Circulating Fluidized Bed Fly Ash Research Articles

Unraveling the interactive effects of Na2O/Al2O3, SiO2/Al2O3 and calcium on the properties of geopolymers from circulating fluidized bed fly ashes

Circulating fluidized bed fly ash (CFB-FA), generated as solid waste from the thermal power plants with different contents of calcium. The reuse of CFB-FA for construction materials is the most promising method for the large-scale transformation of waste to resource. This paper examines the ratio effects of Na2O/Al2O3 and SiO2/Al2O3 on geopolymers from low-calcium fly ash (CFB-FA1) and high-calcium fly ash (CFB-FA2) by using a mixture of Na2SiO3 and NaOH as the alkali activator. The detailed mechanistic study with XRD, FTIR, SEM, and MAS NMR is also conducted for the roles of silicon, aluminum, and calcium in the reaction process and their mutual interaction mechanism, along with the regulating mechanism for the compressive strength and setting time of geopolymers. The results indicate that the increased ratios of Na2O/Al2O3 and SiO2/Al2O3 are favorable for the reactions and reduce the setting time of CFB-FA1 and CFB-FA2 based geopolymer. In CFB-FA1, the higher silicon and aluminum content is conducive to their dissolution by regulating the Na2O/Al2O3 ratio, forming N-A-S-H and resulting in the increased strength. A small amount of calcium mainly exists in the form of CaSO4, which completely participates in the reaction. In CFB-FA2, the silicon and aluminum contents are relatively low, while the calcium content is high. The compressive strength is achieved by regulating the SiO2/Al2O3 ratio to increase the dissolution of silicon and aluminum, allowing CaSO4 and CaO to react and form C-A-S-H and ettringite, with CaCO3 as unreactive. The low calcium is favorable for the long-term strength, but the high calcium acts oppositely, primarily attribute to the decomposition of ettringite, a critical component for maintaining the compressive strength of CFB-FA2 based geopolymers.

High-volume utilization of circulating fluidized bed fly ash for the production of autoclaved aerated concrete: Performance optimization and hydration characteristics

Circulating fluidized bed fly ash (CFA) is a waste emitted from coal combustion process in circulating fluidized bed boilers. The high SO3 and f-CaO content severely limits its application in building materials. The purpose of this study is to explore the possibility of large-scale application of CFA in autoclaved aerated concrete (AAC). The preparation parameters of AAC were discussed and the performance was optimized. Moreover, the workability and environmental properties of AAC were explored. The results indicated that AAC was well adapted to high CFA content, although a high CFA content caused a higher water demand as well as a reduction in the silicon content. The introduction of quartz sand and blast furnace slag effectively addressed the problems of insufficient active silica and high specific gravity. When the CFA doping was 50 %, H4 sample showed the best mechanical properties (3.82 MPa and 641 kg/m3), which meet the requests of A3.5, B06 in the Chinese national standard GB/T 11968–2020. Moreover, the toxicity leaching results showed that H4 sample was green and harmless. During the performance optimization process, more well-crystallized tobermorite was produced, whose crystal form changed from acicular to slat. Massive C-S-H gels and better crystallinity of tobermorite were observed, where were bonded and intercalated with each other, thus enhancing the properties. The paper may provide guidance for high dosing of CFA in AAC.

Process exploration for scale melting and solidifying of municipal solid waste incineration (MSWI) fly ash by horizontal cyclone melting furnace

This study used the horizontal tubular heating furnace to explore the melting potential of circulating fluidized bed (CFB) incinerator fly ash and mechanical grate furnace (MGF) incinerator fly ash. The horizontal cyclone melting furnace was then built to explore further the feasibility of scale melting of MSWI fly ash. The melting characteristic temperature, amorphous content, and heavy metal leaching concentration characterized the melting potential and solidification effect of MSWI fly ash. The experimental results show that the amorphous content of CFB fly ash after melting is up to 92.37%, and the volatilization rate of heavy metals Zn, Pb, and Ni does not exceed 30%. MGF fly ash exhibits the “sintering into shells” phenomenon during heating, and the leaching concentrations of heavy metals Pb in the sintered products still exceed the standard limits. In addition, the volatilization rates of heavy metals Cu, Zn, Cd, Pb, Cr, and Ni in Slag II are above 50%, and the volatilization rate of Cr reaches 85%. So, slag’s amorphous content also affects heavy metals’ volatilization rate. The MSWI fly ash melting characteristic temperature decreases with the decrease of alkalinity value. When the alkalinity value drops to 0.6, the melting characteristic temperature reaches its lowest value. Mixing 80% CFB fly ash or 50% MGF bottom ash into MGF fly ash can significantly enhance the melting potential to reduce hazardous waste. When using the horizontal cyclone melting furnace to process MSWI fly ash on a large scale, MSWI fly ash achieves an excellent melting effect with an amorphous content of over 93% at the positions of the furnace middle section, inner tail cone, slag discharge outlet, and flue gas outlet. The fly ash particles are in motion in the melting furnace, so the particle size distribution affects the melting effect of MSWI fly ash.

Mechanical properties, microstructure and life-cycle assessment of eco-friendly cementitious materials containing circulating fluidized bed fly ash and ground granulated blast furnace slag

To reduce CO2 emission from the cement industry and solve environmental issues caused by the accumulation of circulating fluidized bed fly ash (CFA), this study aimed to prepare eco-friendly cementitious materials using CFA and ground granulated blast furnace slag (GGBS) as supplemental cementitious materials. The hydration properties, reaction kinetics, microstructure and life-cycle assessment of blended cements were investigated. The results showed that the compressive strengths of the blended cement samples containing 10 % CFA and 20 % GGBS increased by 7.3 % and 5 % at 7d and 28d, respectively, compared to the pure cement paste. The hydration process was characterized using isothermal calorimetry and analyzed by the Krstulovic-Dabic (K-D) model. The blended cement paste with the best compressive strength exhibited the highest total hydration heat release throughout the hydration process, with extended reaction times. The synergistic effect of CFA and GGBS promoted the formation of C-(A)-S-H gel, consuming more portlandite and leading to a decrease in the most probable pore size and an increase in gel pore volume in the pore structure. From the perspective of cost, economy, and environment, the normalized strength cost, energy consumption efficiency index, and CO2 intensity index can be reduced by 16.1 %, 27.1 %, and 33.3 % respectively. This work contributes to a better understanding of the synergistic mechanism in the CFA-GGBS blended cement system, and serves as a significant reference for the development of economical and eco-friendly cementitious materials.

In-depth analysis of macro-properties and micro-mechanism of eco-friendly geopolymer based on typical circulating fluidized bed fly ash

Geopolymer is an eco-friendly building material that significantly reduces energy consumption and CO2 emissions. The increase in circulating fluidized bed fly ash (CFBFA) production presents an opportunity to use CFBFA in geopolymer synthesis, mitigating environmental hazards and promoting a sustainable, low-carbon cement industry. However, the complex transformation of amorphous aluminosilicates in CFBFA into geopolymer gels and the impact of calcium (Ca) composition on the macro-properties and micro-mechanism of geopolymers are not well understood, hindering large-scale utilization of the CFBFA-based geopolymer. In this study, low-Ca CFBFA (LCFA) and high-Ca CFBFA (HCFA) were used as precursors, with NaOH as the activator. The workability, mechanical properties, and microstructure evolution of the geopolymer samples were systematically compared. The LCFA-based geopolymer slurry exhibited ideal workability. The high Ca content in HCFA increased the water demand of the slurry, thereby reducing its workability. Compared to LCFA-based geopolymer samples, although Ca-containing components in HCFA accelerated the geopolymerization processes, some gels transformed into irregular crystalline phases during the later stages of curing, compromising the dense structure and slowing the development of mechanical properties. The evolution of the Si–Al coordination structure showed that LCFA-based geopolymers favored the development of Si-rich gels (Q4(1Al) and Q4(2Al)), whereas HCFA-based geopolymers exhibited a preference for Al-rich gels (Q4(3Al) and Q4(4Al)). The results of this research can provide valuable insights for the synthesis of CFBFA-based geopolymers.

Revealing the intrinsic sintering mechanism of high-strength ceramsite from CFB fly ash: Focus on the role of CaO

The preparation of high-strength ceramsite from circulating fluidized bed (CFB) fly ash is an effective approach for commercial utilization. The role of CaO plays a key factor in the performance of ceramsite. The impact of CaO on the sintering mechanism was studied at the sintering temperature from 1200 °C to 1300 °C and the CaO addition ratio from 2 wt% to 8 wt%. The in-situ visual furnace equipped with a high-resolution microscope, incorporated with TG-DSC-FTIR, SEM, XRD, and the semi-quantitative analysis method of XRD, was conducted to investigate the sintering process of ceramsite. The results verified that during the high-temperature sintering, CaO reacted with SiO2 and Al2O3 in CFB fly ash to constitute low-melting minerals like anorthite, and the elevation of sintering temperature expedited the conversion of anorthite and eutectics into the molten liquid phase. The expansion gas was wrapped by the molten liquid phase and reached equilibrium, which contributed to the closure of the pores and the elevation of the shrinkage rate of ceramsite. In addition, the rise in sintering temperature dropped the amorphous phase content and increased the mullite content, further enhancing the compressive strength. When the sintering temperature was in the range of 1250 °C–1300 °C and the CaO content was in the range of 2 wt%-4 wt%, the high-strength ceramsite with excellent physicochemical properties could be obtained. It suggested that the synergistic impacts of sintering temperature and CaO improved the performance of ceramsite.

Mechanical properties of solid waste-based composite cementitious system enhanced by CO2 modification

Utilizing CO2 to modify solid waste and produce high-performance cementitious material is a promising strategy. In this study, circulating fluidized bed fly ash (CFBFA), calcium carbide slag (CS) and flue gas desulphurization gypsum (FGDG) were prepared by mixing CO2 to produce cementitious materials. The impacts of CO2 on mechanical properties and carbonation reaction mechanism of composite materials were investigated. The results shown that the microstructure and mechanical properties of the gelling materials were improved by CO2 modification. When the CO2 mass transfer conditions as followed: CO2 flow rate was 1.5 L/min, stirring rate at 600 r/min, and stirring time at 20 min, compressive strength of modified material was 22 % higher than that of the non-CO2 modified material. Additionally, the density increased by 3.2 %, and porosity was reduced by 34.1 %. Among them, TG/DTG analysis found that the highest effective fixed CO2 rate was 11.1 %. The ultrasonic nondestructive testing also shown that the structure of the CO2-modified cementitious material was significantly better than that of the CFBFA-CS-FGDG composite cementitious system, and the ultrasonic loss rate was reduced by 9.4 %. Therefore, the use of CO2 to modify the composite system was of great significance for the realization of CO2 storage and the improvement of the properties of composite materials. One interesting future possibility is solid waste-based grouting materials production at energy plants: CFBFA, CS and FGDG can be used to capture CO2 from flue gases, thus improving the strength of produced grouting materials.

Formulation determination and performance study of high-volume fly ash-based grouting fire extinguishing materials

Considering the continued advancement of China's industrial sector, a substantial quantity of circulating fluidized bed fly ash (CFBFA) and calcium carbide slag (CS) is generated throughout the production process. Considering the rich calcium content inherent in CFBFA (circulating fluidized bed fly ash) and the inherently alkaline nature of CS(carbide slag), the amalgamation of these two constituents leads to a multitude of hydration reactions, culminating in the synthesis of polymeric grouting materials. This study undertaked a comprehensive examination and regulation of three factors within the binary system: water-cement ratio, CS content, and CS particle size. Through the utilization of scanning electron microscopy (SEM), X-ray diffraction (XRD), and (TG) thermogravimetric analysis, the investigation aims to elucidate the influence mechanism of the composite gelling system comprising fly ash and calcium carbide slag on the compressive strength, setting time, and other pertinent indicators of grouting materials. The data reveals that when the water-cement ratio is set at 0.85, CS content at 30%, and CS ball milling time at 1 h, the resulting grouting material exhibits a fluidity of 279 mm, a water separation ratio of 3.13%, and achieves a strength of 15.6 MPa at 28 days. It boasts a stable and easily transportable slurry, which possesses a certain degree of cementitious performance, effectively sealing off any air leakage channels. Moreover, by subjecting CS to appropriate pre-treatment measures, a suitable quantity of active silicon and aluminum elements can be thoroughly dissolved within the CFBFA (circulating fluidized bed fly ash) slurry system, giving rise to a staggered distribution and dense hydration product structure. Considering the requirements for gangue mountain fire extinguishing slurry, a CFBFA-CS based grouting material formula has been carefully selected. Additionally, simulation grouting fire extinguishing experiments have demonstrated that the injection of the slurry is highly effective in reducing the temperature of the combustion test block, isolating oxygen, and lowering the temperature within the combustion zone. After 5 h, the temperature drops to below 100 °C, and no re-ignition phenomena are observed. The outcomes of this research can provide valuable scientific guidance for the development of high-performance fire extinguishing grouting materials utilizing fly ash and carbide slag.

The effects and solidification characteristics of municipal solid waste incineration fly ash-slag-tailing based backfill blocks in underground mine condition

Currently, there is a limited understanding of the material performance effects and hydration mechanisms of solidified waste-based binder filling materials containing municipal solid waste incineration fly ash in the actual underground filling environment. This study utilized the Circulating Fluidized Bed (CFB) and Grate Furnace (GF) techniques to produce quadri-component uncalcined binder fill materials from municipal solid waste incineration fly ash (MSWI FA), blast furnace slag (BFS), flue gas desulfurization (FGD) gypsum, and tailings. The solidified test blocks were then cured in-situ in a void space of a certain iron underground mine in Hebei with conditions (temperature: 25-30℃, humidity: 18-19%, mine water influx: annual average of 150m3/h). The research results indicate that the mechanical performance and heavy metal solidification outcomes of these blocks surpassed those samples cured under laboratory conditions (temperature: (20±1)℃, humidity: not less than 90%), demonstrating the feasibility of using MSWI FA for underground binding and backfilling in mines. Building upon preliminary research, a binder mix proportion was established: 20wt.% MSWI FA, 70wt.% BFS, and 10wt.% FGD gypsum. Test blocks were fabricated using lead-zinc tailings sand and iron mine tailings as aggregates. All blocks met the 28-day strength requirements for typical mine backfills ranging from 1 to 6.5MPa, with the strength of the XB-M group reaching 24MPa (when cured underground). The leaching concentrations of heavy metals were all below the limits set for class III groundwater standards, with Pb, Zn, Cr, and As achieving a solidification rate of over 98% at 28 days. Microscopic hydration mechanisms indicate that the primary hydration products in the solidified body were ettringite, Friedel's salt, and C-S-H gel. The high content of reactive Al2O3 in the CFB fly ash enables the [AlO4]5- to substitute [SiO4]4-, leading to the formation of a long-chain, high polymerization C-A-S-H phase. Additionally, the abundant Cl- in GF fly ash readily reacted with [Al(OH)6]3−, forming Friedel's salt. The hydration products in the blocks cured under mine conditions significantly increased in their degree of polymerization. This provides a practical application approach for the future treatment and reuse of MSWI FA. Environmental implicationMSWI FA is classified as hazardous waste (HW18) due to its content of harmful components such as chlorides, dioxins, and soluble heavy metals, which pose potential environmental pollution risks. This study, for the first time, solidified test specimens were sent to an underground mining void in a certain iron mine in Hebei, China, for on-site curing (temperature range: 25-30°C, humidity: 18-19%, annual average mine water inflow rate: 150m3/h). The mechanical properties and the solidification performance of heavy metals in the test specimens were superior to those under laboratory curing conditions (temperature: (20±1)°C, humidity not less than 90%).

Mechanical properties and hydration behaviour of circulating fluidised bed fly ash- ground granulated blast furnace slag-lime ecofriendly cementitious material

As a major solid waste in coal-fired power plants, circulating fluidized bed fly ash (CFBFA) has a high content of SO3 and f-CaO, leading to a lack of avenues for its resource utilization. Using the characteristic of it, this study combined it with ground granulated blast furnace slag (GGBFS) to produce an ecofriendly cementitious material (CGL) without cement. At the CFBFA/GGBFS ratio of 3:2, the lime content of 20 wt% and the water–solid ratio of 0.45, the unconfined compressive strength of CGL mortar could achieve 33.44 MPa at 28 d. With the prolongation of curing age, high polymerized C-(A)-S-H gel with a foil-like morphology, as well as ettringite and hydrotalcite were continuously generated and filled the pores, resulting in a continuous decline of the porosity. Simultaneously, the proportion of small pores (<50 nm) increased continuously. In addition, CGL had significant environmental and economic advantages. Compared with Portland slag cement (32.5R), the energy consumption, carbon emissions, and production costs of CGL reduced by 73.51 %, 75.76 %, and 25.93 %, respectively. Therefore, this work offered a practical and novel way for resource utilisation of CFBFA.

Energy effective utilization of circulating fluidized bed fly ash to prepare silicon-aluminum composite aerogel and gypsum

To reduce the cost of Si-Al aerogels preparation, circulating fluidized bed fly ash (CFA) was developed to be as the alternative to synthetic precursors. High energy consumption of alkali-melting and secondary wastes production were the major challenges. Here, a technique characterized by effective energy consumption and non-secondary waste was developed to convert CFA into Si-Al aerogel. The process consists two stages, preparation of Si-Al sol by sintering of CFA and Na2CO3 followed by sulfuric acid leaching, and synthesis of Si-Al aerogel by so-gel with trimethyl chlorosilane modification and ambient pressure drying. The optimization results of proportion and sintering temperature showed that the optimal temperature of sintering of Na2CO3 and CFA with the mass ratio of 0.7 was 750 °C, 100 °C lower than that of most other waste aluminosilicate materials. CaSO4·0.5H2O which meet building gypsum requirement was obtained by specifying the drying temperature of acid-leached residue at 126 °C for 2 h. The modification procedure was explored to obtain Si-Al aerogel with a large specific surface area of 857 m2/g and hydrophobic angle of 139.3°. Thermal and mechanical properties tests indicated that the Si-Al aerogels and gypsum produced from CFA exhibited promising thermal insulation and the potential application in construction.

Insight into the synergic effects of circulating fluidized bed fly ash, red mud and blast furnace slag in preparation of ultrahigh-performance concrete: Reaction mechanism and performance optimization

Ultrahigh-performance concrete (UHPC) is a kind of special concrete with excellent mechanical properties and good flexibility that has attracted much attention. Partial replacement of cement with supplementary cementitious materials (SCMs) can significantly reduce costs and ensure UHPC performance. The composite preparation of cementitious materials using the physicochemical properties of different solid wastes has been proved to be effective. On this basis, the highly alkaline solid waste red mud (RM), highly pozzolanic activity solid waste containing sulfate circulating fluidized bed fly ash (CFA) and high pozzolanic activity solid waste blast furnace slag (BFS) were utilized to prepare low cement clinker UHPC. 20 groups of experiments were designed by response surface methodology (RSM). Then, the optimization and prediction of the performance of low cement UHPC was explored using simplex lattice method. In addition, the hydration degree and pore structure were characterized by X-ray diffraction (XRD), thermogravimetric analysis (TG), mercury intrusion porosimetry (MIP) and back scattered electron (BSE). In the case of 60% cement blending, the best compressive strength (128.15 MPa) occurred in 9% CFA, 28% RM and 3% BFS. The results demonstrated that the increasing strength existed with the addition of RM and a high content of CFA had a negative impact on performance development. In the cases of low cement systems (<45%), however, the compressive strength increased and then decreased as CFA content increased from 0 to 40% or RM content increased from 0 to 70%, which meant that a compound synergistic existed among the three solid wastes. Microstructure analysis demonstrates that the hydration degree and pore structure of UHPC were effectively improved due to synergies among CFA, RM and BFS. These results implied that the compound synergy is more significant in low clinker systems and beneficial to the development of strength. This paper provides a reference for low cement clinker and the low-cost preparation of UHPC.

Preparation, characterization, and rheological analysis of eco-friendly geopolymer grouting cementitious materials based on industrial solid wastes

Geopolymer grouting materials has been applied in underground engineering as a reinforcement material. At the same time, with the enhancement of human environmental awareness, low carbon environmental grouting materials have attracted widely attention. Therefore, this study used industrial solid wastes circulating fluidized bed fly ash (CFBFA), calcium carbide slag (CS), and flue gas desulfurized gypsum (FGDG) as the main raw materials to prepare eco-friendly geopolymer grouting materials. The compressive strength, setting time, fluidity and stone ratio analysis were systematically evaluated. Finally, the microscopic experiments, including X-Ray Diffraction, CT scanner, TG/DTG analysis, and Scanning Electron Microscopy (SEM), were conducted to investigate the mechanism of geopolymer grouting reinforcement materials based on CFBFA, CS and FGDG. The results showed that the solid-wasted grouting materials have a 28d compressive strength of 20.5 MPa, the stone ratio of 99.8%, the fluidity was 250 mm. And adding the 6% alkali activator may enhance the system's fluidity and setting time. In summary, this study analyzed the viability of producing high performance grouting materials for subterranean engineering utilizing industrial solid wastes CFBFA, CS, and FGDG, which are expected to be an alternative for cement as an eco-friendly grouting cementitious material.

The performance of bubble-mixed lightweight soil to relieve expansion of circulating fluidized-bed fly ash

The treatment of circulating fluidized bed (CFB) fly ash with bubble-mixed lightweight soil (BMLS) was proposed. Artificial air bubbles in BMLS were used to restrict the dilatancy of CFB fly ash mixed with cement. The effects of CFB fly ash on pore structure of BMLS with CFB fly ash (BMLS-CF) were investigated. The material composition of BMLS was evaluated by X-ray diffractometry, and the pore structure was characterized by scanning electron microscope and mercury intrusion porosimetry. The results show that sulfur compounds in CFB fly ash react with cement in BMLS-CF to form ettringite and gypsum, which adhere to pore sizes of various sizes. The pore sizes of 50 nm to 50 μm are the most affected by expansive substances. The pores in BMLS-CF can accommodate the formation of expansive substances, and the effect of the accommodation increases with the increases in volume and number of bubbles. When the CFB fly ash content is between 50% and 66.7% with the same air-corbubble content, the effect of the accommodation is the best. The unconfined compressive strength of BMLS-CF first increases, then decreases with the increase of the CFB fly ash content, and the peak arises at a CFB fly ash content of 66.7%. It is therefore recommended that the CFB fly ash content in BMLS-CF be 66.7–75%.

Hydration characteristics of cement with high volume circulating fluidized bed fly ash

Circulating fluidized bed fly ash (CFBFA) is a large industrial solid waste generated by clean circulating fluidized bed coal combustion technology. The coarse and porous particle structure and high content of f-CaO and II-CaSO4 are the main obstacles to the wide application of CFBFA as a supplementary cementitious material. In this study, the core–shell structure of f-CaO and II-CaSO4 were destroyed, and the pozzolanic activity of CFBFA was enhanced by ultrafine grinding. Physical activation can accelerate the mutual excitation of CFBFA and cement, promote the formation of ettringite (AFt) in the early stage of hydration, improve the microstructure, and the later strength of cement with 50 wt% UCFBFA can reach the level of pure cement. The hydration mechanisms of cements with pulverized fuel ash (PFA), CFBFA and ultrafine CFBFA (UCFBFA) were also comparatively analyzed and discussed. This study proves that ultra-fine grinding can effectively improve the reactivity of CFBFA, which provides ideas for the future research on the stability improvement of CFBFA-based cement and the large-scale resource application of CFBFA.

Utilization of coal gangue power generation industry by-product CFA in cement: Workability, rheological behavior and microstructure of blended cement paste

Circulating fluidized bed fly ash (CFA), by-product of coal gangue power generation industry based on circulating fluidized bed combustion technology, differs from ordinary fly ash (OFA) in its physical and chemical properties. This paper aims to investigate the influences of CFA with three particle sizes on the workability and rheological properties of cement paste. The results show that untreated raw CFA (RCFA) can significantly degrade many properties of fresh cement paste. Directly using CFA as a mineral admixture in cement-based materials is not suitable in terms of workability. However, CFA through grinding methods can significantly mitigate the detrimental effect of CFA on the flowability of cement paste. When 15% ultrafine CFA (UCFA) was added, the cement paste exhibited relatively favorable workability and rheological properties. The rheological behavior of cement paste containing CFA with three particle sizes follow the shear thinning, which is in line with the modified Bingham model. Furthermore, the reasons for the impact of CFA on the microscopic level of cement workability are analyzed and discussed. The addition of CFA reduces the amount of free solution in the cement paste, leading to the formation of a flocculated structure and an increase in the thixotropic loop area. Based on the analysis of TOC, BET and TEM, it was determined that numerous polycarboxylate superplasticizer (PCE) molecules are adsorbed onto the rough and porous surface structure of CFA particles, hence the destructive ability of PCE to the flocculation structure caused by cement hydration is weakened. Therefore, the mechanism that CFA affecting the working performance of cement paste can be determined as: the adsorption of free water and PCE molecules.

Preparation of CFB fly ash/sewage sludge ceramsite and the morphological transformation and release properties of sulfur

In order to improve the resource utilization of circulating fluidized bed (CFB) fly ash and sewage sludge, light-weight and high-strength CFB fly ash/sewage sludge ceramsite (CFBS ceramsite) was prepared by using CFB fly ash and original wet sludge as the main raw materials. The effects of dosages of CFB fly ash, sewage sludge, fluxing agent, and foaming agent and sintering temperature on the physical and mechanical properties of CFBS ceramsite were studied by single factor experiment method. Using SEM, XRD, sulfur analyzer, TG-DSC, flue gas analyzer, TG-MS and XPS analyses, the micro morphology and mineral composition of CFBS ceramsite were studied, and the morphological transformation and release properties of sulfur were also revealed. The results show that the optimal preparation conditions are: the mass ratio [CFB fly ash: sludge] = [65 %∼80 %: 35 %∼20 %], addition of 10 % glass powder, addition of 2.5 %∼10 % calcium carbonate, and sintering at 1200 ∼ 1250℃ for 15 ∼ 20 min. The resulting CFBS ceramsite can be obtained with bulk density of 750 ∼ 850 kg/m3, water absorption of 0.27 %∼0.94 % and cylinder compressive strength of 5.11 ∼ 7.86 MPa. Sulfur in the CFBS ceramsite is mainly derived from organic sulfur in the sewage sludge and inorganic sulfur in the CFB fly ash, and it exists in the form of sulfate and sulfide when the sintering temperature exceeds 400℃. The sulfur content of the final prepared CFBS ceramsite is 0.0217 wt%. Decomposition of SO2 from the anhydrite in CFB fly ash is the main reason for the swelling and foaming of the CFBS ceramsite. However, calcium carbonate, derived from either sulfur-fixing agent in CFB fly ash or auxiliary foaming agent, cannot play the role of foaming agent, while the CaO decomposed from calcium carbonate increases the content of fluxing component in the ceramsite and finally affects the properties of CFBS ceramsite.

A non-fired ceramsite construction material with enhanced lightweight high strength properties

The combustion of low calorific value coal in circulating fluidized bed boilers produces a large amount of fly ash with low activity and utilization. In this paper, the lightweight high strength ceramsite was prepared by a non-fired method using low-activity circulating fluidized bed fly ash (CFBFA) as raw material. And the effects of reinforcing agent and foaming agent on the properties of ceramsite were studied. The dosages of cement, quicklime, desulfurization gypsum and hydrogen peroxide were 15.0 wt%, 6.0 wt%, 6.0 wt% and 2.0 wt%, respectively. The hydration reaction in the system can be maximized by steam curing at 70 ℃ for 12 h. The bulk density and cylinder compressive strength of the obtained ceramsite were 695.34 kg/m3 and 5.5 MPa, respectively, which belonged to 700 grade high strength fly ash ceramsite. The corresponding apparent density and apparent porosity of the products were 1091.88 kg/m3 and 51.86 %, respectively. The admixture of cement, quicklime and desulfurization gypsum can all promote the hydration reaction in the system to produce denser needle-like and flocculent CSH. In addition, the admixture of hydrogen peroxide only enhanced the porosity of ceramsite and had no effect on the phase. The experimental process was simple, green and energy saving. In particular, the total solid waste utilization was up to 79 %.

Study on the composite gravel preparation and the synergistic absorption of CO2 by fly ash of CFB boiler

The cooperative treatment of fly ash and fuel gas of coal-fired plants is a promising way. In this study, circulating fluidized bed fly ash (CFBFA), gypsum, Portland Cement (PC) and water were mixed and pelleted, with carbonation curing by fuel gas to prepare the composite gravel. Experimental results showed that CSH gel and ettringite were the main hydration products and calcite was the main carbonation product. The highest strength of the composite gravel reached 11.11 MPa at 28d which was positively related to the absorption rate of CO2, and the CO2 absorption rate reached 4.789 %. Both hydration and carbonation reactions contributed positively to strength. The carbonation process caused a higher Si 2p binding energy, and the addition of gypsum led to a higher binding energy of Ca 2p. The addition of gypsum increased the porosity and main pore diameter. The porosity and main pore diameter determined the strength of composite gravel. It was observed that gypsum acted as a catalyst to promote the formation of hydration products and carbonation products. This study provided a theoretical support for the preparation of composite gravel which was a substitute for river sand and natural gravel.

Properties of Unburned Brick Produced by Entirely Waste-Stream Binder Activated by Desulfurization Gypsum

The massive accumulation of industrial solid wastes such as circulating fluidized bed fly ash (CFA), silicon-calcium slag (SCS), and desulfurization gypsum (FGD) occupy land resources and bring varying degrees of pollution to soil, water, and atmosphere. Unburned brick is a new construction material prepared from industrial waste residues such as fly ash and tailings without high-temperature calcination. It has excellent potential in consuming large quantities of industrial solid waste. In this paper, 70% of CFA and 30% of SCS are used as the primary raw materials, and the FGD is used as the activator to prepare unburned bricks by static pressure forming. The mechanical properties of the specimens at different curing ages were tested by compressive strength test. The hydration mechanism and microstructure of unburned brick were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), thermogravimetric (TG), Fourier transform infrared spectroscopy (FTIR), and inductively coupled plasma-optical emission spectrometry (ICP-OES). The results show that the compressive strength of the specimen increases first and then decreases with the increase of FGD content, and the compressive strength reaches the maximum when the FGD content is 5%. The microscopic test results show that the presence of FGD promoted a higher degree of CFA and SCS dissolution, increasing ettringite formation, which is responsible for strength increase, but extreme doses of FGD resulted in strength degradation. Meanwhile, the higher SiO2/Al2O3 ratio confirms the simultaneous formation of hydrated calcium silicate (C-S-H) gel and hydrated calcium aluminosilicate (C-A-S-H) gel within the hydrated product, while a low SiO2/Al2O3 ratio confirms the simultaneous formation of ettringite.