The Ash from Fluidized Bed Combustion as a Donor of Sulfates to the Portland Clinker

Abstract

The paper deals with possibilities of using solid residues from fluidized bed combustion of coal, bed and filter ash in the production of composite Portland cements. The ash from fluidized bed combustion contains a high amount of CaO, in the form of free lime or CaSO4 (anhydrite), so it could be used as a possible donor of sulfates to the Portland clinker instead of usually used gypsum. At first, the chemical composition of collected ashes was determined by X-Ray Fluorescence and the ongoing hydration process was monitored by isoperibolic calorimetry. Then samples containing mixtures of Portland clinker and ash were prepared. Their respective compressive strength and flexural strength were analyzed and observations were made on the hydration and composition of products of the hydration reaction detected by X-Ray diffraction. Finally, the results of selected mixtures were verified with prepared standardized mortars.

Full text

Procedia Engineering 151 ( 2016 ) 394 – 401
1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Peer-review under responsibility of the organizing committee of ICEBMP 2016
doi: 10.1016/j.proeng.2016.07.392
ScienceDirect
Available online at www.sciencedirect.com
International Conference on Ecology and new Building materials and products, ICEBMP 2016
The ash from fluidized bed combustion as a donor of sulfates to the
Portland clinker
Denisa Hanisková*, Eva Bartoníþková, Jan Koplík, Tomáš Opravil
Brno University of Technology, Faculty of Chemistry, Materials Research Centre, PurkyĖova 464/118, Brno 612 00, Czech Republic
Abstract
The paper deals with possibilities of using solid residues from fluidized bed combustion of coal, bed and filter ash in the
production of composite Portland cements. The ash from fluidized bed combustion contains a high amount of CaO, in the form of
free lime or CaSO4 (anhydrite), so it could be used as a possible donor of sulfates to the Portland clinker instead of usually used
gypsum. At first, the chemical composition of collected ashes was determined by X-Ray Fluorescence and the ongoing hydration
process was monitored by isoperibolic calorimetry. Then samples containing mixtures of Portland clinker and ash were prepared.
Their respective compressive strength and flexural strength were analyzed and observations were made on the hydration and
composition of products of the hydration reaction detected by X-Ray diffraction. Finally, the results of selected mixtures were
verified with prepared standardized mortars.
© 2016 The Authors. Published by Elsevier Ltd.
Peer-review under responsibility of the organizing committee of ICEBMP 2016.
Keywords: Portland clinker; fluidized bed combustion ash; compressive strength; flexural strength; reaction of hydration.
1. Introduction
The main fossil fuel for production of the electric energy in thermal power plants is coal. During a combustion
process, a high amount of solid residues is formed. Chemical, physical and mineralogical properties of the residues
differ depending on a type of coal and a type of combustion process [1,2,3]. There are two main types
of the combustion process, a high-temperature combustion and a fluidized bed combustion [4]. The advantage
* Corresponding author.
E-mail address: xchaniskova@fch.vutbr.cz
© 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Peer-review under responsibility of the organizing committee of ICEBMP 2016

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Denisa Hanisková et al. / Procedia Engineering 151 ( 2016 ) 394 – 401
of the fluidized bed combustion is that the desulfurization process is situated in a boiler. On the other hand at high-
temperature combustion special technological equipment is needed, because desulfurization is situated after
the combustion process [5,6,7,8]. Limestone (CaCO3) is added directly to the boiler as a desulfurization additive.
The calcium sulfate (CaSO4) is formed by the reaction of the limestone particles with sulfur dioxide (SO2). The solid
residues from the fluidized bed combustion contain noncombustible constituents of coal and products
of desulfurization, so particles of CaSO4 [9]. The high amounts of SO3 and CaO are the reason why fluidized bed
combustion ashes differ from high-temperature combustion ashes. For that matter fluidized bed combustion ashes
should not be used as an additive to concrete [10].
There are some studies where hydration behavior and properties of mixtures prepared from cement and fluidized
bed combustion fly ash were observed [11,12]. Ash was added in small amounts and as a replacement of high
temperature fly ash. In both cases the addition of ash caused the increase in mechanical properties.
The main additive, which is mixed with Portland clinker to make Portland cement, is gypsum (CaSO4ǜ2H2O).
It retards hydration of C3A (3CaOǜAl2O3, tricalcium aluminate), one of the main phases of cement, and allows
workability of Portland cement. In this work, the fluidized bed combustion ash is added to the Portland clinker as
a donor of sulfates and also as partially substituent of the clinker. Using the secondary raw materials
in the production of cement has ecological and economic advantages [13]. In mixtures presented in this work mass
ratio between the clinker and the ash from 90:10 to 10:90 was applied. Fluidized bed combustion ashes, bed and
filter ashes, from two thermal power plants in The Czech Republic, was used. The mechanical properties and
the process of hydration reaction were monitored on the prepared mixtures.
2. Experimental
2.1. Methods
Saccharate method was used to determine a content of free lime in collected ashes. A sample of ash was mixed
with saccharose and water. The mixture was filtered and a filtrate was titrated by hydrochloric acid solution
on phenolphthalein. The content of free lime (CaO) was calculated according to the formula:
vm
MVc
CaO 

%
where c is a concentration of hydrochloric acid solution (molǜdmí3), V is a volume of the hydrochloric acid
solution (dm3), M is a molar weight of CaO (gǜmolí1), m is a weight of the sample of ash (g), Ȟ is a stoichiometric
ratio of reaction.
Mechanical properties, compressive and flexural strength, were measured on the complex device for strength
tests on building materials DESTTEST 3310 (Betonsystem). Flexural and compressive strength were measured
on each testing prism. Dimensions of prisms from pastes were 20×20×100 mm, and from mortars 40×40×160 mm.
Prisms were preserved in a humid environment. Strengths were measured after 1, 7 and 28 days.
The process of hydration reaction was observed by isoperibolic calorimetry, on a device constructed and placed
in FCH BUT (Faculty of chemistry, Brno University of technology). Immediately after stirring the mixture, 300 g
of it were placed in a polystyrene cup, which was enclosed in thermo-insulation foam container and a thermocouple
for measuring temperature during hydration reaction was embedded into the testing mixture. The measurements
were ended after 30 hours when the temperature was almost constant.
2.2. Material
Bed and filter ashes from fluidized bed combustion (power plants Tisová and PoĜíþí K8, The Czech republic) and
the Portland clinker (Mokrá, HeidelbergCement, The Czech republic) were used for preparation pastes and
standardized mortars. Bed ashes and Portland clinker were fine grounded. To mortars, the standardized fine,
medium and coarse sand (ýSN 196-1) was used.
(
1
)

396 Denisa Hanisková et al. / Procedia Engineering 151 ( 2016 ) 394 – 401
The chemical composition of used ashes was examined by X-Ray fluorescence on the device Xenemetric EX-
6600 SSD and is given in Table 1. The content of free lime (CaO) in ashes was determined by saccharate method
and is given in Table 2.
Table 1. The chemical composition of used ashes.
Major oxides (%) Tisová PoĜíþí K8
Bed ash Filter ash Bed ash Filter ash
SiO2 31.1 33.9 30.0 31.2
Al2O3 21.7 22.4 15.1 16.5
CaO 28.1 22.8 29.1 31.2
Na2O 0.34 0.66 0.26 0.40
K2O 0.81 0.67 1.48 1.38
MgO 0.47 0.85 0.82 0.90
SO3 7.77 5.19 16.10 8.80
Fe2O3 3.46 7.17 5.41 7.48
TiO2 5.45 5.41 1.21 1.53
P2O5 0.26 0.31 0.14 0.19
Table 2. The content of free lime in used ashes.
Tisová PoĜíþí K8
Bed ash Filter ash Bed ash Filter ash
Content of free lime (wt.%) 22.94 9.68 9.57 13.54
2.3. Samples composition
Four series of mixtures were prepared. The composition of mixtures in each series was the same (Table 3), they
differ in a type of ash added. Following ashes were used: bed ash from fluidized bed combustion from power plant
Tisová (TB), filter ash from power plant Tisová (TF), bed ash from power plant PoĜíþí K8 (PB), filter ash from
power plant PoĜíþí K8 (PF). The amount of water added to various mixtures was determined from consistency
of the paste, made from clinker and water with water to binder ratio 0.35. The water to binder ratio (w/b) is the ratio
between the total amount of water (g) and the total amount of binder (g), which is the sum of an amount of the ash
and the clinker used in pastes or mortars.
Table 3. The composition of mixtures.
Sample CL A B C D E F G H I
Ash (wt.%) 0 10 20 30 40 50 60 70 80 90
Clinker (wt.%) 100 90 80 70 60 50 40 30 20 10
The clinker to ash ratio in mortars was same as in pastes. Fine, medium and coarse sand was used to prepare
mortars. The types of sand were in ratio 1:1:1. The sand to binder ratio in mortars was 3:1.
3. Results and discussion
The time dependence of compressive and flexural strengths of the pastes prepared from the Portland clinker and
fluidized bed combustion ashes measured on testing prisms after 1, 7 and 28 days are presented in Fig. 1, Fig. 2,
Fig. 3 and Fig. 4.

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Denisa Hanisková et al. / Procedia Engineering 151 ( 2016 ) 394 – 401
(a) (b)
Fig. 1. Dependence of flexural (a) and compressive (b) strength of pastes with TB on time.
(a) (b)
Fig. 2. Dependence of flexural (a) and compressive (b) strength of pastes with PB on time.
(a) (b)
Fig. 3. Dependence of flexural (a) and compressive (b) strength of pastes with TF on time.

398 Denisa Hanisková et al. / Procedia Engineering 151 ( 2016 ) 394 – 401
(a) (b)
Fig. 4. Dependence of flexural (a) and compressive (b) strength of pastes with the PF on time.
Generally, we can say, that reached strengths increase with increasing time of curing and the highest values
reached the pastes with a content of ash from 10–50 wt.%.
Flexural strengths of all pastes made with bed ash from Tisová (Fig. 1) after 28 days exceeded value 6 MPa.
The highest value was measured on the mixture with the content of ash 50 wt.% and it is almost 14 MPa.
The compressive strengths (Fig. 1) of all pastes after 28 days were higher than the compressive strength of clinker
itself. This fact is due to an absence of sulfate ions, which are needed to form ettringite, as is also described in work
of Quennoz,et al. [14]. The values of the compressive strength of all pastes after 28 days were higher than 40 MPa,
the highest values exceeded 80 MPa. The strengths increased with growing quantity of ash, but after they exceeded
the 40 wt.% content of ash the strengths were decreasing. The amount of added ash is equal to 3% content
of sulfates in the mixture, what is the optimal quantity which positively influenced the compressive strength of
cement paste prisms, as also published Lerch [15]. So we suggest the similar mechanism in the strength
development due to the addition of fluidized bed combustion fly ash to the clinker in corresponding amounts.
Navazze et al. [16] also deals with an addition of fluidized bed combustion ash to the Portland cement. They
observed that the strengths of pastes made with the addition of ash were after 91 days higher than the cement itself.
The increase was caused by added amount of free lime. Our experiments show higher strengths for pastes with
addition of ash with lower amount of free lime (PoĜíþí), so the effect of addition of free lime to clinker is probably
opposite compared to the cement.
The values of flexural strengths of the pastes made with bed ash from PoĜíþí K8 (Fig. 2) were after 28 days
between 5 and 10 MPa. So they were slightly lower compared to the pastes with ash from Tisová. The values
of compressive strengths (Fig. 2) were for all pastes higher than 60 MPa, and the highest measured value was
112 MPa, i.e. for paste with 30% content of ash. In comparison to bed ash from Tisová, PoĜíþí bed ash has higher
content of sulfates (Table 1) so the quantity of added ash needed to reach an optimum amount of sulfates and
the highest strengths is lower. Again, strengths of all pastes are significantly higher than the strength of the clinker,
which is caused by the absence of sulfate ions [15].
The usage of filter ash to prepare pastes entails lower values of strengths. Pastes made with fluidized bed
combustion filter ash from power plant Tisová (Fig. 3) had after 28 days flexural strengths about 4–5 MPa,
the highest value was 6 MPa, for paste with the content of ash 20 wt.%. The same paste has the highest value
of the compressive strength, 62 MPa after 28 days.
Pastes made from filter ash from power plant PoĜíþí K8 (Fig. 4) had comparable values of the flexural strength
to pastes with filter ash from Tisová. The compressive strengths are comparable too. Lower strengths then clinker
had pastes with the content of ash higher than 50 wt.%.
The lower values of strengths of mixtures with filter ashes are probably due to slightly different chemical
composition and the higher amount of water needed to prepare the mixture with demanded consistence.

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Mixtures with the highest content of bed ash and the highest strength were selected for tests on mortars.
The measured values showed the same trends as in pastes. Flexural strengths of mortars after 28 days were about
7 MPa. Compressive strengths were about 40 MPa, what is due to the high content of fly ash satisfactorily.
(a) (b)
Fig. 5. Differential calorimetric curves of pastes with 0–50% (a) and 50–100% (b) content of the TB.
(a) (b)
Fig. 6. Integral calorimetric curves of pastes with 0–50% (a) and 50–100% (b) content of the TB.
(a) (b)
Fig. 7. Differential calorimetric curves of pastes with 0–50% (a) and 50–100% (b) content of the PB.

400 Denisa Hanisková et al. / Procedia Engineering 151 ( 2016 ) 394 – 401
(a) (b)
Fig. 8. Integral calorimetric curves of pastes with 0–50% (a) and 50–100% (b) content of the PB.
The calorimetric curves presented on Fig. 6 and Fig. 8 show, that process of hydration of ashes is dependent
on the content of free lime, which exothermally reacts with water. The experimental data were in good agreement
with data published here [17]. The temperature and the heat of hydration increase with the increasing content of free
lime. So, the bed ash from power plant Tisová (TB) achieved during hydration reaction higher temperature and
higher amount of heat in comparison to bed ash from PoĜíþí K8 (PB).
The maximum reached temperature (Fig. 5) increases with the increasing amount of the fly ash in the mixture.
For the mixture with 90% content of fly ash the temperature was higher than for the pure fly ash. The total released
heat for all pastes made with bed ash from Tisová (Fig. 6) was similar and was higher than for the pure ash and
the pure clinker.
The behavior of cement mixtures was published elsewhere [17,18,19]. They studied hydration reaction
of mixtures made from cement and fluidized bed combustion ash and observed that the increasing addition of ash
conversely caused the retardation of hydration.
The Fig. 7 and Fig. 8 show that the maximum reached temperature and the total released heat for pastes made
with bed ash from PoĜíþí K8 decrease with increasing amount of the bed ash. It is due to the fact, that the total
released heat for the pure bed is about two times lower than for the pure clinker.
4. Conclusion
The work was focused on the utilization of fluidized bed combustion fly ash from two thermal power plants
in The Czech republic as a donor of sulfates.
From the experimental observations, the highest values of compressive strengths were measured on pastes
with the ash content from 20 to 60 wt. %. Inconsiderable high values of strength reached mixtures with a higher ash
content. The strengths were more than 60 MPa. These values were measured on the mixtures containing 80 wt.%
of ash, too. The highest value of compressive strength was measured on pastes with 30% content of fluidized bed
combustion fly ash from power plant PoĜíþí K8 and it reached 112 MPa after 28 days.
When the filter ash was used, the highest values of compressive strength reached mixtures with the content of ash
from 20 to 40 wt.%. The compressive and flexural strengths measured on the pastes with filter ashes were lower
in comparison with the mixtures with bed ashes. The larger surface area of filter ash caused need of higher volumes
of mixing water, which could be a possible disadvantage of their application.
From measured calorimetric curves, we can see the affected process of hydration reaction of ashes by the content
of free lime. Mixtures made using bed ash from Tisová reached the similar values of total released heat.
The released heat of bed ash from PoĜíþí was about two times lower than released heat for the Portland clinker. We
can also see the dependence of ash content on the calorimetric measurements. So the released heat and
the maximum reached temperature decreased when the content of ash increased.

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Denisa Hanisková et al. / Procedia Engineering 151 ( 2016 ) 394 – 401
Acknowledgement
This work was supported by the project “Materials Research Centre at FCH BUT - Sustainability and
Development, REG LO1211, with financial support from National Programme for Sustainability I (Ministry of
Education, Youth and Sports)”.
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