Rheology, mechanics, microstructure and durability of low-carbon cementitious materials based on circulating fluidized bed fly ash: A comprehensive review

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Construction and Building Materials 411 (2024) 134688

Available online 22 December 2023

Rheology, mechanics, microstructure and durability of low-carbon cementitious materials based on circulating fluidized bed fly ash: A comprehensive review

Wenhuan Liu

^a^,^b

, Xinyi Liu

^a

, Lu Zhang

^a^,^b

, Yongfeng Wan

^a

, Hui Li

^a^,^b^,^*

, Xiaodong Jiao

^a^,^c

aCollege of Materials Science and Engineering, Xi’an University of Architecture and Technology, Xi’an, Shaanxi, 710055, China

bShaanxi Ecological Cement & Concrete Engineering Technology Research Center, Xi’an, Shaanxi, 710055, China

cGuangxi Key Lad of Road Structure and Materials, Guangxi Transportation Science and Technology Group Co., Ltd., Nanning, Guangxi 530007, China

A R T I C L E I N F O Keywords:

Circulating fluidized bed fly ash Pozzolanic activity

Mechanical properties Rheological properties Durability

A B S T R A C T

Fly ash collected from the flue of a power plant’s circulating fluidized bed furnace is referred to as circulating fluidized bed (CFB) fly ash. The high pozzolanic activity of CFB fly ash has potential benefits for the production of environmentally friendly building materials. To actualize the sustainable and large-scale utilization of CFB fly ash in the field of building materials, this paper reviews the pertinent domestic and international literature and introduces the fundamental characteristics of CFB fly ash, including its phase composition and chemical composition. In terms of fresh mixing properties, mechanical properties, and durability, the research advance- ment of CFB fly ash-based cementitious materials (CFBFACM) was reviewed. Particularly, the influence of un- stable factors f-CaO and SO3 in CFB fly ash on the various properties of ecological building materials is examined, because, in the early stage, f-CaO and SO3 will react to promote the dissolution of active silicon and aluminum and generate hydration products that are conducive to the development of strength. The internal tension is insufficient to withstand the crystallization pressure as gypsum and ettringite become increasingly abundant.

Consequently, macro strength loss and inadequate durability are brought about. By calculating the ratio between the elements of AFt and C-(A)-S-H gel, CFBFACM with high strength, good volume stability, and good durability can be designed. In addition, in the collaborative treatment of multi-component solid waste, full consideration of bulk density and particle size distribution is beneficial to the performance of CFBFACM. The method of adding nanomaterials or modifying CFB fly ash may become the way of large-scale utilization of CFB fly ash. In the future, it is still necessary to evaluate the cost and sustainability of the life cycle of CFBFACM to fully understand the economic value, ecological value, and social value of the large-scale utilization of CFB fly ash.

1. Introduction

China is in the international leading position in the application number of CFB boilers and pollution control [1]. At present, there are 3500 circulating fluidized bed boilers in operation in China, with a unit capacity of 35–2000 t/h [2]. In conjunction with the actual circum- stances of coal resources in China, the primary benefits of CFB boiler power generation are centered on the following factors: (1) the use of low calorific value fuels is maximized, resulting in a higher rate of total coal utilization; (2) Emissions of nitrogen oxide and sulfur oxide have been greatly reduced to prevent air pollution; (3) The accumulation of coal gangue is reduced, and a significant quantity of previously occupied land is liberated. The furnace, separator, and return valve make up the

primary circulation system of a CFB boiler, as shown in Fig. 1. Ashes and desulfurizer, in addition to inert bed material, are among the substances that enter the furnace. Particle agglomeration is the most significant characteristic of materials used in CFB boilers. When the wind force exceeds the resistance formed by particle agglomeration, fine particles are blown to the upper portion of the furnace, departing the furnace and entering the gas-solid separation area before being discharged into the separator. The fine particles that are not captured by the separator enter the flue with the airflow as fly ash, or circulating fluidized bed (CFB) fly ash. The bin returns the fine particulates collected by the separator to the furnace for further combustion [2,3]. It is challenging for particles to enter the top portion of the wind and deposit in the bottom bed to travel continuously till discharged from the slag outlet of the furnace when the

* Corresponding author at: College of Materials Science and Engineering, Xi’an University of Architecture and Technology, Xi’an, Shaanxi, 710055, China.

E-mail address: [email protected] (H. Li).

Contents lists available at ScienceDirect

Construction and Building Materials

journal homepage: www.elsevier.com/locate/conbuildmat

https://doi.org/10.1016/j.conbuildmat.2023.134688

Received 5 August 2023; Received in revised form 28 November 2023; Accepted 18 December 2023

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wind force is smaller than the settling gravity of coarse particles. This component of the ash is also known as circulating fluidized bed (CFB) sulfur-fixing slag [4].

With the increasing prevalence of CFB units in China, the ash produced by CFB units has become a significant mass of solid refuse that requires immediate disposal. China’s annual CFB fly ash emissions exceed 280 million tons, whereas annual CFB fly ash production in the United States and Europe is 14 million tons and 1.5 million tons, respectively [5–8]. Numerous experiments have investigated methods to diversify the applications of CFB fly ash in fields such as construction materials, soil remediation, road materials, self-healing agent and low strength backfill, and cement admixture among others [9–21]. Pres- ently, the primary use for CFB fly ash is the production of construction materials; however, the high content of f-CaO and SO3 will cause volume expansion and block fracture. To prevent expansion splitting, CFB fly ash must be pretreated, which increases the economic cost of raw materials and dampens the enthusiasm of businesses for the use of CFB fly ash. In the process of large-scale utilization of CFB fly ash, the rational utilization of f-CaO and SO3 has become an urgent demand for con- sumption, and it is imperative to comprehend the characteristics of building materials prepared from CFB fly ash.

Numerous scientists have examined the pertinent properties of CFB fly ash. He et al., for instance, summarized the physicochemical properties and potential dangers of CFB fly ash, discussed the various applications of CFB fly ash in the field of building materials, and concentrated on the optimal utilization of CFB fly ash – aeration or RCC and geopolymer preparation [5]. Zhang et al. elucidated the physicochemical properties, modification techniques, and application of CFB fly ash in the field of construction materials [23]. Zhang et al. reviewed the physical and chemical properties, mechanical properties, expansion properties, and rheological properties of CFB fly ash eco-cement, pointed out that f-CaO is the main factor limiting the utilization of CFB fly ash, and proposed that f-CaO can compensate the volume shrinkage of eco-cement, and large-scale utilization can reduce CO₂ emission in the cement industry [24]. Although many researchers have examined the physicochemical properties, modification methods, and utilization methods of CFB fly ash, there has been no in-depth discussion of the fresh mixing properties, mechanical properties, and durability of the cementing materials prepared with CFB fly ash. In this paper, the influencing factors of various properties were summarized, the hydration mechanism, the expansion mechanism, and other topics were discussed, as well as the research findings of numerous cementitious materials based on CFB fly ash. Additionally, strategies for enhancing the CFBFACM’s mechanical properties, durability, and fresh mixing

properties were summarized and discussed, offering theoretical support and a focus for future research.

2. CFB fly ash composition 2.1. Chemical Composition

The amorphous phase predominates in CFB fly ash, which is composed primarily of SiO2, Al2O3, and CaO [25]. The calcium oxide in the desulfurized is not completely reacted to form f-CaO in CFB fly ash during the desulfurization procedure. More f-CaO can function as lime, and the pH value ranges from 11 to 13. The pozzolanic activity of CFB fly ash can be enhanced through ultrafine milling. During the pulverizing process, the particle size of CFB fly ash decreases, and the fracture polymerization degree of Si-O bonds and Al-O bonds decreases, increasing in pozzolanic activity [26]. In the current "Cement Chemical Analysis Method (GB/T 176–2017)" in China, three determination methods for f-CaO are clearly given: glycerol method, ethylene glycol method and ethylene glycol extraction-EDTA titration method. Sheng et al. [27] and Fu et al. [25] determine f-CaO by glycerin-ethanol extraction method. The specific operation steps are as follows: 1) 0.5gCFB fly ash and 15 mL glycerin were heated in anhydrous ethanol and boiled for 10 min; 2) The standard benzoic acid anhydrous ethanol solution was used to titrate, and the f-CaO content in CFB fly ash was calculated according to the dosage of benzoic acid anhydrous ethanol solution. The finer the specific surface area is, the higher the f-CaO content released by CFB fly ash is. However, after grinding to a certain extent, the content of f-CaO in CFB fly ash no longer increases, showing a relatively stable trend in Fig. 3. Grinding 50 min is the best time to optimize the physical and chemical properties of CFB fly ash. After grinding, the strength and activity index of CFB fly ash were significantly increased, while the average pore radius, total cumulative pore volume, and pore content were significantly decreased [28]. The fine CFB fly ash has a higher pH value than the unpulverized CFB fly ash.

CaCO3 particles are formed upon contact with air, and the pH value decreases [25]. Fu et al. [25] used a thermal analyzer to determine the contents of CaCO₃and Ca(OH)₂in CFB fly ash. In addition, the desulfurized undergoes a complete reaction with SO2 in the CFB combustion furnace to form calcium sulfate, making CaSO4 an important component of CFB fly ash. Even though the fact that CFB fly ash is also self-hardening, the strength of the test block formed is low. In general, self-hardening is stronger the higher the CaO and SO3 content. Com- ponents with high chemical activity are responsible for the self-hardening of CFB fly ash [29]. As shown in Table 1, the degree of

Fig. 1.Process of FBFA emission [22].

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hydration hardening of CFB fly ash after different blaine-specific sur- faces is different, mainly manifested in the difference in compressive strength and flexible strength. This may be related to the specific surface area of CFB fly ash at different grinding times. Song et al. [29] used GB12957–2005(Chinese standard) modified method to evaluate the self-cementing property of coal ash. In order to adapt to the characteristics of CFB fly ash, Song et al. [29] shortened the curing time to 2d.

Since CFB fly ash contains a lot of gypsum, there is no need to add additional gypsum to the sample. The specific operation steps are as follows: 1) CFBC coal ash and water (normal consistency required) are prepared into a cake sample, and cured in a saturated humid atmosphere at 20℃ for 1 day; 2) Soak in water at 17–25℃ for 1 day, and then test the edge of the sample to evaluate its self-cementation performance: if the edge is complete and clear, the self-cementation performance of the test coal ash is obvious. For the detection of active substances in CFB fly ash, Song et al. also gave detailed experimental steps: 1) Add 50 mL water and 0.5gCFB fly ash into 250 mL beaker; 2) Then add 40 mL of 2 mol /L hydrochloric acid at 10±2℃ and stir continuously for 5 min until the CFB fly ash is completely dispersed; 3) After continuous stirring for 25 min, SiO₂and Al₂O₃are continuously dissolved in the stirring process, and the SiO2 and Al2O3 in the filtrate can be analyzed after filtration [29,30].

The composition of CFB fly ash depends on the desulfurization time, the amount of desulfurizer added, the type of fuel and the combustion conditions of the boiler [5]. CFB boiler load has little effect on particle size distribution, chemical composition and phase composition of fly ash. Because the firing temperature of CFB fly ash is between 800–900℃, the particle morphology is rough and porous, and many cracks will appear, which is conducive to the dissolution of Al2O3 in CFB fly ash [31]. The addition of a large amount of desulfurizer and a long time in the process of CFB boiler desulfurization will increase the content of f-CaO and increase the risk of volume instability. In the combustion process of CFB boiler, the dense bed temperature, fluidization velocity and bed layer pressure drop have significant effects on the carbon content of fly ash. As can be seen from Fig. 2, the higher the dense bed temperature and bed lamination drop, the lower the carbon content of CFB fly ash after combustion. With the increase of fluidization rate, the carbon content in fly ash increases gradually [32].

2.2. Physical properties

The three main mineral phases that make up CFB fly ash calcium are anhydrite, quick lime, and amorphous oxides, as shown in Fig. 4. Quick lime makes up a relatively small portion of the total, usually not more than 20%, and unreacted calcium is present in fly ash as amorphous oxides [25]. Circulating fluidized bed fly ash also contains a higher concentration of anhydrite and a low concentration of crystal phases such as calcium carbonate, quartz, and hematite, whereas the principal mineral phases of fly ash are mullite, quartz, and hematite. The addition of limestone and other sulfur-fixing substances during the fluidized bed combustion process is one of the two main causes of the difference between the two mineral phases. Due to the high-temperature phase transition of clay minerals, it is impossible to form mineral phases such as mullite at low combustion temperatures in fluidized beds. In com- parison to fly ash, CFB fly ash has a loose and porous microstructure and an irregular three-dimensional shape, and has a high surface roughness, primarily in the form of coarse grains, flecks, scum, and irregular particles，as shown in Fig. 5 [34–36]. The primary causes of the loose microstructure of particles are combustion temperature and CO2 produced during decomposition [37]. Consequently, the physical and chemical properties of CFB fly ash dictate that its water demand is greater than that of ordinary ash slag. They will absorb additional water and reduce the amount of available water in the paste system. The material’s water demand will decrease as the particle size decreases [38]. Li et al. [39] determined the water requirements of CFA, PFA and modified CFB fly ash (MCFBCAs), as shown in Table 2. It can be found that grinding 50 mins or adding modifier can reduce the water requirement of modified CFB fly ash, because MCFBCAs has better particle size distribution and dispersion, forming a dense accumulation mode. The modified method reduces the interstitial water of MCFBCAs, breaks the flocculation structure, and releases more free water. Xu et al.

[38] measured the water requirement of CFB fly ash at different grinding times, and the results showed that the water requirement of CFB fly ash at 20 min and 60 min was 39.0% and 34.5%, respectively, which was Table 1

Self-cementitious strength and setting time of CPFAs [27].

Materials W/A Setting time

(min) Flexible strength

(MPa) Compressive

strength (MPa) Initial Final 7 days 28 days 7 days 28 days

DF1 0.6 1545 1767 0.1 0.8 0.4 2.2^a

DF2 20 68 0.6 0.7 1.5 2.1^a

DF3 305 409 0.5 2.7 1.1 9.8

DF4 371 519 0.2 0.4 0.4 0.7

aCracks appeared after 10 days’ curing. The blaine specific surface of DF1、 DF2、DF3 and DF4 are 467 kg/m³、378 kg/m³、321 kg/m³and 249 kg/m³.

Fig. 2. Influence of different factors on carbon content of CFB fly ash: a) bed pressure drop; b) dense bed temperature; c) Fluidization speed[31].

0 10 20 30 40 50 60 70 6

7 8 9 10 11 12

Grinding time (min)

f-OaC)%(

Fig. 3.Variation of f-CaO content in CFBC fly ash with grinding time[25].

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significantly lower than that of non-grinding CFB fly ash (44.3%). This shows that with the increase of the fineness of CFB fly ash, the normal water requirement decreases greatly. This is because the surface of CFB fly ash becomes smoother after grinding, and less water needs to be moistened on the surface, thus reducing the water requirement. Adding CFB fly ash to a cementing material or concrete will result in a continuous increase in the water absorption of the material. Chi et al. [40]

gradually increased the compaction pressure in the process of preparing compacted concrete, thus reducing the water absorption of CFB fly ash based concrete. In summary, the problem of high water requirement of CFB fly ash can be solved by the following methods: (1) grinding 50 min;

(2) Adding modifier and grinding at the same time; (3) Increase the compaction pressure when preparing RCC.

2.3. Hazardous Substances

The presence of harmful substances in CFB fly ash is the primary factor preventing its widespread industrial application. The majority of harmful substances consist of heavy metal elements, f-CaO and SO3, and other harmful substances. (1) Heavy metals: CFB fly ash frequently contains heavy metals such as Zn, Sr, Ba, As, Cr, Cd, Pb, Co, Cu, Hg, Ni, Sn, etc [5,8]. Under the influence of groundwater erosion or rain

erosion, these heavy metal elements will permeate the surface or deep crust, resulting in soil and groundwater contamination. (2) f-CaO and SO3: Due to the presence of f-CaO and SO3, the building materials composed of CFB fly ash frequently exhibit expansibility, which results in specimen cracking and impairs the specimen’s normal working performance, as shown in Fig. 6. The expansion is more evident the higher

Fig. 4. XRD spectra of PCC and CFBC fly ash[33].

Fig. 5. SEM image of CFBFA (a. CFBC ash; b. CFA from Shanxi, China; c. CFA from Sichuan, China; d. Raw CFB fly ash from Sichuan, China; e. Gridding 20 mins; f.

Gridding 60 mins) [37,38,40,41].

Table 2

Self-cementitious properties of CFBCA, MCFBCAs and PFA [39].

No. Water

requirement ratio (%)

Compressive

strength (MPa) Strength ratio of specified samples to MF0 (%) 7 days 28

days 7 days 28

days

MF0 122 3.8 11.4 100 100

MF1(grinding 50 min) 109 11.8 14.3 310 125 MF2(portions of MF1 with

1%Modifier J) 110 13.0 15.4 342 135

MF3(intergrinding admixture of FM-0 with 1% Modifier J for 50 min)

109 14.0 16.0 368 140

MF4(intergrinding admixture of FM-0 with 1% Modifier J and 0.8%

Modifier W for 50 min)

103 16.6 22.4 437 196

PFA 92 0.5 0.44 13 3.85

Fig. 6. Expansive ratio of CFB fly ash [27].

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the concentration of f-CaO and SO3. (3) Other harmful substances: The presence of chloride ions is the main cause of steel corrosion in concrete.

The concentration of Cl in CFB fly ash is approximately 0.06%, a level that cannot be ignored. In addition, dioxins and other organic pollutants will contribute to environmental degradation if they are not removed promptly.

2.4. Chemical properties [38,39]

The self-hydration process of CFB fly ash can be roughly divided into four stages:

Stage 1: Hydration of f-CaO generates Ca(OH)2. The micellar process occurs:

CaO＋H2O →[Ca(OH)2⋅nCa²^＋]²ⁿ^＋⋅(n-x)OH^-⋅xOH^-

Stage 2: Activated alumina, silicate, gypsum, and water reaction or calcium aluminate hydration to produce ettringite:

Ca(OH)2＋3CaSO4⋅2H2O＋Al2O3＋23H2O→Ca6Al2SO18⋅32H2O (AFt).

Stage 3: Anhydrite reacts with water to form gypsum:

CaSO4＋2H2O→CaSO4⋅2H2O.

Stage 4: Reactive silicon and reactive aluminum react with Ca(OH)₂ to form C-(A)-S-H gel or C2S for hydration:

SiO2＋xCa(OH)2＋(y-x)H2O→CaOx⋅SiO2⋅H2Oy(C-S-H gel)

Al2O3＋SiO2＋xCa(OH)2＋(y-x)H2O→CaOx⋅Al2O3⋅SiO2⋅H2Oy(C-A-S-H gel)

The first stage provides the reaction system with high alkalinity, which is a prerequisite for the rapid hydration reaction. A significant amount of hydration products (C-S-H gel, AFt, etc.) were produced for the system in the second, third, and fourth stages, and the paste quickly condensed and hardened, increasing the strength of the test block [38].

When CFB fly ash powder is ground into fine particles, the outer CaSO4

layer is obliterated and CaO is liberated to react with water to produce Ca(OH)2 micelles. In this process, a great deal of hydration heat is released, and a great deal of Ca²⁺, SO42-, active Si, and active Al is dissolved and liberated. CaO and anhydrite facilitate the dissolution of Si-Al which is in an active state [42]. In addition, after the generation of Ca(OH)2, the pH value of the system was roughly 12.8, which accelerated the generation of hydration products such as C-S-H gel and AFt, and increased the thermal evolution of hydration. As the fineness of CFB fly ash increases, the specific surface area of CFB fly ash increases, and so does the effect of hydration on heat release; however, as the slaked lime-CFB fly ash’s hydrated slaked lime content rises, the slaked lime-CFB fly ash’s hydration heat evolution tends to decrease, as shown in Fig. 7 [38]. Therefore, Song et al. believed that the main factors affecting the hydration of CFB fly ash were the presence of free calcium oxide and highly active components [30].

Anhydrite is transformed into gypsum and ettringite as the first chemical reaction that occurs when CFB fly ash comes into contact with

Fig. 7. a. Heat evolution curves of CFB fly ash-lime pastes [43];b. Heat evolution of lime-CFBC ash(Different gridding time) [38];c. Heat evolution of slaked lime-CFBC ash(Different gridding time) [38].

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water or reacts in an environment with high humidity. When there is less water in the system, ettringite forms in the mixture of anhydrite and gypsum. Ettringite typically forms after anhydrite is converted to gypsum. In the first 10 days after hydration, ettringite forms most quickly; if the material’s moisture does not evaporate, ettringite will still form after a year; Ettringite formation is linked to water absorption and volume expansion, both of which are weakened by pre-hydration [44].

3. Workability

A uniform cementitious material formed by good workability can increase the CFB fly ash system’s strength [45]. CFB fly ash differs from pulverized coal furnace fly ash in terms of its physical characteristics.

The water requirement is high and the particle structure is loose, rough, and porous, which will have an impact on how well cementitious materials function. Various studies on the workability of CFB fly ash have been done recently and come to various conclusions. According to studies, the curing temperature, particle size, aggregate content, and admixture content all have an impact on the CFB fly ash’s ability to perform its intended function. The majority of researchers think that adding a lot of CFB fly ash to the slurry will hinder its performance and reduce the time needed for the material to set initially and finally.

Therefore, to modify the working performance, the majority of researchers think about adding auxiliary cementing materials or activators to the mixture.

3.1. Flowability

The fluidity of CFB fly ash is influenced by a wide range of internal and external factors, which can be further divided into two categories.

CFB fly ash has a porous, uneven, and rough surface. The flow performance of cementitious materials made with a higher dosage of CFB fly ash is generally subpar under the same auxiliary cementitious material, water-cement ratio, and curing environment conditions. The

performance of the material flow is significantly influenced by the water cement ratio, curing temperature, alkali activator content, auxiliary cementitious material, and aggregate content when the mix ratio is the same [46].

The rough, loose, and porous surface is the primary cause of the poor fluidity of cementitious material/concrete made with CFB fly ash as the raw material. Because it has a greater specific surface area, more water absorption, and less free water [35,47,48]. The different types of CFB fly ash, such as rough particles, horn particles, flake particles, miscella- neous particles, and irregular particles, are other factor contributing to the poor fluidity [49,50]. On the other hand, CFB fly ash requires more water in the f-CaO reaction process, resulting in poor flow performance [51]. Chen et al. prepared GFA (CFB fly ash +slag) samples and AGFA samples (CFB fly ash +slag +NaOH+Na2SiO3) with different CFB fly ash content, and found that: with the increase of fly ash content of CFB, the fluidity of GFA and AGFA decreased gradually, and the fluidity of GFA was significantly lower than that of AGFA [52]. This is because the alkali solution in AGFA excites the slag, which encourages the formation of hydration products like C-S-H gel. On a macro level, the slurry fluidity decreases as the slurry viscosity increases due to the gel products, the increased friction between the particles, and the slurry. It is more difficult for the slag to be excited to form additional C-S-H gels and other hydration products because CFB fly ash is less alkaline than NaOH solution and Na2SiO3 solution. As a result, GFA has a much higher fluidity than AGFA. The morphology effect of CFB fly ash is another reason for the high water absorption and low fluidity of slurry [53,54]. Hoang-Anh Nguyen et al. added an appropriate amount of high efficiency water reducing agent to the system to increase the fluidity of CFB fly ash concrete [55]. It can be seen from Fig. 8 that there is no linear relationship between fluidity and slump of freshly mixed concrete and the content of fly ash in CFB. When no cement was added, the flow and slump of the samples (C0WA25, C0WA35 and C0WA45) gradually decreased with the increase of the content of CFB fly ash. However, no corresponding linear relationship was found in the other groups. At the

Fig. 8.Slump fluidity and slump of different green super-sulfated cement concretes [55].

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same time, when the CFB fly ash content is 35%, the fluidity of fresh concrete (such as C3WA25, C3WA35, C3WA45) is between 475 mm and 510 mm. This shows that proper addition of Portland cement (OPC) and CFB fly ash can improve the workability of freshly mixed concrete to some extent. When the OPC content range is 0% ~ 5% (such as C0WA25, C3WA25, C5WA25), the fluidity and slump of new concrete can be improved. When the OPC content is 5% ~ 10%, the performance of freshly mixed concrete has no obvious improvement.

All in all, CFB fly ash has high reactivity and can quickly generate gel-like hydration products in an alkaline environment. On the other hand, due to the high specific surface area, rough particle morphology, and high water requirement of unhydrated CFB fly ash, the paste fluidity decreases with the increase of CFB fly ash content. A balance between the degree of reaction and the degree of flow can be achieved by adding appropriate weakly alkaline substances.

3.2. Setting time

The high concentration of f-CaO and SO3 in CFB fly ash results in a different setting time than regular fly ash from pulverized coal furnaces.

The composition, dosage, and fineness of CFB fly ash, as well as the type of activator, external curing temperature, solution pH, and water-to- cement ratio, all have a significant impact on the setting time of cementitious materials made with CFB fly ash. The amount of debate that exists right now surrounds the CFB fly ash cementitious materials’

setting time. The majority of researchers think that as CFB fly ash content rises, material setting time decreases.

The addition of CFB fly ash can shorten the setting time of slurry, which is inversely proportional to the amount of CFB fly ash and closely related to the type of activator in the material system [56]. Chen et al.

prepared two different cementitious material systems: GFA (slag +CFB fly ash) and AGFA (slag +CFB fly ash +NaOH+Na2SiO3) [52]. In com- parison to the AGFA system, the GFA system required more time to set.

The setting time is accelerated in both systems by increasing the CFB fly ash content. The alkali solution in AGFA promotes the dissolution of Si and Al in the raw material, releasing a large amount of AlO4 tetrahedrons and SiO4 tetrahedrons, promoting the formation of C-A-S-H gel and C-S-H gel, and shortening the condensation time in a macroscopic way. In the study of Shon and Nguyen, it was also found that the setting time of mixtures would be shortened with the increase of CFB fly ash.

The addition of OPC does not appear to affect the setting time of concrete, but when the content is high, the free water is quickly consumed, and the continuous generation of hydration products will also reduce the workability of concrete and shorten the setting time of the system. As an example, the initial and final setting times of the new paste are shortened with the increase of CFB fly ash content from 25% to 45% [48,55, 57]. The high water requirement of CFB fly ash is an additional factor contributing to the shortened setting time. The amount of f-CaO has a significant effect on the setting time of the slurry formed from CFB fly ash. In the GFA, CaSO4 quickly coats the f-CaO, making it difficult to dissolve and further aid in the polymerization process. Sheng et al. found that the initial setting time of CFB fly ash with f-CaO contents of 7.55%

and 15.97% is 1545 min and 305 min, respectively, while the final setting time is 1767 min and 409 min, respectively[27]. It can be seen that the CFB fly ash with high content of f-CaO has a fast hydration rate and short setting time, whereas the setting time is long. When the content of f-CaO exceeds 5%, the setting time is shortened sharply, and when the content exceeds 25.5%, the material solidifies rapidly. If only CFB fly ash is used as cementing material, the optimum range of f-CaO content to ensure proper setting time development is 9.0%− 17.0%[58].

Gypsum can promote the hydration of C3S while CFB fly ash contains a large amount of gypsum. The higher the gypsum content in the system, the more likely condensation occurs. In the CLSM system, various raw materials are hydrated together to produce ettringite, which shortens the setting time to some extent [48]. Another factor contributing to the CFB fly ash slurry’s rapid condensation is the rate at which hydration

products form. Curing temperature, solution pH level, CFB fly ash fineness, etc. are some of the variables affecting the rate at which hydration products form. It can be seen from Table 3 that with the increase of CFB fly ash fineness and curing temperature, the material setting time is gradually shortened. Scince ground CFB fly ash can dissolve SiO₂and Al2O3 in large quantities in a short time, more AFt, C-S-H gel, and gypsum can form. The formation of hydration products and the con- sumption of a substantial amount of free water are the primary causes of a reduction in setting time [37,38,59].

In addition, the contact area between f-CaO and free water is larger in the ground CFB fly ash, and the generated Ca(OH)2 increases the pH value of the solution, which is also conducive to the dissolution of active Si and Al in the CFB fly ash, thus accelerating the formation of ettringite and C-S-H gel, and shortening the setting time. Although CFB fly ash stimulated by hydrated lime with a high pH value raises the solution’s pH, the setting time is prolonged, primarily because the solution’s high Ca²⁺content inhibits the dissociation of CaSO4 and the dissolution of Ca²⁺. On the other hand, the dissolved Ca²⁺rapidly forms Ca(OH)₂ micelles and absorbs negatively charged CFBC particles, shortening the setting time. Admixtures may also accelerate the hydration process.

Taking Na₂CO₃as an example, it can react with Ca(OH)₂in the system to generate CaCO3 particles that fill pores, and NaOH can also be generated to improve the alkalinity of the system[51]. The increase in curing temperature can promote the rapid generation of hydration products, improve the amount of water evaporation, and shorten the setting time on the macro level [54,60,61].

Some academics believe that adding fly ash from a coal-fired power plant will lengthen the setting time of slurry. Table 3 demonstrates that the setting time of the slurry gradually increases as the proportion of CFB fly ash increases. Wu et al. found in cement-based composites that the addition of CFB fly ash would delay the initial setting time, and with the increase of CFB fly ash content, the initial setting time accounted for a higher percentage of the total setting time [62–64]. The prolonged setting time may be closely related to the high content of free calcium oxide and anhydrite in CFB fly ash. The anhydrite covering the surface of CFB fly ash particles prevents the reaction of f-CaO with water, and thus prevents the possibility of further reaction of Ca(OH)2 with slag and other active substances, which shows a longer setting time on a macroscopic level. On the other hand, the hydration rate of CFB fly ash is related to the slow transport of H2O molecules through the ettringite barrier and Ca(OH)2 [54,61]. Shen et al. used anhydrite in CFB fly ash as cement retarder, and also found that the gypsum content in CFB fly ash had a significant effect on the initial setting time [65]. In the two systems consisting of clinker-high gypsum content CFB fly ash and clinker -low gypsum content CFB fly ash, the initial setting time of the former was 122 min, which was twice as long as that of the latter. Compared to natural gypsum, anhydrite in CFB fly ash retards the formation of ettringite and is easier to convert as a cement retarder. CFB fly ash can be an effective component of cement with reasonable design proportion and proper addition of ingredients. In the two-phase system of hydrated lime and CFB fly ash, hydrated lime cannot solidify in the system quickly and will release a significant amount of Ca²⁺into the system, preventing the dissolution of Ca²⁺in the CaSO4 layer and f-CaO. Additionally, it is difficult to carry out the agglomeration reaction of Ca(OH)2 micelle in the system, which extends the time for material solidification [38]. Kim et al. believed that the reason for the delay in the setting time of CFB fly ash was related to the formation of silicate products with higher crys- tallinity and unique crystalline form [66]. Compared with C-A-S-H and other gel products, silicate products are difficult to form cross-linked network structures.

The proper development of setting time can be controlled as follows:

(1) the content of free calcium oxide in the system is 9.0%− 17.0%; (2) low-temperature mixing curing; (3) design a reasonable ratio of cementitious materials.

In conclusion, CFB fly ash is strongly related to both the characteristics of the manufactured cementitious material and how well it

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performs in use. The working performance of CFB fly ash can be greatly enhanced by adding water-reducing agents, increasing the water- cement ratio, and delaying the early setting speed. These actions also enhance the workability of cementitious materials.

3.3. Rheological properties

One of the fundamental characteristics of a material is its rheological

property, which greatly affects the stirring ability, stability, perme- ability, and pouring behavior of freshly mixed slurries and reflects the internal link between the fluidity of the slurries and their microstructure. The working characteristics of freshly mixed slurry are immedi- ately characterized by the distinctive rheological property parameters such as yield stress, viscosity, shear thinning, and thickening behavior [68,69]. At present, the rheological properties of CFB fly ash cementing materials are still being studied. CFB fly ash differs from pulverized coal Table 3

Setting time and relative setting time of different CFBFACM.

Material system Mix ratio Initial setting time

Final setting

time Relative

setting time Conclusion Ref.

CFB fly ash-

GGBFS 15%CFB fly ash＋85%

GGBFS

20%CFB fly ash＋80%

GGBFS

6.2 h 6.3 h 6.9 h 6.0 h

11.6 h 11.6 h 12.6 h 12.2 h

/ The irregular shape of CFB fly ash results in low fluidity. The setting time of SCA is 2-3 h longer than that of OPC. The longer setting time is due to the slow hydration rate of CFB fly ash, and CaSO4 is coated with active material of CFB fly ash.

[54]

CFB fly ash(G)- lime(Q)/

slaked lime (H)

GX stands for CFB fly ash grinding time G0＋30%Q G0＋30%H G20＋30%Q G20＋30%H G60＋30%Q G60＋30%H

100 min 1300 min 20 min 410 min 15 min 355 min

/ With the increase of fineness of CFB fly ash, setting time is greatly shortened.

The higher the amount of lime, the shorter the setting time. [38]

CFB fly ash- GGBFS- Cement

60%GGBFS＋10%CFB fly ash＋30%Cement 60%GGBFS＋20%CFB fly ash＋20%Cement 70%GGBFS＋10%CFB fly ash＋20%Cement 70%GGBFS＋20%CFB fly ash＋10%Cement

152 min 168 min 155 min 173 min

/ 78.82%

97.65%

82.35%

103.53%

Adding CFB fly ash can prolong the setting time of cement-based materials.

CFB fly ash can replace gypsum as cement retarder because it contains f-CaO and gypsum. With the increase of fly ash content of CFB, the percentage of relative setting time increases gradually.

[63]

CFB fly ash-

GGBFS 20%CA+80%GGBFS

25%CA+75%GGBFS 6.0 h

5.9 h 13.8 h

13.6 h CFB fly ash hydration rate is slow, resulting in a slow hydration rate. [61]

PC-CFB fly ash 20%CFBC fly ash

（blind coal） 14℃ 23℃ 36℃ 20%CFBC fly ash

（pitch） 14℃ 23℃ 36℃

200 min 560 min 450 min 290 min 240 min 670 min 545 min 340 min

340 min 785 min 620 min 370 min 380 min 905 min 700 min 425 min

/ The setting time of pulverized coal ash paste mixed with CFB is later than that of control group, and the setting time of bituminous CFB fly ash mixed with CFB is the latest. With the decrease of curing temperature, the setting time is gradually extended.

[67]

CFB fly ash-

GGBFS Add sodium silicate 10%CFB fly ash＋90%

GGBFS

GGBFS No sodium silicate 10%CFB fly ash＋90%

GGBFS

The setting time of GFA is longer than AGFA, adding sodium silicate significantly reduces the setting time, and the higher the content of CFB fly ash, the shorter the setting time. Dissolved ion clusters bind to Ca²⁺, accelerating gel product formation and shortening coagulation time on a macroscopic scale.

[52]

OPC-CFB fly

ash-fly ash 30%CFB fly ash＋50%

fly ash＋20%Cement 20%CFB fly ash＋50%

fly ash＋40%Cement

165 min 120 min 125 min 115 min

/ 94.12%

41.18%

47.06%

35.29%

The addition of fly ash and CFB fly ash will delay the initial setting time of materials. The content of free calcium oxide and anhydrite in CFB fly ash is high, so it can replace gypsum as retarding agent. 30%CFB fly ash content has a higher percentage of initial setting time than 20%CFB fly ash content.

[62]

CFB fly ash CFB fly ash(23.9µm) CFB fly ash(15.5µm) CFB fly ash(9.6µm) CFB fly ash(6.1µm)

1136 min 451 min 235 min 64 min

1383 min 507 min 272 min 90 min

/ With the decrease of fineness of CFB fly ash, the initial setting time and final

setting time are gradually shortened. [37]

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fly ash in that it is loose, porous, and comes in a variety of shapes. This negatively affects the rheological characteristics of the slurry. The results of the current study demonstrate a relationship between the rheological properties of CFB fly ash cementing materials and the amounts of CFB fly ash, polycarboxylic acid water reducing agent, activator, and particle size distribution of CFB fly ash. Commonly used rheological models are the Bingham model(τ=τ₀＋γη), the Revised-Bingham model(τ=τ₀＋γη^＋cγ²), and the Herschel-Bulkey model(τ=τ₀＋ηγⁿ). The yield stress τ₀is the minimum shear force required by the flow of fresh slurry, the viscosity coefficient (η) is a measure of the friction force in the slurry, and the pseudo-plastic index n is used to describe the nonlinear relationship between the shear stress and shear rate.

Zheng et al. mixed fly ash, CFB fly ash, and ultra-fine CFB fly ash with cement in different proportions to prepare cementing materials. Rheo- logical fitting models of different samples are shown in Table 4 [34]. The incorporation of CFB fly ash hurts the rheological property of slurry, whereas ultrafine CFB fly ash has a positive impact. When the fly ash content of CFB is less than 50%, for instance, the system displays the Bingham fluid model. When the fly ash content of CFB exceeded fifty percent, the system displayed a modified Bingham fluid model. How- ever, when superfine CFB fly ash was mixed with cement at 1:1, the paste shear thickening showed the Herschel-Bulkey model. The yield stress and plastic viscosity of the paste were reduced by 45% and 54%, respectively, and the flow property of the paste was significantly improved. The structure of CFB fly ash is loose and porous, with a high specific surface area and high surface roughness. In the hydration process, more water is absorbed and retained, and free water in the system is reduced, which hurts rheology.

The most influential variables on rheological properties are particle size distribution and packing density. Table 4 demonstrates that when CFB fly ash is combined with cement, the D50 value increases, the particle size distribution expands, and the paste fluidity decreases. When superfine CFB fly ash is combined with cement, the D50 value decreases, the particle size distribution contracts, and the fluidity of the paste increases. Based on the previous studies, Li et al. further studied the rheological properties of different UCFB fly-cement slurry, and the results showed that: when the fly ash content of UCFB was less than 30%, the slurry fluidity increased positively and the yield stress showed a downward trend, and the reverse was true when the fly ash content of UCFB was more than 30% [35,50]. When the particle size distribution of cement-UCFB fly ash is optimal, small particles are filled between the pores of large particles, and pore water is converted to free water, thereby enhancing fluidity. When the fly ash content of UCFB is high, its irregular porous structure impedes the flow of particles, substantially increases yield stress and plastic viscosity, and reduces material fluidity.

The relative yield stress of CFB fly ash slurry can be increased by appropriately adding lime to the cementing material system of CFB fly ash, which can be done by appropriately grinding CFB fly ash to adjust the appropriate particle size distribution or by adding water reducing agent. In conclusion, the rheological property of freshly mixed CFB fly ash slurry can be improved by reducing the content of CFB fly ash.

When different types of CFB fly ash with differing contents are uti- lized, the shear characteristics, or rheological characteristics, are consistent with various fluid models. The results indicate that the yield stress and plastic viscosity of CFB fly-slag-desulfurized gypsum terpolymer progressively increase with increasing CFB fly ash content, with the yield stress and plastic viscosity being at their lowest when the CFB fly ash content is 20%. When the fly ash content of CFB is 60%, the yield stress and plastic viscosity of the slurry are increased by 15.67%

and 44.23%, respectively, compared with the fly ash content of CFB is 40% [64]. The relative yield stress and viscosity of CNAAC (CFB fly-ash-cement-lime-phosphogypsum-slag five-element cementitious material) increase obviously with the increase of lime content. With the increase in cement content, the relative yield stress of the CNAAC sample decreased slightly [37]. The relative yield stress increases from 32.32Nmm to 36.21Nmm and 46.75Nmm, respectively, as the lime content rises by 5%, 10%, and 15%. The primary causes of this phe- nomenon are charge attraction and particle aggregation. Lime instantly becomes hydrated after being dissolved in water, where it preferentially binds to Ca²⁺in solution to form positively charged Ca(OH)2 micelles.

On the other hand, Si and Al on the surface of CFB fly ash absorb OH- and become negatively charged when it comes into touch with water. The positively charged Ca(OH)2 micelles attract negatively charged CFB fly ash particles to form agglomerated particles, resulting in a significant increase in the relative yield stress and viscosity of the slurry. The reason why the yield stress decreases with the increase of cement content in CNAAC is that the water requirement of CFB fly ash is higher than that of cement, and the greater the cement replacement amount, the more obvious the yield stress reduction effect will be. Wu et al. studied the effects of the water-cement ratio and the content of highly effective water-reducing agents on the rheological properties of the slurry [70].

The experiment showed that the change in the water-cement ratio had no substantial effect on the rheological properties of the slurry in its initial state. The yield stress and viscosity of the slurry significantly increased with the drop in the water-to-cement ratio, and the slurry even displayed varying degrees of pseudoplasticity. The rheological properties of the slurry can be changed, the pore structure can be improved, and the quantity of detrimental pores can be decreased by adding polycarboxylic acid superplasticizer to CFB fly ash-based slurry. However, the low water requirement prevents the formation of C-S-H gel and reduces the formation of tobermolite. After the addition of water reducing Table 4

Rheological parameters fitting results of fly ash with different proportions and cement slurry.

Materials Rheological model Fitting results τ₀_/Pa η_/

(Pa⋅s) R² De (μ_m) _n

Cement I BH τ=1.7195+0.24096γ 1.7195 0.24096 0.9796 25.8965 1.46052

Cement I＋30%PFA BH τ=0.74972+0.22086γ 0.74972 0.22086 0.9943 23.8379 1.47255

Cement I＋50%PFA BH τ=1.64576+0.23125γ 1.64576 0.23125 0.9915 20.8178 1.53869

Cement I＋70%PFA BH τ=1.14383+0.24088γ 1.14383 0.24088 0.9903 15.0977 1.63107

Cement I＋30%CFA BH τ=1.29978+0.47371γ 1.29978 0.47371 0.9770 28.0011 1.46418

Cement I＋50%CFA BH τ=3.03909+0.64773γ 3.03909 0.64773 0.9502 31.6901 1.39164

Cement I＋70%CFA R-BH τ=2.48516+1.92825D-0.01621γ² 2.48516 1.92825 0.9890 36.4879 1.28568

Cement I＋30%UCFA I H-B τ=0.56614+0.16658γ^1.1677 0.56614 0.16658 0.9996 16.8346 1.57111

Cement II＋10%UCFA II BH τ=0.37636+0.10834γ 0.37636 0.10834 0.99595 19.2771 0.9832

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agent, under the double action of electrostatic repulsion and steric hindrance, the gel-water system is in a relatively stable suspension state.

Meanwhile, the flocculation structure dissolves, free water is released, and the fluidity increases in the macro state. Reduces the viscosity, yield stress, and thixotropy of the slurry. When the amount of water-reducing agent reaches saturation, also known as the water-reducing point, the slurry’s rheological properties no longer change significantly. The equilibrium between the quantity of water-reducing agent and water demand can enhance the rheological properties of a slurry with constant water demand.

4. Microstructure of CFB fly ash-based cementitious materials 4.1. Reaction characteristics of CFB fly ash-based cementitious material

The rate of heat release and total heat release of CFB fly ash is closely related to the concentration of f-CaO, particle size, and grinding duration. The hydration process of CFB fly ash mixed with cement is anal- ogous to that of pure cement, consisting of the same five stages: pre- induction, induction, acceleration, deceleration, and stable period [26]. Kang et al. compared the heat release rate and cumulative heat release process of different types of CFB fly ash and cement, and measured that the cumulative heat release of OPC, CFB-B fly ash, and CFB-A fly ash are 184 J/g, 422 J/g, and 64 J/g, respectively [58]. The content of f-CaO is primarily responsible for the variance in heat discharge of fly ash with differing CFB. After 5 min of hydration, the peak heat flow of CFB fly ash is greater than that of cement slurry (about 0.023 W/g), the content of f-CaO in CFB-B is greater, the reaction rate is faster, and the initial heat flow peak reaches 0.185 W/g, whereas the initial heat flow value of CFB-A (about 0.05 W/g) is lower but still greater than that of cement. Zhao et al. studied the effect of grinding time on the heat of hydration of CFB fly ash [59]. They observed that the total heat release of CFB fly ash after grinding for 60 min was 38%

higher than that of the paste formed by the original CFB fly ash. The greater the grinding duration, the greater the rate of heat release from CFB ash paste. As a result of the destruction of the microstructure of CFB fly ash during the grinding process, the surface energy of CFB fly ash was reduced during the hydration process and mineral ions were released, resulting in a rapid increase in the paste heat release rate, which peaked during the early stages of hydration. In addition, the paste hydration rate of cement mixed with 30% CFB fly ash was substantially reduced, the induction period was lengthened, the second peak heat release and the total heat release were reduced, and the induction period was lengthened. Cement’s hydration activity is reduced and its hydration rate is slowed after the addition of CFB fly ash. High-activity CFB fly ash (e.g., CFB fly ash pulverized for 60 min) has little effect on the hydration heat and early heat release rate of cement after it has been mixed with cement. Xun et al. and Zhao et al. also found in the lime-CFB fly ash binary system that the finer CFB fly ash is, the larger its specific surface area is, and the more violent the reaction is to convert Ca(OH)2, resulting in higher hydration thermal evolution, as shown in Table 5 [38,59]. This may be due to the destruction of the CaSO4 layer coated by

CFB fly ash after fine grinding, the accelerated hydration rate of CFB fly ash, and the increased content of soluble Si and Al in CFB fly ash. Ca²⁺ inhibits the dissolution of Ca²⁺in f-CaO and the hydration of CFB fly ash when the Ca²⁺concentration in the system is high. The hydration or pozzolanic reaction rate of CFB fly ash is relatively high, and mechanical scouring destroys the structure of CFB fly ash. The accelerating effect of mechanical lapping on the pozzolanic reaction of CFB fly ash. In addition, the addition of hydrated lime to the binary system can create a high pH environment, promote the dissolution of active Si and Al in CFB fly ash, form hydration products including ettringite and C-S-H gel, and accelerate the hydration reaction rate. The hydration heat rate will increase proportionally to the quantity of hydrated lime present. When the concentration of hydrated lime reaches a certain threshold, the rate of hydration heat evolution decreases with the concentration of CFB fly ash.

Li et al. showed that the total heat release of UCFA (ultra-fine CFB fly ash) after adding cement was 94.5% of pure cement for 72 h, about 10%

higher than that of RCFA [26]. They believed that CFB fly ash was in nearly non-hydrating state during the first 12 h of the hydration process, and that CFB fly ash’s contribution to hydration heat increased as the hydration process progressed. Ultrafine grinding accelerated the hydration process of CFB fly ash mixed with cement, significantly increased the third peak of heat release, and significantly increased both the hydration rate and total heat release. Kim et al. adopted a Na2SiO3

solution to activate the slag-CFB fly ash binary system, and when the CFB fly ash content increased to 20%, the cumulative heat of hydration increased from 135 J/g to 145 J/g, in Fig. 9 [66]. On the one hand, a small amount of f-CaO in CFB fly ash can self-hydrate; on the other hand, anhydrite and Ca(OH)2 in ash have activation effects on slag, which promote the increase of early hydration heat and the acceleration of the hydration heat release rate of cementitious materials. Isothermal calo- rimetry is unable to discern the specific exothermic process and sequence of reactions in cementitious material formed from CFB fly ash.

The volcanic reaction rate and exothermic time of CFB fly ash are both low. Si and Al in CFB fly ash undergo a delayed reaction with calcium hydroxide [43].

4.2. Pore structure of CFB fly ash-based cementitious material

The evolution of pore structure reveals the density, average pore size, and cumulative porosity of the cementitious material, which aids researchers in analyzing the sample’s strength variation and microstructure evolution. The pore structure of the cementitious material based on CFB fly ash is related to the quantity and type of activator, and the pore structure of the cementitious material has a positive correlation with the mechanical strength of the material. As shown in Fig. 10, with the extension of age, the strength of the sample gradually increases, while the cumulative porosity and average pore size gradually decrease. Wu et al. studied the influence of different lime content on the pore structure of CFBFACM, and the results showed that: when the mass ratio of CFB fly ash to lime is 10:1(CF10), the porosity of the material at 90d is 54.0%, which is the highest compared with other samples (the porosity of CF20, Table 5

Characteristics values of hydration heat of different cement pastes [59].

Samples Ending time of induction

period/h Appearing time of the second exothermic

peak/h Second exothermic peak value/

Jg⁻¹h⁻¹ Total heat release/Jg⁻¹

24 h 48 h 72 h

100%Cem 1.69751 8.07049 10.67209 186.3897 239.1244 256.8055

30%-Cem＋70%CFA-

0 min 2.14143 10.14414 8.23100 131.6535 188.3506 215.8100

30%-Cem＋70%CFA-

20 min 2.20832 10.06509 8.38079 146.2895 197.3549 222.5148

30%-Cem＋70%CFA-

40 min 1.98332 9.25630 8.78248 162.5830 210.8021 235.1011

30%-Cem＋70%CFA-

60 min 1.86778 8.81238 9.08355 165.2691 215.1549 237.9546