Effect of activating solution on alkali activated binders based on fluidized
bed combustion fly ash.
Miroslava Drabová, Ivan Brezáni, Juraj Mosej, Martin Sisol
[email protected], [email protected], [email protected], [email protected]
Abstract
In the present paper fluidized bed combustion (FBC) black coal fly ash was used as source material
for alkali activation. Fly ash was alkali activated by the activation solution containing sodium hydroxide,
sodium water glass and water. Pastes were cured in a hot-air drying chamber at 22°C for 20 hours.
Hardened alkali activated binders were test on mechanical strength after 7, 28, 90, 180 and 360 days to
determine the effect of activating solution composition on alkali activation. Factors investigated as
reaction variables were Na2O content, molar ratio SiO2-to-Na2O (named as Ms), water content and age
of activated binders. The experimental results were statistical analyzed by ANOVA test. According to
experimental results statistically significant influence on flexural strength have all observed factors.
Statistically significant influence on compressive strength has all observed factors with the exception of
Na2O content that could not be statistically confirmed.
Keywords: alkali activation; fluid bed combustion fly ash; ANOVA test; compressive strength
1. Introduction
In the last decade, geopolymer binders or alkali activated binders have emerged as one of the
possible alternatives to cement binders for applications in concrete industry. The term “geopolymer”
was first used by Davidovits to designate a new class of amorphous to semi-crystalline threedimensional aluminosilikate materials resulting from polycondensation reaction of metakaoline with
alkaline solutions [1].
Nowadays geopolymer binders are generally understood as alkaline activated aluminosilicates
consisting a reactive solid component and an alkaline activation solution. As a source of aluminosilicate
can be used any material, that contains SiO2 and Al2O3 in sufficient amounts and in reactive form (e.g.
ashes, slags, active clays, pozzolana, etc.) [2] [3]. An alkaline activation solution contains (apart from
water) individual alkali hydroxides, silicates, aluminates, carbonates, and sulphates or combinations
thereof [4].
When the alkaline solution comes into contact with the aluminosilicate solid material the dissolution
step starts. The dissolution of Al and Si ions from aluminosilicate material in high alkaline aqueous
solution is taking place [5]. The greater the amount of hydroxyl ions in the solution, so that silicon and
aluminum ions dissolute faster [6]. The high concentration of OH- ions in the alkaline medium severs
the covalent Si-O-Si, Si-O-Al and Al-O-Al bonds present in the vitreous phase of the ash. The silicon
and aluminum ions released into the medium form Si-OH and Al-OH groups. In a subsequent stage
these monosilicates and aluminates condense to form Si-O-Al and Si-O-Si bonds, giving rise to an
alkaline aluminosilicate gel characterized by its three-dimensional structure. This product is structured
around tetrahedrally co-ordinated silicon and aluminium, forming a polymer chain in which the Al3+ ions
replace the Si4+ ions. The resulting net anionic charge is compensated by the capture of monovalent
alkaline cations [7][8]. There is chemical bonding of geopolymer precursors (oligomers) by gradually
released of water molecules during polykondensation. This process is known as polymerization [1].
Geopolymerization is a complex multiphase process [9]. Reaction rate and chemical composition of
the resulting reaction products depends on a several factors that can be divided as follows: 1)
properties of raw material as chemical and phase composition, particle size [10][12] and 2) composition
of the activating solution as type and concentration of the activating solution [5], presence of soluble
silicate [13], water content [14].
Materials created by alkaline activation of aluminosilicates represent a unique type of materials
possessing an excellent mechanical strength, thermal stability, fire and chemical resistance,
dimensional stability, adhesion to aggregate. Due to these properties, they are viewed as alternative
materials for certain industrial applications [13].
The use of waste materials caused by the requirement to develop new technologies producing
environmentally friendly concrete, where Portland cement will be replaced by environmentally less
demanding materials. Nowadays, waste materials are insufficiently used. Geopolymers created by
alkali activation of an easily available natural also waste aluminosilicates are very attractive
environmentally for a number of reasons [15]. By utilization of ash as a raw material for geopolymer
production waste is converted into valuable material with excellent mechanical and utility properties.
In the present paper, fluidized bed combustion (FBC) fly ash was investigated as source material for
geopolymer synthesis. Fluidized bed combustion technology is one of the promising clean coal
technologies, since lime is used to absorb sulfur. Combustion temperature of FBC unit is only 800 –
900°C. This results in ash with high contents of silica (SiO2), lime (CaO), gypsum (CaSO4) and high
amount of crystalline phase. In addition, the particle size of this ash is approximately 1 – 300 µm with
quite irregular shape. Only few researches have ever been reported on using fluid fly ash as part or full
source materials for geopolymer synthesis [16][19]. This is possibly due to the low geopolymeric
reactivity of fluid fly ash, which results from lower amorphous fraction deriving from the low firing
temperature, making fluid fly ash also an unfavorable raw material for direct geopolymerisation.
2. Experimental Section
2.1. Plan of experiments
To determine the influence of componential factors on mechanical properties of alkali activated
binders based on fly ash, this paper applied method of plan experiments to design mixtures, where
factors as Na2O content (overall Na2O content to the fly ash mass expressed as %), molar ratio SiO2-toNa2O (named as Ms), age were investigated as reaction variables. Values of Na2O content ranged from
11.1 to 14.7 %, Ms from 1.26 to 1.53 and age from 7 to 360 days. Value of water content (expressed as
overall content water to the fly ash mass; named as w) was constant 0.47. Mixture with composition of
Na2O content 14.7 % and Ms = 1.53 is missing because of impossibility to mix this mixture at the same
conditions. Same experiments were performed with reduced water content at value 0.43. When
preparing geopolymer specimens using these mixtures all mixing- and curing processes were fixed.
Composition of all realized mixtures is shown in Table 1.
Na2O [%]
14.7
14.7
13.5
13.5
13.5
12.3
12.3
Table 1 The composition of the realised mixtures
Ms [mol/mol]
w [g/g]
Na2O [%]
Ms [mol/mol]
w [g/g]
1.40
0.47
12.3
1.26
0.47
1.26
0.47
11.1
1.53
0.47
1.53
0.47
11.1
1.40
0.47
1.40
0.47
11.1
1.26
0.47
1.26
0.47
13.5
1.40
0.43
1.53
0.47
13.5
1.26
0.43
Na2O = overall Na2O content to the fly ash mass; Ms = molar ratio
1.40
0.47
SiO2/Na2O; w = water content
2.2. Materials and specimen preparation
Fly ash was derived from black coal combustion in fluidized bed type boilers of EVO Vojany thermal
power plant (Slovakia). Chemical composition of fly ash determined by X-ray fluorescence (XRF)
spectrometer is 42.10 % SiO2, 18.65 % Al2O3, 11.50 % CaO, 5.95 % Fe2O3, 3.72 % SO3, 1.96 % K2O,
1.59 % MgO and other. Grain-size distribution of fly ash determined by sieving shows 80 % particles
passing through a sieve size of 40 microns.
Fly ash was alkali activated by the activation solution that was prepared by mixing a solid sodium
hydroxide in spheres (95-99.5 % content of NaOH) with sodium water glass (aqueous sodium silicate
solution with 36 - 38% of Na2SiO3, density of 1336 kg/m3 and the molar ratio of SiO2/Na2O = 3.37) and
water.
Pastes resulted by mixing fly ash with activating solutions was filled into a forms. Forms were
covered with a film to avoid a water evaporation form pastes during curing process. Pastes were cured
in a hot-air drying chamber at 22°C for 20 hours. Thereafter, the samples were removed from the forms,
marked and stored in laboratory conditions till the moment of the strength test performed.
2.2. Mechanical properties of alkali activated binders based on fly ash
The mechanical strength of resulting products was studied on prismatic specimens with dimensions
40x40x160 mm. The values of a compressive strength and a flexural strength were determined after 7,
28, 90, 180 and 360 days according to the Slovak Standard STN EN 12390-3 using the hydraulic
machine Form+Test MEGA 100-200-10D. In Table average values of flexural and compressive
strength are presented. Mean flexural strength is a result of three measuring and mean compressive
strength is a result of six measuring.
Table 2 Mean values of flexural and compressive strength and water absorption of alkali
activated materials
Mean flexural strength [MPa]
water content [g/g]
0.47
0.43
Age
[day] Ms [mol/mol]
1.40
1.26
1.53
1.40 1.26
Na2O content [%]
14.7 13.5 12.3 11.1 14.7 13.5 12.3 11.1 14.7 13.5 12.3 11.1 13.5 13.5
7
1.7 1.7 3.4 2.3 2.0 1.3 2.5 0.4 NaN 2.1 2.3 2.1 3.8 2.4
28
3.8 4.7 4.5 2.7 3.2 2.0 3.4 3.1 NaN 1.6 3.0 2.3 2.8 4.5
90
4.5 5.6 6.4 3.6 5.6 3.4 4.6 4.5 NaN 3.0 4.6 2.4 3.5 5.2
180 4.5 3.4 5.8 4.3 4.9 3.4 3.7 3.8 NaN 2.4 5.7 3.9 6.4 4.8
360 7.4 4.3 7.8 8.3 5.0 4.0 5.9 5.4 NaN 0.5 6.1 4.1 4.7 4.8
Mean compressive strength [MPa]
7
22.2 19.2 18.9 18.1 20.7 20.4 18.9 16.7 NaN 12.9 16.8 15.5 22.9 28.3
28
28.1 39.0 30.6 27.0 29.9 36.6 29.6 28.1 NaN 28.9 27.4 20.4 30.3 43.8
90
39.7 42.3 42.1 38.7 35.6 42.0 36.5 31.4 NaN 36.0 36.1 20.7 29.6 57.6
180 38.1 44.9 45.6 43.9 41.8 46.9 36.8 39.6 NaN 28.2 40.9 38.6 52.1 61.3
360 58.6 52.5 49.1 42.7 54.9 47.9 43.6 42.5 NaN 37.8 44.8 41.9 42.6 59.3
3. Results and Discussion
The experimental results were statistical analyzed by ANOVA test with given significance level
α=0.05. Statistically significant influence has factor with parameter p<α. In Table 3 are summarized
results of ANOVA test based on influence of Na2O content, Ms, water content and age on flexural and
compressive strength of alkali activated binders.
Table 3 Results of ANOVA test for factors: Na2O content, Ms, water content, age and its
influence on flexural and compressive strength of alkali activated binders with significance level
α=0.05
flexural
compressive
p
significance
p
significance
strength
strength
Na2O
0.0002
S
Na2O
0.1714
NS
Ms
0.0018
S
Ms
0.0028
S
w
0.0126
S
w
0.0396
S
age
0.0000
S
age
0.0000
S
NS – not significant; S – significant
According to experimental results (see Table 3) statistically significant influence on flexural strength
have all observed factors. Statistically the most significant influence has age factor followed by Na2O
content and Ms. Statistically the least significant influence has water content. Statistically the most
significant influence on compressive strength has age. Significant influence has also Ms and water
content. Influence of Na2O content on compressive strength could not be statistically confirmed.
Statistically significant influence of parameters: Na2O content, Ms and water content on water
absorption of alkali activated binders could not be statistically confirmed even though parameter p for
water content factor is near to significance level α.
Figure 1 Effect of age, Ms and Na2O content on flexural strength of alkali activated binders
Figure 2 Effect of age, Ms and Na2O content on compressive strength of alkali activated binders
There are values of flexural and compressive strength of alkali activated binders based on fly ash
depending on age, Na2O content and Ms shown in Figure 1 and Figure 2. Strength of alkali activated
binders is gradually increased with increasing age.
Effect of Na2O content on mechanical strength is not clear. It seems that the value of flexural
strength is the highest when Na2O content is 12.3%. When Na2O content is higher (13.5 %) flexural
strength is reduced. However, further increase of Na2O content to 14.7 % in particular for Ms of 1.26
results in repeated increase of flexural strength. When Ms is 1.26 and 1.4, the compressive strength of
alkali activated binders is increased with increasing Na2O content up to 13.5 % (maximum). Further
increase of Na2O content (14.7 %) results in decrease of compressive strength. Exception is
compressive strength measured after 360 days that achieves the highest value for Na 2O content of
14.7 %.
When Na2O content is 11.1 %, the flexural strength of alkali activated binders is decreased with
increasing Ms. This trend applies to flexural strength measured between 7 and 90 days. When age of
alkali activated binder is older (more than 90 days), maximum of flexural strength is achieved for Ms of
1.4, and further increase of Ms to 1.53 reduces the flexural strength. When Na 2O content is 12.3 %,
maximum of flexural strength is achieved for Ms of 1.4 in full range of age. When Na2O content is 13.5
%, flexural strength alkali activated binders measured in age of 7 day is increased with increasing Ms.
However, when age is more than 28 days included the maximum of flexural strength in Ms of 1.4 is
observed. The compressive strength of alkali activated binders based on fly ash is increased with
increasing Ms from 1.26 to 1.4, but further increase in Ms (1.53) results in reduction of compressive
strength. This trend applies to all Na2O content in full range of age.
When water content in fresh mixture is decreased from 0.47 to 0.43 (Table ), the mechanical
strength alkali activated binders is also decreased for Ms of 1.4 in full range of age. On other hand,
mechanical strength of alkali activated binders is increased with decreasing water content from 0.47 to
0.43 for Ms of 1.26 in full range of age.
4. Conclusions
In submitted contribution the influence of factors as Na2O content, SiO2/Na2O molar ratio (Ms),
water content in activation solution and age (7 to 360 days) on flexural and compressive strength of
alkali activated binders based on fluidized bed combustion black coal ash is investigated. Effect of
mentioned factors is statically evaluated by ANOVA test. According to experimental results statistically
significant influence on flexural strength have all observed factors. Statistically the most significant
influence has age factor. Statistically the least significant influence has water content. Statistically the
most significant influence on compressive strength has age. Significant influence has also Ms and
water content. Influence of Na2O content on compressive strength could not be statistically confirmed.
Acknowledgments
This work was supported by the research grant project VEGA 1/1222/12 and APVV 0423-11.
References
[1].
[2].
[3].
[4].
[5].
[6].
[7].
[8].
[9].
Davidovits J. Geopolymer: Chemistry and applications. 2nd ed. France: Institut Géopolymère;
2008.
Van Jaarsveld JGS, Van Deventer JSJ, Lorenzen L. The potential use of geopolymeric
materials to immobilise toxic metals: Part I. Theory and aplications. Miner. Eng. 1997; 10:659669.
Panias D, Giannopoulou IP, Perraki T. Effect of synthesis parameters on the mechanical
properties of fly ash-based geopolymers. Colloids Surf A Physicochem Eng Asp 2007;
301:246-254.
Ryu GS, Lee YB, Koh KT, Chung YS. The mechanical properties of fly ash-based geopolymer
concrete with alkaline activators. Construction and Building Materials 2013; 47:409-418.
Xu H, Van Deventer JSJ. The geopolymerisation of alumina-silicate materials. Int. J. Miner.
Process 2000; 59:257-266.
Phair JW, Van Deventer JSJ. Effect of silicate activator pH on the leaching and material
characteristics of waste-based inorganic polymers. Miner. Eng. 2001; 14:289-304.
Fernández-Jiménez A, Palomo A, Sobrados I, Sanz J. The role played by the reactive
alumina content in the alkaline activation of fly ashes. Microporous Mesoporous Mater 2006;
91:111-119.
Fernández-Jiménez A, de la Torre AG, Palomo A, López-Olmo G, Alonso MM, Aranda MAG.
Quantitative determination of phases in the alkaline activation of fly ash. Part II: Degree of
reaction. Fuel 2006; 85:1960-1969.
Provis JL, Duxson P, Van Deventer JSJ, Lukey GC. The role of mathematical modeling and
gel chemistry in advancing geopolymer technology. Chem Eng Res Des 2005; 83:853-860.
[10]. Xu H, Van Deventer JSJ. Effect of source materials on geopolymerization. Ind Eng Chem Res
2003; 42:1698-1706.
[11]. Fernández-Jiménez A, Palomo A. Characterisation of fly ashes. Potential reactivity as alkaline
cements. Fuel 2003; 82:2259-2265.
[12]. Kumar S, Kumar R. Mechanical activation of fly ash: Effect on reaction, structure and
properties of resulting geopolymer. Ceramics International 2011; 37:533-541.
[13]. Duxson P, Fernández-Jiménez A, Provis JL, Lukey GC, Palomo A, Van Deventer JSJ.
Geopolymer technology: the current state of the art. J Mater Sci 2007; 42:2917-2933.
[14]. Van Jaarsveld JGS, Van Deventer JSJ, Lukey GC. The effect of composition and temperature
on the properties of fly ash- and kaolinite-based geopolymers. Chem Eng J 2002; 89:63-73.
[15]. Fernández-Jiménez A, Lachowski EE, Palomo A, Macphee DE. Microstructural
characterisation of alkali-activated PFA matrices for waste immobilisation. Cem. Concr.
Compos. 2004; 26:1001-1006.
[16]. Slavik R, Bednarik V, Vondruska M, Nemec A. Preparation of geopolymer from fluidized bed
combustion bottom ash. J. Mater. Process. Technol. 2008; 200:265-270.
[17]. Xu H, Li Q, Shen L, Zhang M, Zhai J. Low-reactive circulating fluidized bed combustion
(CFBC) fly ashes as source material for geopolymer synthesis. Waste Manage. 2010; 30:5762.
[18]. Chindaprasirt P, Rattanasak U. Utilization of blended fluidized bed combustion (FBC) ash and
pulverized coal combustion (PCC) fly ash in geopolymer. Waste Manage. 2010; 30: 667-672.
[19]. Chindaprasirt P, Rattanasak U, Jaturapitakkul C. Utilization of fly ash blends from pulverized
coal and fluidized bed combustions in geopolymeric materials. Cem. Concr. Compos. 2011;
33: 55-60.