Circulating Fluidized Bed Combustion Fly Ash Research Articles

Recycled mineral admixtures based on recycled clay brick

The utilization of construction demolition waste as recycled mineral admixtures can simultaneously reduce cement consumption and the environmental impact on the construction industry. Thus, recycled mineral admixtures based on recycled clay brick powder (CBP) were prepared and circulating fluidized bed combustion (CFBC) fly ash was introduced as the activity compensation and sulfate excitation component. The effects of the hydration of recycled mineral admixtures on the mechanical performance and drying shrinkage of mortar were investigated. The results indicated that the incorporation of CFBC fly ash into the blended cement paste accelerated the hydration of C3S and shortened the induction period. The fly ash also promoted the secondary hydration of C3A and the conversion of ettringite to the monosulfate phase. The CBP consumed portlandite to form a calcium–silicate–hydrate gel at a later age, whereas CFBC fly ash promoted the pozzolanic reaction to occur earlier. The incorporation of the CBP and CFBC fly ash increased the porosity and pore volume of the blended paste.

Long-term dissolution behavior of amorphous aluminosilicate in sodium hydroxide solution for geopolymer synthesis using circulating fluidized bed combustion fly ash

The dissolution behaviors of silicon (Si) and aluminum (Al) in the alkaline solution is vital for revealing the reaction mechanism during geopolymer synthesis from fly ash. Most of the relevant reported studies reported were performed under constant agitation for a short period (several hours), which is insufficient because during geopolymer formation, the Si and Al are generally dissolved slowly and continuously in the static state with no agitation. In this study, the circulating fluidized bed combustion fly ash (CFBFA) was immersed in NaOH solutions in the static state for specified times (1–35 d), and the dissolution behaviors of Si and Al were investigated. The effects of the NaOH concentration, curing temperature and liquid-to-solid ratio (L/S) on the dissolution behavior were investigated. The phase and structure changes of CFBFA before and after dissolution were revealed by X-ray diffraction and nuclear magnetic resonance spectroscopy. The obtained results revealed that the dissolution of Si was slower in the static state than that under the continuous agitation, whereas the dissolution of Al under these two conditions was similar. The dissolution activity of amorphous Si in CFBFA in the NaOH solution was significantly higher than that of amorphous Al. Increasing the NaOH concentration promoted the dissolution of Si more than that of Al. The dissolved Si and Al combined easily to form some precipitates when the temperature was above 60 °C or the L/S was 10:1 mL/g. The formation of precipitates was not conducive to the further dissolution of Si and Al. The rate-determining step for the dissolution of amorphous Si and Al in the static state was the diffusion of OH– ions and the reaction products. The order of the dissolution activity of amorphous aluminosilicates in the NaOH solution was Q4(0Al) > Q4(1Al) > Q4(2Al). These results provide reference information for the evaluation of active Si and Al amounts in CFBFA during geopolymer synthesis.

Performance and Microstructural Evaluation of Rice Husk Ash–Ground Granulated Blast Furnace Slag–CFBC Fly Ash Mixtures Produced as an Eco-Cement

Performance and Microstructural Evaluation of Rice Husk Ash–Ground Granulated Blast Furnace Slag–CFBC Fly Ash Mixtures Produced as an Eco-Cement

Properties of high-volume slag cement mortar incorporating circulating fluidized bed combustion fly ash and bottom ash

This study investigates the incorporation of circulating fluidized bed combustion (CFBC) fly ash as an activator and CFBC bottom ash as fine aggregate in high-volume slag cement mortar. A series of investigations were carried out to analyze the influence of CFBC ash products on the properties of mortar. For comparison, control specimens of ordinary Portland cement and high-volume slag cement were considered. The results obtained from X-ray diffraction and thermogravimetric analyses revealed that the addition of CFBC fly ash activated the hydration of the slag and improved the initial strength of the mortar. Additionally, a pore size distribution test showed that the introduction of CFBC fly ash resulted in decrease of the critical pore diameter and the porosity of the high-volume slag mixes up to 28 days of curing. Similarly, it was found that the compressive strength of high-volume slag mortar manufactured with CFBC fly ash and CFBC bottom ash was improved. Furthermore, the enhanced hydration of high-volume slag mortar due to CFBC ash products resulted in decreased water absorption and fewer total voids. However, inferior resistance to chemical attack was also observed.

Mechanical Activation of the Ca-Rich Circulating Fluidized Bed Combustion Fly Ash: Development of an Alternative Binder System

Mechanical activation of the calcium-rich fly ash formed in circulating fluidized bed combustion (CFBC) boilers was investigated to enhance the compressive strength performance of the pastes. We studied the effect of the activation on the physical, chemical, and mineral characteristics of fly ash and its pastes. Our study shows that already a short mechanical activation yields a 10-fold improvement in the compressive strength of the pastes, reaching 60 MPa after 90 days of curing without any chemical activation or blending. Mechanical activation caused fragmentation of large porous aggregates in the raw ash enhancing the physical properties and reactivity of fly ash particles. Similarly to calcium sulfoaluminate cements, the mechanical strength was provided by the formation of abundant ettringite and possibly C-(A)-S-H gel-like phase that created a highly compact microstructure. Our findings suggest that mechanically activated Ca-rich CFBC fly ash can be successfully used as an alternative binder.

Utilization of low sulfur fly ash from circulating fluidized bed combustion burner as geopolymer binder

Circulating fluidized bed combustion (CFBC) is a newer type of burner that employ a circulating process to burn fuel effectively. CFBC burning process is gaining more popularity due to its compact size, high efficiency and lower burning temperature compared to the pulverized coal combustion (PCC) burner. The CFBC burner produces fly ash with different physical properties compared to the PCC burner, i.e. the fly ash is not rounded, and required higher water content for comparable workability. The CFBC fly ash also has a high sulfur content that is detrimental for hardened concrete. Due to its drawbacks, the CFBC hardly used as cementitious material and geopolymer precursor. This study focuses on comparing variations in the concentration of NaOH solution and variations in the ratio of alkaline activators to the setting time and compressive strength of geopolymer mortars on a new class of CFBC fly ash, which have low sulfur content. The concentrations of NaOH solution were 6M, 8M, 10M, and 12M, while the alkaline activator ratios used were 3.0, 2.5, 2.0, 1.0, and 0.5. It was concluded that the low sulfur CFBC fly ash has a potential to be utilized as geopolymer precursor, however, with a shortcoming in its high water demand. The CFBC fly ash used in this study resulted in a geopolymer matrix with good compressive strength and stability. The water demand varies with the fly ash sampling time shows the challenges in the utilization of the fly ash. The highest mortar’s compressive strength, 33.4 MPa at 90 days was achieved at NaOH concentration of 8M and ratio of sodium silicate solution to sodium hydroxide solution of 2.5 with excellent stability.

Study on hydration characteristics of circulating fluidized bed combustion fly ash (CFBCA)

Circulating fluidized bed combustion fly ash (CFBCA) is a by-product of the coal-fired process in power plants. In this paper, the hydration characteristics of CFBCA are studied. The pozzolanic activity of CFBCA and pulverized coal combustion fly ash (PCA) at different ages was tested, the hydration products, microstructure of CFBCA-cement paste and PCA-cement paste and the elements distribution of Si, Al and Ca on the surface of fly ash particles were studied. Then the effects of pH value and SO3 content on the dissolution of active SiO2 and active Al2O3 from CFBCA and PCA were further discussed. The experimental results show that the pozzolanic activity of CFBCA is higher than that of PCA, and the surface of CFBCA is more easily corroded and the active SiO2 and active Al2O3 are easy to dissolve in the hydration process. CaO and anhydrite (CaSO4•II) contained in CFBCA play an important role in promoting the dissolution of active SiO2 and active Al2O3. In addition, the loose structure of CFBCA and the low degree of polymerization of [SiO4] and [AlO6] are also one of the reasons for its higher hydration activity than PCA. Relevant research revealed the hydration mechanism of CFBCA, and provided a theoretical basis for its application in cement and concrete.

Influence of low calcium fly ash on compressive strength and hydration product of low energy super sulfated cement paste

This research aims to explore the effects of low calcium Class F fly ash (FFA) as an alumina supplier on the compressive strength and hydration products of an ecological super-sulfated cement (SSC) with fluidized bed combustion (CFBC) fly ash (CFA) as a crucial activator. Experimental results showed that partial replacement of ground granulated blast furnace slag (GGBFS/slag) by FFA in range of 10–30 wt% significantly improved the compressive strength of hardened eco-SSC paste. Microstructural examination using scanning electron microscope equipped with energy dispersive X-ray spectrometer (SEM/EDS), X-ray diffraction (XRD), thermogravimetric and derivative thermogravimetric analysis (TGA), and Fourier transform infrared (FTIR) spectroscopy obviously proved that the main hydration products of the hardened paste were ettringite/monosulfate (AFt/AFm) and calcium aluminum silicate hydrate (C-A-S-H). Increase in FFA resulted in higher degree of AFt/AFm precipitation and C-A-S-H with lower Ca:(Si + Al) ratio.

Utilization of CFBC Fly Ash as a Binder to Produce In-Furnace Desulfurization Sorbent

Circulating fluidized bed combustion (CFBC) power generation technology is known to efficiently reduce the emission of air pollutants, such as SO2 and NO2, from coal combustion. however, CFBC coal ash contains high contents of free CaO, making it difficult to recycle. This research has been conducted to find ways to use the self-hardening property of CFBC coal ash, one of its inherent characteristics. As part of these efforts, the present study intended to investigate the properties and desulfurization efficiency of Ca-based desulfurization sorbents using CFBC fly-ash as a binder. Limestone powder was mixed with CFBC fly-ash and Ca(OH)2 to fabricate desulfurization sorbents, and it generated hydrate of cement, including portlandite, ettringite, and calcium silicate, etc. The compressive strength of the desulfurization absorbent prepared by CFBC fly ash and Ca(OH)2 was 72–92% that of the desulfurized absorbent prepared by using general cement as a binder. These absorbents were then compared in terms of desulfurization efficiency using a high-temperature fluidized bed reactor. It was confirmed that the desulfurization absorbents fabricated using CFBC fly-ash as a binder achieved the best performance in terms of absorption time, which reflects the time taken for them to remove over 90% of high-concentration SO2 gas, and the conversion ratio, which refers to the ratio of CaO turning into CaSO4.

Geopolymerization enhanced hydrothermal synthesis of analcime from steel slag and CFBC fly ash and heavy metal adsorption on analcime

ABSTRACTThis work is focused on crystal phase transition of solid wastes and functional application of crystal analcime in waste water purification, which provide a new environment-friendly route. In this paper, analcime was synthesized from steel slag and Circulating fluidized bed combustion (CFBC) fly ash (CFA) by hydrothermal method enhanced via geopolymerization (non-crystallized process). Then the analcimes were used for the removal of heavy metal ions (Pb2+, Cu2+) in aqueous solutions. Both the raw materials and products were characterized by XRF, XRD, FT-IR, SEM-EDS, and TEM. The results showed that non-crystallized process reduced the time of hydrothermal reaction and promoted the purity of analcime. The adsorption kinetics of analcime were all well fitted the pseudo-second-order model, and adsorption isotherms were well described by the Langmuir model. The maximum adsorption capacity of analcime for Pb2+ and Cu2+ were around 75.76, and 21.83 mg/g, respectively. The preference order observed for adsorption is Pb2+ > Cu2+.

Effects of sulfate rich solid waste activator on engineering properties and durability of modified high volume fly ash cement based SCC

The engineering properties and durability of high volume low calcium fly ash (HVFA) cement based self-compacting concrete (SCC) modified with addition of circulating fluidized bed combustion (CFBC) fly ash were explored. Experimental results showed that an addition of CFBC fly ash did not influence the stability and passing and filling abilities of modified HVFA cement based SCCs. But its presence significantly improved the compressive strength, bonding behavior, and durability of the hardened SCC specimens. The added small amount of CFBC fly ash was found to increase the earlier compressive strength of hardened SCC specimen at one day of curing up to 43.8% higher than that of the control SCC specimen, which indicates a remarkable contribution of CFBC fly ash addition to shortening the time of construction. At ages of 3, 7, and 28 days, such increases were up to 30.2%, 22.3%, and 17.8%, respectively.

Binder chemistry of sodium carbonate-activated CFBC fly ash

The use of circulating fluidized bed combustion in coal power plants is dramatically increasing, and the disposal of the resultant ash is a significant environmental concern. The present study investigated the binder chemistry of sodium carbonate-activated CFBC fly ash. Compressive strength tests showed that strength development was relatively faster in samples with 5% sodium carbonate. The main binder gel in samples with and without sodium carbonate was composed of AFt, AFm and C–S–H, while the formation and Al-substitution in C–S–H was enhanced by sodium carbonate-activation. At an excessive dosage of sodium carbonate, the formation of AFt was significantly hindered due to the precipitation of sodium sulfate, and a notable amount of AFm was formed in its place. The results obtained in this study fill a deficit in the understanding of fly ash activated with near-neutral salts, which are used extensively in several industrial applications, including alkali-activated cements.

Ternary binder made of CFBC fly ash, conventional fly ash, and calcium hydroxide: Phase and strength evolution

Coal combustion products present a source of aluminosilicate materials for further utilization. The ternary binder studied here is such an example, consisting of circulating fluidized bed combustion (CFBC) fly ash, conventional fly ash and Ca(OH)2 activator. The paste yields a compressive strength of 32 MPa after 28 days of standard sealed curing. Volumetric evolution of crystalline and amorphous phases during hydration is quantified using XRD analysis, differential thermal gravimetry, porosimetry and electron microscopy. A micromechanical model is applied to interpret the evolution of compressive strength due to the growing proportions of C-S-H and ettringite in the system. This opens the way for further optimization and utilization of this ternary binder.

Mechanism of Mn(II) Absorption and Desorption with CFBC Fly Ash Modified by Alkaline Wet Ball Milling

In this study, the adsorption properties of modified circulating fluidized-bed combustion (CFBC) fly ash by alkali wet millingwere investigated for Mn (II) cations. The effects ofNaOH content,milling speed and milling timeon the modification processof CFBC fly ashwerestudied. Preliminary statistical analysis has indicated thatmetal concentration, time, pH, fly ash dosage and temperaturewere the most importantvariables that affect the adsorption capacity. Results lead towards the conclusion that alkali wet millingtreatment cansignificantly increasethe adsorption capacity of the CFBC fly ash. The adsorption mechanism of the modified CFBCfly ash was determined using Langmuir, Freundlich isotherms and the Lagergren pseudo-first-order, pseudo-second-order, Elovich equation and the intraparticle diffusion equation, which indicated it was not thesole rate determiningstep, and the adsorption process was controlled by physical and chemical adsorption.The results showed that theCFBC fly ash can be utilized as a low-cost adsorbent for the removal of Mn (II) ionsfrom solution.

Optimization of the Composition of the Binder Based on CFBC Fly Ash

This paper is bound to previous research of materials based on fly ash. The main objective was to design an optimal ternary (three-component) binder based on fly ash from Circulating fluidized Bed Combustion (CFBC). The design of the binder is based on the optimization of individual components. Main components of the binder are CFBC fly ash, high-temperature fly ash and slaked lime. The binder was progressively designed and optimized. The strength characteristics of the various binders were measured, evaluated and the results were inserted into computer programs Surfer 8 and Grapher 8 from which ternary diagrams with strength maps were created. The best binder mixture were selected from the optimization process.

Influence of Milling on some Chemical Properties and Reactivity of CFBC Fly Ash

This paper describes influence of milling on chemical properties of fly ash from Circulating fluidized Bed Combustion (CFBC). Specific properties of fly ash was determined using calorimetric measurement. It was determined heat properties and total content of calcium oxide CaO. The following methods of measurement were also performed: granulometric measurement and chemical analysis. The ash properties of non-milled and milled ashes were also described and evaluated and the ash reactivity was compared.

Effect of Elevated Temperature on Engineering Properties of Ternary Blended No-cement Mortar

This paper aims to examine the engineering properties of ternary blended no-cement mortar which subjected to the various elevated temperatures exposure. The mortars were produced by mixing ground granulated blast furnace slag (S), Type-F fly ash (F) and circulating fluidized bed combustion (CFBC) fly ash (C). The water-to-binder ratio was fixed at 0.40 and the CFBC fly ash content was fixed at 15 wt.% of the mixture that acts as the main activator. The specimens were exposed to the elevated temperatures ranging from 200°C to 800°C. The mass loss, compressive strength, and ultrasonic pulse velocity were determined before and after exposure to the elevated temperatures. The obtained results showed after exposed to high temperature, the mortar weight reduction was discovered in the range of 6.0–8.7% when temperature rose from 200°C to 600°C, and decreased significantly up to 12.4% as temperature reached 800°C. The major strength loss occurred after 600°C with the residual compressive strength approximately at 44.2%. At 200°C, increased strength was found on SFC mixture and when temperature rose to 400°C, the specimens still can resist the load reliably with the strength loss less than 8.0%. Consequently, SFC mortar generates good durability and heat resistance below 400°C.

Circulating fluidized bed combustion ash as controlled low-strength material (CLSM) by alkaline activation

The present study is aimed at providing a useful means of treating and utilizing the CFBC ash as controlled low-strength material (CLSM). The CLSM in this study was produced by activating a mixture of CFBC fly ash, bottom ash and blast furnace slag with sodium hydroxide (NaOH) solution. The characteristics of the CLSM were evaluated by the main factors such as flow, bleeding and compressive strength as stated in ACI 229, along with initial setting time and XRD patterns. The CLSM showed delayed strength development due to the reactivity of calcium sulfate in CFBC fly ash and bottom ash. This effect is evidenced by the XRD pattern of the CLSM at 7 and 56d which showed the consumption of gypsum asa source of calcium sulfate, to produce the reaction product ettringite.

Effects of Circulated Fluidized-Bed Fly Ash, Ground Granulated Blast-Furnace Slag and Coal Fly Ash on Properties of Mortars

The amount of circulating fluidized bed combustion (CFBC) ash is increasing and the disposal cost of CFBC ash have also been increasing annually. Hence, developing a better utilization of CFBC fly ash with the by-products ground granulated blast furnace slag (GGBFS) and coal fly ash is an urgent and important task. In this study, seven mixes with different blended cements were prepared to cast mortar specimens. Flow test, setting times test, water absorption rate, compressive strength test and flexural strength test were performed to investigate the properties of mortars. Test results show that the addition of coal fly ash, CFBC fly ash, and GGBFS improves the workability.When the amount of cement replacement by CFBC fly ash, GGBFS, and coal fly ash was recommended less than 20% simultaneously, the compressive strength is higher than that of plain cement mortar. However, CFBC fly ash would results in a lower strength when adding up to 30%. Thus, the amount of CFBC fly ash replacement cement was recommended to be limited below 20% and the total amount of cement replacement was limited below 60%.

Engineering Properties and Microstructural Performance of Low Energy Super-Sulfated Cement Using Industrial Waste Anhydrite

This study aims at proposing the mix proportions of low energy super-sulfated cement (SSC) concrete from industrial waste anhydrite from circulating fluidized bed combustion (CFBC) fly ash (CFA) as an alternative sulfate activator of ground granulated blast furnace slag (GGBFS/slag). The optimized mix proportion of the SSC was carried out by using mixture of different amounts of CFA in range of 25—45 wt.% and various quantities of ordinary Portland cement (OPC) in range of 0 — 10 wt.% to trigger the hydration of slag. Experimental results showed that with the expected slump at values of 190 — 220 mm, the 28-day compressive strengths of the concrete with low energy SSC reached 43.69 MPa which can be feasibly applied for widely advanced construction materials. The OPC in range of 3 — 5 wt.% and 25 wt.% of CFA were considered as the optimum ingredients of the activator and was suggested to be used for fabricating the low energy SSC concrete with the good performance on compressive strength, dynamic Young’s modulus, UPV measurement, and stabilized change of length. The OPC additive up to 10 wt.% was encouraged to be used for producing the SSC concrete with significant reduction on creep.