Properties of concrete incorporating high volumes of class F fly ash and san fibers

The results of an experimental investigation to study the effects of replacement of cement (by mass) with three percentages of fly ash and the effects of addition of natural san fibers on the slump, Vebe time, compressive strength, splitting tensile strength, flexural strength and impact strength of fly ash concrete are presented. San fibers belong to the category of “natural bast fibers.” It is also known as “sunn hemp.” Its scientific (botanical) name is Crotalaria juncea. It is mostly grown in the Indian subcontinent, Brazil, eastern and southern Africa and some parts of the United States (Hawaii and Florida). A control mixture of proportions 1:1.4:2.19 with W/Cm of 0.47 and superplasticizer/cementitious ratio of 0.015 was designed. Cement was replaced with three percentages (35%, 45% and 55%) of class F fly ash. Three percentages of san fibers (0.25%, 0.50% and 0.75%) having 25-mm length were used.The test results indicated that the replacement of cement with fly ash increased the workability (slump and Vebe time), decreased compressive strength, splitting tensile strength and flexural strength and had no significant effect on the impact strength of plain (control) concrete. Addition of san fibers reduced the workability, did not significantly affect the compressive strength, increased the splitting tensile strength and flexural strength and tremendously enhanced the impact strength of fly ash concrete as the percentage of fibers increased.     

Effects of fly ash,phosphoric acid,and nano-silica on the properties of magnesium oxychloride cement

This study investigates the effects of fly ash, phosphoric acid, nano-silica additives on the hydration process, setting time, compressive strength, water resistance, and thermal stability of magnesium oxychloride cement (MOC). MOC samples incorporating different combinations of additives are prepared, and their hydration products and microstructures are studied via X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), thermogravimetry-differential scanning calorimetry (TGA-DSC), and scanning electron microscopy (SEM). Results indicate that the addition of nano-silica to MOC containing fly ash and phosphoric acid reduces initial and final setting times, decreases the thermal stability, and increases compressive strength. Furthermore, the water resistance of modified MOC pastes is significantly improved through the combined use of additives. Hydration mechanisms arising in MOC are elucidated, and the remarkable enhancement of water resistance is attributed to secondary hydration of the 5 Mg(OH)₂·MgCl₂·8H₂O (5·1·8 phase) and the formation of amorphous gel facilitated by nano-silica inclusion.     

Analysis of mechanism on water-reducing effect of fine ground slag, high-calcium fly ash, and low-calcium fly ash

The addition of ultrafine powder (UFP) to concrete can improve the fluidity of concrete, showing a water-reducing effect. The aim of this article was to analyze the water-reducing mechanism of UFP both experimentally and theoretically. Three UFPs—fine ground slag, high-calcium fly ash, and low-calcium fly ash—were chosen for the study. The contrastive experiments were done to investigate the fluidity of mortars with 30%, 45%, 60%, and 75% equivalent cement replaced by fine ground slag, high-calcium fly ash, and low-calcium fly ash, respectively. The results showed the physical and chemical characteristic of the powders, such as their grain morphology, glass phase activities, densities, specific areas, and their grain cumulating conditions, can strongly affect their water-reducing effect.     

Development of novel refractory ceramic continuous fibers of fly ash and comparison of mechanical properties with those of E-glass fibers using the Weibull distribution

This study aimed to develop manufacturing technology for high-strength refractory ceramic fibers (RCFs) using fly ash, which is a highly promising material for the exterior and thermal insulation industry. The technology also contributes to reducing the environmental pollution caused by landfilling fly ash after coal is burned. Fly ash discharged from a thermal power plant, which had aluminosilicate chemical compositions, was used as the main material. As auxiliary materials, basalt, anorthite, feldspar, dolomite, and calcite were used to adjust the melt flowability, and frit, silica sand, and burr stone were used to lower the melting temperature. Moreover, the development of aluminosilicate fly ash fiber has the advantages of lower cost for raw materials and processing. Fly ash and natural rocks are inexpensive, and most of all, unlike the case for glass fiber production, the high cost of B₂O₃ is not a necessary expense. Fly ash is retrieved in powder form, which is advantageous compared to the starting materials for glass; the grinding process of raw materials can be skipped. From the fibrilization index calculation, we showed that the spinnability was influenced by the chemical composition of the salt-forming oxides in the fly ash compounds. We also found a correlation between the winding speed and the fiber diameter. The mechanical properties of a series of fly ash fibers were assessed by the Weibull distribution and then compared with those of the E-glass fibers that were melt-spun under an analogous condition.     

The removal of phosphate ions from aqueous solution by fly ash, slag, ordinary Portland cement and related blends

Phosphate ions have been removed from aqueous solution by fly ash, slag, ordinary Portland cement (OPC) and related cement blends. The rate and efficiency of PO₄³⁻ removal were found to increase in the order: fly ash, slag, OPC, apparently mimicking the order of increasing percent CaO in the adsorbents. Blending OPC with fly ash or slag evidently results in diminished PO₄³⁻ removal efficiency. Better removal was obtained at higher solute concentration, acidic pH and higher temperature. The effect of particle size and the speed of mixing were found not to be significant. A first-order kinetic model was used to obtain values for overall sorption rate constants and intraparticle diffusion constants. The Frumkin isotherm was found to be the appropriate equation for modelling isotherms from the experimental adsorption data, and values have been obtained for the isotherm constants. A 400-mg/l PO₄³⁻ (as P) solution was fed at a steady velocity of 2.0 cm/min through a 2.0-cm fixed-bed column (at pH 9.0 and 25 °C), and breakthrough curves were constructed to obtain estimated adsorption capacity values of 32, 60, 75, 78 and 83 mg PO₄³⁻/g adsorbent for fly ash, slag, OPC+fly ash, OPC+slag and OPC, respectively.     

Crystallisation behaviour of blast furnace slag modified by adding fly ash

Crystallisation of molten blast furnace (BF) slag can increase its viscosity, which can in turn affect the quality of slag fibres. Fly ash was added to BF slag to control its crystallisation and modify its chemical composition. FactSage simulation and analyses using X-ray diffraction (XRD), scanning electron microscope-backscattered electrons (SEM-BSE) coupled to an energy dispersive spectrometer (EDS), and single hot thermocouple technique (SHTT) were performed to explore the crystallisation behaviour of the modified BF slag. The relationship between temperature, mineral precipitation, and added fly ash content was investigated. The minerals contained in the modified BF were melilite, anorthite, clinopyroxene, and spinel. Variation in the fly ash content did not change the composition of the precipitate, but changed its content and the crystallisation temperature of the minerals, which affects the initial crystallisation temperature of the modified BF slag. It decreased as fly ash content increased, and was influenced by the crystallisation of melilite when the added fly ash content was between 5% and 20%. When the added fly ash content increased to 25%, the initial crystallisation temperature was influenced by the precipitation of anorthite. The initial crystallisation temperatures obtained by FactSage simulation, XRD analysis, and SHTT experiments differed due to kinetic effects. The modified BF slag with a fly ash content of 15% is considered suitable for manufacturing of slag fibres due to its low initial crystallisation temperature and cost.     

Thermal expansion of slag and fly ash from coal gasification in IGCC power plant

Integrated gasification in combined cycle (IGCC) is an electrical power generation system which is characterized to be a clean coal technology different than conventional process in combustible treatment. IGCC process gives rise to inorganic solid wastes in the form of vitreous slag and fly ashes with singular thermal properties. The gasification of the fuel takes place at high temperature and pressure in reducing atmosphere. Under that conditions, gases such as H₂, N₂ or CO, which are the main components of the gas mixture in the gasifier, show a high solubility in the melt and during the cooling remain enclosed in the vitreous slag. When these wastes are afterward thermal treated in oxidizing conditions, two phenomena occur. The development of a crystalline phase by devitrification of the glassy matrix and the releasing of the enclosed gas, which starts at temperatures nearly to the softening point. At higher temperatures the bubbles with increasing kinetic energy tend to ascent with difficult through the viscous liquid phase and promotes an expansive reaction, giving rise to a foam glass-ceramic product. This paper has been focused on the study of thermal expansion in slag and fly ash samples from the ELCOGAS IGCC power plant located in Puertollano (Spain).     

Modeling the hydration of concrete incorporating fly ash or slag

Granulated slag from metal industries and fly ash from the combustion of coal are industrial by-products that have been widely used as mineral admixtures in normal and high strength concrete. Due to the reaction between calcium hydroxide and fly ash or slag, the hydration of concrete containing fly ash or slag is much more complex compared with that of Portland cement. In this paper, the production of calcium hydroxide in cement hydration and its consumption in the reaction of mineral admixtures is considered in order to develop a numerical model that simulates the hydration of concrete containing fly ash or slag. The heat evolution rates of fly ash- or slag-blended concrete is determined by the contribution of both cement hydration and the reaction of the mineral admixtures. The proposed model is verified through experimental data on concrete with different water-to-cement ratios and mineral admixture substitution ratios.     

Factors affecting the suitability of fly ash as source material for geopolymers

The suitability of fly ash stock piles for geopolymer manufacturing was studied. The results of chemical analyses, X-ray diffraction (XRD) and particle size distribution (PSD) of five sources of fly ash obtained from coal-fired power generating plants in the US are presented. Geopolymer paste and concrete specimens were prepared from each stock pile. The specimens were subjected to an array of chemical and mechanical tests including XRD, RAMAN spectroscopy, setting time and compressive strength. A correlation study was undertaken comparing the fly ash precursor chemical and crystallographic compositions as well as particle size distribution, with the mechanical and chemical characteristics of the resulting geopolymer. Factors inherent to the fly ash stockpile such as particle size distribution, degree of vitrification and location of the glass diffraction maximum were found to play an important role in the fresh and hardened properties of the resulting geopolymer.     

Use of zeolite, coal bottom ash and fly ash as replacement materials in cement production

In this research, the effects of zeolite, coal bottom ash and fly ash as Portland cement replacement materials on the properties of cement are investigated through three different combinations of tests. These materials are substituted for Portland cement in different proportions, and physical properties such as setting time, volume expansion, compressive strength and water consistency of the mortar are determined. Then, these physical properties are compared with those of PC 42.5. The results showed that replacement materials have some effects on the mechanical properties of the cement. The inclusion of zeolite up to the level of 15% resulted in an increase in compressive strength at early ages, but resulted in a decrease in compressive strength when used in combination with fly ash. Also, setting time was decreased when zeolite was substituted. The results obtained were compared with Turkish Standards (TS), and it was found that they are above the minimum requirements.     

Triboelectrostatic separation of fly ash and charge reversal

Triboelectrostatic separation has been investigated as a method for separating unburned carbon from coal combustion fly ash. It was found that when a fly ash is exposed to moisture before it undergoes separation, the charging properties of the components of the fly ash change significantly. The mineral and carbon components of the fly ash appear to charge oppositely to how they were charged before exposure to moisture. A correlation was found between the degree of charge reversal and the relative amounts of leachable ions, especially calcium and sodium ions, present on the surface of the ash.     

Mineralogy and geochemistry of Greek and Chinese coal fly ash

In this paper the mineralogy and geochemistry of Greek and Chinese coal fly ash are examined. Annual production of fly ash in China is around 160 Mt while in Greece lignite fly ash accounts around 10 Mt. Even though the mineralogical and chemical composition of the fly ashes coming from these two countries differs, there are common questions on the utilization of this material. The variation of the Greek fly ash’ chemical composition, from Ca-poor to Ca-rich fly ash, has resulted to applications such as dam construction, use in cement and possibly in concrete and road construction. The Chinese fly ash, which is rich in mullite, is broadly applied for brick making.     

Enhanced mechanical properties of wood ash and fly ash as supplementary cementitious materials

The incorporation of fly ash (FA) and wood ash (WA) in concrete as supplementary cementitious materials (SCM) was studied. The chemical composition of ordinary Portland cement, FA and WA was determined according to ASTM C-114. SEM and optical microscopy were used for the analysis of concrete. Setting time, compressive strength, water absorption and acid resistance of the concrete with different percentages of SCM ranging from 0 to 60% were evaluated. The results obtained showed that setting time and rate of water absorption increased with the increase in percentage of SCM. After 7 and 28 days, the compressive strength of concrete with 20% FA as SCM was higher than that with substitution with 20% WA. Resistance of concrete against sulphate attack increased with an increase in the percentage of FA. It was found that incorporating more than 20% WA resulted in a decrease in sulphate attack resistance.     

Facile preparation of thermal insulation materials by microwave sintering of ferronickel slag and fly ash cenosphere

A novel approach for preparing thermal insulation materials by microwave sintering of ferronickel slag (FNS) in the presence of fly ash cenosphere (FAC) was proposed and evaluated. The study showed that during microwave radiation, the contact interface between FNS and FAC would preferentially form magnesium iron chromate spinel and magnesium iron aluminate spinel particles as hot spots by absorbing microwave vigorously, promoting decomposition and transformation of the raw materials into the thermal insulation phases, mainly cordierite and enstatite. After sintering at 900 °C by microwave for only 20 min with the addition of 25 wt% FAC, a thermal insulation material with thermal conductivity of 0.41 W/(m·K), bulk density of 1.46 g/cm³, compressive strength of 30.72 MPa, water absorption of 21.07%, and linear shrinkage of 7.06% was obtained. Compared with the conventional sintering method, the temperature was reduced by 300 °C, with the sintering time shortened by 6 times. This study represents a good example for clean and efficient value-added utilization of FNS, FAC and other relavent solid wastes.     

Effect of fibers on expansion of concrete with a large amount of high f-CaO fly ash

The effects of different types of fibers on expansion of cement paste, in which a large amount of high content of f-CaO fly ash (HFA) was added, were investigated and the way to prevent cement paste with HFA from expansion was proposed. The results showed that the effects of different fibers on expansion of cement paste are related to the properties of fibers. Carbon fiber and alkali-resistant glass fiber, which have high elastic modulus, can effectively restrain the expansion, while nylon fiber with low elastic modulus has no restraint effect on the expansion. In addition, the restraint effect of alkali-resistant glass fiber increases with the increase of fiber content.     

The leachability of heavy metals in hardened fly ash cement and cement-solidified fly ash

The effect of mix proportion, leachant pH, curing age, carbonation and specimen making method etc. on the leaching of heavy metals and Cr(VI) in fly ash cement mortars and cement-solidified fly ashes has been investigated. In addition, a method for reducing the leaching of Cr(VI) from cement-solidified fly ashes is proposed. The results mainly indicate that: (1) either Portland cement or fly ash contains a certain amount of heavy and toxic metals, and the leaching of them from hardened fly ash incorporated specimens exists and is increased with fly ash addition and water to cement ratio; (2) the leachability of some heavy metals is greatly dependent on leachant pH; (3) when carbonation of cement mortars occurs the leaching of chromium ions is increased; (4) the amount of heavy metals leached from cement-solidified fly ashes depends more on the kind of fly ash than their contents in fly ash; and (5) with ground granulated blast furnace slag addition, the leaching of Cr(VI) from solidified fly ashes is decreased.     

Influence of additions of coal fly ash and quartz on hydrothermal solidification of blast furnace slag

Blast furnace water-cooled slag (BFWS) has been solidified hydrothermally with tobermorite formation. The experimental results showed that the addition of fly ash and quartz was favorable to the formation of tobermorite, and the strength development of solidified body depended on both of the tobermorite formation and filling degree of formed tobermorite in the spaces between BFWS particles. The fly ash added appeared to have a higher reactivity than the quartz used during the initial hydrothermal processing due to the higher solubility of glassy phase in fly ash. The tobermorite formation seemed to be very sensitive to the fly ash content, e.g., the addition of fly ash 10-20 mass% was favorable to tobermorite formation, while the excessive addition of fly ash (> 20 mass%) appeared to impede the tobermorite formation. The excessive addition of quartz was also shown to exert a negative effect on the tobermorite formation, which causes strength deduction.     

Mineralogy and microstructure of sintered lignite coal fly ash☆

Lignite coal fly ash from the ‘Nikola Tesla’ power plant in Yugoslavia has been characterised, milled, compacted and sintered to form monolithic ceramic materials. The effect of firing at temperatures between 1130 and 1190 °C on the density, water accessible porosity, mineralogy and microstructure of sintered samples is reported. This class C fly ash has an initial average particle size of 82 μm and contains siliceous glass together with the crystalline phases quartz, anorthite, gehlenite, hematite and mullite. Milling the ash to an average particle size of 5.6 μm, compacting and firing at 1170 °C for 1 h produces materials with densities similar to clay-based ceramics that exhibit low water absorption. Sintering reduces the amount of glass, quartz, gehlenite and anhydrite, but increases formation of anorthite, mullite, hematite and cristobalite. SEM confirms the formation of a dense ceramic at 1170 °C and indicates that pyroplastic effects cause pore formation and bloating at 1190 °C.     

Comparison of chars in slag and fly ash as formed in pf boilers from B?dzin Power Station (Poland)

The amount of unburned organic matter in solid residues from coal combustion process is one of the indicators of the process and may influence their usage as by products. During the examination of slag and fly ash from B?dzin Power Station (Poland) the following forms of unburned coal were distinguished: crassispheres, tenuispheres, isotropic and anisotropic networks, tenuinetworks, honeycombs, inertinite and detritus. Slag characterises higher crassispheres and networks contents while high detritus contents are typical of fly ash. Spheres content in both slag and fly ash is always lower than vitrinite content in the feed coal. Fusinite content in the feed coal is lower than inertinite content in the solid residues. Other inertinite macerals probably participated in the formation of honeycombs.     

Utilization of fly ash for treatment of coal mines wastewater: Solubility controls on major inorganic contaminants

Acid mine drainage (AMD) has been reacted with coal fly ash in a 24 h equilibration time using 1:3 and 1:1.5 FA:AMD ratios by weight to produce neutral and alkaline process waters. The capacity of the fly ash to remove the major inorganic contaminants was examined. The elemental concentration trends with time for the two ratios were used to discern which elements have solubility control in the neutralization process. The geochemical computer code PHREEQC and WATEQ4 database was used for geochemical modeling of the process water. The resulting solid residues (SR) were analyzed by X-ray diffraction (XRD), and scanning electron microscopy equipped with energy dispersive X-ray spectroscopy (SEM-EDX) in an attempt to detect the minerals phases controlling the inorganic contaminants concentration in solution. The relative quantities of soluble bases (CaO, MgO) in fly ash and hydrolyzable constituents in AMD dictated whether the final solution at a given contact time will have a dominant acidic or basic character. Concentration of Fe, Al, B were observed to be controlled by mineral solubility for the entire reaction time while mineral solubility control for Ca, Na, Mg, Si and Mn concentrations developed after the initial rapid dissolution. Initial concentration was controlled by precipitation of gypsum and adsorption on iron-oxy-hydroxides at pH > 5.5. Increase of pH in solution with contact time caused the removal of the metal ions mainly by precipitation, co-precipitation and adsorption. Fe was mainly removed as Fe(OH)_3(a), goethite, Al as basaluminite, boehmite and alunite at pH 5.28-6.95 and as gibbsite and diaspore at pH 5.53-9.12. Cu and Zn were removed by adsorption onto the precipitating iron(oxy)-hydroxides and aluminum (oxy)-hydroxides. Si is released by dissolution of SiO_2(a) at pH < 5. Na was removed as Na-jarosite at pH 3.96-6.95 and Ca as gypsum and anhydrite. The treatment of AMD with fly ash was observed to be site-specific, i.e., the effectiveness of the treatment process will depend on the quality of the fly ash and the AMD. The product water meets the DWAF water quality limits for domestic use and irrigation at pH > 8 except for species Na, B, Mg, Ca, Sr and Ba which remain in solution. In addition B, Mg, Sr, Mo and Ba are released from dissolution of fly ash and will be of concern in the proposed treatment process.