Characterization of Spreader Stoker Coal Fly Ashes (SSCFA) for their use in cement-based applications

Fuel 162 (2015) 224–233

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Characterization of Spreader Stoker Coal Fly Ashes (SSCFA) for their use in cement-based applications M. Sow, J. Hot, C. Tribout, Martin Cyr ⇑ Université de Toulouse (UPS, INSA), LMDC (Laboratoire Matériaux et Durabilité des Constructions), 135, avenue de Rangueil, 31 077 Toulouse cedex 4, France

h i g h l i g h t s  100,000 tons/year of Spreader Stoker Coal Fly Ash (SSCFA) produced in Reunion Island.  SSCFA composed of spherical particles, similar to pulverized coal fly ash.  However, large amount of unburned carbon particles, far above the limit of EN 450-1.  The high LOI of SSCFA affects its morphology and consequently the workability.  SSCFA shows pozzolanic activity and increase the strength of mortars.

a r t i c l e

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Article history: Received 2 June 2015 Received in revised form 3 September 2015 Accepted 8 September 2015 Available online 15 September 2015 Keywords: Fly ashes Combustion system Spreader stoker Pulverized coal Particle size Chemical composition

a b s t r a c t The paper presents a comparison between Pulverized Coal Fly Ashes (PCFA) and Spreader Stoker Coal Fly Ashes (SSCFA) by analyzing their chemical, mineralogical and physical characteristics. PCFA have been recognized as being valuable industrial by-products and many research studies have been published on their characteristics, properties and utilizations. On the contrary, relatively little is known about SSCFA due to a lack of research work and their valorisation appears to be a difficult task, mainly due to their high unburned carbon content. Three fly ashes are studied here, two resulting from pulverized coal power plants and one from spreader stoker power plant. The results show that the tested fly ashes have a similar chemical and mineralogical composition whatever the combustion process used. SSCFA presents some specific characteristics approaching those of normalized PCFA. However, the combustion system seems to have an impact on the physical properties and performance of fly ashes as supplementary cementitious materials when blended with Portland cement. SSCFA has indeed a high unburned carbon content, which could be explained by a combustion process not adapted for coal. Rheological and mechanical tests results show that SSCFA/cement Portland-based mortars have interesting properties and could be used in some specific industrial applications. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Generally speaking, fly ash is a by-product of coal combustion (bituminous coal, sub-bituminous coal, anthracite or lignite) but could sometimes design waste derived from the combustion of other materials than coal (e.g. municipal solid waste incinerator fly ashes [1–3], flue gas desulfurization (FGD) wastes [4]). During the thermal process (various combustion processes exist), coal melts and a small part of it falls in the bottom of the boiler producing the bottom ash. The main part is instead carried away by the exhausted gas stream, cooling quickly and solidified as small glassy ⇑ Corresponding author. Tel.: +33 5 61 55 67 07; fax: +33 5 61 55 99 49. E-mail address: [email protected] (M. Cyr). http://dx.doi.org/10.1016/j.fuel.2015.09.017 0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.

spherical particles, which form the fly ash. Fly ash is then recovered from the flue gas by means of mechanical separators followed by electrostatic precipitators or bag filters. Depending on the physical and chemical properties of the coal being burned and on the combustion process, the composition and properties of fly ash may vary considerably [5]. Fly ash appears as a relatively fine powder with rounded particle diameters mainly between 1 and 150 lm [6,7]. It is however a complex inorganic– organic heterogeneous material containing associated and finely dispersed solid, liquid and gaseous elements. The main components of fly ash have been characterized as being silica, alumina, calcium and ferrous oxides [8–10]. In general, fly ashes resulting from the combustion of bituminous coal and anthracite contain a low amount of calcium while subbituminous coal or lignite lead

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to coal ashes composed of more than 10% of calcium. A classification of fly ashes for industrial applications, mostly for use as mineral addition in cement and concrete, is made based on the source of coal and major element oxide content (cf. NF EN 197-1 [11], NF EN 450-1 [12] and ASTM C618 [13]). A method developed by the American Society for Testing and Materials allows a classification of fly ashes for use as mineral admixture in concretes: class F fly ash and class C fly ash [13]. On the one hand, type F fly ash (i.e. low calcium) contains at least 70% of SiO2 + AlO3 + Fe2O3. This fly ash exhibits pozzolanic properties but possesses little or no selfhardening property. On the other hand, type C fly ash (i.e. high calcium) has a minimum of 50% of SiO2 + AlO3 + Fe2O3, contains significant amount of calcium hydroxide (at least 10%) and exhibits both pozzolanic and cementitious properties. Other chemical and physical requirements in this classification include Cl , SO3 and Na2O contents, moisture, particle size and loss on ignition. Due to better burning efficiency, most of thermal power stations use furnaces fired with pulverized coal. Fly ash resulting from this process possesses interesting properties (i.e. type C or F fly ashes) that allow it to be used as supplementary cementitious materials, among other applications such as ceramics, filling materials, fertilizers, soil amendment and geopolymers [10,14,15]. Replacing a portion of Portland cement with fly ash was shown to be beneficial as it improves the handling and performance of concrete materials and in the same time reduces the need to use primary raw materials [8,16] providing thus a real economic advantage. However, one has to keep in mind that European standards limit the volume of fly ash (30%) to be incorporated because of carbonation problems. Moreover, all types of generated fly ash cannot be used as supplementary cementitious materials or valorised in various applications such as those mentioned below. The remainder is considered as waste and has to be landfilled. The need to develop new recycling routes and innovative applications for coal fly ash as well as a better characterization of fly ash available appear essential [17–19]. A spreader stoker burning process is currently used on Reunion Island. Optimized for the combustion of biomass and cocombustion with fossil fuels [20–23], it burns bagasse during the sugarcane harvest and coal the rest of the season. Coal fly ashes resulting from this specific combustion system, referred as Spreader Stoker Coal Fly Ashes (SSCFA), are currently sent to landfill. In fact, they cannot be used in civil engineering applications due to unburned carbon content higher than the one recommended in the European Standard NF EN 450-1 (less than 5–9%, depending on the category of fly ash) [12]. The lack of general knowledge about this kind of fly ash, its properties as well as the lack of adequate standard regulations concerning its potential applications are the dominant barriers to its valorization. Because of a large SSCFA production (100,000 tons/year), a limited space (2512 km2) and restrictive regulatory guidelines, Reunion Island is thus currently facing SSCFA storage problem. The burning process seems to play an important role in the classification of fly ashes. SSCFA undergo physical transformation leading to changes compared to traditional Pulverized Coal Fly Ashes (PCFA). The aim of this paper is thus to present a new kind of fly ash, SSCFA, and highlight the similitudes and differences between three kinds of fly ashes: SSCFA and PCFA1 from Reunion Island, and PCFA2, which is a normalized fly ash currently used in cement production according to European Standard NF EN 197-1 [11]. These fly ashes are characterized through chemical, mineralogical and physical analyses and a comparison is made to determine the particularities of SSCFA. Moreover, a study on fresh and hardened mortars composed of 75% of Portland cement and 25% of fly ashes, the three fly ashes being tested, is presented to give an insight into the behavior of these materials in industrial applications.

2. Spreader stoker compared to pulverized coal boilers 2.1. Pulverized coal boilers A pulverized coal-fired boiler is an industrial boiler that generates thermal energy by burning pulverized coal. This kind of boiler currently dominates the electric power industry and can have a size of up to 1000 MW (cf. Fig. 1) [10]. Coal is transferred from coal stock equipment to a coal pulverizer where it is dried and ground into a fine powder (i.e. pulverized) before feeding a burner. A vertical roller mill is mainly used as grinder. The pulverized coal is then blown with carrier primary air (i.e. air used to carry the coal and to dry it before entering into combustion chamber) into a boiler and burned by ejecting secondary air (i.e. air supplied separately to complete the combustion). Combustion takes place at temperatures between 1200 and 1700 °C, depending largely on coal source [14]. The heat from coal combustion is used to generate steam to supply a steam turbine driving a generator to produce electricity. Combustion conditions induce the liberation and fragmentation of minerals, which undergo thermal decomposition. During combustion, the inorganic matter of coal is transformed to particulate matter known as ashes and volatiles. Bottom ash designs particulate matter fallen out of the gas stream and collected on the furnace bottom. On the contrary, fly ash is carried away with the combustion gases through the boiler and collected by electrostatic precipitators [5,16,24]. 2.2. Spreader stoker boilers Spreader stoker power systems are typically around 50 MW in size and are capable of converting biomass as well as all ranks of coal into energy that will in turn be used to generate electricity and/or heat (cf. Fig. 2) [21,22,25]. Fuel is evenly distributed into the boiler and to a uniform depth over the entire moving grate surface by using a spreader that propels fuel particles into the air above the grate. As the fuel is thrown into the boiler, the high temperature around 1000 °C and the upward flow of flue gases help flash drying of the fuel and combustion of volatiles. Fine particles ignite and burn while suspended in the combustion air, while coarser particles fall onto the grate and are burned in a thin bed of fuel on the grate. Primary combustion air is uniformly supplied from beneath the grate and diffuses through the bed of coal. A portion of the total combustion air is also admitted through ports above the grate to complete the combustion process [22,26,27]. Securing proper distribution of air through a stoker fuel is fundamental as combustion is governed by the

Fig. 1. Pulverized coal combustion system.

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specified in European Standard NF EN 197-1 [11]. Its chemical composition is given in Table 1. Laboratory grade calcium hydroxide (Ca(OH)2) without calcium carbonate (as verified by XRD) was used to evaluate the pozzolanic activity of the ashes. The aggregate used in mortar production was a normalized quartz sand conformed to NF EN 196-1 with particle size ranging between 0 and 2 mm [29]. 3.2. Mortars preparation

Fig. 2. Spreader stoker combustion system.

physical properties of the fuel bed and air distribution through the bed [28]. Due to differences in both processes (e.g. temperature, coal dispersion, etc.), SSCFA needs to be characterized in order to evaluate possible routes for their valorization, for instance in cement-based applications such as normalized PCFA.

Fly ash/Portland cement-based mortars were composed of 75% of cement and 25% of fly ashes (weight percentages). Three different mortars were prepared for workability test (one for each fly ash). For compressive strength measurements, PCFA2-cement based mortar was not tested. A reference mortar was also made with 100% of Portland cement. Water to binder ratio was set at 0.50 and sand to binder ratio at 3. Mixing and specimen preparation were carried out at ambient temperature and relative humidity of 55%. Mortars were first mechanically mixed (normalized mixing according to NF EN 1961 [29]) during 4 min (including 90 s break) and then cast in PVC molds (6 specimens of 4  4  16 cm for each mortar) with the aid of a vibrating table. They were kept covered in a room at constant temperature and humidity. They were finally demolded after 24 h and kept in sealed plastic bags at 20 °C before testing. 3.3. Pastes preparation

3. Materials and methods 3.1. Materials 3.1.1. Fly ashes Three fly ashes were studied in this paper: two fly ashes from Reunion Island resulting from different combustion processes – Spreader Stoker Coal Fly Ash (SSCFA) and Pulverized Coal Fly Ash (PCFA1) – and one normalized siliceous Pulverized Coal Fly Ash (PCFA2) used in cement production according to European Standard NF EN 197-1 [11]. Fly ashes from Reunion Island (SSCFA and PCFA1) were collected in dry state from the power thermal plants and were then wetted during transportation from Reunion Island to the mainland to limit their dispersion in the atmosphere. PCFA2 came from Cordemais power station in France. In the following, it was considered as a reference class F fly ash representative of siliceous pulverized coal fly ash available on the market in terms of chemical, mineralogical and physical characteristics. For most characteristics studied in this paper, precise values were reported following tests performed in LMDC laboratory. Otherwise, as this kind of fly ash is the object of a lot of research work, average values from literature were sometimes used. Coal burned in thermal power plants located in Reunion Island came from South of Africa and was representative of bituminous coal used in the market. 3.1.2. Other materials The cement used for the evaluation of the performance of fly ashes in cement-based materials was CEM I 52.5 N CE CP2 NF as

Fly ash/Portland cement-based pastes were composed of 75% of cement and 25% of fly ashes (weight percentages). SSCFA and PCFA1 ashes were tested for standard consistence and setting time measurements. Mixing was performed at ambient temperature and relative humidity of 55% during 3 min according to NF EN 196-3 [30]. Fly ash/portlandite-based pastes were composed of 75% of fly ash and 25% of portlandite (Ca(OH)2). Water-to-binder ratio was set at 0.60. Mixing by hand and specimen preparation were carried out at ambient temperature and relative humidity of 55%. Pastes were cast in cylindrical plastic recipients, which were then hermetically sealed and placed in a drying oven at a temperature of 60 °C and moisture content of 100% before being pulverized and studied by thermogravimetry analysis. 3.4. Methods 3.4.1. Fly ashes characterization Fly ashes were characterized through chemical, mineralogical and physical analyses. The properties investigated as well as the experimental procedures used are summarised in Table 2. 3.4.2. Mortars properties Mortars were subjected to rheological and mechanical tests. Mortar flowability was evaluated under vibration using LCL apparatus according to NF P 15-437 [31] and compressive strength was measured at 7 and 28 days using a press according to NF EN 196-1 [29] (cf. Table 2).

Table 1 Chemical composition of the cement used. Oxide (wt.%)

LOI

SiO2

Al2O3

Fe203

CaO

TiO2

MgO

P2O5

K2O

Na2O

Mn2O3

SO3

CEM I 52.5 N CE CP2

2.16

20.41

5.61

2.25

65.08

0.27

1.78

0.27

0.53

0.24

0.05

3.25

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M. Sow et al. / Fuel 162 (2015) 224–233 Table 2 Methods. Characteristics

Methods

Chemical composition

Inductively coupled plasma-optical emission spectrometry (ICP–OES), Optima 7000 DV X-ray diffraction (XRD), Siemens D5000 diffractometer with Co Ka radiation, k = 1.789 Å, steptime: 10 s, stepsize: 0.02°(4–70°) Scanning electron microscopy (JEOL JSM-6380 LV) coupled with elemental analysis by energy dispersive X-ray spectroscopy (SEM + EDX) Hydrostatic weighing of powder in non-reactive liquid Blaine specific surface area (SSB) according to NF EN 196-6 [31] Brunauer, Emmett and Teller (BET) method, using nitrogen sorption Loss on ignition (LOI) at 1000 °C according to NF EN 196-2 [32] Laser granulometry CILAS 1090 LD, Mie (1.73: real part, 0.10: imaginary part), wet method with water Optical microscope coupled with a particle analyzer, Malvern, Morphology G3S

Mineralogy

Morphology

Absolute density Specific surface

Amount of unburned particles Particle size distribution Granular morphological analysis Workability

Standard consistence and setting time Strength Activity Index

Pozzolanic activity

Flow under vibration using LCL apparatus according to NF P 15-437 [30] Each value is an average of 2 tests Vicat apparatus according to NF EN 196-3 [33] Each value is an average of 2 tests Compressive strength of 4  4  16 cm prisms (crosshead speed 2 400 N/s ± 200 N/s according to NF EN 196-1 [29]) Hydration time: 7–28 days Each value is an average of 6 tests Thermo-gravimetric analysis (TGA), STA 449 F3 Jupiter-Simultaneous Thermal Analyzer TGA DSC

4. Results and discussion 4.1. Comparison between pulverized coal and spreader stoker fly ashes 4.1.1. Chemical characteristics The chemical compositions and the amount of unburned particles (LOI: Loss on ignition) of fly ashes selected for this study are detailed in Table 3. The sample preparation used for chemical analyses was based on borate fusion. A mixture of fly ash, lithium tetraborate and lithium metaborate was placed into a muffle furnace at 1100 °C for 30 min. The melt was then carefully dissolved in diluted nitric acid before ICP–OES analyses. Chemical analyses were performed on fly ash samples after LOI test (the sum of the oxides was around 100%). The LOI values will be discussed later (cf. Section 4.1.3). The three fly ashes are essentially composed of silica, alumina and ferrous oxides. PCFA2 chemical composition is consistent with what is reported in literature for siliceous pulverized coal fly ash [8,10]. As expected, it contains more than 70% (wt.%) of SiO2 + AlO3 + Fe2O3 and less that 10% (wt.%) of CaO. The same is true for PCFA1, which as PCFA2 is composed of 86% (wt.%) of SiO2 + AlO3 + Fe2O3 with the same amount of SiO2 but more AlO3
SSCFA has SiO2 + AlO3 + Fe2O3 and CaO contents similar to the ones of PCFA1 and PCFA2. Minor differences in composition appear when looking

at the other oxides contents, particularly for Na2O, MgO, Mn2O3 P2O5 and SO3. SO3 in SSCFA was in the mineralogical form of anhydrite (cf. Section 4.1.2). It is well accepted in literature that variability in chemical analysis of the same kind of fly ashes may be observed as the chemical composition of fly ashes depends on the characteristics and coal rank burned in thermal power plants [8,10]. Despite their different origin, the chemical analyses reported here for the three fly ashes reveal limited quantitative variations, which are not significant to distinguish SSCFA and PCFA1 from normalized PCFA2. SSCFA can thus be considered equivalent to a siliceous normalized pulverized coal fly ash in terms of chemical composition. Generally speaking, it is quite important to assess the variability of a material in terms of chemical composition, particularly when used as mineral additions in cementitious systems or in any kind of industrial applications. As few studies deal with SSCFA, one can ask if the chemical composition shown in Table 3 can be considered as being representative of this new fly ash. Fig. 3 brings a positive response to this interrogation. The oxide percentages of the SSCFA sample studied here are consistent with values obtained from a database monitoring the chemical compositions of random samples of SSCFA collected on site. Box plots are used to represent relevant information from the database for the past 7 years. The variations may come from the coal, the storage conditions and the sample preparation for analysis.

4.1.2. Mineralogical characteristics According to the literature, fly ashes are composed of constituents which can be classified as primary and newly formed minerals or phases [34–36]. They contain: – original coal minerals or phases that have not undergone phase transformations during coal as they have relatively high decomposition or melting temperatures (such as quartz or feldspars), – secondary new phases formed during coal combustion when temperature is increasing (such as hematite, mullite or amorphous phase), – and tertiary new phases that can be formed during transportation and storage. It is the case for instance for lime that can be transformed to portlandite, of anhydrite to form gypsum. These tertiary phases were not found in the present work. The X-ray diffractograms reported in Fig. 4 for 2b ranging from 10° to 70° (for the sake of clarity, the curves are offset) show that the selected fly ashes have a polyphasic structure composed of an amorphous phase and several major crystallized phases such as mullite (Al6Si2O13), quartz (SiO2), magnetite (Fe3O4) and hematite (Fe2O3). These observations are consistent with literature for PCFA [8,10,37]. Minerals present in SSCFA are similar to those usually found in normalized PCFA [8,38]. However, a difference in the amount of the amorphous phase can be observed between the three fly ashes with SSCFA having the highest proportion of amorphous phase. This glassy amorphous phase due to the rapid cooling of burned coal is mainly composed of silica and alumina for a siliceous fly ash and guarantees a certain pozzolanic activity of the fly ash. The reactivity of the glass depends on the calcium content of the

Table 3 Chemical composition of coal fly ashes (wt.%) obtained by ICP–OES.

SSCFA PCFA1 PCFA2

SiO2

Al2O3

Fe2O3

CaO

K2O

Na2O

MgO

Mn2O3

TiO2

P2 O5

SO3

Total

LOI

50.05 53.12 53.65

26.14 29.45 26.05

7.27 4.34 6.26

5.28 6.66 5.29

1.08 1.04 1.33

2.40 0.27 0.04

2.44 1.25 0.90

0.12 0.08 2.27

1.81 1.80 1.32

0.71 2.35 0.33

1.30 0.29 0.72

98.60 100.65 98.16

27.9 19.3 3.2

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Chemical composition (wt. %)

60%

Max Third quartile Median Mean First quartile

50% 40%

Min

SSCFA sample (cf. Table 3)

30% 20% 10% 0% SiO2

Al2O3 Fe2O3

CaO

K2O

Na2O

MgO

TiO2

Mn2O3 P2O5

SO3

Fig. 3. Chemical composition of SSCFA sample tested in this study compared to values from a database monitoring oxides composition of SSCFA samples collected on site for the last 7 years.

Fig. 4. XRD of SSCFA, PCFA1 and PCFA2.

fly ash and is generally more important for a high calcium fly ash [16,39]. The exact composition of the amorphous phase observed between 2b = 17° and 2b = 40° for the three fly ashes cannot be determined by using XRD only. However, an interesting observation can be made when comparing the X-ray diffractograms of SSCFA before and after treatment (which consists in heating SSCFA at 800 °C): the proportion of the amorphous phase significantly decreases without any significant changes in peaks (cf. Fig. 5). Only small new peaks seemed to appear and could be due to the crystallization of feldspars. This result could suggest that a part of the amorphous phase is composed of unburned carbon particles, which are removed when heating at 800 °C (as confirmed by the lower LOI value equal to 0.78% for the treated SSFCA). From a quantitative point of view, the amount of amorphous phase can be determined by a method based on the measurement of intensities (i.e. surfaces). The amorphous phase amount is defined by the ratio between the intensity of the amorphous hump and the sum of the diffraction intensities of the characteristic peaks of all the crystallized minerals (i.e. the area between the base line and the XRD diffractogram) [40]. The surfaces are calculated for 2b ranging from 15° to 70°. The obtained results are 71%, 75% and 58% for SSCFA, PCFA1 and PCFA2 respectively. The value for the SSCFA after treatment is 50%.

4.1.3. Physical characterization The physical characterization includes morphology, loss on ignition, specific gravity, specific surface Blaine and particles size analysis. These parameters were selected as they have an important impact on the reactivity of fly ashes. An insight on the morphological properties of fly ashes through SEM images coupled with EDX analyses is shown in Fig. 6. The other results are reported in Table 4. As reported in literature through particle size distribution and SEM analyses, fly ash can be considered as a fine power with particles size varying between 0.5 and 300 lm, and spherical particles having a diameter ranging from 1 to 150 lm [7,19,25,41]. Larger and irregular particles can also be found and are generally associated to agglomerates or unburned carbon particles [9]. SEM images obtained from SSCFA and PCAF1 samples shown in Fig. 6 are in line with these trends. As determined by EDX, the predominant elements in the fly ash samples were aluminum, silicon, carbon and, in a lesser extent sodium, magnesium, potassium, calcium and iron. The number of irregularly shaped particles observed for PCFA1 and particularly SSCFA was however much higher than in the case of normalized pulverized coal fly ash. One reasonable explanation could be the presence of many unburned carbon particles, as suggested in the literature [8,9,14,17]. PCFA1 particles seemed to be more rounded than SSCFA (cf. Fig. 6, images 5 and

M. Sow et al. / Fuel 162 (2015) 224–233

229

Fig. 5. XRD of SSCFA and SSCFA after treatment (heating at 800 °C).

6 vs. images 2 and 3) but were far from being equivalent to normalized pulverized coal fly ash particles due the higher unburned content (cf. Table 4 and 19.3% vs. 3.2%). The shape of the particles can be related to their size. Larger particles appeared to be more irregular in shape and smaller particles more spherical. This trend was confirmed by granular morphology analysis, as it will be discussed later (cf. Section 4.2.1). Some hollow particles were also observed (cf. Fig. 6, image 2), as for normalized PCFA [42]. Moreover, the color of SSCFA images was darker than the one for PCFA1. This difference could be also found its origin in the higher carbon content of SSCFA [17]. From the results in Table 4, it can be seen that LOI values are quite high for SSCFA and PCFA1 compared to PCFA2 (27.9% and 19.3% vs. 3.2%). The high LOI values obtained for SSCFA and PCFA1 can be explained by a spreader stoker burning process optimized for biomass and co-combustion with fossil fuels leading to a shorter combustion time and probably a lower combustion temperature compared to pulverized coal process [20–23]. In fact, apart from the coal rank, the morphology of a fly ash is mainly controlled by combustion temperature and cooling rate. The low LOI value for PCFA2 indicates that the performance of the boiler in the power plant is well adapted. Moreover, as stated above, larger particle sizes generally correspond to higher unburned carbon concentrations. This is consistent with mean diameter values obtained for SSCFA and PCFA1 with PCFA1 having a lower mean diameter and LOI value. However, the same is not true for PCFA2, which has the lowest LOI value but the highest mean diameter. This could be explained by the presence of agglomerated particles with some small particles stick to the outer surface of larger particles (cf. Fig. 6, image 6), as already observed by some authors [9]. The specific surface area values reported in Table 4 were measured using the Blaine technique (SSB). The values obtained by this technique are generally lower that the ones with BET technique [8]. SSB values for SSCFA and PCFA1 are similar (8000 cm2/g). However, a lower SSB value is measured for PCFA2 (3400 cm2/g), this value being consistent with other ones found in the literature [8,38]. BET method was also used to determine the specific surface area of SSCFA and treated SSCFA (cf. Section 4.1.2). As said previously, treated SSCFA has a low LOI value and no significant changes in XRD compared to SSCFA. BET values are 17.57 and 2.20 m2/g for SSCFA and treated SSCFA, respectively. This difference in specific area measured by BET points out the effect of unburned carbon particles, which could affect gas adsorption.

Concerning specific gravity (SG), values reported in literature varies over a wide range depending on the coal source and oxide content of the fly ash [8,10]. SG value measured for SSCFA are consistent with values obtained for sub-bituminous or bituminous coal fly ashes (1.90–2.96 g/cm3) [8,19,38]. In terms of physical characteristics, it can be seen that the main difference between SSCFA and normalized PCFA can be found in the LOI value, a parameter which could as well affect the BET value and the particle size distribution. 4.2. SSCFA addition in mortars 4.2.1. Fresh state Before molding, the workability of mortars composed of 75% of Portland cement CEM I 52.5 N and 25% of fly ash was assessed by measuring the flowing time with the LCL apparatus according to NF P 15-437 [31]. The workability is defined as the ability of a mixture to flow under vibration, the higher is the flowing time, the worse is the workability. Fig. 7 reports the flowing times obtained for 100%CEM I 52.5 N, 75%CEM I 52.5 N + 25%SSCFA, 75%CEM I 52.5 N + 25%PCFA1 and 75%CEM I 52.5 N + 25%PCFA2. It is generally observed that a partial substitution of Portland cement by fly ash in mortar or concrete mixtures improves the workability by decreasing the water demand [43]. This effect depends on the quality of fly ash and the amount of cement replaced [43,44] and can be explained by three mechanisms: the adsorption of fine fly ash particles on cement grains preventing the formation of flocs trapping water; the spherical shape and smooth surface of fly ash particles reducing inter-particle friction and the ‘‘particle packing effect” with fly ash particles filling the voids [16]. The rheological results obtained here depend on the type of fly ash used as cement replacement. The reference mortar and the mortar containing 25%PCFA1 have the same flowing time (5 s) while the mortar prepared with 25%SSCFA has the worst flowing time (13 s). As expected, mortar containing 25%PCFA2 shows a workability improvement with the lowest flowing time (2.5 s) [8,16,43]. Explanations could be found in the shape and size of fly ash particles and in the unburned carbon content. As observed in Table 4, SSCFA has the highest LOI (27.9%) value meaning that more incompletely burned carbon matter present in the mortar could favor the consumption of water in its higher intern porosity. PCFA1 has a lower LOI value (19.3%) but still relatively high compared to normalized PCFA2 (3.2%). The workability of mortar

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Fig. 6. SEM images of SSCFA (1–4) and PCFA1 (5–8) coupled with EDX analyses.

prepared with PCFA1 is however quite acceptable as similar to the one of the reference paste. The limited effect of unburned carbon particles on workability in this case could be explained by the shape and size of PCFA1 particles. In fact, PCFA1 has small spherical particles (cf. Table 4 and Fig. 6) leading to a positive effect on the workability and thus limiting the negative impact due to the presence of unburned carbon particles. On the contrary, SSCFA particles tend to be larger (cf. Table 4) and irregular in shape (cf. Fig. 6), which in addition to the high volume of carbon particles worsen workability. To go further into the understanding of the workability results, granular morphological analysis was performed on each fly ash, SSCFA, PCFA1 and PCFA2, to provide more information about the existence and concentration of spherical particles. This method

Table 4 Physical characteristics of tested coal fly ashes. Fly ash

LOI (%)

SG (g/cm3)

SSB (cm2/g)

D10 (lm)

D50 (lm)

D90 (lm)

Dm (lm)

SSCFA PCFA1 PCFA2

27.9 19.3 3.2

2.19 2.24 2.30

7900 8000 3400

4.5 2.3 2.7

16.9 13.2 17.1

49.2 45.2 61.1

22.1 19.2 25.5

LOI: loss on ignition; SG: specific gravity; SSB: specific surface Blaine; D10, D50, D90: 10%, 50% and 90% passing; Dm: mean diameter (particle size distribution by volume).

uses an optical microscope coupled with image analysis software to generate information regarding particle sizes and shapes. Here, a constant mass (3 mg) of each fly ash was scanned, which means

M. Sow et al. / Fuel 162 (2015) 224–233

231

Fig. 7. Flowing time of tested mortars.

that the number of particles for each sample was not the same as it depends on the bulk density of particles. Fig. 8 shows plots of the circularity against particle size for each fly ash along with the particle size distribution by number. The circularity is defined as the ratio of the equivalent perimeter to the real perimeter of a particle, where the equivalent perimeter is the perimeter of a disk with the same area as the particle. Fig. 9 represents the population

cumulative frequency versus the circularity for each fly ash. It can be seen from the circularity versus diameter of particles graphs (cf. Fig. 8) that PCFA2 seems to be more circular than SSCFA and PCFA1, with less particles having a circularity inferior to 0.90 (less angular particles). Moreover, the larger particles appear to be less spherical as the points are offset to the right side of the graphs (i.e. larger diameter) when circularity decreases. The population

Fig. 8. Circularity vs. diameter (dots) and particle size distribution by number (triangles) for the tested fly ashes (SSCFA in red, PCFA1 in blue and PCFA2 in light pink). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 10. Strength activity index (SAI) at 7 and 28 days of mortars containing 25% of fly ash (the straight line correspond to the minimum requirement at 28 days (0.75) given by EN 450-1 [12]).

Fig. 9. Population cumulative frequency vs. circularity for the tested fly ashes.

Table 5 Normalized consistency (expressed as W/B) and setting time of cement pastes containing 25% of fly ash compared to a reference cement paste prepared with 100% CEM I 52.5 N.

CEM I 52.5 N 75%CEM I 52.5 N + 25%SSCFA 75%CEM I 52.5 N + 25%PCFA1

W/B

Setting time (h)

EN 450-1 limit (h) [12]

0.31 0.39 0.37

2.4 5.1 4.7

4.8 (2  Reference value)

cumulative frequency results show that a high proportion of PCFA2 particles have a circularity ranging from 0.95 to 1.00, meaning that they are close to a spherical shape: 68.4%, 60.2%, 46.8% for PCFA2, PCFA1 and SSCFA respectively. The difference in shape observed between the three tested fly ashes could be a reasonable explanation for the increase in flowing time when using SSCFA and are consistent with observations from SEM analyses. The previous observations on workability of mortars can be confirmed by setting time measurements realized on fly ash/ Portland cement pastes with normalized consistency according to NF EN 196-3 [30]. It can be seen from Table 5 that the normalized consistency and setting times values of cement pastes with 25% of fly ash as cement replacement are higher than the reference Portland cement CEM I 52.5 N. The PCFA1-based cement paste only just met the EN 450-1 requirement in terms of setting time [12] (i.e. it shall not be more than twice as long as the initial setting time of a 100% test cement paste) while an increase of 18 min is observed for SSCFA-based cement paste. These results could be explained by different mechanisms [45]: a dilution effect of the cement since the pastes contained 25% less cement than the reference, resulting in a decrease of the quantity of hydrates formed in the first few hours; an increase of the water to binder ratio due to the high water demand, known to have an effect on the setting time; a harmful effect of the fly ashes themselves maybe due to the presence of minor elements (e.g. Zn, P. . .) or unburned carbon particles perturbing the hydration of the cement [46].

Fig. 11. TGA of SSCFA-portlandite systems.

cement by SSCFA or PCFA1 does not affect significantly the compressive strength of mortars even if the LOI value of these fly ashes is high. The increased rate of strength development between 7 and 28 days in mortars is attributed to the pozzolanic reaction caused by the amorphous silica and alumina content of tested fly ashes. The consumption of portlandite was followed by TGA and results presented in Fig. 11 shows indeed a pozzolanic activity for SSCFA. The formation of C–S–H and calcite was also observed through this test (the carbonation of samples during preparation explain the presence of calcite). Due to its pozzolanic nature, the SSCFA, as normalized PCFA, reacts with portlandite (Ca(OH)2) produced during cement hydration resulting in an increase in long-term strength [47,48]. Moreover, the delay in setting time observed at early age (cf. Table 5) does not seem to have a significant effect on later age strength. SAI measured at 90 days for SSCFA-based mortar even reaches the quite satisfactory value of 0.97. 5. Conclusion

4.2.2. Hardened state Fig. 10 reports the compressive strength results at 7 and 28 days as strength activity index (SAI). This index is defined as the ratio of the compressive strength of fly ash-based mortar (75%CEM I 52.5 N + 25%SSCFA or 25%PCFA1) and reference mortar (100%CEMI 52.5 N). It can be observed that SAI values (at 7 and 28 days) are higher than the 28-days minimum value required in the European Standard EN 450-1 (i.e. 75%) [12]. The replacement of 25% of

The aims of this paper were to compare the chemical, mineralogical and physical characteristics of spreader stoker and pulverized coal fly ashes and give an insight into the rheological and mechanical properties of SSCFA-based mortars. A better understanding of the characteristics of fly ash particles is indeed essential to the development of value-added fly ash utilization technologies. Three fly ashes were studied, one derived from

M. Sow et al. / Fuel 162 (2015) 224–233

spreader stoker process and two from pulverized coal burning system. The following conclusions can be drawn: – As class F pulverized coal fly ash, spreader stoker coal fly ash is composed of silica, alumina and ferrous oxides. It has several major crystallized phases and an amorphous phase, which guarantees a certain pozzolanic reactivity. – Spreader stoker fly ash is partly composed of spherical particles, similar to those encountered in pulverized coal fly ash currently used in cements and concretes. However, this fly ash contains a large amount of unburned carbon particles, far above the one recommended in the European Standard NF EN 450-1. – Spreader stoker-based systems have degraded rheological properties and retarded setting time, probably due to the high unburned carbon content. – Spreader stoker-based mortar has interesting mechanical properties with a strength activity index much higher than the minimum value specified in standard EN 450-1, probably due to a its high pozzolanic activity. Specific industrial applications are thus worth considering in order to verify if the high carbon content is detrimental or not.

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