Issue 38

U. Haider et alii, Frattura ed Integrità Strutturale, 38 (2016) 305-318; DOI: 10.3221/IGF-ESIS.38.41 306 well as to reuse the already disposed fly ash [3]. Fly ash, which used to be known as a waste, is recognized as a useful chemical substance and it is a REACH registered product, having several useful applications [4-7]. There are about 861 billion tonnes of coal reserves in the world of which half consists of brown coal deposits [8-9]. Morphology of fly ash is highly affected by the source of pulverised coal and amount of water present in the pores of the coal particles, which are combusted to produce it [10]. Morphology of fly ash consists of pseudo spheres and irregular particles containing: aluminium, silicon, calcium, iron, carbon from unburned coal, and lesser abundance of other twenty six elements, mostly consisting of heavy metals and some rare earth elements [11-12]. Different morphological and rich chemical composition possessed by fly ash are a hurdle for its own utilization in certain applications, where one part of it is more desirable than others, which can be resolved by separating its particles by some technique [13]. Therefore, separation of particles of fly ash is required in finding new applications. Various studies have been carried out to extract and analyse hollow spheres which float on the surface of water, when wet separation methods are employed. Kolay and Singh [14] and Ngu et al. [15] carried out studies on physico-chemico- mineralogical properties of hollow spheres recovered from surface of fly ash lagoons, separated by water. They showed the presence of oxides of Si, Al, and Fe to be between 50-60%, 30-35%, and 5-10% respectively. Kolay and Bhusal [16] obtained cenospheres / hollow spheres of two different densities by wet separation using water and Lithium Metatungstate solution, the later employed in order to recover some percentage of hollow spheres even contained in the sinked part during water separation. Such microspheres represent about 70 to 75% of fly ash, depending on: origin of coal, combustion method, and cooling rate [17]. Over of these microspheres, hollow spheres, which have densities less than water, are recovered from top of water surface by sink float method. About 1-5% of the fly ash is hollow spheres [17-18]. ASTM C618 classifies fly ashes according to their chemical composition in two types: Class C and Class F [19]. Class C fly ashes are characterized by the fact that when added to water react and rapidly harden, whereas this phenomenon is not observed for Class F fly ashes [16]. The later type is characterised by a separation of its particles based on their densities, when mixed in water. It is well known that finer and amorphous materials are able to accelerate the pozzolanic reaction in order to improve the strength characteristics of mortars and concretes [20]. The pozzolanic reactivity of pulverized coal fly ash is due to the presence of active silica and alumina, which initiates pozzolanic reaction by reacting with calcium hydroxide, derives from cement hydration [21]. However due to larger size of raw fly ash particles as compared to cement particles, concrete and mortar strength is not significant increases if high volume fly ash is used. Therefore, efforts are made in order to obtain finer particles, such as for example air classifier systems [22] or wet separation systems. Usually with the air classification methods two types of products are obtained: underflow and overflow products, or coarse and fine products [23]. Air classifiers have some drawbacks [24]. Firstly, the percentage of overflow / fine products recovered is very low, while significant percentages of underflow / coarse products are produced simultaneously [25]. Secondly, the yield of overflow / fine products is very low. Thirdly, the cost of air classification machines is considerably high [25]. The strength and durability of cementitious structures are important properties since they define the service life of the structure [26]. As a matter of fact, finer particles of fly ash, due to their smaller particle size and larger surface area, have the ability to increase strengths and increase long-term durability in cementitious materials [27]. Studies regarding effects of fly ash on strength and durability of cementitious structures are available in literature [28-30]. The use of fly ash to replace cement in cementitious materials reduces the consumption of cement and thereby reduces CO 2 emissions related to cement production [31, 32]. Fly ash has several uses, but still its global utilization is 25% of the world’s fly ash production [33]. However, there are some obstacles for utilizing fly ash in large volumes in cementitious composites, such as slow development of strength, quality and composition of fly ash, setting time issues etc. [34]. Furthermore, it is not clearly known how ASTM Class F fly ashes affect strength, and resistance to ingress of chemical when added in large volumes in cementitious materials. Most of the studies that have been carried out on wet separation of ASTM Class F brown coal fly ash using water focus on the hollow spheres, which float on the water surface being a minute fraction of the particles in fly ash. A detailed physical, morphological, chemical, and mineralogical analysis on remaining 95-99% of the fly ash which sinks in water after wet separation is not available in the literature. Therefore, the purpose of this research is to carry out wet separation of ASTM Class F brown coal fly ash using water and then examine the physical, morphological, chemical, and mineralogical properties of the separated parts of raw fly ash. Then, the paper is focussed on strength and durability of high volume cementitious mixtures when some types of the above parts are added to the mixture.