P. HRUBÝ et al.: STABILITY OF BASALT-FIBRES REINFORCEMENT IN ALKALI-ACTIV ATED SYSTEMS
203–210
STABILITY OF BASALT-FIBRES REINFORCEMENT IN
ALKALI-ACTIV ATED SYSTEMS
STABILNOST OJA^ITVE Z BAZALTNIMI VLAKNI V ALKALNO
AKTIVIRANIH SISTEMIH
Petr Hrubý
1
, Luká{ Kalina
1
,Ji øí Másilko
1
, Jaromír Poøízka
1
, Franti{ek [oukal
1
,
Vlastimír Nevrlý
2
, Magdalena Kimm
3
, Thomas Gries
3
1
Brno University of Technology, Faculty of Chemistry, Purkyòova 118, 612 00 Brno, Czech Republic
2
Brno University of Technology, Faculty of Mechanical Engineering, Technická 2896/2, 612 00 Brno, Czech Republic
3
Institut für Textiltechnik (ITA) of RWTH Aachen University, Otto-Blumenthal-Straße 1, 52074 Aachen, Germany
Prejem rokopisa – received: 2019-07-15; sprejem za objavo – accepted for publication: 2019-11-26
doi:10.17222/mit. 2019.151
Basalt fibres (BFs) represent suitable reinforcing materials for cementitious materials because of their mechanical and thermal
properties as well as good price-performance ratio. The crucial issue limiting the possibility of using these fibres in
alkali-activated systems (AASs) is their stability and durability in a highly alkaline environment. There is a significant
inconsistency across the publications in this case. Hence, the verification of the BFs stability in an environment simulating the
AAS is fundamental for a successful implementation as a reinforcement in the matrix. Accelerated leaching tests in
water-glass/NaOH and water-glass/Na 2CO 3 solutions with the evaluation of loss of mass, loss of tensile strength as well as
chemical composition changes of the fibres and leaching solutions were made using various techniques, such as XPS, SEM or
ICP-OES. A noticeable loss of mass as well as a deterioration of the tensile properties accompanied by a simultaneous
dissolution of the fibres was observed. Additionally, the transition-zone properties were studied using a SEM-EDX in this study.
Poor or no adhesion between the BFs and the AAS matrix was found.
Keywords: alkali-activated materials, alkali resistance, basalt fibres, accelerated leaching tests, durability testing
Bazaltna vlakna (BF) so primerna oja~itev za cementne materiale, ker imajo dobre mehanske in termi~ne lastnosti, kakor tudi
ugodno razmerje med ceno in u~inkovitostjo. Precej{nja omejitev je njihova uporaba v alkalno aktiviranih sistemih (AAS) ter
njihova stabilnost in trajnost v mo~no alkalnem okolju. V objavah na to temo najdemo {tevilne neskladnosti. Zato je verifikacija
stabilnosti bazaltnih vlaken v simuliranem AAS okolju osnova za njihovo uspe{no uporabo, kot oja~itev alkalno aktivirane
matrice. Avtorji so izvajali pospe{ene teste lu`enja v raztopinah: vodno steklo/NaOH in vodno steklo/Na 2CO 3, z ovrednotenjem
izgube na masi, zmanj{anja natezne trdnosti in sprememb kemijske sestave vlaken. Mikrokemijske analize so izvajali z
razli~nimi metodami, kot so: XPS, SEM in ICP-OES. Avtorji na osnovi preizkusov ugotavljajo pomembno raztapljanje vlaken
in s tem povezano izgubo na masi, kot tudi isto~asen padec natezne trdnosti. Dodatno so z uporabo SEM-EDX {tudirali lastnosti
v prehodni coni vlaken. Na{li niso nobene prave kohezijske vezi med bazaltnimi vlakni in AAS matrico.
Klju~ne besede: alkalno aktivirani materiali, odpornost proti alkalijam, bazaltna vlakna, pospe{eni lu`ilni testi, preizkusi
trajnosti
1 INTRODUCTION
The more widespread application potential of AAS
could be supported by the utilization of fibre or fabric
reinforcement since it is related with the improvement of
mechanical properties along with ductility and overall
durability. Besides favourable properties like earlier and
higher initial mechanical strengths, lower hydration heat,
better durability when exposed to high temperatures or
aggressive environments they are also more environ-
mentally friendly than OPC
1
, which is an important
factor since the OPC production accounts for7%o f
global CO
2
production.
2
The reduction in carbon dioxide
emission of about 50 % to 100 % for one tonne of binder
produced can be achieved by appropriately adjusting
the composition of the AAS.
3
One of the most common
ways is the utilization of secondary raw materials like
blast furnace slag or various types of ashes from
incineration.
Basalt fibres (BFs) represent suitable reinforcing
materials for AAS because of their mechanical and
thermal properties as well as a good price/properties’
ratio, as stated in
4–6
. The crucial issue limiting the
possibility of their utilization in AAS is their stability
and durability in a highly alkaline environment. The
superior durability of BFs was found under acid con-
ditions in the studies.
7,8
However, there is a significant
inconsistency across the publications in the case of
stability in a highly alkaline environment. Some of them
4
stated good stability, meanwhile others like
3,6–11
reported
low stability when exposed to alkaline solutions. The
dissolution process starts with the -Si-O-Si- and
–Al-O-Si- bonds breakage and continues with dissolu-
tion through the pores and microcracks on the surfaces of
the fibres.
10,11
As the dissolution of the aluminosilicate
network occurs the gel layer is formed on the fibre sur-
face and its thickness increases with time. Meanwhile,
Materiali in tehnologije / Materials and technology 54 (2020) 2, 203–210 203
UDK 67.017:004.942:676.017.62:678.067 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 54(2)203(2020)
*Corresponding author's e-mail:
xchrubyp@fch.vut.cz (Petr Hrubý)

the dissolution reaction moves forwards to the fibre core,
leaving insoluble oxides, hydroxo-carbonates and hydro-
xides as crystalline and amorphous phases behind. The
formation of this phases retards further the diffusion rate
of hydroxyl ions to the core of the fibre, as stated in
8
.
The overall dissolution rate of the glassy structure may
also be influenced by factors like the aqueous transport
of chemical species in the near-surface region or the
tendency to re-precipitate a solid if the equilibrium is
approached, as described in
12
. The dissolution process of
BFs can be limited by the application of a proper surface
coating, as reported in
9,10
. The mechanism of alkali-
resistance improvement is related to the formation of a
thin stable hydrated surface layer with a high content of,
e.g., zirconium, which retards the hydroxide ions’ diffu-
sion rate. The formation of poorly soluble compounds
containing Mg
2+
,T i
4+
or Fe
3+
is responsible for the
retardation of the dissolution process in the absence of
Zr.
10
2 EXPERIMENTAL PART
2.1 Raw materials
2.1.1 Basalt fibres (BFs)
The basalt fibres were provided by the Institut für
Textiltechnik (ITA) of RWTH Aachen University.
Table 1 describes the chemical composition of the BFs,
originally from Kammeney Vek, determined by X-ray
fluorescence (XRF). The amorphous structure of the BFs
was confirmed using X-ray diffraction (XRD).
Table 1: Chemical composition of the BFs from Kammeney Vek
determined by XRF analysis
Chemical composition in w/%
SiO 2
Al
2
O
3
Fe
2
O
3
CaO MgO Na
2
OK
2
OT i O
2
Other
55.7 15.4 10.8 7.4 4.1 2.4 1.5 1.2 1.4
2.1.2 Blast-furnace slag (BFS)
Blast-furnace slag with a Blaine specific surface area
of 400 m
2
·kg
–1
was obtained from Kotou~ [tramberk,
Ltd. Its chemical composition is given in Table 2. The
BFS was 84 % amorphous, while the rest was crystal-
line–akermanite (9.5 %); calcite (3.7 %); merwinite
(2.3 %) and quartz (0.5 %) according to the XRD using
the Rietveld method.
Table 2: Chemical composition of the BFS determined by X-ray
fluorescence (XRF) analysis
Chemical composition in w/%
CaO SiO 2
MgO Al
2
O
3
SO
3
TiO
2
K
2
O MnO Na
2
OFe
2
O
3
41.1 34.7 10.5 9.1 1.4 1.0 0.9 0.6 0.4 0.3
2.1.3 Alkaline activators
Liquid sodium water-glass (WG) was obtained from
V odní sklo, Inc., silicate modulus (M
s
) = 2.24; sodium
hydroxide (SH) was obtained from PENTA Ltd. (pure);
sodium carbonate (SC) was obtained from INCHEMA
Ltd. (pure).
2.2 Tests and methods
2.2.1 Accelerated leaching tests
The accelerated leaching tests were designed to
predict the long-term degradation behaviour of the BFs.
The test is based on the immersion of specific BF
fragments in various solutions at increased temperature
(60 °C) for a defined time period. The immersion
solutions were designed with the goal to simulate the
most aggressive environment formed in alkali-activated
blast-furnace slag systems, thus the initial mixing
solution (without BFs) where the concentration of alkalis
as well as the pH is the highest. The summary of used
solutions is as follows: WG/SC or WG/SH, M
s
= 0.5;
activator dosage (6, 8, 10 and 12) % Na
2
O; water-to-slag
ratio (w/s) = 0.49. The amount of solution was adjusted
to the respective tests. The pH values of the immersion
solutions were about 11.6 for WG/SC 6–12 % Na
2
O
mixtures. The pH values of the WG/SH solutions were
slightly higher at 11.8–12.0 (the pH decreased with
increasing activator dosage).
2.3 Loss of mass
The basalt fabric fragments with dimensions of about
2×2c mw e r ep r epared, weighed using an analytical
balance and placed into polypropylene containers with a
defined amount of immersion solutions, so the samples
were immersed and covered with a 1-cm layer above
them. These containers were kept at 60 °C. Three frag-
ments were taken out in defined time intervals
(1, 7, 14, 21 and 28) d, washed in distilled water for 24 h
and then dried at 60 °C until the constant weight of the
sample was reached. The loss of mass was determined as
the percentage of the difference between the initial
weight and the weight after leaching.
2.4 Loss of tensile strength
The leaching, washing and drying procedures were
the same as in the previous case, but with a different
sample size (bundle of filaments with length 35 cm). The
testing procedure was designed with reference to ASTM
D2256. The tests were performed using the Zwick Z010
universal testing machine (deformation rate 5 mm/min;
gauge length between jaws changed from 100 mm to
150 mm due to different disintegration of immersed
testing samples). The clamps were pneumatic and
designed for the fibres testing and originated from
Zwick. The measured and evaluated parameter was the
maximum strength before the fibre broke. The results are
expressed as a percentage change against the reference
(kept at 60 °C without immersion), thus as a tensile
strength maintenance.
P. HRUBÝ et al.: STABILITY OF BASALT-FIBRES REINFORCEMENT IN ALKALI-ACTIV ATED SYSTEMS
204 Materiali in tehnologije / Materials and technology 54 (2020) 2, 203–210

2.5 Chemical composition changes
The chemical composition changes were studied on
the fragments after the loss of mass testing and consisted
mainly of XRD and XPS analyses, as will be specified
more closely. The changes in the concentration of the
selected elements (Al, Fe, Ca and Mg) in immersion
solutions due to the fibres leaching were determined by
means of the Inductively Coupled Plasma Optical Ato-
mic Emission Spectroscopy (ICP-OES) – Horiba scien-
tific Ultima 2. A defined amount of immersion solutions
was taken in the time intervals of (1, 7, 14, 21, 28, 56
and 84) d of leaching, filtered through the 0.45-μm
syringe filter and 500-times diluted with distilled water.
2.6 X-ray photoelectron spectroscopy and X-ray dif-
fraction
The XPS analyses were carried out with an Axis
Ultra DLD spectrometer using a monochromatic Al-K
 (h = 1486.7 eV) X-ray source operating at 75 W (5 mA,
15 kV). The spectra were obtained using an analysis area
of approximately 300×700 μm. The Kratos charge
neutralizer system was used for all the analyses. The
high-resolution spectrum was measured with a step size
0.1 eV and a pass energy of 20 eV. The instrument base
pressure was2·10
–8
Pa. The spectra were analysed using
CasaXPS software and charge corrected to the main line
of the carbon C 1s spectral component (C\C; C\H) set to
284.80 eV. A standard Shirley background was used for
all the sample spectra. The XPS measurements were
made with the goal to characterize the changes of the BF
surface layer due to the immersion in alkaline solutions.
The XRD analyses were carried out with a diffracto-
meter Empyrean from the PANalytical corporation. The
measurement parameters were as follows: tube current
30 mA and voltage 40 kV; scan axis gonio; step size
0.0013 nm; time per step 96 s. The results were eval-
uated using the software HighScore plus. The
semi-quantitave analysis was only carried out due to the
possible amorphous content in the samples.
2.7 Interfacial zone characterization – SEM-EDX
The Zeiss Evo LS 10 scanning electron microscope
with EDX analysis was used for the characterization of
the interfacial zone. The samples were AABFS pastes
(w/s = 0.36) reinforced with one yarn of basalt fibres.
They were broken after 7 d and 28 d of hydration and
excessive fibres were removed. These fragments were
placed on the carbon tape, sputter coated with gold and
analysed (accelerating voltage 10 kV; probe 100 pA).
3 RESULTS AND DISCUSSION
3.1 Loss of mass of the fibres
The weight changes of the BFs due to the immersion
in WG/SC and WG/SH solutions are shown in Figure 1.
The immersion during the initial 14 days was accom-
panied by the most significant loss of mass and it slowed
down thereafter (except for 12 % Na
2
O WG/SC
mixture). It should be related to the formation of an
insoluble or protective layer containing Fe
3+
,M g
2+
and
Ti
4+
, as indicated in Y. V. Lipatovstudy.
10
The dissolution
rate of the fibres depends on factors like the diffusion
rate or tendencies to re-precipitate a solid if the
equilibrium is reached, as presented in
12
, hence the
oversaturation of the a immersion solution with parti-
cular ion can retard the further dissolution rate as well.
This presumption was confirmed by the ICP-OES, as
will be discussed later. There was a definite trend of the
loss of mass dependence on the activator dosage – the
higher activator dosage was the more significant loss of
mass observed for WG/SC. The dependence of the loss
of mass on the activator dosage was not so conclusive for
P. HRUBÝ et al.: STABILITY OF BASALT-FIBRES REINFORCEMENT IN ALKALI-ACTIVATED SYSTEMS
Materiali in tehnologije / Materials and technology 54 (2020) 2, 203–210 205
Figure 1: Dependence of BFs’ weight maintenance on the time of
immersion for activator dosages from: a)6%to12%Na
2
O – WG/SC,
b) WG/SH

the WG/SH solutions. However, it seems that WG/SH
forms a more aggressive environment since the loss of
mass was more intense for the 6–10 % Na
2
O dosages.
The weight loss values for 6–10 % Na
2
O were (16.1,
38.1 and 41.7) % for WG/SC, and 41.5, 45.8 and 48.7 %
for WG/SH solutions after 28 d, respectively. These
results are in a good correlation with those presented in
J. J. Lee’s paper
11
, where a weight loss of about 40 %
when immersed in 10 % NaOH for 80 days at a
laboratory temperature was stated. H. Funke et al.
13
stated a lower weight loss of BFs due to the leaching in
2-M NaOH at a laboratory temperature for 28 d.
3.2 Loss of tensile strength of the fibres
The results obtained were expressed as tensile
strength maintenance (comparison with the reference
without leaching) after the leaching in WG/SC and
WG/SH solutions and can be seen in Figure 2. The most
significant deterioration of the tensile strength was
noticed during the first 24 h (reduction of tensile strength
to: (33.9, 45.1, 20.7 and 17.2) % against the reference for
6–12 % Na
2
O) and continued up to 7 d. Still, the
simultaneous deterioration continued up to 28 d, when
none or very low remaining tensile strengths were
measured (2.1 % for6%N a
2
O) in the case of the
WG/SC solutions. There is a good correlation with the
results obtained from the loss of mass testing, because
the increase of the activator dosage led to a worsening of
the remaining tensile strength too. It should be born in
mind, that the loss of mass almost stopped within the
standard deviations (6–10 % Na
2
O) after 21 d; however,
the loss of tensile strength continued thereafter. This is
related to the formation of some precipitates on the
surface of the fibres (confirmed using the XRD), which
became ineffective for the tension carrying, as also stated
in
11
. From the long-time point of view, a considerably
more aggressive environment was formed using the
WG/SH solutions, since none of the tensile strength
could have been measured due to the advanced
disintegration of the samples after 7 d and thereafter.
Thus, the only ones which could be measured were the
ones after1dofleaching (46.9, 28.2, 26.2 and 23.2) %
for (6, 8, 10 and 12) % Na
2
O, respectively). It seems that
the instability of the BF in the WG/SH solutions
originates from the high dissociation constant of the
hydroxyl ions, as stated in J. J. Lee et al.
11
, who also
stated that the immersion of BF in saturated Ca(OH)
2
had not been accompanied by a significant loss of mass
but a rapid deterioration of the tensile strength was
observed. The deterioration of the tensile properties was
also stated in
7
and
8
.
3.3 Chemical composition changes
3.3.1 ICP-OES
The dissolution of the fibres should be related to the
release of the main constituents into the immersion
solutions, and thus with the increase of their concen-
tration in the solutions. The concentration changes of Al,
Fe, Ca and Mg were analysed with regard to the com-
position of the fibres, as well as the potential application
in the AAS. The dependence of the concentration
changes of chosen analytes for 6–12 % Na
2
O WG/SC
and WG/SH solutions are shown in Figures 3 and 4, res-
pectively. First, it should be mentioned that all the
analysed elements were present in the initial solution
before the leaching took place. It can be assumed that the
concentration increase of those elements was related to
the dissolution of the fibres. The background for the
ICP-OES measurements was water.
More clear and persuasive results were obtained for
the WG/SC solutions. The higher the activator dosage
(6–12 % Na
2
O) was, the more intense was the
dissolution process of the fibres (the increase of the
concentration of analytes in the immersion solution). The
general trend can be described as follows. The concen-
tration of elements simultaneously increases with the
time of immersion up to certain concentration, but after
that a gradual decrease was observed in most cases. This
can be explained by the simultaneous dissolution of the
fibres until the chemical equilibrium was reached,
followed by the oversaturation of the solution with some
P. HRUBÝ et al.: STABILITY OF BASALT-FIBRES REINFORCEMENT IN ALKALI-ACTIVATED SYSTEMS
206 Materiali in tehnologije / Materials and technology 54 (2020) 2, 203–210
Figure 2: Dependence of BFs’ tensile strength maintenance on the
time of immersion for activator dosages from: a) 6 to 12 % Na
2
O–
WG/SC, b) WG/SH

P. HRUBÝ et al.: STABILITY OF BASALT-FIBRES REINFORCEMENT IN ALKALI-ACTIV ATED SYSTEMS
Materiali in tehnologije / Materials and technology 54 (2020) 2, 203–210 207
Figure 4: Chosen elements concentration development as a function of activator dosage and time of immersion of BFs in WG/SH solutions
at 60 °C
Figure 3: Chosen elements concentration development as a function of activator dosage and time of immersion of BFs in WG/SC solutions
at 60 °C

elements and ended with the precipitation/recrystalli-
zation of the least-soluble component (e.g. CaCO
3
). The
comparison with the loss of mass results brings useful
information such as the results of the Al concentration. A
significant loss of mass was observed up to 14 d, the
ICP-OES validated this with the most significant
increase of Al concentration in the immersion solution
also up to 14 d for 6–10 % Na
2
O. Moreover, the immer-
sion in 12 % Na
2
O WG/SC was accompanied by the
highest loss of mass, which continued up to 28 d, which
is the same trend as was observed for the Al concen-
tration increase in the immersion solution. A positive
effect of Mg or Fe presence on the formation
10
of the
stable layer was not confirmed, since the increase of
their concentration immersion solution was also ob-
served in the corresponding period of leaching.
The results obtained for the WG/SH solutions were
not so definite, still they confirmed that WG/SH forms a
more aggressive environment for the fibres and their
dissolution is more intense (a larger amount of Al and Fe
leached). Another difference lies in the fact that there are
no signs of recrystallization or precipitation of the
elements from the solution (please refer to the Al
concentration change – the concentration simultaneously
increases up to 14 d of immersion and almost stops after
that; it indicates that all the Al is released from the
fibres). A general trend of analytes concentration
changes was also different. In the majority cases (Mg,
Ca, Fe) the increase of the activator dosage led to a lower
increase of concentration in the immersion solution, but
more intense as well at some point (Al). The solid-state
compounds that precipitated in the solutions were
subjected to XRD analysis and the presence of sodium
carbonate in two modifications (low temperature – 91 %
and high temperature – 2 %), Pirssonite (6 %) and some
aluminate silicate hydrate (1 %) for WG/SC solutions
and sodium silicate (63 %) along with sodium carbonate
(37 %) in the case of the WG/SH solutions were de-
tected.
These results seem to be unsatisfactory, but a real
AAS would include slag as well, thus the concentration
of Ca, Al, Fe or Mg in the pore solution would be higher
due to the slag dissolution, which should retard any
further dissolution of the fibres. Another factor influen-
cing the dissolution rate is the pH, which would also be
lower in real systems. This was also affirmed by the
SEM-EDX analysis of the BF in the AABFS paste,
where no signs of such a rapid dissolution of the BF
were noticed.
3.3.2 XPS and XRD
The chemical composition of the BF was determined
using XPS after 28 d of treatment under alkaline con-
ditions at elevated temperatures. The analysed samples
were 10 % Na
2
O WG/SC and WG/SH solutions. A
special emphasis was towards the specification of the
oxidation state of the present iron because of its
significant influence on the dissolution resistance. The
monovalent and divalent iron cations have a lower bond-
ing energy to the glassy structure; hence they are more
extensively and rapidly exchanged from the surface of
the fibres than the trivalent cations. Ferric ions act as
intermediates and favour the formation of the glassy
structure, due to the smaller ionic radius over the Fe
2+
,a s
more discussed in my detail by T. Förster et al.
12
. Ferrous
and ferric compounds were calculated by the multiplet
peaks fitting of a high-resolution 2p
3/2
spectra envelope
by the Gupta and Sen model.
14
The presumptions of better resistance of Fe
3+
over the
Fe
2+
against the dissolution was not fully confirmed. The
immersion in 10 % Na
2
O WG/SH was related to
complete the dissolution of the iron from the fibre
surfaces since the Fe 2p photoelectric peak between
710.6–711.2 eV
15
was not detected (Figure 5). The
WG/SC solution with the same activator dosage forms
less aggressive environments. The presence of iron in
both oxidation states was observed (Fe
3+
at 60.5 at. %;
Fe
2+
at 39.5 at. %) along with Al 2s and Al 2p photo-
electric peaks, which were not detected for samples
leached in WG/SH (the fact of total Al releasing from
the fibres was also predicted based on the ICP-OES). It
indicates that the dissolution of the glassy structure in
the WG/SC solution proceeds to a lower extent than in
the case of WG/SH. The Si 2s and Si 2p photoelectric
peaks were detected for both samples.
BF samples after the immersion were subjected to the
XRD as well. The immersion in 10 % Na
2
O WG/SC was
accompanied by the formation of calcium carbonate
along with the amorphous hump. In the case of the
WG/SH solution the presence of an alumina-silicate
compound containing Na, Ca and Mg without an exact
stoichiometry was detected.
3.3.3 Interfacial zone characterization
The formation of the transition zone as well as the
quality of the adhesion between the matrix and the BFs
P. HRUBÝ et al.: STABILITY OF BASALT-FIBRES REINFORCEMENT IN ALKALI-ACTIV ATED SYSTEMS
208 Materiali in tehnologije / Materials and technology 54 (2020) 2, 203–210
Figure 5: Results of XPS for BFs after the immersion in 10 % Na
2
O
WG/SC and WG/SH solutions up to 28 d at 60 °C

are crucial for the reinforcement to be effective, and thus
their further characterization was made. Very poor
quality adhesion was achieved in the case of the 10 %
Na
2
O WG/SC system since the SEM images (Figure 6)
showed relatively smooth surfaces of pastes under the
fibres. Furthermore, the core of the fibres remained
unattached by any paste fragments, as would be expected
if some sort of interaction had taken place. The signs of
significant degradation were not observed. Some sort of
interaction was noticed after 7 d in case of the 10 %
Na
2
O WG/SH (Figure 7), where the presence of many
attached paste fragments was detected (EDS analysis
showed that the composition agreed with the compo-
sition of the surrounding matrix); however, this effect
was no longer visible after 28 d. Thus, it was perhaps
related only to the slightly more etched surface (WG/SH
systems have an higher initial pH) of the fibres
accompanied by the higher interaction with the matrix
during the initial period. The fact that no chemical
interactions between the BFs and the AAS (based on fly
ash) were formed, was also stated in
16
.
The major hydration product for both tested AASs
was C-A-S-H gel with the Ca/Si molar ratio of about
0.9–1 and the Al/Si ratio of about 0.2. The composition
of transition zone (1–50 μm from the fibres) for the
WG/SH system was quite like the reference matrix far
from the fibres (at least 3 mm), but with some minor
changes of Ca/Si ratios. Still, it is likely the place with a
lower degree of hydration than with other hydration
products. This was confirmed by the elimination of the
composition differences after 28 d. The composition of
the hydration products in the zone surrounding the fibres
(less than 1 μm) was influenced by the BF composition
due to the interaction volume at the chosen accelerated
voltage, so there were some minor nuances in the
composition, but no major influence can be attributed to
them.
4 CONCLUSIONS
The applicability of the basalt fibres as reinforcing
materials to the AAS was studied through various
experiments, with the goal to determine their durability
under the conditions simulating a highly alkaline
environment like the one in the AAS matrix. The
following conclusions were made:
P. HRUBÝ et al.: STABILITY OF BASALT-FIBRES REINFORCEMENT IN ALKALI-ACTIV ATED SYSTEMS
Materiali in tehnologije / Materials and technology 54 (2020) 2, 203–210 209
Figure 7: SEM images showing the fibre/matrix interfacial zone
microstructure of WG/SH 10 % Na
2
O activated system after:
a) 7 d and b) 28 d of hydration
Figure 6: SEM images showing the fibre/matrix interfacial zone
microstructure of WG/SC 10 % Na
2
O activated system after: a) 7 d
and b) 28 d of hydration

• Immersion of BF in 6–12 % Na
2
O WG/SC and
WG/SH solutions at 60 °C caused the dissolution of
the fibres.
• A significant loss of mass of the fibres and the
deterioration of the tensile strength were observed
after the leaching of the BF in various solutions.
• The WG/SH solutions form a more aggressive
environment, as can be seen from the results of the
XPS, and others as well.
• No signs of quality adhesion or chemical interaction
between the BF and AAS matrix were observed.
• Further research of the BFs’ behaviour in these
systems is necessary for their successful utilization as
reinforcing material in various systems based on the
alkali activation of blast furnace slag or various types
of ashes. This research should focus especially on the
possibilities of the surface coatings utilization on the
basalt fibres, which could help to protect the core of
fibres against the highly aggressive alkaline environ-
ment, such as the one in the AAS.
Acknowledgement
This outcome was achieved with the financial support
by the project of specific research FCH-S-18-5194 and
FCH/FSI-J-19-5794.
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210 Materiali in tehnologije / Materials and technology 54 (2020) 2, 203–210