Steam hydration of CFBC ash and the effect of hydration conditions on reactivation

A detailed study has been carried out on how hydration methods and conditions influence the sulphur capture potential of ash from a 165 MWe circulating fluidized bed combustion boiler firing a petroleum coke and coal blend. Both bed ash and fly ash were hydrated with saturated steam at various saturation conditions for different periods of time. Samples of the hydrated residues were then analyzed for free lime and calcium hydroxide content after the hydration process. Some size fractions of the steam-hydrated samples and those hydrated with liquid water in previous work were re-sulphated for 90 min using synthetic flue gas in a thermogravimetric analyzer at 850 °C to investigate how reactivation conditions affect the final sulphur capture behaviour of the ash. This work confirms that either hydration method is effective for reactivating the bed ash fractions tested but not fly ash, which should either be re-injected directly or reactivated in some other manner to improve its sulphur capture potential.     

Reactions of fly ash with calcium aluminate cement and calcium sulphate

The hydration processes in the ternary system fly ash/calcium aluminate cement/calcium sulphate (FA/CAC/C$) at 20 °C were investigated; six compositions from the ternary system FA/CAC/C$ were selected for this study. The nature of the reaction products in these pastes were analysed by X-ray diffraction (XRD) and infrared spectroscopy (FTIR). At four days reaction time, the main hydration reaction product in these pastes was ettringite and the samples with major initial CAC presented minor ettringite but calcium aluminates hydrates. The amount of ettringite developed in the systems has no direct relation with the initial components.     

Sulphur dioxide removal using South African limestone/siliceous materials

This study presents an investigation into the desulfurization effect of sorbent derived from South African calcined limestone conditioned with fly ash. The main aim was to examine the effect of chemical composition and structural properties of the sorbent with regard to SO₂ removal in dry-type flue gas desulfurization (FGD) process. South African fly ash and CaO obtained from calcination of limestone in a laboratory kiln at a temperature of 900 °C were used to synthesize CaO/ash sorbent by atmospheric hydration process. The sorbent was prepared under different hydration conditions: CaO/fly ash weight ratio, hydration temperature (55-75 °C) and hydration period (4-10 h). Desulfurization experiments were done in the fixed bed reactor at 87 °C and relative humidity of 50%. The chemical composition of both the fly ash and calcined limestone had relatively high Fe₂O₃ and oxides of other transitional elements which provided catalytic ability during the sorbent sorption process. Generally the sorbents had higher SO₂ absorption capacity in terms of mol of SO₂ per mol of sorbent (0.1403-0.3336) compared to hydrated lime alone (maximum 0.1823). The sorbents were also found to consist of mesoporous structure with larger pore volume and BET specific surface area than both CaO and fly ash. X-ray diffraction (XRD) analysis showed the presence of complex compounds containing calcium silicate hydrate in the sorbents.     

Effect of fly ash on the kinetics of Portland cement hydration at different curing temperatures

This paper describes the effect of fly ash on the hydration kinetics of cement in low water to binder (w/b) fly ash-cement at different curing temperatures. The modified shrinking-core model was used to quantify the kinetic coefficients of the various hydration processes. The results show that the effect of fly ash on the hydration kinetics of cement depends on fly ash replacement ratios and curing temperatures. It was found that, at 20 °C and 35 °C, the fly ash retards the hydration of cement in the early period and accelerates the hydration of cement in the later period. Higher the fly ash replacement ratios lead to stronger effects. However, at 50 °C, the fly ash retards the hydration of the cement at later ages when it is used at high replacement ratios. This is because the pozzolanic reaction of the large volumes of fly ash is strongly accelerated from early in the aging, impeding the hydration of the cement.     

Steam hydration-reactivation of FBC ashes for enhanced in situ desulphurization

Bed and fly ashes originating from industrial-scale fluidized bed combustors (FBCs) were steam hydrated to produce sorbents suitable for further in situ desulphurization. Samples of the hydrated ash were characterized by X-ray diffraction analysis, scanning electron microscopy and porosimetry. Bed ashes were hydrated in a pressure bomb for 30 and 60 min at 200 °C and 250 °C. Fly ash was hydrated in an electrically heated tubular reactor for 10 and 60 min at 200 °C and 300 °C. The results were interpreted by considering the hydration process and the related development of accessible porosity suitable for resulphation. The performance of the reactivated bed ash as sulphur sorbent improved with a decrease of both the hydration temperature and time. For reactivated fly ash, more favourable porosimetric features were observed at longer treatment times and lower hydration temperatures. Finally, it was shown that an ashing treatment (at 850 °C for 20 min) promoted a speeding up of the hydration process and an increase in the accessible porosity.     

Utilization of fly ash coming from a CFBC boiler co-firing coal and petroleum coke in Portland cement

Fly ash coming from a circulating fluidized bed combustion (CFBC) boiler co-firing coal and petroleum coke (CFBC fly ash) is very different from coal ash from traditional pulverized fuel firing due to many differences in their combustion processes, and thus they have different effects on the properties of Portland cement. The influences of CFBC fly ash on the strength, setting time, volume stability, water requirement for normal consistency, and hydration products of Portland cement were investigated. The results showed that CFBC fly ash had a little effect on the strength of the Portland cement when its content was below 20%, but the strength decreased significantly if the ash content was over 20%. The water requirement for normal consistency of cement increased from 1.8% to 3.2% (absolute increment value) with an addition of 10% CFBC fly ash; and the free lime (f-CaO) content of CFBC fly ash affected the value of increasing. The setting time decreased with an increase of CFBC fly ash content. The volume stability of the cement was qualified even when the content of SO₃ and f-CaO reached 4.48% and 3.0% in cement, respectively. The main hydration productions of cement with CFBC fly ash were C-S-H (hydrated calcium silicate), AFt (ettringite), and portlandite.     

激发剂对蒸养水泥一粉煤灰胶凝材料性能的影响

通过测定不同龄期净浆化学结合水量和抗压强度,并结合SEM,研究了在蒸养条件下激发剂对水泥-粉煤灰复合胶凝材料水化性能的影响。结果表明：蒸养条件及激发剂提高了水泥-粉煤灰复合胶凝材料的水化速度与粉煤灰的活性。     

Thermal properties of hydrating calcium aluminate cement pastes

As the hydration of calcium aluminate cements (CAC) is highly temperature dependent, yielding morphologically and structurally different hydration products that continuously alter material properties, a good knowledge of thermal properties at early stages of hydration is essential. Thermal diffusivity and thermal conductivity during CAC hydration was investigated by a transient method with a numerical approach and a transient hot wire method, respectively. For hydration at 15 °C (formation of mainly CAH₁₀), thermal diffusivity shows a linear decrease as a function of hydration degree, while for hydration at 30 °C there is a linear increase of thermal diffusivity. Converted materials exhibited the highest values of thermal diffusivities. The results on sealed converted material indicated that thermal conductivity increased with an increase in temperature (20-80 °C), while thermal diffusivities marginally decreased with temperature. The Hashin-Shtrikman boundary conditions and a simple law of mixtures were successfully applied for estimating thermal conductivity and heat capacity, respectively, of fresh cement pastes.     

Steam reactivation of 16 bed and fly ashes from industrial-scale coal-fired fluidized bed combustors

The hydration behaviour of sixteen ashes, obtained from different commercial-scale fluidized bed combustors, has been investigated. Hydration is important for both ash disposal and reactivation of excess lime present in the ashes for further use in flue gas desulphurization. The techniques used were instrumental and conventional chemical analysis, thermogravimetry and X-ray diffraction. The ashes comprised both fly ash and bottom ash, with particle size less than 2 mm. The ashes were heat treated in air to oxidize free carbon and then hydrated with pressurized steam at about 170 °C, alone and with addition of pure CaO.It has been shown that steam hydration is effective in quantitatively converting CaO to Ca(OH)₂, but in most cases the free lime content (i.e. CaO+Ca(OH)₂), expressed as CaO, decreases and added CaO enters into pozzolanic reactions with coal ash components, in part or even completely. Both the chemical evidence and X-ray phase analyses indicate that hydrated silicates and silicoaluminates are formed. The hydrated ashes are all able to take up additional SO₂ and it appears that the presence of amounts of Ca(OH)₂ detectable by phase analysis is not necessary for such capture.     

Effects of three Turkish fly ashes on the heat of hydration of PC-FA pastes

Effects of the type and amount of fly ash substitution on the heat of hydration of portland cement-fly ash pastes were investigated. Three Turkish fly ashes were used. One of them was a high-calcium and the other two were low-calcium fly ashes. The specimens contained 0, 10, 20, and 40% fly ash by weight of portland cement. The tests were carried out as described in ASTM C 186 however one separate set of specimens were first subjected to an early external temperature of 67±2°C for six hours followed by the standard temperature until time of test. The results revealed that the low-calcium fly ashes, regardless of their type, reduce the heat evolution when used for partial cement replacement. The high-calcium fly ash, on the other hand, does not produce significant changes in the heat of hydration.     

A study of fly ash-lime granule unfired brick

In this paper, the properties of fly ash-lime granule unfired bricks are studied. Granules were prepared from mixtures of fly ash and lime at fly ash to hydrated lime ratios of 100:0 (Ca/Si = 0.2), 95:5 (Ca/Si = 0.35) and 90:10 (Ca/Si = 0.5). After a period of moist curing, the microstructure and mineralogy of the granules were studied. Microstructure examination reveals that new phases in the form of needle-like particles are formed at the surface of granule. The granules were used to make unfired bricks using hydrothermal treatment at temperature of 130 ± 5 °C and pressure of 0.14 MPa. The microstructures, mineralogical compositions, mechanical properties and environmental impact of bricks were determined.The results reveal that the strengths of unfired bricks are dependent on the fineness of fly ash. The strength is higher with an increase in fly ash fineness. The strengths of the fly ash-lime granule unfired brick are excellent at 47.0-62.5 MPa. The high strength is due to the formation of new products consisting mainly of hibschite and Al-substituted 11 Å tobermorite. The main advantage of utilization of granule is the ability to increase the pozzolanic reaction of fly ash through moisture retained in the granule. In addition, the heavy elements, in particular Cd, Ni, Pb and Zn are efficiently retained in the fly ash-lime granule unfired brick.     

Mechanical activation of fly ash: Effect on reaction, structure and properties of resulting geopolymer

Geopolymerisation of mechanically activated fly ash was studied at ambient (27 °C) and elevated (60 °C) temperatures by isothermal conduction calorimeter. Under both the conditions, mechanical activation enhanced the rate and decreased time of reaction. It was interesting to observe that in the samples milled for 45 min (median size ∼5 μm), a broad peak corresponding to geopolymerisation initiated at 27 °C after 32 h. The rate maxima at 60 °C, a measure of fly ash reactivity, showed a non-linear dependence on particle size and increased rapidly when the median size was reduced to less than 5-7 μm. Improvement in strength properties is correlated with median particle size, and reactivity of fly ash. The characterisation of the geopolymer samples by SEM-EDS, XRD and FTIR revealed that mechanical activation leads to microstructure and structural variations which can be invoked to explain the variation in the properties.     

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.     

Hydration of fly ash-portland cements

Hydration products of fly ash-portland cements were studied with x-ray diffraction (XRD), differential thermal analysis (DTA) and scanning electron microscopy (SEM) as part of a continuing research effort to understand the pozzolanic activity of fly ashes. It was found that the amount of calcium hydroxide crystals in the cement pastes is diminished due to the addition of fly ash to the cement. Ettringite was produced in the early age, and the consumption of sulfate by the formation of ettringite was accelerated by the addition of fly ash. A partial conversion of ettringite to monosulfate within the first 7 days of hydration in the fly ash-portland cement pastes, but the formation of ettringite continued to form up to at least 28 days of hydration in the pastes without fly ash. Examination of the fly ash bearing pastes showed, in all cases, varying amounts of calcium hydroxide and unreacted portland cement, with minor quartz and gehlenite hydrate. It appears that hydration reactions actually occur in the fly ash cement pastes more or less on a particle-by-particle basis.     

Estimation of the degree of hydration of blended cement pastes by a scanning electron microscope point-counting procedure

A scanning electron microscope (SEM) point-counting technique was employed to study the hydration of plain portland and blended cement pastes containing fly ash or slag. For plain portland cement pastes, the results for the degree of cement hydration obtained by the SEM point-counting technique were consistent with the results from the traditional loss-on-ignition (LOI) of nonevaporable water-content measurements; agreement was within ±10%. The standard deviation in the determination of the degree of cement hydration via point counting ranged from ±1.5% to ±1.8% (one operator, one sample). For the blended cement pastes, it is the first time that the degree of hydration of cement in blended systems has been studied directly. The standard deviation for the degree of hydration of cement in the blended cement pastes ranged from ±1.4% to ±2.2%. Additionally, the degrees of reaction of the mineral admixtures (MAs) were also measured. The standard deviation for the degree of fly ash reaction was ±4.6% to ±5.0% and ±3.6% to ±4.3% for slag. All of the analyses suggest that the SEM point-counting technique can be a reliable and effective analysis tool for use in studies of the hydration of blended cement pastes.     

利用城市垃圾焚烧飞灰开发新型生态水泥混合材料

研究了城市垃圾焚烧飞灰的化学成分及矿物组成，以及掺入水泥后对水泥浆体的凝结时间和力学性能的影响；并借助SEM、XRD和ICP等测试手段对硬化水泥浆体的微观结构、水化产物和重金属的浸出与固化机理进行了研究，探索了将城市垃圾焚烧飞灰开发成新型混合材料的可行性。     

粉煤灰对水泥浆体的电阻率与化学收缩的影响

研究了粉煤灰掺量分别为0、20%和40%,水胶比为0.4的水泥浆体的抗压强度、电阻率、化学收缩以及水化产物的变化规律.电阻率采用无电极电阻率法进行测试,化学收缩采用ASTM C1608-12规定的膨胀测定法进行测试.结果表明,在250 d龄期时,粉煤灰掺量为20%的硬化水泥浆体抗压强度仅比空白组低5%;当粉煤灰掺量增大时,水泥浆体在3 d龄期时的强度、电阻率和化学收缩均减小,抗压强度与电阻率之间具有很好的线性关系.XRD及热重分析表明,随着粉煤灰掺量增加,水化硅酸钙含量减少,在3 d龄期时水化产物中出现了钙矾石.     

The effect of BaCO3 addition on the sintering behavior of lignite coal fly ash

The effect of BaCO₃ (witherite) addition on the sintering behavior of lignite coal fly ash taken from the Seyitömer power plant of Kütahya/Turkey was examined at temperatures of 1100, 1150 and 1200 °C in air atmosphere. Bloating of the fly ash samples sintered at 1150 °C was prevented, that is, the decomposition temperature of CaSO₄ in the fly ash is shifted to a higher temperature, and their physico-mechanical properties (porosity, water absorption, bulk density and bending strength) were improved with BaCO₃ addition. Positive effects of BaCO₃, however, were not seen on the fly ash samples sintered at 1100 °C. All the fly ash samples sintered at 1200 °C were bloated due to the gas evolving and also they melted. During the thermal treatment at 1150 °C a phase transformation from CaSO₄ (anhydrite) to BaSO₄ (Barite) occurred in the fly ash with BaCO₃ addition as seen from the X-ray diffraction (XRD) patterns and the bar shaped fly ash samples with BaCO₃ saved their structural integrity up to 1150 °C.     

Synthesis of belite cement from lignite fly ash

Synthesis of belite cement from lignite fly ash is studied as it can be produced using low temperature between 750 and 1200 °C leading to energy saving and low carbon dioxide emission. Two synthesis methods viz., clinkerization and hydrothermal processes assisted by calcinations are studied. Lignite fly ash is used as a main starting material. For the clinkerization process, the firing temperatures, types of additives and calcium oxide/silicon dioxide ratios (Ca/Si) are studied. In this process, the reaction between fly ash and calcium carbonate produces gehlenite (2CaO·Al₂O₃·SiO₂) which is undesirable due to its poor hydraulic property. A slightly higher belite (2CaO·SiO₂) phase is obtained using sulfate ion as a dopant and using high Ca/Si ratio. The strength of gehlenite bearing belite cement is, however, rather poor. For the hydrothermal–calcination process, the alkaline concentrations and calcining temperatures are studied. The final products are belite phase and mayenite (12CaO·7Al₂O₃) which are desirable as they possess hydraulic properties. The reasonable 28-day compressive strength of the belite cement mortar of 9.5 MPa is obtained. The hydrothermal process assisted by calcination is, therefore, suitable for use in the synthesis of belite cement from lignite fly ash.     

压制低水胶比条件下硅酸盐水泥-粉煤灰体系的水化分析

在压制低水胶比条件下,研究硅酸盐水泥和粉煤灰复合胶凝材料体系的水化程度和水化速率,并利用SEM和XRD分析了水化产物的微观结构.结果表明,在压制低水胶比条件下,水泥的水化受到限制,掺加粉煤灰后能够改善硅酸盐水泥的水化条件,提高水化程度.