Circulating Fluidized Bed Combustion Ash Research Articles

Evaluating the potential of high free-CaO fluidized bed boiler ash as a cement admixture

This study investigates the feasibility of utilizing ash from circulating fluidized bed combustion (CFBC) boilers as a cement admixture. Two types of CFBC ash, distinguished by their free-CaO contents, are examined to evaluate their impact on the hydration reactions and mechanical properties of cement. The incorporation of CFBC ash with elevated free-CaO content into cement facilitates a hydration reaction, which results in accelerated setting and reduced compressive strength. To effectively utilize CFBC ash as a cement admixture, it is necessary to reduce the free-CaO content to an appropriate level, and carbonation curing is found to be an efficient method. It is confirmed that even CFBC ash with a high free-CaO content can effectively be utilized as a cement admixture through pretreatment with carbonation curing. This study may have valuable implications for the ability of CFBC ash to reduce carbon dioxide via mineral carbonation and its potential use as a cement admixture.

Experimental study on dynamic resilient modulus of subgrade with circulating fluidized bed combustion ash as filler

Using circulating fluidized bed combustion ash (CFBCA) as roadbed filler can solve the problem of secondary utilization, and dynamic resilient modulus (DRM) can match the dynamic performance of roadbed filler under driving load. In order to explore the dynamic performance of the subgrade with CFBCA as filler under driving load, based on the dynamic triaxial test and mercury intrusion porosimetry test, the DRM of the sample under different curing ages, compaction, moisture content, stress, dry-wet and freeze–thaw cycle conditions were studied, and the pore distribution characteristics of the sample were analyzed. The results show that the DRM of CFBCA samples increases significantly in the 0 ∼ 7d age curing period, and increases slowly in 7 ∼ 28d; with the increase of f-CaO and SO3 content, the DRM of CFBCA samples increased; after the dry-wet cycle and freeze–thaw cycle, the DRM of the CFBCA sample decreased, and the decrease was more obvious under the dry-wet cycle. With the increase of f-CaO and SO3 content, the total porosity and macropore of CFBCA samples decreased; with the decrease of moisture content, the total porosity and macropore of CFBCA samples increased, the DRM becomes smaller. Under different moisture content and compaction, the DRM fitting of the CFBCA sample shows that the prediction results of the three-parameter theoretical model with volume stress and octahedral shear stress as variables are compared with the test results with a high correlation (R2 > 0.96), it can effectively predict the DRM of the subgrade with CFBCA used as filler.

Effect of carbonation curing on physicochemical properties of mineral admixture blended belite-rich cement

The present study investigates the effect of CO2 curing on carbonation degree, CO2 uptake, mechanical and physicochemical properties of mineral admixtures blended with belite-rich cement (BRC). Pulverized coal combustion (PCC) ash, circulating fluidized bed combustion (CFBC) ash, and blast furnace slag (SG) were utilized as mineral admixtures. The results revealed that the PCC ash blended with BRC attained the highest compressive strength under CO2 curing. CFBC ash was comparable to PCC ash in terms of the ability to improve the mechanical and physicochemical properties. TG analysis revealed that CFBC ash blended BRC was excellent for promoting carbonation and CO2 uptake. TG, MIP, XRD, and 29Si NMR results showed that the SG was the least reactive mineral admixture with BRC under CO2 curing and negatively affects the strength attainment outcome, carbonation degree, and CO2 uptake. 29Si NMR confirmed the potential of PCC ash and CFBC ash in enhancing the reactivity of BRC under CO2 curing than the SG. Therefore, it is concluded that the blending of PCC ash and CFBC ash is beneficial in promoting the CO2 sequestration characteristics and compressive strength of BRC.

Utilization of circulating fluidized bed combustion ash to prepare supersulfated cement

The present study aims to introduce the circulating fluidized bed combustion (CFBC) ash as the source of aluminosilicate material for supersulfated cement (SSC). Contour maps were designed to optimize the proportion of CFBC ash-anhydrite-clinker by comprehensive considerations of strength, expansion and pore structure. The hydration products were characterized by XRD, TG-DSC and SEM methods. The CFBC ash content in SSC could be increased up to 70%, the corresponding clinker and added anhydrite contents should be set to approximately 20% and 10%, respectively. The main hydration products of SSC are ettringite, C-S-H, gypsum and portlandite. A value of [SO3]/[Al2O3] molar ratio in the range 0.8–1.2, is recommended for proportion optimization of SSC.

The Impact of Variable Curing Conditions on the Properties and Microstructures of Mixtures of Ground Granulated Blast Furnace Slag and Circulating Q1 Fluidized Bed Combustion Ash

The use of Portland cement, an important construction material, is encountering growing challenges because of its adverse environmental impacts. In this study, a new hydration system without Portland cement, which blends ground granulated blast furnace slag (GGBFS) and circulating fluidized bed combustion (CFBC) ash, was developed and tested. The variables included the effect of different types of CFBC ash, the mass ratios between CFBC ash and GGBFS, and different curing temperatures. The mortar properties were determined through the compressive strength test, absorption test, shrinkage test, X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The main results were as follows: (1) The compressive strength of mortar specimens made with this ecologically friendly hydration system reached at least 40 MPa under a 35°C curing temperature. (2) Mortar with finer CFBC ash as an activator had higher compressive strength and lower absorption. (3) A proper mass ratio of GGBFS to CFBC ash is necessary for engineering applications. The present findings indicate that to maximize compressive strength, the particle size and the mass ratio of CFBC ash to GGBFS must be considered. According to the result, the type A ash with 3:7 mass ratio of CFBC ash to GGBFS is recommended.

Effect of ground rice husk ash on engineering properties and hydration products of SRC eco‐cement

AbstractThe effect of ground rice husk ash (GRHA) (R) on engineering properties and hydration products of eco‐cements containing ground granulated blast furnace slag (GGBFS) (S) and circulating fluidized bed combustion ash (CFA) (C) was studied. Four mixture proportions of SRC eco‐cements with GRHA replacement at levels of 0%, 15%, 30%, and 45% by mass of binder were investigated. A reference mixture proportion of paste with 100% ordinary Portland cement (OPC) was prepared for comparison purposes. A series of laboratory tests including setting time, compressive strength, water absorption, porosity, thermal conductivity, scanning electron microscope coupled with energy dispersive spectroscopy, X‐ray diffraction, and Fourier‐transform infrared spectroscopy analysis was carried out. Measured results showed that the GRHA increased setting time and porosity in the SRC eco‐cements having a water‐to‐powder (w/p) of 0.4, leading to the decrease in compressive strength and thermal conductivity while the increase in water absorption. The GRHA increased the cristobalite amount and decreased the portlandite amount in the SRC eco‐cements at the age of 28 days, resulting in the more significant long‐term compressive strength development when compared with the reference paste with 100% OPC. Consequently, the GRHA could be used at a level of 15% by mass of binder to produce the SRC eco‐cement with the compressive strength at 28 days of higher than 30 MPa and the thermal conductivity of 0.713 W/mK, resulting from the formations of AFt, C–S–H, and C–A–S–H gels.

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.

Granular crystallization using CFBC ash for phosphorus removal via hydroxyapatite

Abstract To prevent eutrophication and remove phosphorus, circulating fluidized bed combustion (CFBC) ash was utilized to elute Ca2＋ and OH − for hydroxyapatite (HAP) crystallization. CFBC ash was compared with the gypsum byproduct of a domestic waste incineration plant and pure CaO reagent. CFBC ash proved to be a suitable raw material to supply Ca2＋ and OH − for HAP crystallization. CFBC ash was reprocessed as a granular crystallization agent (GCA) to improve its applicability to non-point pollution source control. In addition, a manufacturing procedure for a GCA utilizing 4.8% blast furnace slag as a binder was developed based on durability and the ion elution rate in water. Column tests with sedimentation showed that 93% and 98% of T-P and PO4-P were removed after 12 h of settling, respectively. In on-site filtration trench tests, filtration following crystallization with a 4 h retention time achieved a 63.5% and 52.8% removal efficiency with respect to T-P and PO4-P, respectively. After operation for 13 d, an increase in porosity with pore distribution on the surface of the GCA was observed. Through their long-term on-site operation, crystallization–filtration trenches using GCA provide an effective alternative process for the management of agricultural non-point source pollution.

Development of a cementless eco-binder as an alternative to traditional Portland cement in construction activities

In this research, the performance of a cementless eco-binder, a mixture of waste materials including slag, circulating fluidized bed combustion ash (CFA), and rice husk ash (RHA) was investigated, in which CFA acted as an activator. One hundred and twenty paste samples were prepared by using the RHA/(slag + RHA) ratios of 0, 15, 30, 45% while keeping a constant ratio of CFA/(slag + RHA) at 25%. The setting period, compressive strength, the ultrasonic pulse velocity (UPV), and drying shrinkage of paste samples were determined at the samples’ age of up to 91 days. In addition, the microstructures of all paste samples were also characterized by scanning electron microscopy (SEM). It was found that the use of cementless eco-binder significantly increased the setting times, lower compressive strength, drying shrinkage, and UPV values compared to the control OPC sample. The maximum 91-day-old compressive strength gained by the binary binder of slag and CFA (R00C25) was 90% of that of the control specimen. Incorporation of RHA with higher replacement levels up to 45% resulted in a significant decrease in compressive strength up to 50%. Moreover, the SEM analysis revealed that there was a large difference in the microstructures of the control and the cementless eco-binder samples, in which the main hydration products were C-S-H/C-A-S-H gels and ettringite (AFt) due to relatively high amount of SO3 and SiO2 in the CFA and RHA, respectively. Thus, it can be realized that the potential for the use of slag, CFA, and RHA as a sustainable cement-free binder is promising in the construction industry, especially for lower strength or no required early high strength structures.
 Keywords:
 cementless eco-binder; circulating fluidized bed combustion; rice husk ash; slag; microstructure; compressive strength; drying shrinkage; setting time; ultrasonic pulse velocity.

Evaluation of time to shrinkage-induced crack initiation in OPC and slag cement matrices incorporating circulating fluidized bed combustion bottom ash

The effect of bottom ash produced from circulating fluidized bed combustion (CFBC) on the shrinkage-induced cracking potential of ordinary Portland cement (OPC) and slag cement mixtures were investigated by evaluating the time to crack initiation, predicted by comparing tensile strength obtained from direct tensile tests with shrinkage-induced stress obtained from shrinkage strain and elastic modulus results. Although replacing OPC and slag cement amounts with CFBC bottom ash delayed cracking times due to its shrinkage compensation, the effect varied with the drying condition. Therefore, for effective application of CFBC ash in cement mixtures, boundary conditions affecting drying rates need careful consideration.

Effect of CFBC ash as partial replacement of PCC ash in alkali-activated material

The present study investigates the effect of partial replacement of conventional pulverized coal combustion (PCC) fly ash with circulating fluidized bed combustion (CFBC) ash in alkali activated material. The alkali activated material was produced by using sodium hydroxide (4M, 5M and 6M) and sodium silicate solution. The characteristics of alkali activated materials were assessed by evaluating compressive strength, along with microstructure and physiochemical properties. A strong correlation was observed between the mole concentration of sodium hydroxide and compressive strength of alkali activated materials containing CFBC ash. The microstructural and physiochemical results revealed that the calcium rich CFBC ash significantly affects the formation of polymerization products. A hybrid form of polymerization product was formed which resulted in densified microstructure. Furthermore, higher molarity of sodium hydroxide can unlock the polymerization potential of CFBC ash. The findings also revealed that incorporation of lower amount of CFBC ash is an efficient way to reduce the formation of sulfoaluminates in alkali activated materials incorporating CFBC ash.

Mechanical Properties and Durability of High‐Performance Concretes Blended with Circulating Fluidized Bed Combustion Ash and Slag as Replacement for Ordinary Portland Cement

This study investigates the mechanical properties and durability of three families of high‐performance concrete (HPC), in which the first was blended with fly ash, the second with circulating fluidized bed combustion (CFBC) ash, and the third with CFBC slag. In addition to each of the three mineral additives, silica fume and a superplasticizer were also incorporated into the HPC. Hence, three families of HPC, containing 10%, 20%, and 30% mineral admixtures and 9% silica fume of the binder mass, respectively, were produced. The microstructure and hydration products of the HPC families were examined by scanning electron microscopy (SEM) and X‐ray diffraction (XRD) to explore the influence of fly ash, CFBC ash, and CFBC slag on the compressive strength and frost resistance of HPC. The experimental results show that the compressive strength of HPC could reach 60 MPa at 28 d age. When the fly ash content was 30%, the compressive strength of HPC was 70.2 MPa at 28 d age; after the freeze‐thaw cycle, the mass loss and strength loss of HPC were 0.63% and 8.9%, respectively. When the CFBC ash content was 20%, the compressive strength of HPC was 75 MPa at 28 d age. After the freeze‐thaw cycle, the mass loss and strength loss of HPC were 0.17% and 0.81%, respectively.

Investigation of high volume of CFBC ash on performance of basic magnesium sulfate cement

Basic magnesium sulfate cement has the advantages of fast setting, high strength, high toughness, water resistance, corrosion resistance, etc. But the cost has become a reason for limiting its widespread application. With the widespread application of circulating fluidized bed combustion (CFBC) technology in China, the accumulation of CFBC ash is increasing. Reasonable use of CFBC ash can not only reduce cost of basic magnesium sulfate cement but also protect environment. In this paper, the effect of a high volume of CFBC ash on fluidity, flexural strength and compressive strength of basic magnesium sulfate cement is studied. Hydration products and micromorphology analyses are measured by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results reveal that the strength of basic magnesium sulfate cement with 20% CFBC ash is the highest, and its microstructure is the model that CFBC ash and MgO fill a three-dimensional network structure established by needle-shaped 5·1·7 phase. When the amount of CFBC ash is more than 40%, the formation of 5·1·7 phase is affected severely, which greatly reduces the strength.

Incorporation of CFBC ash in sodium silicate-activated slag system: Modification of microstructures and its effect on shrinkage

Circulating fluidized bed combustion (CFBC) ash was applied in sodium silicate-activated slag system as a secondary activator and/or for the substitution of commercialized expanding admixtures. To understand the effects of this ash on such alkali-activated slag (AAS) system, a series of experiments relating to hydration kinetics, hydration products and microstructure, and engineering properties were performed. It was found that the introduction of the ash resulted in different types of hydration products in AAS, leading to increases in both the size and volume of the pores. Although the setting time was retarded, the autogenous and dry shrinkages of the AAS decreased by the increase in the ash content. The shrinkage and strength of AAS affected by the external moisture conditions during curing could be controlled by substituting the slag with the ash.

Development of non-sintered zero-OPC binders using circulating fluidized bed combustion ash

The aim of this study was to develop zero-OPC binders using two circulating fluidized bed combustion (CFBC) boiler ashes with different free-CaO contents. For this purpose, heat of hydration, setting time, compressive strength, microstructure assessment, and thermal analysis tests were conducted to analyze the effect of changing the mixture of zero-OPC binder on the hydration characteristics. Furthermore, the use of mineral admixtures such as fly ash, silica fume, and ground-granulated blast-furnace slag (GGBS) to improve the material’s strength was investigated. The hydration products of the zero-OPC binder were ettringite, portlandite, and calcium silicate hydrates components, which are similar to those of existing cements. From the test results, it was found that an optimal free-CaO content of 9.0%–17.0% ensured appropriate compressive strength and setting characteristics. Pre-hydration of CFBC ashes was found to effectively control the amount of free-CaO. The effect of mineral admixtures on improvements in strength confirmed that the CFBC ash-based zero-OPC binder is a highly promising alternative to existing cements.

Use of slurry fluorogypsum (FG) with controlled pH-adjustment in FG-based blends

This study investigates the mechanical and durability properties of blends made of fluorogypsum (FG) with the pH adjusted by using controlled amounts of circulating fluidized bed combustion ash (CFBCA) and denoted as C-FG, class C fly ash (FA), and type II Portland cement (PC). A series of pH tests was conducted on samples of C-FG to develop an analytical relationship between acidity and CFBCA content, which can be used to determine the optimal amount of CFBCA needed to obtain a specified pH value. Two compositions of C-FG-based blends were investigated in detail to identify the effects of CFBCA content on compressive strength, modulus of elasticity, Poisson’s ratio, relative volumetric expansion, unit weight, and setting times. The obtained properties were compared with those of FG-based blends having the same composition and made using FG with the pH adjusted by using uncontrolled amounts of CFBCA (U-FG). Results suggest that the amount of CFBCA can have significant effects on the properties of C-FG-based blends, depending on the composition. In addition, C-FG-based blends generally achieve a higher compressive strength and initial stiffness than the corresponding U-FG-based blends.

Utilization of circulating fluidized bed combustion ash in producing controlled low-strength materials with cement or sodium carbonate as activator

The present study investigates the fresh and hardened properties of controlled low-strength materials (CLSM) utilizing circulating fluidized bed combustion (CFBC) ashes, activated with cement or sodium carbonate. Engineering properties such as flowability, settlement, setting behavior, compressive strength, and hydration characteristics were evaluated. The results provided new insights, demonstrating that CFBC ashes can be effectively utilized in producing CLSM with suitable material design as the CLSM could achieve the required performances specified in the ACI 299R-13. The CLSM mixtures showed a self-destruction characteristic due to the formation of ettringite, while it showed stable strength development in a dry condition.

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%.

순환유동층 석탄재의 활용 기술과 광산 채움재 관련 규격 동향

순환유동층(CFBC) 보일러 방식의 석탄화력발전소에서 발생하는 석탄재는 유리 석회와 무수석고의 함량이 풍부하고 자경성을 가지는 것을 특징으로 하는 물질로서 유용하게 활용해야 할 자원 중 하나이다. 최근 CFBC 석탄재를 포함하여 석탄화력발전소 부산물을 활용한 광산 채움 기술 및 광해 관리에 대한 관심이 점차 증가하고 있는 가운데, CFBC 석탄재를 광산 채움 분야에 적용하기 위해서는 기술에 대한 검토와 함께 관련 규격에 대해 향후 충분히 논의를 거쳐야 할 여지가 있다. 이를 위해, 본 논문에서는 CFBC 석탄재의 유효 활용 기술에 대한 연구동향을 소개하고, 광산 채움 분야의 활용을 위해 국외의 규격 동향을 고찰한 후 연구방향 및 시사점에 대해 정리하였다. Circulating fluidized bed combustion (CFBC) ash is one of useful mineralogical resources having abundant content of free lime and anhydrite, and has a self-cementitious property. Recently, considerable interest has been gained regarding the utilization of CFBC ash, along with its use in mine backfill and reclamation. Prior to adopt the use of CFBC ash in the mine backfill technology, discussion on the technology and related specification is prerequisite in the future. This paper presents a review on studies in the emerging technology of CFBC ash utilization, and provides useful information with regard to the specifications for mine backfills utilizing CFBC ash.

Adsorption properties of CFBC ash–cement pastes as compared with PCC fly ash–cement pastes

Circulating fluidized bed combustion (CFBC) ash can be potentially used as supplementary cementitious materials for concrete production due to its desirable pozzolanic activity. The adsorption properties of CFBC ash–cement pastes were studied, and ordinary pulverized coal combustion (PCC) fly ash–cement pastes were used as control. The water-adsorption and superplasticizer (SP)-adsorption properties of the pastes were evaluated by water demand and UV-visible absorption spectroscopy respectively. The results show that CFBC ash–cement system has greater compressive strength as compared with PCC fly ash–cement system at a given curing age, although the water demand of the former is significantly higher than that of the latter. CFBC ash–cement pastes possess higher adsorption ability of aliphatic SP than PCC fly ash–cement pastes and the adsorption amount increases with an increase in ash replacement ratio. CFBC ash–cement pastes exhibit lower workability with higher slump loss. It is concluded that CFBC ash can be potentially used as supplementary cementitious material in concrete production, but the mix design of CFBC ash concrete needs to be appropriately adjusted. It is suggested that CFBC ash is used for the production of the concrete needing low flowability.