Accelerated Carbonation Assessment of High-Volume Fly Ash Concrete

The issue of concrete carbonation has gained importance in recent years due to the increase use in supplementary cementing materials (SCMs) in concrete mixtures. While there is general agreement that concrete carbonation progresses at maximum at a relative humidity of about 60%, the rate may differ in the case of cements blended with SCMs, especially with high-volume fly ash replacements. In this study, the effect of high-volume fly ash concrete exposed to low ambient relative humidity (RH) conditions (57%) and accelerated carbonation (4% CO2) is investigated. Twenty-three concrete mixtures were produced varying in cementitious contents (310, 340, 370, and 400 kg/m3), water-to-cementitious materials ratio (0.45 and 0.50), and fly ash content (0%, 15%, 30%, and 50%) using a low and high-calcium fly ash. The specimens were allowed 1 and 7 days of moist curing and monitored for their carbonation rate and depth through phenolphthalein measurements up to 105 days of exposure. The accelerated carbonation test results indicated that increasing the addition of fly ash also led to increasing the depth of carbonation. Mixtures incorporating high-calcium fly ash were also observed to be more resistant against carbonation than low-calcium fly ash due to the higher calcium oxide (CaO) content. However, mixtures incorporating high-volume additions (50%) specimens were fully carbonated regardless of the type of fly ash used. It was evident that the increase in the duration of moist curing from 1 day to 7 days had a positive effect, reducing the carbonation depth for both plain and blended fly ash concrete mixes, however, this effect was minimal in high-volume fly ash mixtures. The results demonstrated that the water-to-cementitious ratio (W/CM) had a more dramatic impact on carbonation resistance than the curing age for mixtures incorporating 30% or less fly ash replacement, whereas those mixtures incorporating 50% showed minor differences regardless of curing age or W/CM. Based on the compressive strength results, carbonation depth appeared to decrease with increase in compressive strength, but this correlation was not significant.

Enhanced visible-light photocatalytic oxidation capability of carbon-doped TiO_2 via coupling with fly ash

A carbon‐doped TiO2/fly ash support(C‐TiO2/FAS)composite photocatalyst was successfully synthesized through sol impregnation and subsequent carbonization.The carbon dopants were derived from the organic species generated during the synthesis of the C‐TiO2/FAS composite.A series of analytical techniques,such as scanning electron microscopy(SEM),attenuated total reflection‐Fourier transform infrared(ATR‐FTIR)spectroscopy,X‐ray photoelectron spectroscopy(XPS),and ultraviolet‐visible diffuse reflectance spectroscopy(UV‐Vis DRS),were used to characterize the properties of the prepared samples.The results indicated that C‐TiO2 was successfully coated on the FAS surface.Coupling between C‐TiO2 and FAS resulted in the formation of Si–O–C and Al–O–Ti bonds at their interface.The formation of Si–O–C and Al–O–Ti bonds gave rise to a positive shift of the valence band edge of C‐TiO2 and enhanced its oxidation capability of photogenerated holes as well as photodegradation efficiency of methyl orange.Moreover,the C‐TiO2/FAS photocatalyst exhibited favorable reusability and separability.This work may provide a new route for tuning the electronic band structure of TiO2.

Adsorption of residual amine collector HAY from aqueous solution by refined carbon from coal fly ash and activated carbon

Refined carbon(RC) derived from coal fly ash(CFA) as well as powdered activated carbon(PAC) was investigated as adsorbent to remove residual amine collector HAY from aqueous solution.The RC and PAC were characterized by scanning electron microscopy(SEM),surface area measurement,Zeta potential measurement and Fourier transform infrared(FTIR) spectroscopy.The effect factors and mechanisms of HAY adsorption onto RC and PAC were studied in detail.The results show that the experimental kinetic data agree well with the pseudo second-order equation,and the Langmuir isotherm model is found to be more appropriate to explicate the experimental equilibrium isotherm results than the Freundlich model.The adsorption capacities of PAC and RC increase with pH.It is found that alkaline condition is conducive to the adsorption of HAY onto PAC and RC and the adsorption efficiency of RC is close to PAC at pH near 11.Zeta potential variation of adsorbents suggests that HAY generates electrostatic adsorption onto RC and PAC.FTIR analysis shows that the adsorption is dominantly of a physical process.The Box-Behnken design optimization conditions of process are RC 1 g/L,pH 11,temperature 302 K and initial HAY concentration 100 mg/L.Under these conditions,the measured adsorption ratio and adsorption capacity are 87.91%and 87.91 mg/g,respectively.Thus,the RC is considered to be a potential adsorbent for the removal of residual amine from aqueous solution.

Research on Existing Pattern of Carbon and Its Removal from Fly Ash

Fly ash is a fine and dispersed powder discharged from power station after the coal being burned. With the deepening of people’s recognition about the pollution problem of fly ash, the ways of utilizing fly ash are gradually increasing. Utilizing value of fly ash is closely related to the unburned carbon content.On the basis of analysis of modern testing method,a fundamental thinking is theoretically posed for decreasing unburned carbon content from fly ash by a dry removing carbon technology. The triboelectric separation method shown that the above mentioned thinking of dry removing carbon from fly ash is practical.

Characterization of LDPE Reinforced with Calcium Carbonate—Fly Ash Hybrid Filler

The synergetic effect of calcium carbonate (CC)-fly ash (FA) hybrid filler particles on the mechanical and physical properties of low density polyethylene (LDPE) has been investigated. Low density polyethylene is filled with varying weight percentages of FA and CC using melt casting. Composites are characterized for mechanical, thermal, microstructural and physical properties. Results show that the flexural strength increases with increases in FA content of the hybrid filler. It is evident from the study that to achieve optimum density a certain combination of both fillers need to be used. The optimum combination of CC and FA for a higher density (1.78 g/cm3) is found to be at 20 wt% FA and 30 wt% CC. An increase of 7.27% in micro-hardness over virgin polyethylene is obtained in composites with 10 wt% FA and 40 wt% CC. The presence of higher amount of CC is seen to be detrimental to the crystallinity of composites. X-ray, FTIR and DSC results show that composite with 45 wt% CC and 5 wt% FA exhibits a typical triclinic polyethylene structure indicating that the composite is amorphous in nature. There was the synergy between FA and CC fillers on flexural strength and crystallinity of composite. However, the fillers show the antagonistic effect on energy at peak and micro-hardness.

Mechanical Properties of Cement Mortar Containing Fine-Grained Fraction of Fly Ashes

This paper presents the effect of fly ash grain-size fractions on Portland-fly ash cement hydration and its properties. Siliceous fly ashes of size fraction of 0 - 16 and 16 - 32 μm, separated from initial fly ash samples from 1st, 2nd and 3rd hopper in ESP system, were analysed. Cement hydration was investigated by determination of hydration heat and content of Ca(OH)2 and C3S in cement samples. Water to cement ratio and initial setting time of cement pastes as well as compressive strength and microstructure of cement mortars were also analyzed. Results showed that the same amount of the same size ash fraction can give cement of lower or higher early strength and its lower or higher increase with time. Incorporation of 20 wt% of ash fraction of 0 - 16 μm can produce Portland-fly ash cement CEM II/A-V of strength class 42.5R (from 2nd hopper) or 52.5N (from 3rd hopper). Cement containing 40 wt% of ash fraction of 0 - 16 μm from 2nd and 3rd hopper can be classified as pozzolanic cements CEM IV/A-V of strength class 42.5 and normal or rapid early strength, respectively. Different development of strength of cement with addition of the same size ash fraction separated from the initial ash sample from the next hopper in ESP system is connected with higher depolymerization degree of SiO4 units in ash glass, resulting from the greater amount of AlO4 units replacing SiO4 units. Ash fraction of 16 - 32 μm shows lower depolymerisation of glass network and as a consequence lower hydration degree of C3S to portlandite and calcium silicate hydrates (C-S-H).

Analysis of Carbon Nanotubes Produced by Pyrolysis of Composite Film of Poly (Vinyl Alcohol) and Modified Fly Ash

We determined the catalytic function of chemically modified fly ash (MFA) for the growth of carbon nanotube (CNT) ropes with ~54% yield by the pyrolysis of the composite film of poly (vinyl alcohol) (PVA) at 500°C for 10 min under 2 L/min flow of nitrogen. Fly ash was treated with 2M sodium hydroxide to have MFA and used with PVA to fabricate the composite film by aqua casting. CNT was analyzed using SEM, TEM, XPS and Raman spectroscopy. The growths of CNT on MFA surfaces were visualized with different geometric self-assembly, e.g., bundles of CNT in ropes, twisted ropes, Y-branch ropes and staked-cone sheet. Thus, the mixtures of CNT ropes and MFA are a potential filler material for fabricating composites with polymer and metal.

Growth of Carbon Nanotubes by Pyrolysis of Composite Film of Poly (Vinyl Alcohol) and Modified Fly Ash

We found carbon nanotube (CNT) materials by the pyrolysis of the composite film of poly (vinyl alcohol) (PVA) reinforced with modified fly ash (FA) at 500°C for 10 min under 2 L/min flow of nitrogen. Fly ash was treated with 2M sodium hydroxide and used with PVA to fabricate the composite film by aqua casting. CNT materials were analyzed using XPS, Raman, SEM and TEM. The admixtures of CNT materials and FA are a potential filler material for fabricating composites with polymer and metal. The process is an eco-friendly recycling paradigm for using value-added advanced products for the proper management of sustainable waste materials, plastic and FA.

Vapor-phase elemental mercury adsorption by residual carbon separated from fly ash

The adsorption capacity for vapor-phase elemental mercury(Hg0) of residual carbon separated from fly ash was studied in an attempt for the control of elemental mercury emissions from combustion processes. At low mercury concentrations(<200 μg/m3), unburned carbon had higher adsorption capacity than commercial activated carbon. The adsorbality of unburned carbon was also found to be source dependent. Isotherms of FS carbon(separated from fly ash of a power plant of Shishi in Fujian Province) were similar to those classified as typeⅡ. Isotherms of XJ carbon(separated from fly ash of a power plant of Jingcheng in Shanxi Province) were more like those classified as type Ⅲ. Due to the relatively low production costs, these residual carbons would likely be considerably more cost-effective for the full-scale removal of mercury from combustion flue gases than other technology.

Enhancing fly ash utilization in backfill materials treated with CO_(2)carbonation under ambient conditions

The environmental concerns resulting from coal-fired power generation that produces large amounts of CO_(2)and fly ash are of great interest.To mitigate,this study aims to develop a novel carbonated CO_(2)-fly ash-based backfill(CFBF)material under ambient conditions.The performance of CFBF was investigated for different fly ash-cement ratios and compared with non-CO_(2)reacted samples.The fresh CFBF slurry conformed to the Herschel-Bulkley model with shear thinning characteristics.After carbonation,the yield stress of the fresh slurry increased significantly by lowering fly ash ratio due to gel formation.The setting times were accelerated,resulting in approximately 40.6%of increased early strength.The final strength decreased when incorporating a lower fly ash ratio(50%and 60%),which was related to the existing heterogeneous pores caused by rapid fluid loss.The strength increased with fly ash content above 70%because additional C-S(A)-H and silica gels were characterized to precipitate on the grain surface,so the binding between particles increased.The C-S(A)-H gel was developed through the pozzolanic reaction,where CaCO_(3)was the prerequisite calcium source obtained in the CO_(2)-fly ash reaction.Furthermore,the maximum CO_(2)uptake efficiency was 1.39 mg-CO_(2)/g-CFBF.The CFBF material is feasible to co-dispose CO_(2)and fly ash in the mine goaf as negative carbon backfill materials,and simultaneously mitigates the strata movement and water lost in post-subsurface mining.

Comparisons of Fly Ash and Deposition Between Air and Oxy-Fuel Combustion in Bench-Scale Fluidized Bed with Limestone Addition

In Oxy-fuel circulating fluidized bed,the residual Ca O particles may react with high concentration of CO2 in flue gas to form bonded deposit on heat transfer surfaces in backpass when limestone is used as a sorbent to capture SO2.In this paper,experiments were designed on ash deposition in a bench-scale fluidized bed under oxy-fuel and air atmosphere. A novel ash deposit sampling probe was used to simulate the tubes of tail surfaces.The chemical composition of fly ash and ash deposit from both air-firing and oxy-fuel firing cases were analyzed by Inductively Coupled Plasma-Atomic Emission Spectrometry( ICP-AES) and Scanning Electron Microscopy( SEM),respectively. The degrees of carbonation reaction of ash deposits were measured by Thermo Gravimetric Analysis. The results showed that there are distinct differences in fly ash deposition rate between oxy-fuel and air firing cases,and oxy-fuel combustion with limestone addition can affect chemical composition of fly ash and ash deposit,especially for elements of Ca,Na,K,and S. However,the carbonation reaction degree of ash deposits is found weak,which is due to the relatively low Ca O content in ash deposit or not long enough of the sampling time.

Main influencing factors and coal fly ash characteristics of gasoues fuel reburning process

The unburned carbon concentration in fly ash and the influence of main factors on the reduction of nitrogen oxides during gaseous fuel reburning process were experimentally studied in a 36 kW down-fired furnace when five typical coals with different qualities were served as the primary fuel. It is found that the higher nitrogen oxide reduction efficiency can be obtained by reburning process when the coal used as the primary fuel contains more volatile matter. But under the optimizational operating conditions, both above 50% nitrogen oxide reduction and low carbon loss can be achieved by reburning process even though the primary fuel is the low-volatile coal. The experimental results show that the reasonable residence time in reburn zone is 0.6-0.9 s, the appropriate gaseous reburn fuel percentage is 10%-15% and the optimal average excess air coefficient in reburn zone is 0.8-0.9. These results extend the ranges of the key parameter values for reburning process with respect to that the low-volatile coals are used as the primary fuel.

Soil Properties in Coniferous Forest Stands Along a Fly Ash Deposition Gradient in Eastern Germany

Physical, chemical, and microbial properties of forest soils subjected to long-term fly ash depositions were analyzed in spruce (Picea abies (L.) Karst.) stands of eastern Germany on three forest sites along an emission gradient of 3 (high input), 6, and 15 km (low input) downwind of a coal-fired power plant. Past emissions resulted in an atypical high mass of mineral fly ash constituents in the organic horizons at the high input site of 128 t ha-1 compared to 58 t ha-1 at the low input site. Magnetic susceptibility measurements proved that the high mineral content of the forest floor was a result of fly ash accumulation in these forest stands. Fly ash deposition in the organic horizons at Site I versus III significantly increased the pH values, effective cation exchange capacity, base saturation and, with exception of the L horizon, concentrations of mobile heavy metals Cd, Cr, and Ni, while stocks of organic C generally decreased. A principal component analysis showed that organic C content and base status mainly controlled soil microbial biomass and microbial respiration rates at these sites, while pH and mobile fractions of Cd, Cr, and Ni governed enzyme activities. Additionally, it was hypothesized that long-term fly ash emissions would eventually destabilize forest ecosystems. Therefore, the results of this study could become a useful tool for risk assessment in forest ecosystems that were subjected to past emissions from coal-fired power plants.

MSWI Fly Ash Based Novel Solidification/Stabilization Matrices for Heavy Metals

The possibilities of MSWI fly ash as a major constituent of novel solidification/stabilization matrices for secure landfill were investigated by mixing MSWI fly ash with rich aluminum components, which was added as bauxite cement or metakaolinite instead, to form Friedel and Ettringite phases with high fixing capacities for heavy metals. The physical properties, heavy metals-fixing capacity, mineral phases and its vibration bands in the novel matrices were characterized by compressive strength, TCLP（toxic characteristic leaching procedure）, XRD （x-ray diffraction） , DTG （derivative thermogravimetry）, and FTIR （fourier transform infrared spectroscopy）, respectively. The Tessier＇s five-step sequential extraction procedure was used to analyze the fractions of chemical speciation for Pb, Cd and Zn ions. The experimental results indicate that Friedel-Ettringite based novel solidification/stabilization matrices can incorporate Pb, Cd and Zn ions effectively by physical encapsulation and chemical fixation, and it exhibits a great potential in co-landfill treatment of MSWI fly ash with some heavy metals-bearing hazardous wastes.

Effects of Carbonation and Freeze-thaw Cycles on Microstructure of Concrete

The hydraulic concrete durability under the alternation of freeze-thaw and carbonation has been systematically investigated in this work, where both the micro part and the microscopic characteristics of concrete interface were analyzed based on computed tomography（CT） test and scanning electron microscopy（SEM）. Average CT numbers of each section, declined at water-cement ratio of 0.35, increased at 0.45, and changed a little at 0.55. The specimen in the absence of fly ash exhibited less types of hydration products and the surface was observed to be a needle-like ettringite, with a relatively dense overall structure. However, with the increase of fly ash content, pores and micro-cracks of specimen structure increased. Hexagonal flake calcium hydroxide, present in the specimen after the first carbonation, was negligible in the test pieces of the first freezethaw where the main hydration products were ettringite and calcium silicate gel. Regular hexagonal plates of calcium hydroxide exhibited in the interior of the specimen in which charring first occurred but calcium hydroxide rarely existed in the interior of the specimen in which freeze-thaw first occurred.

Toughness Improvement of Geothermal Well Cement at up to 300^&deg;C: Using Carbon Microfiber

This study aimed at assessing the usefulness of carbon microfiber (CMF) in improving the compressive-toughness of sodium metasilicate-activated calcium aluminate/Class F fly ash foamed cement at hydrothermal temperatures of up to 300&deg;C. When the CMFs came in contact with a pore solution of cement, their surfaces underwent alkali-caused oxidation, leading to the formation of metal (Na, Ca, Al)-complexed carboxylate groups. The extent of this oxidation was enhanced by the temperature increase, corresponding to the incorporation of more oxidation derivatives at higher temperatures. Although micro-probe examinations did not show any defects in the fibers, the enhanced oxidation engendered shrinkage of the interlayer spacing between the C-basal planes in CMFs, and a decline in their thermal stability. On the other hand, the complexed carboxylate groups present on the surfaces of oxidized fibers played a pivotal role in improving the adherence of fibers to the cement matrix. Such fiber/cement interfacial bonds contributed significantly to the excellent bridging effect of fibers, resistance to the cracks development and propagation, and to improvement of the post-crack material ductility. Consequently, the compressive toughness of the 85&deg;-, 200&deg;-, and 300&deg;C-autoclaved foamed cements reinforced with 10 wt% CMF was 2.4-, 2.9-, and 3.1-fold higher than for cement without the reinforcement.

Comparison of Prairie and Eroded Agricultural Lands on Soil Organic Carbon Retention (South Dakota)

The primary objective of this research was to predict changes in soil organic carbon (SOC) and total soil nitrogen (TSN) stocks as a result of land use change from prairie to agricultural land if the mesic-frigid temperature line moved north in the US and the former frigid soils were cultivated. The conversion of prairie to agricultural use, as a result of climate shift, would release SOC to atmosphere and enhance greenhouse gas emissions. The SOC and TSN differences between the prairie site and agricultural land were compared in South Dakota. The agricultural land had 18% less SOC and 16% less TSN or only half of the expected loss from prairie levels. An attempt was made to document the land use history of the prairie site to understand why SOC and TSN losses were less than anticipated. The fly ash concentration levels on prairie side slopes suggested that the prairie was historically disturbed and eroded. Intensive grazing and burning contributed to the disturbance. The SOC and TSN stock losses appear to represent the minimal change that would occur in the next 100-year time period if a prairie was shifted to agricultural use as a result of climate shift and the mesic-frigid temperature line in US was to move north.

Bond Durability of Carbon-Microfiber-Reinforced Alkali-Activated High-Temperature Cement Adhering to Carbon Steel

The study aims at evaluating the bond durability of a carbon microfiber (CMF)-reinforced alkali-activating calcium aluminate cement (CAC)/fly ash F (FAF) blend cementitious material adhering to carbon steel (CS) under stresses induced by a 350℃ heat-25℃ water cooling cycle. This cementitious material/CS joint sample was originally prepared in an autoclave at 300℃ under a pressure of 8.3 MPa. For comparison, two reference geothermal well cements, Class G modified with silica (G) and calciumaluminum phosphate (CaP), were employed as well reinforced with CMF. In the CAC/FAF blending cement systems, the CAC-derived cementitious reaction products preferentially adhered to CS surfaces, rather than that of FAF-related reaction products. CMF played a pivotal role in creating tough interfacial bond structure of cement layer adhering to CS. The bond toughness also was supported by the crystalline cementitious reaction products including sodalite, brownmillerite, and hedenbergite as major phases, and aragonite, boehmite, and garronite as minor ones. The brownmillerite as an interfacial reaction product between cement and CS promoted the chemical bonding of the cement to CS, while the other phases served in providing the attractive bonding of the cement to CS. The post-stress-test joint samples revealed the formation of additional brown-millerite, aragonite, and garronite, in particular brownmillerite as the major one. The combination of chemical bonding and self-advancing adherence behavior of the cement was essential for creating a better interfacial bond structure. A similar interfacial bond structure was observed with CaP. The crystalline phase composition of the autoclaved cement revealed apatite, zeolite, and ferrowyllieite as major reaction products, and aragonite and al-katoite as the minor ones. Ferrowyllieite was identified as cement/CS interfacial reaction product contributing to the chemical bond of cement, while the other phases aided in providing the attractive bond of cement. After a stress test, two phases, ferrowyllieite and aragonite, promoted the self-advancing adherence of cement to CS. However, the effectiveness of these phases in improving adherence performance of cement was less than that of CAC/FAF blend cement, reflecting the fact that the bond durability of CAC/FAF blend cement was far better than that of the CaP. In contrast, the autoclaved silica-modified G cement consisting of xonotlite, and 0.9 nm-to-bermorite and riversideite, with calcite as the crystalline reaction products, had no significant effect on improving the shear bond strength and the bond’s toughness. No interaction product with CS was found in the cement adhering to CS. After a stress test, the calcite phase acted only to promote the self-advancing adherence of cement, but its extent was minimal compared with that of the other cements, thereby resulting in poor bond durability.

Benefaction from Carbonation of Flue Gas CO_(2) as Coal Mining Filling

CO2 capturing, transport and sequestration by pressurized water dissolution and reacting by natural alkali lime and magnesia in coal fly ash or other sources become an industrial advantageous sequestration option resulting in green waste solutions or solid fines. Mg and Ca containing minerals are reacting with CO2 to form carbonates. Various types of fly ash materials may react with CO2 to form carbonate regarding ash composition and reaction parameters. Mineral sequestration of CO2 will also allow using the products in cement industry or as cement material in constructions with low cost. This paper discussed progress on coal mining filling by carbonation method using coal fly ash of Soma, Yatagan, Afsin Elbistan Power Stations. Other filler materials containing coal mine waste shale, fly ashes and foam concrete, and additives were searched for pretreatment methods to enhance cement reactivity;and in analyzing the structural changes to identify reaction paths and potential barriers.

Modeling mercury adsorption on carbon particles in simulated flue gas

A model was developed to describe the adsorption characteristic of mercury in flue gas based on one residual carbon sample and one activated carbon sample. The differential equations were established with mass balance of mercury in the gas phase and in the solid phase. Then the model was solved using a Matlab program with a Runge-Kutta process. The mercury adsorption isotherms of these two adsorbents were obtained by breakthrough column experiments. The results show that at low gas phase mercury concentrations （ 〈 0. 3 mg/ m^3）, the adsorption equilibrium of residual carbon is in accord with the case of a type Ⅱ isotherm of the Freundich theory. Whereas the data of activated carbon falls into the Langmuir relationship, it is the case of a type Ⅲ isotherm. The experimental data were fitted to the Freundlich model by Matlab software. The variances of mercury concentration are smaller than 0. 81 which implies the agreement between measurements and simulation is quite agreeable considering the wide scatter of the measurements. This model is useful for forecasting mercury removal efficiency and is helpful to the mechanism analysis of mercury adsorption on carbon-based adsorbent.