By decreasing the density while maintaining strength and without adversely affecting cost. Introducing new aggregate sin to the mixed sign is a common way to lower a concrete’s density. Normal concrete contains four components, cement, coarse aggregate, fine aggregate and water.
The coarse aggregate (granite or gravel) and fine aggregate (sand) are the components that are usually replaced with lightweight aggregates.
CENOSPHERE
Fly Ash Cenosphere (FAC) is spherical and hollow morphology, chemical characteristics, as well as their mechanical and energy-attenuating properties can be incorporated with conventional cement to form lightweight workable materials. Fly ash cenospheres are hollow alumino-silica spherical shaped particles, with shell thickness of several microns. The size ranges from a few microns to 400 µm.
Due to its lower bulk density ranging from 400 to 800 kg/m3, can be used as lightweight fillers in cement-based composite.
PHYSICAL AND CHEMICAL PROPERTIES
The key physical properties of lightweight aggregate (LWA) for use in concrete are specific gravity, shape bulk density, surface texture, moisture content, water absorption, and porosity 2,5,14. As the FACs are obtained in a dry state 15 therefore, moisture content and water absorption are irrelevant and not applicable. Further, the properties of FACs may vary from batch-to-batch even from the same power plant 16.
Size and morphology FAC particles are spherical and silky textured 15. The micro-morphological features are shown in the scanning electron microscopic (SEM) is given in Fig 1. It may be seen that these are most rounded and hollow or blank from the inside with a shell thickness of several microns. The size range may change in different groups and resulted as 1–100 mm 15, 1–400 17–19, 1–300 mm 22, and 1–600 mm 21.
The speci?c surface area has been found by Blaine air permeability testing and Braunauer Emmet-Teller (BET) analysis as 2.50–4.57 m2/g 20 and 6.02m2/g 17, respectively. Particle size has conflicting correlation with the speci?c surface area, meaning that a FAC batch with a higher percentage of ?ner particles would affect and lead to a higher visual surface area. Fig. 1 SEM images of FAC at different magnification; (a) 10 µm, and (b) 200 µm 14,19.
Bulk density and speci?c gravity The bulk density of FAC may range between 400 to 800 kg/m3 19,21,28 whilst the speci?c gravity (measured by helium pycnometer) is measured about 2.48 21.
Chemical and phase-mineral composition The chemical composition of FAC (thru X-ray Fluorescence Spectroscopy (XRF)) resembles closely that of typical ?y ash, primarily due to the same origin of both. Typical oxide compositions of FAC, as reported by many researchers, are given in Table 1. It can be seen from Table 1 that silicon dioxide and aluminum oxide collectively form the majority portion; approximately 90%, whereas calcium oxide and other elemental oxides are present in small amounts. The phase-mineral compositions of FAC acquired by X-ray. Amorphous silica minerals in different forms like quartz and cristobalite have been identi?ed in the patterns.
Further, alumina and mullite are also noticed whilst alumina is a predominant mineral present in FAC. Table 1. Oxide composition (weight %) of FAC from XRF Decription Hanif et al. 17,19 Wang et al. 22 Kolay and singh 20 Blanco et al. 25 SiO2 73.10 60.10 52.53 57 Al2O3 16.70 28.40 30.01 24 Fe2O3 1.96 4.80 7.53 6.8 CaO 1.06 0.80 1.15 4.3 K2O 3.94 3.50 1.98 3.3 TiO2 3.94 – 1.79 1.2 MgO – 1.50 3.2 2.2 MnO 0.05 – – – Na2O 2.42 0.90 0.02 0.37 SO4 0.42 0.03 0.02 – 2.2.1.4.
Pozzolanic activity and degree of reactivity of FAC The attendance of amorphous silica and a small amount of lime may perhaps helpful in pozzolanic reactions in the cement-based system. Wang et al. 22 studied the potential pozzolanic activity of FAC by thermogravimetric methods. Two cement pastes, one contained FAC and one not contaiend FAC, were prepared with the same water to cementitious materials ratio and tested for Calcium Hydroxide (CH) and non-evaporable water content (Wc).
Testing ages were designated and selected at 7, 28, 91, 182, and 365days. The CH and Wc amounts were obtained from thermogravimetric analysis (TGA) where the weight loss was determined in dehydroxylation (disintegration of CH) and decarbonation (decomposition of Calcium Carbonate) processes in the cement hydration products while subjected to high temperature. Reduction in the amounts of CH and Wc for pastes with FAC showed their degree of reactivity, however the particle sizes carefully chosen by 22 were less than 106 mm, henceforward the pozzolanic activity results were not symptomatic of all the FAC particle range. In another study, Hanif et al. 19 determined the pozzolanic activity of FAC for the complete range of particle sizes and corroborated the partial reactivity of FAC in cementitious systems by quantifying the degree of reactivity. This is a significant outcome as it justi?es the better mechanical properties of FAC incorporated composites, even at lesser density levels.
MECHANICAL PROPERTIES
The mechanical properties of FACC are the compressive strength, ?exural strength, tensile strength, elastic modulus and speci?c strength (strength to density ratio) as reported in the literature, are summarized in the subsequent sections.
Density and compressive strength Compressive strength is the most usually and commonly studied property of cement-based materials because several other mechanical properties of cement based-composites like elastic modulus, abrasion resistance ?exural strength, splitting tensile strength, etc. are linked to this property 23. Concrete with contained FAC has been found to have suitably low density with sufficiently and effectively higher strength as compared to contemporary concretes with conventional LWA.
Depending on the mixture proportioning, the water to binder ratio, the use of SCM and admixtures, various grades of strength of concrete contained FAC have been reached. Although Montgomery and Diamond 24 were the major in using FAC in cementitious composites, its use as LWA developed in late 20th century. Afterwards, Blanco et al. 25 developed a concrete contained FAC with density of 1510 kg/m3 and corresponding compressive strength as 33.03MPa.
This triggered increased interest from researchers working on LWC with different LWA because even though the conventional LWA could help to produce LWC, but with lower strength levels associated, low their use. It can be understood that the density of concrete contained FAC drops directly through the addition of FAC as ?ller material and the resulting composites follow with the speci?cations of structural lightweight concrete 2,6. In additional, it has been observed that the concrete contained FAC combined or incorporate with SCM like ?y ash, silica fume and iron ore tailings have better mechanical strength and packing properties as elaborated by 26–27. Some researchers also have a try to further reduce the density of concrete contained FAC by incorporating other LWA, like aerogels but the density reduction was not significant and corresponding strength reduction was quite considerable 17.
The current research on concrete contained FAC, Wu et al. 26 and Wang et al. 27 achieved the most promising and encouraging results with speci?c strength (strength per unit weight) values of 40.14, 47.18, and 41.03 kpa/kgm3, respectively. The resulting cement composites were termed as ultra-lightweight cement composites (ULCC). Founded on experimental research, Wang et al.
Succeeded in recommending a method to design the mix proportions of ULCC for objective (desired) unit weight, compressive strength and workability. The proposed methodology was based on the correlation between the spacing among spherical FAC particles (when packed together) and the water to binder (cementitious materials including cement and SCM, if any) ratio for attaining the aimed workability. If the FAC particle size distribution and particle density is known, the method can be successfully employed in achieving the desired unit weight and strength for the concrete contained FAC. This was a closely important step towards the development of concrete contained FAC as it minimizes the laboratory trials needed for achieving the target properties of fresh and hardened concrete contained FAC.
Flexural and tensile strength in order to improve of LWC performance and ductility, the use of discontinuous micro reinforcement is vital, which is why in the past many of the research studies focused on ?ber reinforced concrete contained FAC. FAC have been incorporated into cementitious composites with polyvinyl alcohol (PVA) ?bers 17,21, polyethylene ?bers (PE) 26,29, polypropylene ?bers (PP) 28 and steel ?bers 29. PVA ?bers were found to enhance ultimate ?exural strength with toughness indices I5 and I10 double than those for concrete contained FAC incorporated with PP ?bers 28. Use of relatively higher volume fraction (2%) of PVA ?bers led to much pronounced strain hardening (up to 4.5% strain) with excellent multiple cracking under uniaxial tensile tests 21. A tensile strength of up to 6 MPa was achieved.
These results indicate clearly that PVA ?bers have good compatibility with concrete contained FAC which is primarily due to their hydrophilic nature and the presence of hydroxyl group (in the PVA ?bers) resulting in strong chemical bonding leading to high Gd 30. 2.3 COAL BOTTOM ASH Coal Bottom Ash (CBA) can be used as replacement for fine aggregates in concrete. Recent studies noted the possibility of using industrial by-products to produce normal concrete and high strength concrete used as partial or whole replacement became evident 31. Also incorporating industrial wastes is confirmed to have superior properties compared to conventional concrete in terms of strength, performance and durability 32. In this chapter, the literature review, the first section contains general information on the production of CBA and the second section concentrates on durability of CBA. There are about 75-85 % of the crushed coal burned are FA that conveyed into the pipe gases and later extracted from the gas by electronic precipitators 33, However the remaining 15-25 % BA will compile from the combustion compartment as coarser material.
BA is kept in a molten state and collected when it flows into the ash hopper below. The water then turned the molten material into crystallized particles. Flue gases exits along with the remaining CCPs.
PHYSICAL AND CHEMICAL PROPERTIES
CBA has porous surface texture and regular in shape, the ash is usually a well-graded material with size that may vary. The study showed that bottom ash are from the well graded size distribution ranging from fine gravel to fine sand size and the majority of the sizes occurred in the range of 0.075 mm and 20 mm 34, Also a study showed that the bottom ash sample from the same power plant showed that the grain size distribution of bottom ash size occurred in a range between 0.03 and 2.00 mm 35. CBA is dark grey in colour and the characteristics are gritty, porous and mainly sand size material that is collected at the bottom of the furnace 36.
As mentioned above, the “crystallized” pallets formation is referred as Boiler Slag; a hard, black and glassy material. Using X-ray energy dispersive spectrometry (EDS) or X-ray fluorescence (XRF) on CBA will reveal that the main chemical compounds include Silicates (SiO3), Aluminates (Al2O3) and Iron oxide (Fe2O3) with a host of other compounds in smaller percentages 38. The analysis on chemical composition conducted by independent researches on three thermal power stations in Malaysia which is Kapar thermal power station, Taung Bin and Jimah Power plant can be seen in Table 2. According to 34,35 the major components of Tanjung Bin power plant’s CBA were Silica, Alumina and Iron oxide; and CBA used is Class F grade because the sum of SiO2 + Al2O3 + Fe2O3 exceeds 70% and according to ASTM C618 this can be attributed to the use of Bituminous or Anthracite Coal which produces low calcium content. Potassium, magnesium and sodium percentages are also present with traces of barium, manganese and zinc. BS 3892: Part 1: 1993 specified an SiO3 content to be less than 2.5% while ASTM C618 specified maximum of 5.0 % and Na2O alkali of not more than 1.5 %.
Table 2. Chemical analysis results from different power plants in Malaysia. Bottom Ash Percentages Description Muhardi et al., (2010) 35. Fawzan (2010) 39. Naganathan et al., (2012) 40. SiO2 42.7 49.4 9.78 Al2O3 23 22.3 20.75 Fe2O3 17 13.7 30.71 CaO 9.8 9 11.1 K2O 0.96 1 – TiO2 1.64 2.2 – MgO 1.54 0.87 3.2 P2O5 1.04 0.65 – Na2O 0.29 0.13 – SO3 1.22 0.68 – BaO 0.19 – – MnO – 0.08 – ZnO – – 1.8 2.3.2 MECHANICAL PROPERTIES The use of CBA and CFA in concrete has been carried out by replacing fly ash with cement partially and turned bottom ash particles to smaller sizes one way is by grinding it into Pozzolanic materials.
Strength Properties A study cited from previous authors that the use of bottom ash in the amounts of 10 – 40% as replacement for fine aggregate 40. Test showed that the compressive and tensile strength of bottom ash concrete generally increases with the increase in replacement ratio of fine aggregate and curing age. Investigations by showed that it is possible to manufacture lightweight concrete with saturated-surface-dry condition (SSD) in the range of 1560 ??? 1960 kg/m3 and a 28 days compressive strength in the range of 20 -40 N/mm2. 2.3.2.1 Durability of concrete using various ratio of replacement The influence of various ratio mixtures of CBA, 31 observed that the compressive strength and density of concrete decrease due to increase of CBA replacement compared to control concrete. The strength of bottom ash concrete gains a slower rate in the initial period and faster rate after 2 days due to the pozzolanic reaction on bottom ash.
Study shows five tests has been done with different replacement percentages as 10%, 20%, 30%, 40% and 50% of CBA based on weight of the fine aggregate with 90 days compressive strength 41. In additions studies conducted the research with 25%, 50% and 100% ratio of sand replacement at 7, 2, 60 and 90 days age and 7, 14, 28 and 60 days age. The test results showed the increment of CBA replacement will increase the porosity of the concrete, So it can be concluded that the compressive strength and the values of slump test decrease due to the increment of CBA replacement. If too much CBA content however, the strength will decrease.
According to 31, the optimum replacement levels of CBA are 20% for a good strength at 7, 28, 56 and 112 days. But at 30% to 100% replacement of CBA the compressive flexural, split and water permeability test are approximately same as that of the controlled concrete and the compressive strengths were decreased. Besides that, a study stated the bottom ash replacement at 10% and 20% could be used in real condition without insulation and with limited moistening. This finding was supported by 40, the maximum strength for concrete containing CBA occurred at the replacement level of 20%. The main factors affecting the concrete durability was a chemical and physical property of BA.
So, it is possible to produce durable concrete by using CBA as fine aggregate replacement. Studies suggested that the water absorption and porosity of concrete increased when bottom ash is used as sand replacement 31. The higher level of replacement of the fine aggregates with CBA will affects the workability of concrete which is due to the amount of water absorption. In additions a study concludes that the more CBA is being introduced to the concrete mix will reduce the performance of the concrete in term of strength but produces more ductile and lightweight concrete with high workability of mixture which is suitable for application of structures that are more resistant to high impact and able to sustain failure longer compared to the ordinary concrete.
