Strength Properties of Untreated Coal Bottom Ash as Cement Replacement

Coal Bottom Ash (CBA) is a mineral by-product of thermal power plants obtained from the combustion of coal. In many countries, CBA wastes are identified as hazardous materials. The utilization of CBA can help in alleviating environmental problems; thus, this research was carried out to explore the possibility of its use as cement replacement in concrete manufacturing. Presently, In Malaysia, research that concerns about the use of CBA as cement replacement is very limited. Therefore, this study was aimed to investigate the properties of CBA as cement replacement and to identify the optimum percentage of untreated CBA as cement replacement. The CBA used in this study were taken from the Tanjung Bin power plant. In this research, the amount of CBA in the concrete mixture varied from 20% to 40% to replace cement. The properties of concrete containing CBA as cement replacement was examined through slump test, sieve analysis, concrete compressive strength test and splitting tensile strength test. The compressive strength test and splitting tensile strength test were performed at 7 and 28 days of curing time. Based on this research, it can be concluded that the optimum percentage of CBA as cement replacement is 25% for a curing time of both 7 and 28 days with the concrete compression strength of 45.2 MPa and 54.6 MPa, respectively. Besides, the optimum percentage for tensile strength is also at 25% CBA for a curing period of both 7 and 28 days with the tensile strength of 2.91 MPa and 3.28 MPa, respectively.


INTRODUCTION
One of the most important sources of energy is coal, which is fueling almost 40% of electricity worldwide. In Malaysia, some power plants use coal as a raw material to generate electricity since 1988. One of the thermal power plants byproducts is the Coal Bottom Ash (CBA) that faces an increasing production running into hundreds of thousand tones in Malaysia. CBA is produced due to the presence of rock detritus in the fissures of the coal seams. In the combustion process, the combustible materials were burned; meanwhile, the incombustible material such as rock detritus then resulted in CBA. It is one of the renowned industrial wastes produced at the bottom of coal furnaces. It is originated from agglomerated ash particles that are not lighter and finer to be found in the flue gasses.
CBA extracted from the boiler flue gases is called fly ash (FA) and the coarser part collected at bottom of the furnace is called CBA. Degree of pulverization of coal, firing temperature in the furnace, and type of furnace affect the properties of CBA (Singh and Siddique, 2016). The fly ash has been widely used in the cement industry but the bottom ash is still not largely utilized in Malaysia (Marto and Tan, 2016). The CBA was disposed to the landfills without any other commercial usage (Abubakar and Baharudin, 2012). In many countries, CBA was identified as hazardous materials.
Therefore, safe, innovative, environmentally friendly, sustainable, and economical solutions to recycle these industrial by-products are required.
The replenishment of CBA that has different mineralogical and physical characteristics than cement in the concrete mixture may have a remarkable effect on the concrete mechanical properties compared with the plain concrete in the hardened state (Khan and Ganesh, 2016). The use of waste materials such as CBA as cement supplementary material in concrete can result in alleviating the environmental problems and cost savings (Sua-Iam and Makul, 2014). By using CBA, it is possible to produce lightweight concrete with density varies from 1560-1960 kg/m 3 and compressive strength varies from 20-40 N/mm 2 at 28 days (Bai, Darcy and Basheer, 2005). Concrete made from CBA exhibited a reduced drying shrinkage in comparison with that of the control samples (Cai, Ghafoori and Yuzheng, 1998).
Due to increased demand for mixing water, Bottom Ash (BA) mixture might display a much higher degree of bleeding than the control concrete. Besides, it has high fire resistance for protection against fire. Those materials that retain a large quantity of water are more desirable since when they are exposed to fire, part of this water evaporates and it is transported from the fire exposed surface to the interior of the material, where the water cools and condenses again (García Arenas et al., 2011).
The porosity of hydrated paste influenced the strength development of concrete which is controlled by water-cement ratio and the presence of bond cracks at the interface of aggregate and hydrated paste. Besides, the strength of individual constituent material of concrete also has influenced the strength of the concrete mixture. The compressive strength of CBA concrete mixture was developed in a comparable manner to that of control concrete mixture (Singh and Siddique, 2016). Besides, the compressive strength of CBA decreased as CBA content in concrete increased (Andrade, Rocha and Cheriaf, 2009).
The tensile strength of CBA concrete progresses in similar to normal concrete. Besides, when compared to the concrete compressive strength of, CBA had more influence on the development of split tensile strength. CBA concrete containing 50% CBA showed a maximum increase of 11.01% in splitting tensile strength (Singh and Siddique, 2016). There was no change in split tensile strength and it decreased considerably with increase in bottom ash content if the sand replacement used up to 10%. If the percentage of cement content was not changed, the tensile strength decreased with increase in CBA content (Cai, Ghafoori and Yuzheng, 1998). This study was emphasized to determine the properties (compressive and tensile strength) the optimum percentage of untreated CBA as cement replacement.

Materials
Ordinary Portland Cement (OPC) was used in this research. Fine aggregate for the concrete mixture was clean, hard and well graded. Therefore, in this study, the fine aggregate that passed through 2 mm sieve was selected. Freshwater and tap water was used in this research. The Coal Bottom Ash (CBA) was obtained from Tanjung Bin power plant in Pontian, Johor as shown in Figure 1. The quantity of CBA in the concrete mixture was made in several variations i.e. 0% (as a control), 20%, 25%, 30%, 35%, and 40% to find the optimum percentage of CBA as a cement replacement.   Table 2 shows the concrete mix design used in this research For this research, the slump test was performed to control the consistency of the fresh concrete made in the laboratory. In order to prevent water loss in the concrete samples, the water curing method was applied as shown in Figure 2. In this type of curing, all of the samples were soaked in the water tank for 7 days and 28 days.

Slump Test
A Slump test was carried out to determine the flowability of the concrete. Figure 3 shows the result of the slump test in the concrete mixture containing a different percentage of CBA. From the result in Figure 3, there is a gradual decreasing of slump between control specimen and concrete containing 35% of CBA. This indicated that the workability of concrete is decreasing. The slump remains constant between 35% and 40% which indicated the lowest slump among the different percentages of CBA in concrete.
The workability of concrete containing CBA decreases as the CBA content increases due to increasing particle friction. Rough textured, irregular shaped and porous particles of CBA lead to particle friction (Singh and Siddique, 2016).

Density of Concrete
The density of concrete was examined at 7 and 28 days. Table 3 and Table 4 show the density of concrete containing CBA as cement replacement. Figure 4 shows the details of the concrete compressive strength that contains different percentages of CBA in 7 and 28 days. The data presented in Figure 4 is an average result of 3 concrete cubes. Figure 4 shows that concrete cube with 25% of CBA produces the highest compression strength compared to others. The average compression strength at 7 and 28 days is 45.2 MPa and was 54.6MPa, respectively. Based on the compressive strength test result, it can be concluded that the optimum percentage of CBA as cement replacement is 25%. The use of CBA in the concrete mixture as cement replacement can reduce the usage of cement. It is suitable to be used in the construction industry. Figure 5 shows the result of the splitting tensile strength test of the concrete with different percentages of CBA at 7 and 28 days of curing time. The value presented in Figure 5 is an average of 3 concrete cylinders.   Based on the result of this research, the optimum content of CBA as cement replacement is 25% for both compressive and tensile strength.

Tensile Strength
Compressive test results showed that at the age of 28 days 20% and 25% of CBA mixes have achieved strength higher than 50MPa. For the splitting tensile test, it can be seen that 25% of CBA as cement replacement is the optimum percentage. However, the value is still lower than their control specimen. This might due to high amount of water absorption by the high percentage of replaced materials as CBA have relatively highwater absorptions. It seems that, water cement ratio of these mixes will affect their strength greatly.
Besides, this research proved that the waste produced by Tanjung Bin power plant is suitable to be used as cement replacement. As for the conclusion, the use of CBA in concrete as cement replacement may help to reduce the use of cement and maintain the sustainable use of resource.

DISCLAIMER
The authors declare no conflict of interest.