed when the curing time increased due to the crystal growth.  Table 3 shows the summarized results of XRD analysis of coal ash mixtures with curing time. The results show that at 0 day curing period, crystalline compounds present in coal ash mixtures of 0% to 50% fly ash content were Mullite, Quartz, Calcium Phosphide, Calcite, Cristobalite and Hematite. The same compounds exist in 70% to 100% of fly ash content except for Cristobalite. However, at 100% fly ash content the Calcium Phosphide also could not be traced.

E. Mechanical Properties of Ash Mixtures
Standard compaction, hydraulic conductivity, onedimensional compression, and triaxial tests were performed

Hydraulic Co
The hydraulic asured by mpaction-mou 990). Each a meameter to 9 standard Pr mpaction R=9 intained at a ort.
V Gradation Fig. 1 Table 2, shows the summarized of results of XRD analysis of coal ash mixtures. Results of XRD analysis shows that crystalline compounds present in coal ash mixtures increasing from 0% to 50% of fly ash content were Mullite, Quartz, Calcium Phosphide, Calcite, Cristobalite and Hematite. However, Cristobalite does not exist in 70% to 100% of fly ash content also the Calcium Phosphide not existed in 100% FA.

D. Morphology Characteristic
The chemical composition of the coal ash mixtures samples was investigated using XRF analysis. The percentage of the chemical contents for Tanjung Bin coal ash mixtures are given in table 3. The major of contents of Tanjung Bin coal ash mixtures (reported as oxides) were silica, alumina, iron and calcium oxide. Smaller percentage of magnesium, kalium, barium, potassium, sodium, and titanium oxides were also found. As can be seen from this table, Tanjung Bin coal ash mixtures has relatively low lime (CaO) content, less than 10 percent. According to [10], class of Tanjung Bin fly ash is class F which is ash is typically obtained from bituminous and anthracite coals and consists primarily of an alumina silicate glass, with quartz, mullite, and magnetite also present.

E. Specific Gravity
The values of specific gravity of coal ash mixtures are summarized in table 4 and fig.3. These values ranged from 2.19 to 2.36. The wide range in specific gravity can be attributed to two factors: (1) Chemical composition, and (2) presence of hollow fly ash particles or particles of bottom ash with porous or vesicular textures. The low specific gravities of Tanjung Bin coal ash mixtures are explained by their low iron oxide contents and, conversely, the high specific gravities of Tanjung Bin coal ash mixtures by their high iron oxide contents. Different amounts of hollow particles present in fly ash also cause a variation in apparent specific gravity. Obviously, a fly ash containing a large percentage of hollow particles would have a lower apparent specific gravity than one with mostly solid particles. In fact, the two factors affecting the specific gravity of fly ash may be related. [9] examined the chemical compositions of hollow and solid fly ash particles separately, and the data revealed that hollow-particle fly ash had significantly lower iron content (4.5%) than solid-particle fly ash (25.1%). The apparent specific gravity of bottom ash is also affected by the porosity of its particles. Comparing the specific gravities reported in Table 2 for fly ash composition, the bottom ash has a highest specific gravity than the fly ash, and indicates that slightly higher iron content may be present in the bottom ash. This may be due to the presence of highly porous popcorn-like bottom ash particles.   Table 8 shows the summarized results of unconfined compression tests for coal ash mixtures. The results show that, the 50% of fly ash composition had the highest shear strength compared to the other mixtures. This may be due to the optimum bonded between fly ash and bottom ash particles. The results also show that, without the addition of fly ash, the shear strength seemed to be the lowest because the bottom ash exhibits small cohesive characteristics. In addition, which might be due to the pozzolanic reactions and also some agglomerates bonded particles formed due to the crystal growth.

VI. RESULTS AND DISCUSSIONS
A variety of tests were performed on fly ash composed of fine, nearly spherical particles ranging in size from silt to fine sand, bottom ash composed of coarse angular particles ranging in size from sand to small gravel, and mixtures of the two. Both fly and bottom ash exhibit some special morphological characteristics that are distinctly different from typical soils. The fly ash particles in this study were mostly hollow spheres with thin walls. Some bottom ash particles had complex pore structures. Also, some of the fly ash or bottom ash particles were agglomerations of finer particles. The morphological characteristics of fly and bottom ash affected their specific gravity, particle strength, and consequently, other mechanical properties to varying degrees.
Fly/bottom ash mixtures (with mixture ratios ranging from 50% to 100% fly ash content) were found to exhibit relatively well-defined moisture-density relationships, and the relationships varied with the mixture ratio. However, overall, the values of  (d, max) of ash mixtures were found to be lower than those of typical compacted soils.
The hydraulic conductivity of compacted ash mixtures were found to decrease slightly with increasing fly ash content. This is primarily due to the increasing specific surface with increasing fines content, which generates more resistance to water flow through voids between particles. The overall range of the values was similar to that of a fine sand/silt mixture or silt.
From a mechanical point of view, the fly/bottom ash mixtures compared favourably with conventional sandy soils. They may be more compressible than typical compacted sands at the same compaction levels, mainly due to the higher crushability of bottom ash. At the low to moderate stress levels expected in typical highway embankments, however, the compressibility of compacted ash mixtures is similar to that of typical compacted sands. Moreover, samples of compacted ash mixtures at a moderately high compaction level (e.g., 95% relative compaction) exhibited comparable or even higher shear strength than that of compacted sands at similar compaction levels.
In order of decreasing significance, the degree of relative compaction, the confining stress, and the mixture ratio all affect the stress-strain and volumetric behavior of an ash mixture under shearing, and therefore its peak shear strength. Ash mixtures at 95% relative compaction typically exhibited a behavior similar to that of sandy soils in dense states (i.e., dilatant behaviour), whereas those at 90% relative compaction resembled sand in loose states. Increasing confining stress decreased dilatancy, and thus decreased peak friction angles. Increasing bottom ash content also tended to decrease dilatancy, primarily due to crushing of bottom ash particles during shearing. At 95% relative compaction, however, the effect of fly/bottom ash mixture ratio on the peak shear strength was found to be relatively minimal. The values of critical state friction angle of the ash mixtures were found to be in the same range observed for typical sands.
Based on the results obtained in this study, it appears that high volume fly ash mixtures are suitable for use as backfill in embankments construction, if proper design and construction procedures are followed. Prior to use, the materials must pass the appropriate environmental requirements set by state regulatory agencies. If the environmental requirements are satisfied, the fly/bottom ash mixtures can provide fill materials of comparable strength and compressibility to most soils typically used as fill materials, while having the advantage of smaller dry unit weights.