materials
Article
Photocatalytic Activity and Mechanical Properties of
Cements Modified with TiO2/N
Magdalena Janus 1,2,* , Szymon M ˛adraszewski 2, Kamila Zaj ˛ac 1, Ewelina Kusiak-Nejman 3,
Antoni W. Morawski 3and Dietmar Stephan 2
1Faculty of Civil Engineering and Architecture, West Pomeranian University of Technology, Szczecin, al.
Piastów 50, 70-311 Szczecin, Poland; [email protected]
2Building Materials and Construction Chemistry, Technische Universität Berlin, Gustav-Meyer-Allee 25,
3
Faculty of Chemical Technology and Engineering, West Pomeranian University of Technology, Szczecin, ul.
*Correspondence: [email protected]; Tel.: +48-91-449-4083
Received: 30 September 2019; Accepted: 12 November 2019; Published: 14 November 2019
Abstract:
In this paper, studies of the mechanical properties and photocatalytic activity of new
photoactive cement mortars are presented. The new building materials were obtained by the addition
of 1, 3, and 5 wt % (based on the cement content) of nitrogen-modified titanium dioxide (TiO
2
/N)
to the cement matrix. Photocatalytic active cement mortars were characterized by measuring the
flexural and the compressive strength, the hydration heat, the zeta potential of the fresh state, and the
initial and final setting time. Their photocatalytic activity was tested during NOx decomposition.
The studies showed that TiO
2
/N gives the photoactivity of cement mortars during air purification
with an additional positive effect on the mechanical properties of the hardened mortars. The addition
of TiO
2
/N into the cement shortened the initial and final setting time, which was distinctly observed
using 5 wt % of the photocatalyst in the cement matrix.
Keywords: photoactive cement; TiO2/N; NOx decomposition; mechanical properties
1. Introduction
In the last few decades, nanoparticles have been considered as an additive to the concrete
and related cement products in order to improve the properties of building materials [
1
]. The first
documented addition of the nanoparticles to a cement-based system occurred in 1964 when the
nano-SiO
2
facilitated a faster and more complete hydration of cement [
2
]. However, the application
of various nanoparticles, such as nano-TiO
2
, nano-Al
2
O
3
, and nano-Fe
2
O
3
in cement and concrete
materials has developed intensively since circa 2004 [
3
–
5
]. Combining TiO
2
nanoparticles with
cementitious binders appeared to be one of the most promising ways to obtain environmentally
friendly products [
6
]. Namely, a TiO
2
photocatalyst, when activated by the suitable light, is capable of
supporting the chemical reactions, which can degrade an atmospheric pollutant and give a self-cleaning
property [
7
]. It is worth pointing out that the building surfaces are exposed to the highest levels of air
pollution and at the same time to the solar radiation, which is necessary in photocatalytic processes.
In the urban areas, NOx (NO +NO
2
) is one of the most common pollutants from the external
sources (traffic, industry) [
8
]. NOx contributes to the formation of the photochemical smog, and it is
associated with lung problems and asthma [
9
]. The potential of cementitious materials containing
photocatalysts to decrease the NOx concentration was proven many times [
10
–
12
]. For example,
Lee et al. [
13
] studied changes in NO and NO
2
concentration using TiO
2
-containing cement-based
materials during UV irradiation, suggesting that the materials are capable of oxidizing both gases
Materials 2019,12, 3756; doi:10.3390/ma12223756 www.mdpi.com/journal/materials
Materials 2019,12, 3756 2 of 12
efficiently. It was observed the similar amounts of NO and NO
2
gases were degraded at 3 h, regardless
of the variations in the water/cement ratio. The mechanism of NOx degradation consists of a series of
reactions that take place during the photocatalytic process. Typically, it can be described as a sequential
oxidation process, as follows: NO →HNO2→NO2−→NO3−[8,11].
Many works in the photocatalytic branch are directed at modification of the base TiO
2
structure
through doping of the photocatalyst with the non-metals or the metal ions [
14
,
15
]. Mainly, it can
enhance its activation in the visible light [
16
], but other advantages are also observed. In the treatment
of NOx, modifications of TiO
2
can improve the catalytic selectivity toward nitrate rather than the more
toxic NO2[7].
Although the photocatalytic cements and concretes have been extensively studied [
17
], it is
still controversial whether the added photocatalyst enhances building properties. On the one hand,
the presence of TiO
2
nanoparticles can have a positive filler effect in cement mortars, increasing the
mechanical strengths. One of the reported best performance enhancement results of the inclusion
of TiO
2
nanoparticles in the cementitious materials included a 45% increase in the compressive
strength [
18
] and an 87% increase in the flexural strength [
19
]. Yang et al. [
20
] indicated that the
addition of 0.5 wt % TiO
2
to cement slag pastes allowed achieving approximately 10%, 15%, and 9%
higher compressive strengths in comparison to the reference material at 3 d, 7 d, and 28 d, respectively.
Meanwhile, the flexural strengths of the same photocatalytic materials were 25%, 25%, and 38% higher
than the reference sample after the same range time of the curing. On the other hand, researchers also
observed [
21
,
22
] a slight decrease in the mechanical strength of the photocatalytic cement mortars,
which has been attributed to the decline of the sample’s homogeneity and the formation of weak zones
in the structure of hardened mortars.
Some authors [
7
] indicated that the effective use of photocatalysts in cement is highly connected
with the assurance of the optimized dispersion of TiO
2
particles in the cement matrix. The agglomeration
of TiO
2
particles can interfere not only with the mechanical strength, but also block access to an internal
surface of TiO
2
, limiting the photocatalytic efficiency. The degree of repelling between particles of
cement mortars has a direct relationship with “zeta potential”, which shows the electrokinetic behavior
of particles and gives a valuable indication of the surface charge state, achieving values from
−
30 mV
to +30 mV [
23
,
24
]. When the constituent particles of mortars have the same charge, they tend to
repeal each other, and no agglomeration occurs. Up to now, the zeta potential measurements of
cementitious materials have been performed with a low fraction of solid. Therefore, it is challenging
to obtain information about the zeta potential values of real fresh cement mortars and even more so
referring to photocatalytic cement mortars. Lowke and Gehlen [
24
] considered the zeta potential of
Portland cement and mineral additions in cement suspensions with high solid fractions. They found
a continuous increase in the zeta potential of cementitious suspensions with the increasing w/c ratio
(water/cement). Moreover, the determining factor on the zeta potential appeared to be the molar
Ca/SO4concentration ratio, which was more crucial than the effect of the type of addition.
The absolute values of zeta potential may vary not only with mortar composition or w/c ratio but
also with the time of hydration [
25
]. The hydration mechanism of cement consists of the reactions of
cement components (e.g., alite or tricalcium silicate, belite or dicalcium silicate) with water. The formed
crystalline calcium hydroxide and calcium–silicate–hydrate (C-S-H) comprise over 60 wt % of the
hydration products in the total mass [
26
]. As the reactions continue with time, the hydration products
gradually bind together and with other components of concrete to form a solid mass. The hydration of
cement is an exothermic chemical reaction. The generation of heat is highly determined by the chemical
composition of the cement mixture. It was reported that nanoparticles of TiO
2
could significantly
change the hydration of cement and influence the rates of heat evolution [
27
,
28
]. The cement hydration
is directly related to the setting time of cement mortars. Mostly, the photocatalytic cements showed
a shortened initial and final setting time for the samples with higher TiO
2
contents, which is attributed
to the acceleration of the hydration rate [29].
Materials 2019,12, 3756 3 of 12
The aim of this paper is to present the results of our study on the influence of a prepared TiO
2
/N
photocatalyst on the properties of the fresh and hardened cement mortars. TiO
2
/N was chosen as the
photocatalyst because there is the possibility of producing this material in the amount of 0.5 kg per
day. Moreover, the technological project of installation for TiO
2
/N production exists and may be used
for photocatalyst production at a large scale. The photocatalyst has been added in different dosages
(1, 3, and 5 wt % to cement mass) to cement mortars. The measurements of the initial and the final
setting time, the flexural and the compressive strength, the hydration heat, and the zeta potential were
conducted. The photocatalytic activity was monitored during the NOx degradation process.
2. Materials and Methods
2.1. Materials
Ordinary Portland cement CEMI 42.5 N from Holcim, Germany was used in this study. Standard
sand, according to EN 196-1, was used for all mortars.
The preparation of the photocatalyst (TiO
2
/N) was carried out using HEL Ltd. “Autolab” E746
installation. The commercial titanium dioxide supplied by Grupa Azoty Zakłady Chemiczne ‘Police’
S.A. (Poland) was used as a starting material. First, 600 g of TiO
2
and 350 mL of NH
4
OH with
a concentration of 2.5% were placed in an autoclave. The reactor was closed, and the mixture was
blended using a magnetic stirrer and heated up to 100
◦
C for 4 h. Afterwards, the catalyst was dried
in air for 4 h at 100
◦
C. Finally, the obtained photocatalyst TiO
2
/N was ground with mortar to form
a fine powder. The structural and the textural parameters on N-modified TiO
2
in Table 1were placed.
The results of TEM (transmission electron microscope), XRD (X-ray powder diffraction), FTIR/DRS
(fourier transform infrared spectroscopy/diffuse reflectance), XPS (X-ray photoelectron spectroscopy),
and Raman spectroscopy in our earlier publication were presented [
30
]. The presence of nitrogen in
the modified titania sample was confirmed by FTIR analysis. The narrow bands at 1640 cm
−1
and
1440 cm
−1
are attributed to the hydroxyl (OH) and ammonium (NH
4+
) groups, respectively, while
the band at 1536 cm
−1
could be assigned to either NH
2
or NO
2
and NO groups. The sample was also
studied by Raman spectroscopy. The Raman spectra of the sample exhibit four distinct peaks located at
145 cm
−1
, 393 cm
−1
, 514 cm
−1
, and 646 cm
−1
; those bands correspond to the anatase phase of TiO
2
[
30
].
Table 1. Structural and textural parameters of N-modified TiO2.
Photocatalyst
Local Mean Crystallite
Size According to TEM
[nm]
Global Mean Crystallite
Size According to XRD
[nm]
Mean Particle Size According to
DLS (Dynamic Light Scattering)
[nm]
SBET
[m2/g]
TiO2/N 6.1 10.8 167.6 235
2.2. Specimen Preparation
The specimens 40
·
40
·
160 mm
3
and 80
·
40
·
10 mm
3
were produced according to EN 196-1 with
a water to binder ratio (w/b) =0.4 and cement to standard sand ratio of 1:3. Cement was replaced
by catalyst in 1, 3, and 5 wt % by mass of cement. Samples without replacement were produced as
a reference. For each type of mortar, 6 specimens were produced. Masses needed for the preparation
of 3 specimens are presented in Table 2.
Table 2. Mass of materials used for the production of three 40·40·160 mm3mortar specimens.
Materials Mass of Used Materials [g]
1% 3% 5%
CEM I 42.5 N 444.5 436.5 427.5
TiO2/N 4.5 13.5 22.5
Standard sand 1350 1350 1350
Water 180 180 180
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A standard mixer with a stainless steel bowl with a capacity of 5 dm
3
according to EN 196-1 was
used. First, water was poured into a bowl, and cement was added. The mixer was started immediately
at low speed (rotation 140 min
−1
) and after 30 s, standard sand was steadily added for the next 30 s.
Afterwards, the mixer was switched to the higher speed (285 min
−1
) for an additional 30 s. To remove
all the mortar adhering to the wall and the bottom part of the bowl, the mixer was stopped for 90 s.
In the end, the mixing was continued at high speed for 60 s. The specimens were molded immediately
after the preparation. The first layer of mortar was poured into the mold situated on the jolting
table and then compacted. The second layer of mortar was poured on the first layer and compacted.
The excess mortar was struck offwith the straight metal edge. Casting molds containing fresh samples
were wrapped with stretch film and stored at room conditions for 24 h. All specimens were demolded
after 1 day and were cured in tap water for the next 27 days.
2.3. Compressive and Flexural Strength Measurements
After 28 d, specimens were tested for their flexural and the compressive strength. The flexural
and the compressive strength measurements were carried out following EN 196-1. For each mortar
type, six 40
·
40
·
160 mm
3
specimens were tested for the flexural strength. The prism halves (after the
test of flexural strength) were tested for compressive strength, so for each mortar type, 12 specimens
were tested. A standard testing machine (ToniNORM 2010.040, Toni/Technik, Berlin, Germany) was
used both for the flexural and the compressive strength measurements.
2.4. Setting Time (Vicat Needle Test)
The setting of cement and its rate affects the open time of the mortar. In this study, the influence
of the addition of the catalyst on the setting time of cement was tested. The Vicat Apparatus is a device
that is used to determine the setting time of the cement paste. In this study, an automatic device
ToniSET COMPACT version 05/00, which did 6 parallel tests, was used for determining the setting time.
For each mortar type, 2 specimens were tested. Mortar preparation and the setting time measurements
were run according to the EN 196-3 standard. During the measurement, the specimens were kept at
20
◦
C. The water to binder ratio of paste used for the setting time test was w/b=0.3. The time when
the needle stopped 6 mm from the base plate was recorded as the time for the initial setting. The final
setting was defined as the time when the needle only made a 0.5 mm mark on the surface.
2.5. Hydration Heat Measurements
Calorimetry data were obtained from externally mixed pastes containing 40 g of cement and
16 g of water, in at least a twofold determination. Data points were recorded every 60 s at 20
◦
C
(Isothermal heat flow calorimeter MC-CAL100, C3 Analysentechnik, C3 Prozess und Analystechnik,
Haar, Germany).
2.6. Zeta Potential Measurements
A Zeta and Titration 310 instrument from Dispersion Technology was used for the zeta potential
measurements without the dilution of samples, which to some extent avoided the differences in the
hydration and surface properties between diluted and original samples. First, 16 g of water was added
to 40 g of cement and mixing for 20 s. As the background, the centrifuged water from such prepared
mortars was used. The average particle sizes of cement amounted to 9 µm.
2.7. NOx Decomposition
The photocatalytic activity of the prepared plates of cement mortar toward the degradation of air
pollutions was also proved. In our previous works [
31
,
32
], the gaseous NO (1.989 vppm
±
0.040 ppm,
Air Liquide) was used as model pollution in photocatalytic tests. NOx removal was evaluated using
the experimental installation, whose scheme is presented in Figure 1.
Materials 2019,12, 3756 5 of 12
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Figure 1. The scheme of installation to the photocatalytic removal of NOx (S – the source of pollution;
M – mass flower; H – humidifier; R – photocatalytic reactor with irradiation source; A – NOx analyzer).
The studied plate of cement mortar (one at dimensions of 80·40·10 mm³) was placed in the central
part of a cylindrical reactor (Pyrex glass; Ø × H = 9·32 cm²), and the reactor was tightly closed. The
NO was diluted with humidified synthetic air in a ratio of 1:1. The oxygen and water molecules were
necessary for the formation of oxidative species, which are essential in the photocatalytic reactions.
The polluted air flowed through the reactor continuously with a rate of 500 cm3/min. At the beginning
of the process, the dark conditions were maintained until NO concentration reached equilibrium
(about 1 ppm during about 35 min). Then, the UV lamps were turned on for 30 min. The irradiation
sources surrounded the reactor and were characterized by the cumulative intensity of 100 W/m2 UV
and 4 W/m2 VIS. The temperature of the whole system was stable at the level of 22 °C by using a
thermostatic chamber. The NO and NO2 concentrations were continuously measured in the outlet of
the reactor using chemiluminescent NOx analyzer (T200, Teledyne). All measurements were repeated
three times, and errors were 2%.
3. Results
3.1. Compressive and Flexural Strength
The compressive and the flexural strength of pure cement and cement with the addition of 1, 3
and 5 wt % TiO2/N specimens were measured. The obtained results are presented in Figure 2a and b.
As it can be seen in Figure 2a, the value of the compressive strength of unmodified cement amounted
to 53 MPa (red line), while the addition of 1, 3, and 5 wt % of TiO2/N increased the compressive
strength of the specimens in all cases. The highest value of the compressive strength was observed
for specimens with 1 wt % of TiO2/N and amounted to 57.4 MPa. The lowest increase of the
compressive strength was found for a specimen with the addition of 5 wt % of TiO2/N. Similar
behaviour occurred during the flexural strength measurements. As can be seen in Figure 2b, the value
of the flexural strength of unmodified cement amounted to 6.92 MPa (red line). Analogous as in the
case of the compressive strength, the addition of 1, 3 and 5 wt % of TiO2/N increased the flexural
strength of the specimens in all cases. The highest value of the flexural strength was observed for a
specimen with 1 wt % of TiO2/N and amounted to 7.60 MPa. The lowest increase of flexural strength
was obtained using specimen with addition of 5 wt % of TiO2/N.
The mechanical properties (the compressive and the flexural strength) of cement strongly
depend on the amount of used titanium dioxide. Wang et al. [33] discovered that with the
incorporation of TiO2 nanoparticles, the strength firstly showed a fast increase compared with the
ordinary mortar until the dosage of TiO2 nanoparticles reached up to 2 wt %, and then the rate of this
increase slowed down. The strength of the cement mortar is closely related to the amount of ettringite
and C-S-H gels, and the existence of nanoparticles facilitates the cement hydration, thereby
producing more hydration products. In addition to the filler property of nanoparticles to fill the pores
in C-S-H gels, it is well known that nanoparticles have a large surface area to volume ratio, and hence,
the additional surface area turns out to be an appropriate place for hydration products to precipitate.
Nanoparticles enable the formation of a bond between itself and C-S-H gels. As a result, the strength
can be accordingly improved. However, there is also an undesirable effect due to the large ratio of
Figure 1.
The scheme of installation to the photocatalytic removal of NOx (S—the source of pollution;
M—mass flower; H—humidifier; R—photocatalytic reactor with irradiation source; A—NOx analyzer).
The studied plate of cement mortar (one at dimensions of 80
×
40
×
10 mm
3
) was placed in the
central part of a cylindrical reactor (Pyrex glass; Ø
×
H=9
×
32 cm
2
), and the reactor was tightly closed.
The NO was diluted with humidified synthetic air in a ratio of 1:1. The oxygen and water molecules
were necessary for the formation of oxidative species, which are essential in the photocatalytic reactions.
The polluted air flowed through the reactor continuously with a rate of 500 cm
3
/min. At the beginning
of the process, the dark conditions were maintained until NO concentration reached equilibrium
(about 1 ppm during about 35 min). Then, the UV lamps were turned on for 30 min. The irradiation
sources surrounded the reactor and were characterized by the cumulative intensity of 100 W/m
2
UV
and 4 W/m
2
VIS. The temperature of the whole system was stable at the level of 22
◦
C by using
a thermostatic chamber. The NO and NO
2
concentrations were continuously measured in the outlet of
the reactor using chemiluminescent NOx analyzer (T200, Teledyne). All measurements were repeated
three times, and errors were 2%.
3. Results
3.1. Compressive and Flexural Strength
The compressive and the flexural strength of pure cement and cement with the addition of 1, 3 and
5 wt % TiO
2
/N specimens were measured. The obtained results are presented in Figure 2a,b. As it can
be seen in Figure 2a, the value of the compressive strength of unmodified cement amounted to 53 MPa
(red line), while the addition of 1, 3, and 5 wt % of TiO
2
/N increased the compressive strength of the
specimens in all cases. The highest value of the compressive strength was observed for specimens
with 1 wt % of TiO
2
/N and amounted to 57.4 MPa. The lowest increase of the compressive strength
was found for a specimen with the addition of 5 wt % of TiO
2
/N. Similar behaviour occurred during
the flexural strength measurements. As can be seen in Figure 2b, the value of the flexural strength of
unmodified cement amounted to 6.92 MPa (red line). Analogous as in the case of the compressive
strength, the addition of 1, 3 and 5 wt % of TiO
2
/N increased the flexural strength of the specimens in
all cases. The highest value of the flexural strength was observed for a specimen with 1 wt % of TiO
2
/N
and amounted to 7.60 MPa. The lowest increase of flexural strength was obtained using specimen with
addition of 5 wt % of TiO2/N.
The mechanical properties (the compressive and the flexural strength) of cement strongly depend
on the amount of used titanium dioxide. Wang et al. [
33
] discovered that with the incorporation of
TiO
2
nanoparticles, the strength firstly showed a fast increase compared with the ordinary mortar until
the dosage of TiO
2
nanoparticles reached up to 2 wt %, and then the rate of this increase slowed down.
The strength of the cement mortar is closely related to the amount of ettringite and C-S-H gels, and the
existence of nanoparticles facilitates the cement hydration, thereby producing more hydration products.
In addition to the filler property of nanoparticles to fill the pores in C-S-H gels, it is well known that
nanoparticles have a large surface area to volume ratio, and hence, the additional surface area turns out
to be an appropriate place for hydration products to precipitate. Nanoparticles enable the formation of
a bond between itself and C-S-H gels. As a result, the strength can be accordingly improved. However,
there is also an undesirable effect due to the large ratio of surface area to volume, since nanoparticles
Materials 2019,12, 3756 6 of 12
can glue together, many nanoparticle clusters, and the strength that can be generated is very weak,
leading to a heterogeneous microstructure.
Materials 2019, 12, x FOR PEER REVIEW 6 of 12
surface area to volume, since nanoparticles can glue together, many nanoparticle clusters, and the
strength that can be generated is very weak, leading to a heterogeneous microstructure.
Beyond 3 wt % nano-TiO2, the cementing system seems to be saturated, and the poor dispersion
of the nanoparticles generated by their high surface area may create weak zones in the system. In
addition, it could also enhance the particle packing density of the blended cement by filling up the
nanopores and reducing both the larger pores as well as the overall porosity of the mix. This
decreased the total specific volume of the pores; the refinement of the pore structure when up to 3 wt
% nano-TiO2 is used as a partial replacement of cement was also reported by Praveenkumar et al. [34]
and Nazari and Riahi [35,36].
(a)
(b)
Figure 2. (a) Compressive and (b) flexural strength of CEM I 42.5 N with the addition of 1, 3, and 5
wt % of photocatalyst TiO2/N. In the red line, the compressive strength (53 MPa) and flexural strength
(6.92 MPa) of pure CEM I 42.5 was presented.
3.2. Setting Time
The initial and the final setting time of tested specimens is presented in Table 3. As can be seen,
with the increasing addition of TiO2/N to cement, the initial setting time decreased. In the case of the
specimens modified by the addition of 5 wt % of TiO2/N, the initial setting time was 40 minutes faster
than that for unmodified cement. The same behavior was observed for the final setting time. With an
increasing addition of TiO2/N to cement, the final setting time decreased. Specimens of cement with
an addition of 5 wt % of TiO2/N showed a final setting time that was about 57 minutes faster in
comparison to the unmodified cement. A similar observation was made by Hernández-Rodríguez et
al. [37]; they added commercial TiO2 P25 to CEM I 52.5 R and the results showed that the
photocatalysts act as a setting accelerator.
Table 3. The values of initial and final setting time of CEM I and CEM I with the addition of 1, 3, and
5 wt % of TiO2/N photocatalysts.
Samples
The initial setting time [min]
The final setting time
[min]
CEM I 42.5N
218
305
CEM + 1 wt %TiO2/N
217
310
CEM + 3 wt %TiO2/N
207
275
CEM + 5 wt %TiO2/N
178
248
3.3. Hydration heat
In Figure 3, the isothermal calorimetry results of unmodified cement and cement modified by
the addition of 1, 3, and 5 wt % of TiO2/N photocatalysts were presented.
Figure 2.
(
a
) Compressive and (
b
) flexural strength of CEM I 42.5 N with the addition of 1, 3, and 5 wt
% of photocatalyst TiO
2
/N. In the red line, the compressive strength (53 MPa) and flexural strength
(6.92 MPa) of pure CEM I 42.5 was presented.
Beyond 3 wt % nano-TiO
2
, the cementing system seems to be saturated, and the poor dispersion of
the nanoparticles generated by their high surface area may create weak zones in the system. In addition,
it could also enhance the particle packing density of the blended cement by filling up the nanopores
and reducing both the larger pores as well as the overall porosity of the mix. This decreased the total
specific volume of the pores; the refinement of the pore structure when up to 3 wt % nano-TiO
2
is
used as a partial replacement of cement was also reported by Praveenkumar et al. [
34
] and Nazari and
Riahi [35,36].
3.2. Setting Time
The initial and the final setting time of tested specimens is presented in Table 3. As can be seen,
with the increasing addition of TiO2/N to cement, the initial setting time decreased. In the case of the
specimens modified by the addition of 5 wt % of TiO
2
/N, the initial setting time was 40 min faster
than that for unmodified cement. The same behavior was observed for the final setting time. With an
increasing addition of TiO
2
/N to cement, the final setting time decreased. Specimens of cement with an
addition of 5 wt % of TiO
2
/N showed a final setting time that was about 57 min faster in comparison to
the unmodified cement. A similar observation was made by Hern
á
ndez-Rodr
í
guez et al. [
37
]; they
added commercial TiO
2
P25 to CEM I 52.5 R and the results showed that the photocatalysts act as
a setting accelerator.
Table 3.
The values of initial and final setting time of CEM I and CEM I with the addition of 1, 3, and
5 wt % of TiO2/N photocatalysts.
Samples The Initial Setting Time [min] The Final Setting Time [min]
CEM I 42.5N 218 305
CEM +1wt %TiO2/N 217 310
CEM +3wt %TiO2/N 207 275
CEM +5wt %TiO2/N 178 248
3.3. Hydration Heat
In Figure 3, the isothermal calorimetry results of unmodified cement and cement modified by the
addition of 1, 3, and 5 wt % of TiO2/N photocatalysts were presented.
Materials 2019,12, 3756 7 of 12
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According to the literature, there are five stages of heat for a typical Portland cement [38,39]. The
addition of modified titanium dioxide into the cement influences hydration heat; the paste with the
addition of TiO2/N showed less heat generated up to 20 h compared to the unmodified cement.
Figure 3. Isothermal calorimetry results for cement modified by the addition of 1, 3, and 5 wt % of
TiO2/N to deionized water at a water to binder ratio (w/b) = 0.4.
3.4. Zeta Potential Measurements
The average value of zeta potential amounted to –5.01 mV for unmodified cement mortar and –
4.90 mV, –4.69 mV and –5.94 mV for cement mortars modified by the addition of 1, 3, and 5 wt % of
TiO2/N, respectively.
It is worth pointing out that TiO2 photocatalysts are characterized by a negative charge in high pH
medium. In our previous work [40], it was proven that the point of zero charge of TiO2/N is about 5.8.
Namely, the TiO2/N surface appeared to be positively charged at pH <5.8, whereas it was negatively
charged at pH >5.8. The application of TiO2/N with highly alkaline cement resulted in the presence of a
negatively charged form of TiO2/N particles.
Zingg et al. [41] concluded that the phases C3S and C-S-H are positively charged, whereas the
ettringite is negatively charged. During the initial stage of cement hydration, the aluminate reacts with
water and sulfate, forming a gel-like material (ettringite) surrounding the cement grains. The negative
values of zeta potential at the beginning of the hydration process confirmed it.
3.5. NOx Decomposition
In Figure 4, the photocatalytic activity of unmodified and modified cements is presented. The
activity of obtained materials during NO removal was tested. The mechanism of photocatalytic NO
removal is as follows [42]. Initially, active oxidizing groups are generated at the TiO2 surface
(reactions 1–3):
O2 + e- → O2-
(1)
OH- + h+ → ·OH
(2)
H+ + O2– → HO2.
(3)
The action of these moieties on NO molecules leads to their oxidation to the form of NO2,
followed by the formation of nitric(III) and (V) acids (reactions 4–6):
NO + HO2·
→ NO2 + ·OH
(4)
0 6 12 18 24 30 36 42 48
Amount of heat evolved
[mWatts/g]
time [h]
CEM I+5%TiO2/N
CEM I
CEM+1%TiO2/N
CEM I+3%TiO2/N
CEM I + 1%TiO2/N
CEM I
CEM I + 5%TiO2/N
CEM I + 3%TiO2/N
Figure 3.
Isothermal calorimetry results for cement modified by the addition of 1, 3, and 5 wt % of
TiO2/N to deionized water at a water to binder ratio (w/b) =0.4.
According to the literature, there are five stages of heat for a typical Portland cement [
38
,
39
].
The addition of modified titanium dioxide into the cement influences hydration heat; the paste with
the addition of TiO2/N showed less heat generated up to 20 h compared to the unmodified cement.
3.4. Zeta Potential Measurements
The average value of zeta potential amounted to
−
5.01 mV for unmodified cement mortar and
−
4.90 mV,
−
4.69 mV and
−
5.94 mV for cement mortars modified by the addition of 1, 3, and 5 wt % of
TiO2/N, respectively.
It is worth pointing out that TiO
2
photocatalysts are characterized by a negative charge in high pH
medium. In our previous work [
40
], it was proven that the point of zero charge of TiO
2
/N is about 5.8.
Namely, the TiO
2
/N surface appeared to be positively charged at pH <5.8, whereas it was negatively
charged at pH >5.8. The application of TiO
2
/N with highly alkaline cement resulted in the presence of
a negatively charged form of TiO2/N particles.
Zingg et al. [
41
] concluded that the phases C
3
S and C-S-H are positively charged, whereas the
ettringite is negatively charged. During the initial stage of cement hydration, the aluminate reacts with
water and sulfate, forming a gel-like material (ettringite) surrounding the cement grains. The negative
values of zeta potential at the beginning of the hydration process confirmed it.
3.5. NOx Decomposition
In Figure 4, the photocatalytic activity of unmodified and modified cements is presented.
The activity of obtained materials during NO removal was tested. The mechanism of photocatalytic
NO removal is as follows [
42
]. Initially, active oxidizing groups are generated at the TiO
2
surface
(reactions 1–3):
O2+e−→O2
−(1)
OH−+h+→ ·OH (2)
H++O2
−→HO2. (3)
The action of these moieties on NO molecules leads to their oxidation to the form of NO
2
, followed
by the formation of nitric(III) and (V) acids (reactions 4–6):
NO +HO2
−→NO2+·OH (4)
Materials 2019,12, 3756 8 of 12
NO2+·OH →HNO3(5)
NO +·OH →HNO2. (6)
In Figure 4a, the decreasing of NOx concentration [ppm] is presented. During the first 40 min,
the equilibrium of NO was obtained. After 40 min, the UV light was switch on, and it is possible
to observe that the NOx concertation decreased. The irradiation takes 30 min, and after this time,
the light was switched off. Figure 4b presents the NOx degradation in percent after 30 min of UV light
irradiation. The reference sample, pure CEM I, showed the removal of NOx on the level of about 6.3%.
The same observation concerning the blank sample was presented by Xu et al. [
43
]. They found that
using reference cement composites without any TiO
2
, the NOx concentration slowly decreased by 6%
during 15 min of irradiation. It is worth pointing out that in our studies, the photolysis of tested gas
amounted to 1.3% under the same conditions and the same irradiation source. The application of
nitrogen-modified TiO
2
in cement mortars involved the degradation of NOx on the photocatalytic
path, which can be observed as the unambiguous decrease of NOx concentration directly after turning
on the irradiation. The increase of TiO
2
/N loading in cement matrix caused the increase of the NOx
degradation rate from 14.2% for CEM I +1 wt %TiO
2
/N to 22.9% for CEM I +5 wt %TiO
2
/N. Apart
from the influence of the photocatalyst dose in the cement matrix, the accessible surface area of the
photocatalyst is essential for the photocatalytic effectiveness [
10
]. Therefore, we did not observe
a proportional increase of NOx degradation rate with the higher TiO
2
/N loading. However, it appeared
that the nitrogen-modified photocatalyst might be used as an additive to cement materials to increase
its air purification properties. Moreover, in Table 4, the NO removal and NO
2
formation during the
photocatalytic oxidation of NO are presented.
Materials 2019, 12, x; doi: FOR PEER REVIEW www.mdpi.com/journal/materials
Table 4. The NO removal and NO2 creation during NO photooxidation with cement modified by
TiO2/N.
Sample NO removal [ppm] NO
2
formation [ppm] NOx removal [ppm]
Photolysis 0.023 0.013 0.010
CEM I 0.057 0.009 0.048
CEM I + 1% TiO
2
/N 0.141 0.030 0.111
CEM I + 3% TiO
2
/N 0.179 0.025 0.154
CEM I + 5% TiO
2
/N 0.211 0.032 0.179
In Table 5, the initial photodegradation rates are presented. It was calculated 5 minutes after
switching on the UV light. This value was calculated as µg of NO removal, NO
2
creation, and NO
x
total removal on the surface of modified cement plates [cm
2
] during the time of UV light irradiation
[h]. As it can be seen, the highest vales of NO removal, NO
2
creation, and NO
x
total removal were
when the cement was modified by the addition of 5 wt % of TiO
2
/N.
The similar results of NOx photocatalytic degradation on cement materials were observed by
other authors as well. It was reported [13] that 5% TiO
2
replacement by the mass of cement in cement
pastes allowed decreasing the NO concentration from 1 ppm to about 0.7 ppm. The results were
calculated after 3 hours of exposure to UV irradiation, because it was the necessary time to achieve
the relative stasis in NO concentration. Jimenez-Relinque et al. [21] applied 2% of commercial TiO
2
with different types of cement in normalized mortars. NO gas diluted in the air was used as model
pollutant with an initial concentration of 1 ppm ± 50 ppb. After 1 hour of UV irradiation, they
obtained NO photocatalytic degradation on the level of 15%–30% and NOx removal in the range of
18–25%, depending on the applied cement type.
(a)
(b)
Figure 4. (a) Graph of NO
x
[ppm] decomposition and (b) NO
x
degradation [%] on CEM I samples,
and cements modified by the addition of 1, 3, and 5 wt % of TiO
2
/N under UV light irradiation
In Figure 5, the lifetime of tested modified cement plates was presented. As it can be seen, there
was no decrease in the photocatalytic activity of the modified cement plates. NO removal and total
NO
x
removal are on the same level. There are only small differences between NO and NO
x
concentration, and this behavior suggests that with time (increasing the number of cycles), more NO
2
is produced.
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0 10203040506070
8
NOx [ppm]
Irradiation time [min]
CEM I
CEM I + 1% TiO₂/N
CEM I + 3% TiO₂/N
CEM I + 5% TiO₂/N
irradiation
Figure 4.
(
a
) Graph of NOx [ppm] decomposition and (
b
) NOx degradation [%] on CEM I samples,
and cements modified by the addition of 1, 3, and 5 wt % of TiO2/N under UV light irradiation.
Table 4.
The NO removal and NO
2
creation during NO photooxidation with cement modified by
TiO2/N.
Sample NO Removal [ppm] NO2Formation [ppm] NOx Removal [ppm]
Photolysis 0.023 0.013 0.010
CEM I 0.057 0.009 0.048
CEM I +1% TiO2/N 0.141 0.030 0.111
CEM I +3% TiO2/N 0.179 0.025 0.154
CEM I +5% TiO2/N 0.211 0.032 0.179
Materials 2019,12, 3756 9 of 12
In Table 5, the initial photodegradation rates are presented. It was calculated 5 min after switching
on the UV light. This value was calculated as
µ
g of NO removal, NO
2
creation, and NOx total removal
on the surface of modified cement plates [cm
2
] during the time of UV light irradiation [h]. As it can be
seen, the highest vales of NO removal, NO
2
creation, and NOx total removal were when the cement
was modified by the addition of 5 wt % of TiO2/N.
Table 5.
The initial photodegradation rate for modified cement during NO removal, NO creation, and
NOx total removal.
Sample NO Removal
[µg/cm2/h]
NO2Formation
[µg/cm2/h]
NOx Total Removal
[µg/cm2/h]
Photolysis 0.289 0.038 0.251
CEM I 0.315 0.091 0.224
CEM I +1% TiO2/N 2.530 0.622 1.908
CEM I +3% TiO2/N 3.145 0.496 2.649
CEM I +5% TiO2/N 3.403 0.651 2.752
The similar results of NOx photocatalytic degradation on cement materials were observed by other
authors as well. It was reported [
13
] that 5% TiO
2
replacement by the mass of cement in cement pastes
allowed decreasing the NO concentration from 1 ppm to about 0.7 ppm. The results were calculated
after 3 h of exposure to UV irradiation, because it was the necessary time to achieve the relative stasis
in NO concentration. Jimenez-Relinque et al. [
21
] applied 2% of commercial TiO
2
with different types
of cement in normalized mortars. NO gas diluted in the air was used as model pollutant with an
initial concentration of 1 ppm
±
50 ppb. After 1 h of UV irradiation, they obtained NO photocatalytic
degradation on the level of 15–30% and NOx removal in the range of 18–25%, depending on the applied
cement type.
In Figure 5, the lifetime of tested modified cement plates was presented. As it can be seen, there
was no decrease in the photocatalytic activity of the modified cement plates. NO removal and total NOx
removal are on the same level. There are only small differences between NO and NOx concentration,
and this behavior suggests that with time (increasing the number of cycles), more NO2is produced.
Materials 2019, 12, x FOR PEER REVIEW 2 of 2
Figure 5. The lifetime of CEM I + 5 wt %TiO
2
/N under six cycles of irradiation .
4. Conclusions
The nitrogen-modified titanium dioxide (TiO
2
/N) may be used as an additive to cement mortars
to produce the cement with photocatalytic properties. All photocatalytic samples degraded regarding
the NO
x
concentration during irradiation time, achieving a higher NO
x
removal rate with a higher
TiO
2
/N dosage in cement materials. The addition of TiO
2
/N up to 5 wt % into the cement mortar did
not decrease the mechanical properties but even slightly increased the compressive and the flexural
strength.
Nanoparticles of TiO
2
/N appeared to have an influence on the cement hydration. Acceleration
of the initial and the final setting time indicated that the photocatalytic particles might act as seeds
for the precipitation of C-S-H. The addition of 5 wt % of TiO
2
/N into the cement mortar shortened the
setting time by about 57 minutes. Moreover, the presence of TiO
2
/N in the cement matrix caused less
heat to be generated during the hydration process.
The negative charge of high solid cement mortar, which was determined based on the zeta
potential, was amplified using a higher amount of TiO
2
/N photocatalyst, from –4.3 mV to –5.5 mV at
the beginning of hydration. High TiO
2
/N loading in the cement matrix resulted in more negative zeta
potential, because the very fine TiO
2
is negatively charged at a high pH.
Author Contributions: Conceptualization, M.J. and S.M.; methodology, M.J. and S.M.; investigation, M.J., S.M.,
and K.Z.; writing—original draft preparation, M.J., S.M., and K.Z.; photocatalyst preparation, A.W.M. writing—
review and editing, M.J., S.M., K.Z., and D.S.; supervision, M.J and D.S.; funding acquisition, M.J. and E.K-N.
Funding: This research was funded by the Polish National Agency for Academic Exchange within the Bekker
program.
Conflicts of Interest: The authors declare no conflict of interest.
Figure 5. The lifetime of CEM I +5 wt %TiO2/N under six cycles of irradiation.
4. Conclusions
The nitrogen-modified titanium dioxide (TiO
2
/N) may be used as an additive to cement mortars
to produce the cement with photocatalytic properties. All photocatalytic samples degraded regarding
Materials 2019,12, 3756 10 of 12
the NOx concentration during irradiation time, achieving a higher NOx removal rate with a higher
TiO
2
/N dosage in cement materials. The addition of TiO
2
/N up to 5 wt % into the cement mortar
did not decrease the mechanical properties but even slightly increased the compressive and the
flexural strength.
Nanoparticles of TiO
2
/N appeared to have an influence on the cement hydration. Acceleration of
the initial and the final setting time indicated that the photocatalytic particles might act as seeds for the
precipitation of C-S-H. The addition of 5 wt % of TiO
2
/N into the cement mortar shortened the setting
time by about 57 min. Moreover, the presence of TiO
2
/N in the cement matrix caused less heat to be
generated during the hydration process.
The negative charge of high solid cement mortar, which was determined based on the zeta
potential, was amplified using a higher amount of TiO
2
/N photocatalyst, from –4.3 mV to –5.5 mV at
the beginning of hydration. High TiO
2
/N loading in the cement matrix resulted in more negative zeta
potential, because the very fine TiO2is negatively charged at a high pH.
Author Contributions:
Conceptualization, M.J. and S.M.; methodology, M.J. and S.M.; investigation, M.J., S.M. and
K.Z.; writing—original draft preparation, M.J., S.M. and K.Z.; photocatalyst preparation, A.W.M. writing—review
and editing, M.J., S.M., K.Z. and D.S.; supervision, M.J and D.S.; funding acquisition, M.J. and E.K.-N.
Funding:
This research was funded by the Polish National Agency for Academic Exchange within the
Bekker program.
Conflicts of Interest: The authors declare no conflict of interest.
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2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
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