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Citation: Sanjuán, M.Á.; Argiz, C. A
Ten-Year Study on Alkali Content of
Coal Fly Ash. Fuels 2022,3, 365–374.
https://doi.org/10.3390/
fuels3020023
Received: 17 May 2022
Accepted: 16 June 2022
Published: 19 June 2022
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Article
A Ten-Year Study on Alkali Content of Coal Fly Ash
Miguel Ángel Sanjuán1, * and Cristina Argiz 2
1Construction Materials Unit, Civil Engineering School, Technical University of Madrid, C/Profesor
Aranguren, 3, Ciudad Universitaria, 28040 Madrid, Spain
2
Materials Chemistry Unit, Civil Engineering School, Technical University of Madrid, C/Profesor Aranguren,
3, Ciudad Universitaria, 28040 Madrid, Spain; cg.argiz@upm.es
*Correspondence: masanjuan@ieca.es; Tel.: +34-914429166
Abstract:
After years of decline, coal consumption has risen significantly in the last year (2021),
driven mainly by the ever-increasing demand in fast-growing Asian countries and fostered by rising
gas prices in Europe and the United States. Coal is both the largest electricity production source and
the largest source of carbon dioxide emission. Coal-fired plants produce electricity by generating
steam by burning coal in a boiler, but also large amounts of coal fly ash. Coal fly ash contains essential
constituents for cement production, such as Ca, Si, Al, and Fe. Application of coal-fired ash to
produce clinker at high doses may reduce the limestone content in the raw mix. Furthermore, coal
fly ash is one of the industrial source materials utilized in the development of low-carbon cements
and concretes on account of its chemical characteristics. The monitoring methodology is based
fundamentally on the analysis of a set of variables (Na
2
O
e
, Na
2
O, K
2
O, free CaO, and reactive silica
content and fineness) over time. Weak relations between Na
2
O and K
2
O, and Na
2
O
e
, and reactive
silica content were found. This applied research has been done to verify previously done research.
The scope of this paper is to assess the alkaline content of coal fly ash over a period of 10 years. The
Na
2
O-equivalent of coal fly ash ranged from 0.35% to 2.53%, with an average value of 0.79%. These
values should be taken into account producing concretes made with potentially reactive aggregates
in order to mitigate the alkali–silica reaction (ASR).
Keywords: coal fly ash; alkalis; grate furnace combustion; cement constituent
1. Introduction
Coal-fired electricity generation process has the disadvantage that generates large
amounts of coal fly ash. Its properties strongly depend on the firing conditions and the
original source of coal. It should be noted that the worldwide coal ash exploitation is poor
causing a landfilling problem. World total production in 2020 was 7575 Mt (China: 3760 Mt;
India: 760 Mt; Indonesia: 564 Mt; Australia: 493; USA: 485 Mt; Russia: 398 Mt; UE27:
301 Mt). China was the major producer, and the production grew up in Russia, Indonesia,
India, and Turkey, while it declined in the United States and the European Union.
Following a record of over 10,000 TWh in 2018, in 2020 global coal-fired generation
fell to 9440 TWh (35.2% world share in 2020) [
1
]. The use of renewables in many countries
reduced the share of coal in the electricity mix prompted the 2020 drop in coal-fired
generation. Nevertheless, coal-fired generation is showing strong recovery in 2021 by the
rising gas prices. In addition, retrofitting coal-fired power plants with Carbon Capture, Use
and Storage (CCUS) can help prevent the closure of coal-fired power plants in the context
of the Net Zero Emissions by 2050.
Coal-fired power generation increased worldwide by almost 5% in 2021 and by a
further 3% in 2022. The coal consumption in Spain for power generation increased a 9% in
2021. Spain consumes 10.4 million tons of coal (2020) leading to around 147,000 tons of coal
bottom ash and 1.0 Mt of coal fly ash [2].
Fuels 2022,3, 365–374. https://doi.org/10.3390/fuels3020023 https://www.mdpi.com/journal/fuels
Fuels 2022,3366
The European Circular Economy Action plan sets forth the principles, goals, and
actions dealing with resource exploitation and waste [
3
]. In this respect, it is striking that
the utilization of some industrial wastes, such as coal fly ash, as cementitious constituents
in the production of Portland cements and concretes is a lever for achieving net-zero carbon
dioxide emissions set in the cement industry roadmap to beyond net zero emissions [
4
].
In addition, the European construction sector is responsible of approximately 35% of the
global waste generation. Accordingly, construction sector, in general, and cement sector, in
particular, play a major role in circularity.
The main advantages of using coal fly ash in cements are the improvement of the
concrete durability and the climate change mitigation by lowering the clinker factor. Fur-
thermore, concrete carbonation is a physicochemical process that absorb carbon dioxide
and, therefore, helps to tackle climate change and to enhance sustainable development. In
addition, coal fly ash cement-based materials carbonate faster than common cement-based
materials made without supplementary cementitious materials [
5
]. To summarize, coal fly
ash utilized to produce Portland cements and concretes is a critical decarbonization lever.
The American Society for Testing and Materials (ASTM) classifies coal fly ashes as
Class C and Class F based on the silica, alumina, iron oxide, and calcium oxide contents [
2
].
Class C originates from lignite and sub-bituminous coals, whereas Class F is produced from
bituminous and anthracite coals. The amount of silica, alumina, and iron oxide is between
50% and 70% in Class C coal fly ash, while Class F contains more than 70%. In addition,
Class C contains more than 20% calcium oxide, and, therefore, it presents pozzolanic activity
itself, without the presence of any activator. By contrast, Class F has less than 10% CaO, and,
then, it requires an activator for the pozzolanic reaction, such as Portland cement. According
to the European standard EN 197-1:2011 [
6
], coal fly ash content in common Portland cement
ranges from 6% to 20% (CEM II/A-V), from 21% to 35%, (CEM II/B-V), from 11% to 35%
(CEM IV/A (V)) and from 36% to 55% (CEM IV/B (V)).
The advantages of coal fly ash as a cement component derive from its pozzolanic
activity, i.e., it combines with calcium hydroxide generated by cement hydration reac-
tions to form C-S-H gel, which is a calcium silicate hydrate that contributes to Portland
cement strength [
5
]. Coal bottom ash also exhibits a good pozzolanic activity when it is
grounded [
7
]. By contrast, one disadvantage of some coal fly ashes is a high level of alkalis,
limiting its use in the Portland cement and concrete production.
The primary compounds for all types of coal fly ashes are silica (SiO
2
), alumina
(Al
2
O
3
), iron oxide (Fe
2
O
3
), and calcium oxide (CaO). Bituminous coal fly ash has higher
contents of SiO
2
, Fe
2
O
3
and ignition loss, by contrast it has lower amounts of CaO and
MgO than the rest of ashes. Table 1shows the average coal fly ash alkali content in Europe,
USA, China, India, and Australia and in function of the coal type [
8
,
9
]. The values range
from 0.1% to 4.7%. Accordingly, low-alkali content coal fly ash would be recommendable
to produce concrete made with potential reactive aggregates, such as slates, quartzite,
hornfels, granites, gneiss, and serpentine, to minimize alkali-aggregate reactivity (AAR).
As was well known, the alkali ions content in the concrete pore solution affects negatively
concrete durability with regard to the alkali–silica reaction (ASR), when potentially reactive
aggregates are used. ASR generates swelling gel products from the reactive aggregates
leading to a significant expansive pressure in the concrete promoting the appearance of
cracks. Furthermore, alkalis enhance coal fly ash reactivity [
10
], but also, promote the
generation of a passive layer on the steel reinforcement preventing the steel corrosion.
Then, it should be taken into account that coal fly ash alkali content also influences the final
amount of the alkalis in the concrete pore solution.
Table 1. Coal fly ash alkalis by region and coal type [8,9].
Alkali Europe USA China India Australia Bituminous Sub-Bituminous Lignite
Na2O 0.1–1.9 0.3–1.8 0.6–1.3 0.5–1.2 0.2–1.3 0–4 0–2 0–6
K2O 0.4–4 0.9–2.6 0.8–0.9 0.8–4.7 1.1–2.9 0–3 0–4 0–4
Fuels 2022,3367
Finally, a critical review on the application of coal fly ash is provided in reference [2].
It is now widely acknowledged that the level of risk of degradation and loss of service-
ability induced by the alkali–silica reaction (ASR) is mainly related to the alkali-reactivity
degree of the aggregates, the total alkali content of the concrete and the environment
severity. Furthermore, it is also a well-known fact that, when using potential reactive
aggregates, the most important criteria to consider while defining a mix design to minimize
or to prevent the deleterious alkali-silica reaction (ASR) expansion in concrete are [11]:
•
Restricting the concrete pore solution alkalinity by utilizing blended cements contain-
ing active mineral additions such as coal fly ash. They must meet a well-defined limit
of alkali content;
•
Constraining the ASR gel expansion by modifying the gel nature by using lithium-
containing products;
•The use of low-alkali cements, i.e., Na2Oe= Na2O + 0.658 K2O < 0.6%.
In this work, fly ashes from the combustion of a South African coal were investigated
for use in concrete and Portland cement. Assessment of the aggregates with potential
alkali–silica reactivity is currently a widespread practice owing to the limited availability
of natural aggregates free of potentially alkali-reactive constituents. Therefore, the main
objective of this project is to gain more in-depth knowledge of the alkali content variability
in the coal fly ash that occur along the time in the combustion process for a correct selection
of the coal fly ashes as one of the most appropriate alkali–silica reaction (ASR) preventive
measure. In addition, the authors will seek to achieve various relations between some data
(Na2Oe, Na2O, K2O, free CaO and reactive silica content and fineness).
2. Materials and Methods
2.1. Coal Fly Ash Sampling
In total, 765 spot coal fly ash samples were collected at the point of release for 10 years.
The aim of this research was to characterize the alkali content of samples generated from
South African coal at Carboneras power plant. Almería, Spain (Table 2). Samples were
gathered with the coal-fired power plant running at full load (Table 3). Henceforth, alkali
content of these samples is truly representative of coal-fired power plants under similar
combustion conditions and by using a similar equipment.
Table 2. South African coal analysis.
Ultimate Analyses Analyses Method Not Dried Dried with Air Dry Basis
Moisture (%) ASTM D 5142:2004 4.97 2.15 -
Carbon (%) ASTM D 5373:2008 67.21 69.20 70.72
Hydrogen (excluido el H de la humedad) (%)
ASTM D 5373:2008 3.62 3.73 3.81
Nitrogen (%) ASTM D 5373:2008 1.54 1.59 1.62
Total sulphur (%) ASTM D 4239:B:2005 0.35 0.36 0.37
Ash content (%) ISO 1171:1997 14.97 15.41 15.75
Volatile matter (%) ISO 562:1998 23.38 24.07 24.06
The percentage of oxygen is found by
difference (excluding moisture oxygen from
the analysis) (%)
7.34 7.56 7.73
Gross calorific value (kcal/kg) ISO 1928:2009:E 6265 6451 6593
Hard grove grindability index (HGI)
with a moisture content of (%) ISO 5074:1994 54
1.41 - -
CO2emission factor (Directive 2003/87/EC)
(tCO2/TJ) 97.0
Fuels 2022,3368
Table 3. Data with regard to the power station.
Equipment/General Part Group 1 Group 2
General Power 576.9 582
Year 1985 1997
Boiler
Type Combustion in tangential
swirl burners
Combustion in tangential swirl
burners
Producer Combustion Engineering ABB
Coal burners 24 24
Main steam flow-rate 1679 t/h 1679 t/h
Superheated steam flow-rate 1525 t/h 1525 t/h
Initial steam pressure 176 bar 176 bar
Superheated steam pressure 41 bar 41 bar
Feedwater temperature 253 ◦C 253 ◦C
Main steam inlet temperature 541 ◦C 541 ◦C
Energy yield of the steam boiler 89.42% 89.42%
Condenser
Type Dual-pressure condenser Dual-pressure condenser
Cooling fluid Seawater in an open circuit
cooling system
Seawater in an open circuit
cooling system
Condenser water flow rate 60.408 m3/h 60.408 m3/h
Turbine
Manufacturer Bazán/G.E.E. data
Maximum power 577 MW 577 MW
Number of bodies 4 4
Number of removals 7 7
Operating speed 3000 rpm 3000 rpm
Alternator Manufacturer General Electric General Electric
Power 636 MVA 636 MVA.
Transformer Manufacturer Westinghouse. Westinghouse.
Power 209.5 MVA 209.5 MVA
The Litoral thermal power plant (Endesa), located in 1,788,547 square metres in
Carboneras, Almería, Spain, was built in 1979 to cover the need to increase electrical power.
It consists of two generation groups generating 1159 MW of power in all (Table 3). Each
of these groups consists of the following equipment: a boiler, a turbine, and an alternator.
Group 1 presents a capacity of 577 MW, while group 2, has a capacity of 582 MW. From
1985 to 2021, the installation has generated more than 180,000 GWh. The plant has a Port
Terminal for the purpose of unloading coal for the Litoral Thermal Power Plant.
Currently, the coal fired power plants generate the majority of the electricity worldwide
and produce the highest rate of CO2per kilowatt hour (Table 4) [12].
Table 4. CO2emissions from several power generation technologies [12].
Technology CO2Emissions (Kg/MWh)
Pulverised coal-fired subcritical 850
PC-fired supercritical 800
670
Figure 1shows a SEM photograph of coal fly ash particles. The hollow spherical
particles present lower density than compact spherical particles (high density). They are
wastes of power plant coal combustion produced in large quantities.
2.2. Alkali Content, Reactive Silica, Free-CaO, and Fineness Determination
Alkali content was determined by atomic absorption spectrophotometry (AAS) [
13
].
Reactive silica, free CaO and fineness (residue on 45
µ
m) were determined according to the
standards UNE 80225:2012 [14], UNE 80243 [15] and EN 196-6:2010 [16], respectively.
Fuels 2022,3369
Fuels 2022, 3, FOR PEER REVIEW 5
Figure 1. SEM image of coal fly ash particles.
2.2. Alkali Content, Reactive Silica, Free-CaO, and Fineness Determination
Alkali content was determined by atomic absorption spectrophotometry (AAS) [13].
Reactive silica, free CaO and fineness (residue on 45 µm) were determined according to
the standards UNE 80225:2012 [14], UNE 80243 [15] and EN 196-6:2010 [16], respectively.
2.3. Coal Fly Ash Chemical Analyses
Coal fly ash average chemical composition is given in Table 5. X-ray fluorescence
spectrometry (XRF) was utilized to determine the SiO2, Al2O3, Fe2O3, CaO, MgO, SO3, K2O,
Ti2O5, and P2O5 content by using a spectrometer Bruker S8 Tigger 4 kW model.
Insoluble residue, IR, loss on ignition, LOI, and chloride content determination were
performed according to the European standard EN 196-2 [13]. Table 5 shows that the coal
fly ash collected for this research program, which is a common by-product of a low-lime
bituminous coal combustion, meet the European standard EN 450-1 [17].
Table 5. Average chemical compositions of the tested coal fly ash (%).
Parameter SiO2 Al2O3 Fe2O3 CaO MgO SO3 K2O Ti2O5 P2O5 LOI IR
1 Cl-
Content
(%) 50.5 28.9 4.7 5.0 1.8 0.21 0.8 1.56 0.76 3.6 71.3 0.001
1 Insoluble residue determined by the Na2CO3 method (European standard EN 196-2).
3. Results and Discussion
3.1. Alkali Content over the Years
Figure 2 shows the alkali content results obtained in 765 coal fly ash spot samples
taken for 10 years at the Carboneras coal-fired power station. The alkali content refers to
the content of Na2O and K2O in cement. In almost all cases, the percentages were lower
than 2% and most of the samples displayed percentages lower than 1%.
Figure 1. SEM image of coal fly ash particles.
2.3. Coal Fly Ash Chemical Analyses
Coal fly ash average chemical composition is given in Table 5. X-ray fluorescence
spectrometry (XRF) was utilized to determine the SiO
2
, Al
2
O
3
, Fe
2
O
3
, CaO, MgO, SO
3
,
K2O, Ti2O5, and P2O5content by using a spectrometer Bruker S8 Tigger 4 kW model.
Table 5. Average chemical compositions of the tested coal fly ash (%).
Parameter SiO2Al2O3Fe2O3CaO MgO SO3K2O Ti2O5P2O5LOI IR 1Cl−
Content
(%) 50.5 28.9 4.7 5.0 1.8 0.21 0.8 1.56 0.76 3.6 71.3 0.001
1Insoluble residue determined by the Na2CO3method (European standard EN 196-2).
Insoluble residue, IR, loss on ignition, LOI, and chloride content determination were
performed according to the European standard EN 196-2 [
13
]. Table 5shows that the coal
fly ash collected for this research program, which is a common by-product of a low-lime
bituminous coal combustion, meet the European standard EN 450-1 [17].
3. Results and Discussion
3.1. Alkali Content over the Years
Figure 2shows the alkali content results obtained in 765 coal fly ash spot samples
taken for 10 years at the Carboneras coal-fired power station. The alkali content refers to
the content of Na
2
O and K
2
O in cement. In almost all cases, the percentages were lower
than 2% and most of the samples displayed percentages lower than 1%.
The necessary condition for the initiation of the alkali–silica reaction in concrete is that
the Portland cement must contain a high level of alkali. Normally, alkali–silica reaction is
due to the reactivity between hydroxyl (OH
−
) ions generally associated with alkalis (Na
2
O
and K
2
O) in the concrete constituents, such as Portland cement, and silica minerals in the
concrete mix, which can result in the expansion and microcracking of concrete [18].
When reactive aggregates are used, the pozzolanic blended cements with low alkali
content should be used [
18
]. Pozzolanic constituents reduce the Ca/Si ratio of C-S-H gel,
and this allows more alkalis to be incorporated in the calcium silicate hydrates [
19
]. Some
national standard specifies that the alkali content in Portland cement, calculated by Na
2
O +
0.658 K
2
O, should not exceed 0.60%. Therefore, the coal fly ash alkali content in blended
cements should be as low as possible. The Na
2
O-equivalent of coal fly ash in Figure 2
Fuels 2022,3370
ranges from 0.35% to 2.53%, with an average value of 0.79%. Despite being the same coal
origin, the Na
2
O-equivalent results for coal fly ash showed a relatively high degree of
dispersion, ranging from 35% to 2.53%. This fact reflects the variability in the coal and
burning conditions over the time.
Fuels 2022, 3, FOR PEER REVIEW 6
Figure 2. Maximum, minimum, and average values of Na2O, K2O, and Na2Oe in coal fly ash.
The necessary condition for the initiation of the alkali–silica reaction in concrete is
that the Portland cement must contain a high level of alkali. Normally, alkali–silica reac-
tion is due to the reactivity between hydroxyl (OH−) ions generally associated with alkalis
(Na2O and K2O) in the concrete constituents, such as Portland cement, and silica minerals
in the concrete mix, which can result in the expansion and microcracking of concrete [18].
When reactive aggregates are used, the pozzolanic blended cements with low alkali
content should be used [18]. Pozzolanic constituents reduce the Ca/Si ratio of C-S-H gel,
and this allows more alkalis to be incorporated in the calcium silicate hydrates [19]. Some
national standard specifies that the alkali content in Portland cement, calculated by Na2O
+ 0.658 K2O, should not exceed 0.60%. Therefore, the coal fly ash alkali content in blended
cements should be as low as possible. The Na2O-equivalent of coal fly ash in Figure 2
ranges from 0.35% to 2.53%, with an average value of 0.79%. Despite being the same coal
origin, the Na2O-equivalent results for coal fly ash showed a relatively high degree of dis-
persion, ranging from 35% to 2.53%. This fact reflects the variability in the coal and burn-
ing conditions over the time.
Coal fly ashes are utilized worldwide to manufacture blended cements and concretes.
This pozzolanic constituents provides enhanced properties including better durability in
attacking environments, long-term mechanical strength, and a lower hydration heat [2,5].
These properties are affected directly by the alkali content in the coal fly ash [20].
3.2. The Na2O-Equivalent Relationships
Figure 3 shows the relationship between the Na2O-equivalent and K2O, reactive sil-
ica, 45 µm residue and free lime in coal fly ash. Increasing Na2O percentage will increase
K2O content. In addition, increasing Na2O-equivalent percentage will increase reactive sil-
ica amount. It was suggested that alkali content in coal fly ash mainly depends on reactive
silica amount [20]. By contrast, when the 45 µm residue or free lime in coal fly ash are
linked to the Na2O-equivalent, there is no clear equation between both variables.
0
1
2
3
Jan-04 May-05 Oct-06 Feb-08 Jul-09 Nov-10 Apr-12 Aug-13 Dec-14 May-16 Sep-17
Alkali content (%)
Time (days)
K2O
Na2O
Na2Oe
Figure 2. Maximum, minimum, and average values of Na2O, K2O, and Na2Oein coal fly ash.
Coal fly ashes are utilized worldwide to manufacture blended cements and concretes.
This pozzolanic constituents provides enhanced properties including better durability in
attacking environments, long-term mechanical strength, and a lower hydration heat [
2
,
5
].
These properties are affected directly by the alkali content in the coal fly ash [20].
3.2. The Na2O-Equivalent Relationships
Figure 3shows the relationship between the Na
2
O-equivalent and K
2
O, reactive silica,
45
µ
m residue and free lime in coal fly ash. Increasing Na
2
O percentage will increase K
2
O
content. In addition, increasing Na
2
O-equivalent percentage will increase reactive silica
amount. It was suggested that alkali content in coal fly ash mainly depends on reactive
silica amount [
20
]. By contrast, when the 45
µ
m residue or free lime in coal fly ash are
linked to the Na2O-equivalent, there is no clear equation between both variables.
It is well-known that knowing the relationship among variables will guide further
analyses. A weak relationship seems to exist between Na
2
O-equivalent and reactive
silicon. Accordingly, with the rise in reactive silicon a higher alkali content is expected.
Nevertheless, the knowledge of one parameter cannot be used to deduce the other.
Itskos et al. found a considerable deviation in the coal fly ash chemical composition.
Therefore, they believed necessary an intense monitoring in terms of its possible utilization,
i.e., effective homogenization. In addition, not only alkalis content but also variations of
CaO, SiO
2
, and SO
3
contents of the coal fly ash are decisive for its utilization as a constituent
in Portland cement [21].
An early acceleration of Portland cement constituent’s hydration by alkali has been
found [
22
], leading to a shorter induction period [
23
]. Alkalis increase the initial and
second heat evolution peaks of Portland cement hydration [
24
]. The acceleration effect
of the alkali can be attributed to the increase in the pH value of the concrete pore so-
lution, and the subsequent Ca(OH)
2
solubility decrease. Furthermore, alkalis enhance
the calcium silicate dissolution and the formation of Ca(OH)
2
, as well as a faster rate
of nucleation and crystallization of hydrates, but suppresses the formation of ettringite,
(CaO)
6
(Al
2
O
3
)(SO
3
)
3·
32H
2
O [
22
]. The high silicon concentration in the concrete pore solu-
tion promotes the C–S–H gel generation [
25
], showing a higher compressive strength at
Fuels 2022,3371
early age [
26
]. By contrast, hydration becomes decelerated, and the mechanical strength
of concrete is negatively affected with advancing age [
22
,
27
]. However, the compressive
strength decrease in cement-based materials is related to other factors, which need further
research. Accordingly, the mechanism is not yet clear.
Fuels 2022, 3, FOR PEER REVIEW 7
(a) (b)
(c) (d)
Figure 3. Relationship between the Na2O-equivalent and: (a) K2O; (b) reactive silicon; (c) 45 µm
residue; (d) Free lime.
It is well-known that knowing the relationship among variables will guide further
analyses. A weak relationship seems to exist between Na2O-equivalent and reactive sili-
con. Accordingly, with the rise in reactive silicon a higher alkali content is expected. Nev-
ertheless, the knowledge of one parameter cannot be used to deduce the other.
Itskos et al. found a considerable deviation in the coal fly ash chemical composition.
Therefore, they believed necessary an intense monitoring in terms of its possible utiliza-
tion, i.e., effective homogenization. In addition, not only alkalis content but also variations
of CaO, SiO2, and SO3 contents of the coal fly ash are decisive for its utilization as a con-
stituent in Portland cement [21].
An early acceleration of Portland cement constituent’s hydration by alkali has been
found [22], leading to a shorter induction period [23]. Alkalis increase the initial and sec-
ond heat evolution peaks of Portland cement hydration [24]. The acceleration effect of the
alkali can be attributed to the increase in the pH value of the concrete pore solution, and
0
5
10
15
20
25
30
35
40
45
0123
Residue on a 45 µm sieve
Na
2
Oe (%)
0
1
2
3
0123
Free-CaO (%)
Na
2
Oe (%)
Figure 3.
Relationship between the Na
2
O-equivalent and: (
a
) K
2
O; (
b
) reactive silicon; (
c
) 45
µ
m
residue; (d) Free lime.
3.3. Reactive Silicon versus 45 µm Residue Relationship
Figure 4plots the relationship between reactive silicon and 45
µ
m residue. No corre-
lation was found between these two variables. According to reference [
28
], the average
fineness (and standard deviation) on 45
µ
m, was 22.5% (3.7%), and the minimum, max-
imum and average values are 26.40%, 44.00%, and 34.95%, respectively (Figure 3b). It
is assumed that the coal fly ash retained on a 45
µ
m sieve is an indirect indicator of the
residues on the 63 µm, 90 µm and 200 µm mesh sieves [28].
Fuels 2022,3372
Fuels 2022, 3, FOR PEER REVIEW 8
the subsequent Ca(OH)2 solubility decrease. Furthermore, alkalis enhance the calcium sil-
icate dissolution and the formation of Ca(OH)2, as well as a faster rate of nucleation and
crystallization of hydrates, but suppresses the formation of ettringite,
(CaO)6(Al2O3)(SO3)3·32H2O [22]. The high silicon concentration in the concrete pore solu-
tion promotes the C–S–H gel generation [25], showing a higher compressive strength at
early age [26]. By contrast, hydration becomes decelerated, and the mechanical strength
of concrete is negatively affected with advancing age [22,27]. However, the compressive
strength decrease in cement-based materials is related to other factors, which need further
research. Accordingly, the mechanism is not yet clear.
3.3. Reactive Silicon versus 45 µm Residue Relationship
Figure 4 plots the relationship between reactive silicon and 45 µm residue. No corre-
lation was found between these two variables. According to reference [28], the average
fineness (and standard deviation) on 45 µm, was 22.5% (3.7%), and the minimum, maxi-
mum and average values are 26.40%, 44.00%, and 34.95%, respectively (Figure 3b). It is
assumed that the coal fly ash retained on a 45 µm sieve is an indirect indicator of the
residues on the 63 µm, 90 µm and 200 µm mesh sieves [28].
Figure 4. Relationship between reactive silicon and 45 µm residue.
Although no overall connection was found in this study between reactive silicon and
45 µm residue, it is well-known that the finer the coal fly ash is and the higher reactive
silicon content in it, the more effective it becomes in terms of mechanical strength and
durability [29,30]. This enhancement in the mechanical properties with the fineness in-
crease has also been found in other pozzolanic materials [31].
10
15
20
25
30
35
40
25 30 35 40 45
Reactive silicon (%)
Residue on a 45 µm sieve
Figure 4. Relationship between reactive silicon and 45 µm residue.
Although no overall connection was found in this study between reactive silicon and
45
µ
m residue, it is well-known that the finer the coal fly ash is and the higher reactive
silicon content in it, the more effective it becomes in terms of mechanical strength and
durability [
29
,
30
]. This enhancement in the mechanical properties with the fineness increase
has also been found in other pozzolanic materials [31].
Furthermore, the lack of relationship found between reactive silicon and 45
µ
m residue
suggests that the reactive silicon is uniformly distributed in the coal fly ash regardless of its
particulate size.
We know well that active silica is the fraction of the total silica which is involved
in the pozzolanic reactions producing calcium silicate hydrates with a low Ca/Si ratio
(C-S-H gel) to which the strengthening of blended cements is attributed. An empirical
correlation between compressive strength, fineness and soluble silica of coal fly ash has
been reported [
32
]. Reactive silica is a non-crystalline phase, present in the amorphous
part of the coal fly ash, which is proportional to the compressive strength gain in blended
cements [
33
]. In addition, it has been reported that total silica in coal fly ash can vary from
32% to 42% over 10 years [21].
4. Conclusions
Evaluation of the Na
2
O, K
2
O, reactive silica, 45
µ
m residue and free lime content in
765 coal fly ash samples taken for 10 years was performed. The findings are concluded
as follows:
1.
The Na
2
O-equivalent of coal fly ash ranged from 0.35% to 2.53%, with an average
value of 0.79%. These values should be taken into account producing concretes
made with potentially reactive aggregates in order to mitigate the alkali–silica reac-
tion (ASR);
Fuels 2022,3373
2.
There is a positive correlation between Na
2
O and K
2
O of coal fly ash and there is only
a weak relationship between the Na
2
O-equivalent and reactive silica. Therefore, the
mathematical relationship between the two variables is such that knowledge of one
key cannot be used to deduce the other;
3.
Conversely, this study confirms no correlation between the reactive silicon and the
45
µ
m residue. In addition, non-correlation between the Na
2
O-equivalent content
and the 45 µm residue or the free lime in coal fly ash were found;
4.
The most significant performance characteristics of coal fly ash in concrete are the
particle size (percentage retained on a 45
µ
m sieve), reactive silica content and the
Na
2
O-equivalent amount. They should be determined independently since a weak or
non-correlation was found between them.
Author Contributions:
Conceptualization, M.Á.S. and C.A.; methodology, M.Á.S. and C.A.; software,
M.Á.S.; validation, M.Á.S.; formal analysis, M.Á.S. and C.A; investigation, M.Á.S. and C.A.; resources,
M.Á.S.; data curation, M.Á.S. and C.A.; writing—original draft preparation, M.Á.S.; writing—review
and editing, M.Á.S. and C.A.; visualization, M.Á.S.; supervision, M.Á.S. All authors have read and
agreed to the published version of the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: The paper data are available upon request.
Acknowledgments:
The authors would like to express their gratitude for providing necessary help
for sample collection and testing by the staff of Carboneras thermal Power station.
Conflicts of Interest: The authors declare no conflict of interest.
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