DOI: 10.5604/01.3001.0054.

3217 Volume 121 • Issue 2 • December 2023

International Scientific Journal

published monthly by the

of Achievements in Materials World Academy of Materials

and Manufacturing Engineering and Manufacturing Engineering

Eco-friendly production of foamed

geopolymers based on mine waste
B. Figiela *, A. Bąk **, M. Hebda, K. Korniejenko
Faculty of Materials Engineering and Physics, Cracow University of Technology,
Al. Jana Pawła II 37, 31-864 Kraków, Poland
* Corresponding e-mail address: beata.figiela@pl.edu.pl
** C orresponding e-mail address: agnieszka.bak@pk.edu.pl
ORCID identifier: https://orcid.org/0000-0001-5493-0914 (B.F.);
https://orcid.org/0000-0002-1984-1219 (A.B.)

ABSTRACT

Purpose: The presented work aims to show an alternative solution for replacing mineral
resources used in producing geopolymers with waste materials with good thermal conductivity
parameters and a stable structure.
Design/methodology/approach: In the present study, a material that is coal mine waste
from the Wieczorek mine in Silesia was used to produce geopolymers. In the preparation phase
of geopolymer foams, the ground and calcined material were combined with a 10M sodium
silicate hydroxide solution and a foaming agent reinforced with hemp fibres. The curing process
was carried out in a laboratory dryer at 75°C for 24 h. After 28 days after the preparation of the
samples, the strength and thermal insulation properties were tested.
Findings: The tested effect of adding fibres on the mechanical strength of foamed geopolymers
proves the strength improvement. The use of short hemp fibres increases strength compared
to samples without reinforcement. Hemp fibre in 0.5% also reduced the thermal conductivity
coefficient by almost 15%.
Research limitations/implications: Wastes generated during coal mining are interesting
research material regarding their use as precursors in geopolymerisation. An important issue
is to improve their preparation process from crushing to geopolymers' preparation phase. The
chemical composition of shales is a limitation in their use on an industrial scale. Therefore, it is
also recommended to test with various types of nano additives that can effectively affect the
properties of the finished product.
Practical implications: The properties of coal waste from the Wieczorek mine in Silesia show
its high potential for use in geopolymer synthesis. The use of the waste is ecological as well as
economical, making the material competitive.
Originality/value: A novelty in the production of geopolymers is the production of porous
materials using waste from the mining industry.
Keywords: Geopolymers, Composites, Foams, Fiber-reinforced, Waste, Mining, Industry,
Thermal conductivity, Mechanical properties
Reference to this paper should be given in the following way:
B. Figiela, A. Bąk, M. Hebda, K. Korniejenko, Eco-friendly production of foamed geopolymers
based on mine waste, Journal of Achievements in Materials and Manufacturing Engineering
121/2 (2023) 341-349. DOI: https://doi.org/10.5604/01.3001.0054.3217

CLEANER PRODUCTION AND BIOTECHNOLOGY

© Copyright by International OCSCO World Press. All rights reserved. 2023 Research paper 341
341
 Journal of Achievements in Materials and Manufacturing Engineering

1. Introduction
1. Introduction than in the case of traditional construction using concrete
(estimated 4-8 times) [6]. In addition, during the production
Water is the substance most frequently used by the of geopolymers, there is a much lower emission of CO2 and
world's population. Cement is in second place, making it the other greenhouse gases [7]. In addition to the fly ashes
most popular material in the construction sector. The mentioned above, other waste raw materials can be used to
production of this material is growing year by year, and thus produce those materials, characterised by a high silicon and
the global consumption of CO2 is also increasing. Figure 1 aluminium content [8-10].
shows the global CO2 emissions in 1950-2030. The photo Geopolymer materials can also be used to produce foams
shows that in recent years, especially immediately after that are similar in properties to conventional insulation
World War II, global CO2 emissions were not very high ‒ materials [11]. "Geopolymer foam" is a foamed lightweight
less than 10 billion tons, but over the years, the level of geopolymer that is highly porous. This material is produced
emissions has been increasing, and after 2020, the initial using chemical or mechanical methods [12]. In order to
value was exceeded four times [1-3]. create pores, foaming agents such as zinc or aluminium
powder, sodium hypochlorite, silica fume, sodium perborate
and hydrogen peroxide are added. Those substances react with
alkaline activators used in the geopolymerisation process,
and as a result, a porous structure is produced [13,14].
As mentioned, the geopolymer foaming process can be
done using a mechanical or chemical method. In the
chemical foaming method of geopolymers, the foaming
agent must be mixed with other ingredients to create air
voids from gas release. The most commonly used substance
for chemical foaming is hydrogen peroxide or aluminium
powder. The second method, mechanical foaming, can be
done by mixed foaming and pre-foaming. In the mixed
method, foam is produced when surfactant is added. During
Fig. 1. Global CO2 emissions from cement production and prefoaming, the foam is mixed with the grout that forms it
fossil fuel combustion [based on 2,3] [15,16]. The foaming process is illustrated in Figure 2.

Therefore, the world's cement producers have been asked

to answer how to reduce carbon dioxide emissions on our
planet. Radical ecological actions were needed for this.
Therefore, the recovery logistics principle was adopted,
manifested by a complete departure from type I cements
(CEM-I) in favour of multi-component and slag CEM II-V
types of cement. The second proposed solution is the use of
alternative fuels that will replace the traditional ones and be
used to obtain high temperatures in cement kilns. Currently,
there is very dynamic technological progress in the world,
and this evolution also applies to changes in the field of
cement binders. The dynamic development of construction
has resulted in the creation of new materials that can
completely or partially replace the cement used so far [4,5]. Fig. 2. Foaming process [based on 16]
The material that can be successfully used is geopolymer
and various types of geopolymer binders. Producing Foamed geopolymer materials can potentially be used
geopolymers is much more economical and ecological due for thermal insulation [17,18]. Recently, much research has
to two aspects. The first one uses by-products of coal been devoted to equivalently acoustic insulation [19-21].
combustion (commonly considered waste raw materials), The production of geopolymer porous materials also
the most popular being fly ashes. The second is the process introduces some limitations in their structural use, namely a
of producing geopolymer materials, which is characterised decrease in strength properties [22-24]. One of the ideas to
by energy efficiency – much less electricity is consumed increase the mechanical strength of foamed materials is to

342 Research paper B. Figiela, A. Bąk, M. Hebda, K. Korniejenko

 Volume 121 • Issue 2 • December 2023

introduce various additives to their mixture, also in the form AlO4 tetrahedrons are connected alternately with oxygen
of fibres: glass fibre, low input grass, kenaf fibre, PVA fibre, atoms. Reactive aluminosilicates in strongly alkaline
and steel fibre [25-30]. aqueous solutions decompose successively in the process of
This study synthesises geopolymer foams using waste polycondensation of orthosilicates [33].
generated during hard coal mining and post-process waste of Preparing a 10-molar solution combined the following
hemp fibre. Post-process natural fibres are an additional components: flakes of technical sodium hydroxide, water
advantage to be used in a geopolymer whose main and aqueous sodium silicate R-145 (molar module 2.5,
component is another waste material, making it density 1.45g/ cm3). The ratio of NaOH to water glass was
environmentally friendly and conducive to the idea of a 1:2.5. The mixed solution was left for 24 hours to reach
circular economy. Another advantage is the change in the constant concentration and temperature. After this time, the
geopolymer cracking pattern from brittle to more ductile, proper procedure for preparing porous geopolymers follows.
which improves mechanical strength [31]. Two types of samples were prepared: reference samples with
The presented work aims to show an alternative solution 100% precursor material content and samples with recycled
in the field of building materials, which in this case, can be hemp fibres.
used as an insulating material. The production of the The first step is to weigh the right amounts of
material is economical and simple, and the process itself will ingredients. The waste precursor was combined with the
be presented in the next section. solution in an L/S ratio of 0.4. After about 10 minutes of
mixing in a low-speed mixer, the structure stabiliser ‒
hydroxyethyl cellulose (Glentham Life Sciences, UK) was
2. Materials
Materialsand
2.  andmethods
methods added, and in the second type of sample, hemp fibres, added
in the form of chopped fibres with a length of about 1 cm.
Anthropogenic waste ‒ coal shale from the "Wieczorek" The foaming agent used to prepare the samples was 35%
hard coal mine (Silesia, Poland) was used to produce hydrogen peroxide (Azoty Group, Puławy, Poland), added
samples and post-process hemp fibres. As a result, material in quantity 0.75%. The samples were cured in a laboratory
was obtained whose production process is economical and dryer SLW 750 (POLEKO, Poland) at 75°C for 24 hours.
whose properties do not differ from the standards of similar A simplified concept of product life is shown in Figure 3.
insulation solutions. The base precursor for producing The diagram illustrates the process from extraction to the
geopolymer foams was mine tailings from the Wieczorek finished geopolymer sample.
mine in Poland. Gangue is a by-product of coal mining [32]. Table 1 shows the designations of the samples along
The main components of this type of waste are quartz, illite, with an indication of the amount of components used in their
and kaolinite, which contain large amounts of aluminium production.
oxides and silicon oxides. The composition potentially After 28 days from the preparation of the foam samples,
predisposes gangue to the base material in the production of they were tested for density (physical properties), bending
geopolymers. In order for the material to react with an and compressive strength (mechanical properties) and
alkaline solution, resulting in gelation and geo- measurements of the thermal conductivity coefficient
polymerisation, proper waste treatment is needed. The (thermal properties).
process begins with the crushing and grinding of the mine The density of the tested boards was checked using the
waste, which is in a large form. After the grinding step, geometric method, which is based on measuring the mass
below the particle size 0.75 µm, calcination took place at a and volume of the samples. The weight for each geopolymer
temperature of 700°C, and the material was heated for 24 foam was determined as the average of five measurements.
hours in a fireclay kiln. The next step in the process of The dimensions of the samples were measured using a
preparing foamed geopolymers is the preparation of an laboratory caliper (measuring accuracy up to 0.01 mm),
alkaline solution, which, when combined with the calcined while the weight of the samples was determined using the
precursor, will allow the formation of SiO4 and AlO4 RADWAG PS 200/2000.R2 analytical balance (measuring
tetrahedrons in the spatial lattice of geopolymers. SiO4 and accuracy up to 0.001/0.01 g).

Table 1.
Characteristics of samples for testing based on coal shale from the "Wieczorek" mine
Sample ID Matrix, g Structure stabilizer, g Hemp fiber, g Foaming agent, g Alkaline activator, g
W 1000 3 - 7.5 400
W+hemp fiber 1000 3 5 7.5 400

Eco-friendly production of foamed geopolymers based on mine waste 343

 Journal of Achievements in Materials and Manufacturing Engineering

Fig. 3. Simplified diagram of the product life cycle - coal shale from the “Wieczorek” mine [based on 34]

Flexural strength tests were performed on an MTS The thermal conductivity coefficient λ was checked on
Criterion 43 testing machine with TestSuites 1.0 software a plate apparatus HFM 446 (Netzsch, Wittelsbacherstrasse,
(MTS System Corp., Eden Prairie, MN, USA) with a Germany). The conductivity range of the device is in the
measuring range of up to 30 kN. The test was carried out range from 0.007 to 2.0 [W/m*K] with accuracy of ± 1-2%.
based on the PN-EN 196-1:2016-07 standard (Cement test The obtained boards were subjected to coefficient tests in the
methods - Part 1: Determination of strength - point 9.1) [35]. temperature range of 0-20°C.
The bending strength is determined according to the formula Microscopic observations were performed using an
for the three-point method (1): optical microscope Keyence VHX-7000 at different
ଵ,ହ‫כ‬୊೑ ‫כ‬୪ magnifications.
R௙ ൌ ሾMPaሿ (1) In addition, an XRD phase analysis of the base material
௕య
was performed, which is shown in Table 2. The study was
where:
carried out on a PANalytical Almelo X-ray diffractometer
R௙ – flexural strength [MPa],
and the Rietveld analysis using the HighScore Plus software
b– side length of the section [mm], database.
F௙ – maximum load [N],
l – length between supports [mm]. Table 2.
Five samples with dimensions of 50x50x200 mm were Identified phases in material from “Wieczorek” coal mine
prepared for the tests.
Identified Percentage,
Compressive strength tests were performed on a Chemical formula
phase %
MATEST 3000 kN testing machine (Matest, Treviolo,
Quartz SiO2 34.1
Italy). The test was carried out based on the PN-EN 196-
1:2016-07 standard (Cement test methods - Part 1: Mullite Al6Si2O13 3.8
Determination of strength – point 9.2) [35]. The maximum Muscovite (K,H3O)Al2Si3AlO10(OH)2 39.5
load is the basis for calculating the compressive strength of Albite NaAlSi3O8 22.6
the concrete material according to the formula (2):

R௖ ൌ
୊೎
ሾMPaሿ (2) The study shows the presence of minerals: quartz,
ଵ଺଴଴
muscovite, albite, and mullite. Based on the diffraction
where: patterns obtained with this method, it can be concluded that
R ௖ – compressive strength [MPa], the sample contained a large amount of minerals from the
1600 – surface of tiles (or auxiliary tiles) [mm2], group of silicates with a multilayer structure – muscovite
F௖ – maximum load [N]. 39.5%. Quartz is the second most abundant phase – 34%; in
Five samples with dimensions of 50x50x50 mm were third place is albite – 22.6%, included in the group of alkali
prepared for the tests. feldspars.

344 Research paper B. Figiela, A. Bąk, M. Hebda, K. Korniejenko

 Volume 121 • Issue 2 • December 2023

3. Results
Resultsand
3.  anddiscussion
discussion

3.1. TTesting
3.1.  of physical
esting of physicalproperties
properties- density
- density of
of samples
samples

The apparent density of tested reference sample “W” was

0.69 g/cm3, which is typical for this type of material. Their
density is usually in the range of 0.2-0.8 g/cm3 [36]. Based
on the presented test results, adding hemp fibres reduced the
density of geopolymer plates; after adding fibres, the density
value decreased by almost 6%. Table 3 shows the
Fig. 5. Compressive strength of the samples based on coal
measurement results.
shale from the “Wieczorek” mine
Table 3.
Such an assumption is confirmed by the results of other
Density of samples based on coal shale from the “Wieczorek”
scientists' tests. The compressive strength of the reference
mine
sample corresponds to the compressive strength of the
Sample ID Average density, g/cm3
porous geopolymer based on fly ash, also foamed with H2O2,
W 0.69 ± 0.08
which obtained 26% higher strength [37].
W + hemp fiber 0.65 ± 0.03 Senff et al. tested the effect of the addition of glass fibre
on foamed geopolymers based on fly ash and found an
3.2. Mechanical
Mechanical properties
3.2.  properties––flexural
flexuralstrength
strengthand increase in flexural and compressive strength by 10% and
and compressive
compressive strength tests
strength tests 19%, respectively, for foams containing 1% fibre and 0.1%
aluminium powder (Al), which gives a result of 1.1 MPa for
The results of the mechanical properties tests are shown bending. By increasing the proportion of both fibre and
in Figures 4 and 5. blowing agent (2% and 0.2%, respectively), he observed an
increase in flexural strength by 23% and compressive strength
by 30%, resulting in a compressive strength of 3 MPa [26].
Glass fibre is characterised by a brittle structure in
relation to hemp fibre, which may explain the differences in
the compared studies. Zhang et. All also confirm the trend
of increasing compressive strength using fibres, in this case,
kenaf. H2O2 was used as a foaming agent, and metakaolin in
the matrix. The compressive strength increased by about 1.2
to 2.7 times, of which a smaller increase was noted for the
higher content of the blowing agent [28]. Repeatedly testing
the effect of the addition of fibres on the mechanical strength
of geopolymers proves the improvement of strength [31,38],
Fig. 4. Flexural strength of the samples based on coal shale also in the case of tests carried out in this article
from the “Wieczorek” mine
3.3. TThermal
3.3.  properties- thermal
hermal properties - thermal conductivity
conductivity
The expected result of the strength of the foams was an coefficient
coefficient
increase in the value of fibre-reinforced samples compared
to reference geopolymers. Adding hemp fibre in 0.5% by Table 4 shows the test results of two measurements for
weight increases the bending strength from 0.6 MPa to 0.8 thermal conductivity. The thermal conductivity decreases by
MPa. The compressive strength increased by more than 0.187 W/m*K when hemp fibre is added to the geopolymer
100%, according to the reading from the testing machine. material relative to the reference sample, which has a
However, the error of the MATEST machine sensor should thermal conductivity of 0.13 W/mK. Such a difference can
be assumed at the level of ± 0.5 MPa below 1MPa. Hence, it be attributed to the structure of pores and their number in the
can be assumed that the strength results for bending and samples, which is also correlated with the obtained apparent
compression are correlated. density [39].

Eco-friendly production of foamed geopolymers based on mine waste 345

 Journal of Achievements in Materials and Manufacturing Engineering

Table 4.
Measurement of the thermal conductivity coefficient of the samples based on coal shale from the “Wieczorek” mine
Sample ID First measure, W/m*K Second measure, W/m*K Average thermal conductivity, W/m*K
W 0.13127 0.13295 0.13211 ± 0.0008
W+hemp fiber 0.11211 0.11469 0.11340 ± 0.0003

a) b)

Fig. 6. Optical microscope image of a geopolymer sample with hemp fibre: a) fibre “pulled from the sample”, b) fibre in a
geopolymer matrix

a) b)

Fig. 7. Optical microscope image of a reference geopolymer: a) porous structure and its brittle nature, b) cracks around the
pores

The reasons for the differences in thermal conductivity 3.4. Evaluation

3.4.  of the
Evaluation of the appearance
appearanceofofthe
theporous
porous
can be sought in the packing of fibres in the geopolymer geopolymer structure
geopolymer structure
matrix, with the size of the fibres used being an important
parameter. Less dense packing of longer fibres leads to more Microscopic observations were carried out on an un-
empty pores [25]. In order to obtain the lowest possible damaged sample (reference sample) and after a compression
thermal conductivity for foamed geopolymers based on coal test (sample with fibre), hence possible deformations and
shale while maintaining the permissible compressive irregularities in the pore structure (Figs. 6 and 7).
strength, the characteristics of the pores in the material Foamed geopolymers with fibre are characterised by a
should be improved [37,40]. larger number of smaller pores, for which additional bridges

346 Research paper B. Figiela, A. Bąk, M. Hebda, K. Korniejenko

 Volume 121 • Issue 2 • December 2023

in the form of fibres are formed. The above photos confirm environmentally friendly and safe insulation materials and
the previously drawn conclusions. The increase in strength ability to accumulate heat based on the alkaline activation of
for this type of sample results from the good coherence of anthropogenic raw materials (LIDER/31/0168/L-
the matrix and fibre, which changes its character from a 10/18/NCBR/2019).
brittle structure to a ductile one.
The brittle nature of the structure of the reference sample
can be observed, in which crack propagation begins in the References
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