Sustainable antibacterial performance in cementitious systems using Ag-modified TiO₂ compounds

Abstract

The increasing focus on health and hygiene has expanded the need for protective measures on material surfaces. In this regard, developing antibacterial concrete and mortar capable of eliminating viruses and bacteria is crucial. However, a key challenge in cementitious systems is the inability to maintain long-term antibacterial effectiveness when titanium dioxide (TiO₂) is used as the sole photocatalyst. To address this limitation, this study aimed to enhance the antibacterial properties of TiO₂ by modifying it with silver (Ag) using a planetary ball mill. Concrete and mortar samples incorporating the modified material were produced, and their antibacterial performance was evaluated over both short and long durations. So the originality of this study was to evaluate the performance of cementitious system surfaces against repeated bacterial attacks using a specific mechanical alloying method in the modification of TiO₂ with Ag. Additionally, the modified products were characterized through X-ray diffraction (XRD), fourier transformed infrared spectroscopy (FTIR), scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) imaging, grain size analysis, and band gap energy measurements. The impact of the components on antibacterial efficiency was statistically analyzed using analysis of covariance (ANCOVA). The results demonstrated that Ag-containing samples achieved a 100% bacterial killing rate in all experimental replicates. These findings confirm that Ag-TiO₂ alloying was successfully achieved via planetary ball milling, providing concrete with sustained antibacterial properties in both early and long-term applications.

Graphical abstract

Keywords

concrete / mortar / titanium dioxide / silver / antibacterial performance

1 Introduction

Concrete stands out as one of the most widely used materials in the ever-increasing global demand for construction materials. Researchers have recently developed many strategies to meet the need for clean, green, functional and durable concrete, especially in the wake of COVID-19. One of the important strategies is the use of photocatalytic and antimicrobial agents to functionalize traditional concrete. In this direction, antibacterial properties can be obtained with a photocatalyst incorporated into the structure of the material [1,2]. Microorganisms such as viruses and bacteria not only threaten human health, but also significantly shorten the life of the material on which they grow [3]. It is therefore very important that antibacterial building materials maintain their performance over time [4–6]. In this study, the idea of modifying titanium dioxide (TiO₂), which is often used as a semiconductor in concrete, with metals such as silver (Ag), which are known for their excellent antibacterial activity, has gained importance in order to make the antibacterial performance permanent in the long-term.

Antibacterial concrete is gaining increasing attention due to its potential to enhance durability and resistance to microbial deterioration [7]. By incorporating inorganic or organic antimicrobial agents, such concrete systems offer improved longevity; however, challenges related to retention, dispersion, environmental impact, and microbial resistance necessitate further investigation and real-world validation for broader implementation [8]. In parallel, the development of smart and multifunctional antiviral civil engineering materials through nanotechnology and biotechnology holds promise for strengthening infrastructure resilience against viral threats [9]. Nonetheless, issues concerning material compatibility, dispersion, safety, and practical application require interdisciplinary research and technological advancements [10]. Additionally, there are studies showing that the incorporation of nano TiO₂ improves microstructural properties by inhibiting microbial growth [11].

Nano TiO₂ has a wide range of applications. One study reported that modified TiO₂ significantly improved the biological, physical and chemical corrosion resistance of wastewater infrastructure in reactive powder concrete. By inhibiting microbial growth, improving compaction, reducing deterioration and increasing mechanical strength, it is emerging as a promising material for the sewerage, water treatment and antimicrobial infrastructure applications [12]. In addition, nano titanium dioxide plays an important role in the development of cementitious composites by optimising hydration and improving microstructure, thus enhancing their mechanical, durability and functional properties [13]. However, challenges related to dispersion, production, environmental impact and large-scale application should be addressed by further research to fully unlock its potential in high-performance and multifunctional building materials [14]. In addition, nano-SiO₂-coated TiO₂ contributes to the mechanical strength and microstructural improvement of reactive powder concrete by accelerating cement hydration, reducing calcium hydroxide (CH) crystal formation through pozzolanic reactions and improving dispersion. In particular, an optimum nano-SiO₂ coated TiO₂ content of 3% has been identified to achieve maximum strength and durability [15].

TiO₂ is the most widely used semiconductor material as a photocatalyst in photocatalytic reactions [16]. TiO₂ occurs in nature in three different phases. These are anatase, rutile and brookite [17,18]. The most photocatalytic active of these phases is the anatase phase. However, the available bandgap energy can only respond to 4%–5% of the total solar spectrum [19]. The need to utilize more of the solar spectrum has led to the development of TiO₂.

Ag is one of the most effective metals in terms of antibacterial effect [20]. It is known to be a very good antibacterial material, especially in ionic form or in the form of oxidized nanoparticles [21]. To briefly explain the antibacterial effect mechanism of Ag, ions tear the cell membranes of microorganisms with the effect of the positive electric charge they carry. By adhering to deoxyribonucleic acid (DNA) and ribonucleic acid (RNA)’s in the cell, it prevents the proliferation of these microorganisms [22]. Recently, Ag has been frequently used in modifying TiO₂ to obtain an antibacterial surface due to its effect on disrupting the bacterial structure [23].

There are studies in the literature that optimize the reduction of E. coli and S. aureus strains in Ag-TiO₂ reinforced cementitious systems over time, as well as studies that investigate the effectiveness of unmodified TiO₂ in completely reducing bacteria in cementitious systems by 100% in the short and long-term. There is a very limited number of studies investigating the effect of the mechanical alloying method selected as the modification method on the antibacterial performance. In this study, the powder materials we produced by modification with a specific protocol were mixed directly into the cementitious system structure. In addition, unlike the literature, antibacterial performance measurements were performed on the same surface at different time intervals and different repetitions to determine the persistence of the antibacterial properties. In addition, in this study, antibacterial performance tests were performed on cementitious systems exposed to S. aureus bacteria in the range of 10–60 min to avoid early contamination. Unlike the antibacterial activity studies with only TiO₂, it was aimed to increase the antibacterial performance against repeated effects in the short and long-term by reducing the band gap energy of TiO₂. In this study, we aimed to enhance the cementitious system’s antibacterial performance by functionalizing them with Ag-modified TiO₂. Initially, TiO₂ was modified with different amounts of Ag through a planetary ball mill during the alloying process. Concrete and mortar samples were produced by adding the mixture of Ag-modified TiO₂, with varying concentrations based on the amount of cement used. The surface properties, morphological changes, elemental analysis, and functional groups were studied by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and fourier transformed infrared spectroscopy (FTIR), respectively. The antibacterial activities were evaluated against both Gram-negative and Gram-positive bacteria. Furthermore, the sustainability of the antibacterial performance was assessed by repeating the antibacterial tests after 6 months.

2 Experimental study

2.1 Materials

Cement (CEM) I 42.5R was used as the binder in the mixture. Chemical and physical properties of cement are given in Tab.1.

TiO₂ powder was supplied from the Refsan Company (Kütahya, Türkiye), and Ag powder was obtained from the Nanokar Company (Istanbul, Türkiye). The properties of these powders are shown in Tab.2.

The crushed sand (0–4 mm) and crushed stones (4–8 and 8–16 mm) were used in this study, and city mains water was utilized for the mixing process. The water, cement, and sand amounts in concrete mixtures are 1%, 0.5%, and 3% by weight, respectively. The properties of the mixing water are detailed in Tab.3.

2.2 Method and tests

Planetary ball mill alloying method was used to modify TiO₂ with Ag. Alloying from powders with different structures is an efficient method for producing materials with new structures [24]. The planetary ball mill alloying method offers practical solutions for the production of powders to be used in cementitious systems [25]. High energy is generated inside the containers in the planetary ball mill due to the collision movements of the balls against each other and the container surfaces [26]. With this high energy, the powders in the container; It undergoes elastic and plastic deformation with cold-boiling, breaking and annealing processes. During the alloying process. Many variables such as alloying conditions, alloying speed and time, temperature, chemical structure of the powders used and the properties of the grinding tools are effective [27]. These variables are directly effective in the best performance of alloying processes [28,29].

The alloying process was carried out in experimental studies using a planetary ball mill. When modifying TiO₂ with Ag, Ag was loaded on TiO₂ at 5% and 15% of the TiO₂ ratio. The study used a planetary ball mill consisting of two steel vials. Each vial was filled with the same number of balls of the same diameter. Three different diameter balls (8, 14, 20 mm) were used for alloying. The three different diameters were chosen to increase the effective collision amounts between the balls and the powders. The powders, balls and planetary ball mill used in the alloying process are shown in Fig.1.

The weights of TiO₂ and Ag powders to be added to the vials were calculated after determining the ball weights in the vials. Following the preliminary studies, the weight ratio of ball/powder was selected as 10:1. Balls with a total weight of 1082.78 g were added to one vial, and 1082.29 g were added to the other, measured with a precision scale. The planetary ball mill was operated at 200 r/min for 4 h. The weights of the balls and powders are listed in Tab.4.

Concrete (7 cm × 7 cm × 7 cm) and mortar (4 cm × 4 cm × 16 cm ) samples were produced for the antibacterial test by adding a mixture of Ag-modified TiO₂ with various concentrations depending on the cement amount (0%, 5%, and 15%). Samples were coded as 0T0Ag, 5T0Ag, 5T5Ag, and 5T15Ag, respectively, as summarized in Tab.5.

For concrete production, a concrete mix was prepared using the mix proportions given in Tab.6 At least 3 samples of each mix were taken for each test. For mortar production, a mortar mix was prepared using the proportions in Tab.7. The specimens were compacted using a table vibrator. After one day, the specimens were demolded and kept in a standard curing environment, (23 + 2) °C, in lime-saturated water. The tests were carried out on concretes with 28 d strength.

To investigate how the alloying process in the planetary ball mill affected the grain size of TiO₂, grain size analyses of samples were performed using Mastersizer 2000 (Malvern Instruments Ltd., Worcestershire, UK), which has laser diffraction properties [30]. Laser diffraction grain size analysis is based on the inverse relationship between the size of the grains and the angle of refraction of the incident rays on the grains [31]. This device can measure grain sizes ranging from 20 nm to 2000 μm and operate within a temperature range of 10 to 35 °C. All measurements were carried out at room temperature.

To determine the functional groups of the powders before and after the modifications, FTIR analysis was carried out on the samples in the wave number range of 350–4000 cm⁻¹ using a Perkin Elmer Spectrum Two model FTIR spectrometer.

The surface properties and morphological changes were characterized by scanning electron microscope (SEM) and Energy-dispersive X-ray spectroscopy (EDS). Hitachi Regulus 8230 SEM was used for the images at room temperature. It has a cold field emission electron gun and an image resolution of 0.9 nm at 1 kV.

In this study, the samples were kept in standard laboratory conditions (25 °C, 50% relative humidity) for a certain period of time before the test to ensure surface stabilization. Cement-based materials have an alkaline surface due to their hydration process. To increase the applicability of ASTM E2149 and to correctly evaluate the interaction of bacterial activity with the surface, the surface pH was measured before the test and the tests were performed after the stabilization process was completed. In addition, since ASTM E2149 is recommended to be used mainly on neutral or slightly alkaline surfaces, the samples were tested by keeping them in deionized water for a certain period of time in order to minimize the effect of high alkalinity. The test solution and samples were prepared to be close to the 1:50 (g/mL) ratio specified in ASTM E2149 to ensure homogeneous contact of the bacterial solution with the surface.

For this study, one of the most important objectives is to investigate the short- and long-term antibacterial activity of modified concrete surfaces. The fast killing of bacteria is critical to prevent possible contamination from the concrete surface, while long-term antibacterial performance is essential for the material to retain its antibacterial effect. To examine the performance of the antibacterial effect over short and long periods, samples containing TiO₂ alloyed with Ag, a metal with a high antibacterial effect, were tested repeatedly at different times. The antibacterial activities of samples were evaluated according to the ASTM E2149 standard test method against Gram-negative Escherichia coli (ATCC 25922) and Gram-positive Staphylococcus aureus (ATCC 6538) bacteria. The bacterial concentration of about 10⁵ CFU/mL was prepared, and samples were immersed in 50 mL bacterial suspension. They were placed in a wrist-action shaker and shaken at 37 °C for 24 h. After the incubation period, the number of viable bacteria colonies was counted, and the antibacterial activity was expressed as a percentage reduction of the bacteria. The reduction rate (R%) was calculated using the formula Eq. (1).

(1)\mathrm{R}(\mathrm{\%})=[(B-A)/B]\times 100,

where A is the number of bacteria recovered from the inoculated sample for 24 h, and B is the number of bacteria recovered from the inoculated sample at “0” contact time. After 6 months, the sustainability of the antibacterial performance was tested by repeating the same antibacterial procedures performed earlier.

3 Results and discussion

3.1 X-ray diffraction analysis

XRD analysis on TiO₂, 5T5Ag and 5T15Ag powders is shown in Fig.2. The XRD pattern in Fig.2 clearly shows the characteristic peaks of TiO₂. This study was carried out using the anatase phase of TiO₂. It has been observed in the literature that there are peaks at different angles in the XRD diffraction pattern for different phases and components of TiO₂. After modification, new Ti-Ag peaks appeared at 2θ = 39, 54.2, 70.3, and 86.5 for 5T5Ag and 5T15Ag samples. The results clearly proved that Ag is present in the 5T5Ag and 5T15Ag structures and the alloying method was successful in obtaining the desired amount of alloyed TiO₂. This can be explained as a modification resulting in a change in the bonding structure of TiO₂.

3.2 Grain size analysis

Grain size analysis was performed to classify the powders and ascertain their particle size distribution. Subsequently, the particle size distribution results were plotted on a graph for each sample, as shown in Fig.3. Based on the grain-size distribution curves, the graph indicated that 40%, 25%, and 15% of particle sizes were smaller than 1 µm for 5T0Ag, 5T5Ag, and 5T15Ag, respectively. Upon observing the grain size analysis of the modified products, it was observed that the sample 5T15Ag had the largest grain size. This phenomenon can explain how TiO₂ forms compounds with increased grain dimensions when alloyed successfully with Ag.

3.3 Fourier transformed infrared spectroscopy analysis

FTIR was utilized to identify the functional groups of the powders before and after modifications, and the resulting spectra are shown in Fig.4. TiO₂ exhibits prominently visible three distinct bands. The characteristic bands at 3414, 1624, and the range of 1000–400 cm⁻¹ correspond to the stretching and bending vibrations of the -OH groups of TiO₂ and vibrational stretching of the Ti-O-Ti groups, respectively [32–34]. In the FTIR spectrum, the stretching of metal-oxygen and metal-carbon appear around 600–500 and 1100–1000 cm⁻¹, respectively, and the exact positions of these peaks may vary depending on the specific Ag size [35–38]. For 5T0Ag, the sharp bands at 1430 (1), 1015 (2), 880 (3), and 670 (6) cm⁻¹, and small bands at 750 (4), 730 (5), and 465 (7) cm⁻¹ can be clearly observed in the spectrum. The bands between 880 and 400 cm⁻¹ can be attributed to the vibrational stretching of the TiO₂ groups. The lower intensive bands at 1430, 1015, and 880 cm⁻¹, the disappeared bands at 750, 730, and 670 cm⁻¹, and the new broadband at 670–465 cm⁻¹ were detected between the spectra of powders after modifying TiO₂ with Ag. Although these spectrum changes became more evident as the Ag concentration in the mixtures increased from 5% to 15%, unfortunately, due to the relatively weak IR absorbance of the Ag in the mixtures, few differences could be identified between the spectra of the powders. At the same time, these changes might have been weak due to the overlapping TiO₂ and Ag bands. Finally, FTIR spectra confirmed the presence of TiO₂ and Ag in powder mixtures.

3.4 Band gap analysis

An effective way to change the bandgap of anatase TiO₂ is to dope it with foreign elements. Metals such as Ag lead to lattice deformation and the formation of oxygen vacancies, creating an impurity state in the band gap of TiO₂, which narrows the band gap of TiO₂ and increases the absorption of visible light. By narrowing the band gap, TiO₂ can be excited by photon energy of lower wavelength. This increases the photocatalytic and antibacterial activity of TiO₂. Studies show that the bandgap energy of TiO₂ is approximately 3.2 eV. As shown in Fig.5, as a result of modification with silver in the planetary ball mill, the Ti-Ag band gap decreased to 2.9 eV as measured by UV spectroscopy. Thus, antibacterial performance can be achieved with lower wavelength photons.

3.5 Scanning electron microscope-energy dispersive X-ray spectroscopy analysis

The surface properties, morphological changes, and elemental analyses were characterized by SEM and EDS. The EDS analysis was performed on SEM micrographs magnified 500 ×, while the internal structure was analyzed using SEM micrographs magnified 5000 ×. The images and results of the elemental analysis are presented in Fig.6 and Tab.8, respectively. It was observed that despite the micro voids in the control concretes, the internal structure was filled, micro cracks were formed in the case of TiO₂ addition and the concretes containing modified TiO₂ consisted of smaller grained structures. It was observed that micro voids were present in the control specimens, but the macro voids were filled due to the hydration reactions in the internal structure. The studies reported that irregularly shaped aggregates, spheroidal and oblate spheroidal TiO₂ particles and Ag modified TiO₂ structures can be seen on the surface of the concretes by SEM analysis [39].

EDS analysis results on 0T0Ag, 5T0Ag and 5T15Ag concrete samples are shown in Tab.8. When the table is examined, it is seen that Ag and Ti elements are present on the sample surfaces containing Ag modified TiO₂. Considering that antibacterial reactions take place on the sample surfaces, the presence of Ti and Ag elements on the surface is very important. This situation also shows that the powder materials are homogenously alloyed with the planetary ball mill.

Fig.7 shows the SEM images of the mortar samples. When Fig.7 is analyzed, it can be seen that the control mortars have more filled structures and relatively larger voids between them, while the filling in the microstructure increased in the case of TiO₂ addition. However, many microcracks were observed. It was observed that the internal structures of the mortars containing Ag-modified TiO₂ consisted of small grains and the gap size between them decreased. SEM images showed that the 5T15Ag mortar sample had the smallest pore size and the most homogeneous cell distribution among the mortar samples. In addition, the interconnected voids in this sample are smaller than those in the other samples.

Tab.9 shows the EDS analyses of the mortar samples. When the EDS analysis results were examined, Ti element was clearly seen on the surfaces containing TiO₂, and Ti and Ag elements were clearly seen on the surfaces containing modified TiO₂. This is very important for antibacterial performances on material surfaces.

3.6 Antibacterial activity analysis

Investigating the antibacterial activity and sustainability of concrete surfaces before and after modifications is one of the most critical aspects of this study. Antibacterial activity was evaluated according to the ASTM 2149 method to determine its effectiveness against S. aureus and E. coli. For a product to be considered effective against any bacteria, it must have a bacterial reduction of over 90%. Once this threshold is exceeded, it is accurate to claim that the product possesses antibacterial properties. The antibacterial activity tests were first performed against S. aureus; the results are depicted in Fig.8. At the starting time, the number of viable colonies of S. aureus was 3.5 × 10⁵ cfu/mL (log 5.55). 5T15Ag and 5T5Ag killed all bacteria quickly within 10 and 60 min, respectively, and test results demonstrated that the antibacterial activity increased with the increasing Ag content in the mixture. These results are unsurprising since Ag⁺ ions interact with the thiol groups in bacteria proteins, affecting the replication of DNA, and consequently, they have a strong biocidal effect on many bacteria [40–42]. Generally, Ag is an antibacterial agent with mechanisms of action similar against both Gram-negative and Gram-positive bacteria. These mechanisms include inhibiting cell walls, protein, and nucleic acid synthesis. Additionally, Ag is known to cause damage to the cell surface and disrupt the respiratory chain [43]. Ag has a positive charge, and fortunately, the bacteria cell wall naturally has a negative charge because of electron release caused by catalysis activity during cell respiration. The electrostatic attraction phenomenon between Ag and the bacteria cell membrane will help the released Ag⁺ ions directly penetrate the bacteria cell wall and easily attach to the cell membrane [44–46]. These ions have strong antibacterial properties and will interact with cell membrane and cell wall components. This is one of the crucial mechanisms of Ag toxicity toward bacteria [47]. However, 5T0Ag displayed surprisingly weak antibacterial activity (12.8%) after 60 min, while 0T0Ag had no antibacterial activity (36.2%), as expected. Here, 5T5Ag and 5T15Ag eventually indicated excellent antibacterial activity. The sustainability of the antibacterial performance was tested by repeating the same antibacterial procedures performed earlier. Remarkably, 5T5Ag and 5T15Ag showed superior performance against S. aureus even after 6 months, demonstrating ongoing sustainability [48].

The antibacterial activity tests were secondly performed against E. coli and the results are displayed in Fig.9. At the starting time, the bacteria concentration was 4.0 × 10⁵ cfu/ml. 5T5Ag and 5T15Ag killed all viable colonies rapidly within 3 h, respectively, mirroring the trend observed with S. aureus. The primary interaction of the Ag-TiO₂ with the bacteria is probably an electrostatic attraction between the concrete surface and positively charged regions of the extracellular domain of integral membrane proteins on the cell surface. The ionic structures penetrate the outer and inner bacterial membranes. Protrusions, pits, or holes in bacterial cell walls could be associated with internalized particles [49–53]. Minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) assays showed also that Ag works better against E. coli than S. aureus; Ag has a more potent antibacterial effect on Gram-negative bacteria [54]. One theory that supports this finding is that Gram-negative bacteria have thinner peptidoglycan cell walls, while Gram-positive bacteria have thicker cell walls [55,56]. The number of Ag⁺ ions that successfully penetrate the Gram-positive bacteria is small, which shows a strong interaction between Ag and Gram-negative bacteria [57]. Moreover, TiO₂ is known to have excellent photocatalytic performance, which can generate strong oxidizing power under certain conditions and continuously exhibit an antimicrobial effect when illuminated [58–61]. The average absorption range is less than 388 nm. This wavelength corresponds to the absorption threshold, exhibiting a photocatalytic activity [62]. The valence band of TiO₂ is composed of the orbitals of oxygen (O). In contrast, the conduction band comprises the 3D orbital, which results in a band gap with 3.2 eV of energy [63]. After TiO₂ suspensions are irradiated and activated, reactive oxygen species such as OH, {\mathrm{O}}_2^{-}, and H₂O₂ can be generated on the irradiated TiO₂ surface. Those ionic species may attack and decompose polyunsaturated phospholipids in bacteria [64]. However, 0T0Ag and 5T0Ag showed the lowest antibacterial efficiencies, with 45.0% and 44.8%, respectively after 6 h, similar to those in S. aureus. Samples containing TiO₂ exhibited surprisingly weak antibacterial activity against both bacteria. Considering sustainability, 5T5Ag and 5T15Ag still performed superior antibacterial activity against E. coli after 6 months. As a result, samples containing Ag-modified TiO₂ showed the highest antibacterial activity against both bacteria. These impressive findings demonstrate the remarkable potential for preventing bacteria’s rapid growth and spread.

Fig.10 shows the antibacterial performance of the mortar samples against E. coli bacteria. For any product to be antibacterial effective, it must have killed more than 90% of bacteria during antibacterial testing. When this threshold value is exceeded, it can be said that the product has antibacterial activity. The unmodified samples, such as 0T0Ag and 5T0Ag, had the lowest antibacterial effectiveness, with 3.75% and 42.5% bacterial reduction rates after 24 h. It is stated in the literature that the use of certain amounts of silver is important in terms of antibacterial effectiveness. Samples containing Ag-modified TiO₂ showed the best performance, and 5T5Ag and 5T15Ag killed all bacteria on the surface in 3 h. These findings are very significant in terms of preventing the early proliferation of bacteria on the building materials. To test the sustainability of the antibacterial performance, antibacterial experiments with the same samples were repeated against E. coli six months later. It can be easily realized that 5T5Ag and 5T15Ag still exhibited excellent performance even after a period of six months, and their sustainabilities have evidently appeared to be ongoing.

Fig.11 represents antibacterial activity results of mortar samples against S. aureus. The unmodified samples had a similar trend to E. coli in that 0T0Ag and 5T0Ag had the lowest antibacterial effectiveness, with 29.79% and 27.66% bacterial reduction rates after 24 h. However, 5T5Ag and 5T15Ag killed all S. aureus bacteria on the surface in 60 min. As the Ag amount increases, antibacterial activities will naturally be increased for both types of bacteria. Considering sustainability, 5T5Ag and 5T15Ag still showed superior performance against S. aureus even after a period of six months.

3.7 Analysis of covariance analysis

Tab.10 and Tab.11 shows the ANCOVA analysis performed on the data obtained with mortar and concrete samples against E. coli bacteria. When analyzing the table, it can be seen that TiO₂ has no contribution to the total variance in concrete samples, while the contribution of Ag to the total variance is very high. In mortar samples, the contribution of TiO₂ to the total variance is very small compared to Ag. This shows that Ag structures have much more effect on the antibacterial performance than TiO₂. While small PR (> F) values indicate that the components are significant, Ag was the component with the highest F value. This shows the importance of the effect factor of Ag.

Tab.12 and Tab.13 shows the ANCOVA analysis performed on the data obtained against S. aureus bacteria. Analysis of the table shows that Ag is highly effective against S. aureus bacteria in concrete. It can be seen that the variable time is at least as effective as Ag against S. aureus bacteria. The fact that the PR (> F) values of the variables are quite low shows that they are significant variables in the antibacterial activity. In mortar samples, TiO₂ was ineffective against S. aureus bacteria. In conclusion, both ANCOVA analyses showed that TiO₂ should be modified to achieve superior and sustainable antibacterial performance.

3.8 Literature comparison

Tab.14 shows the striking results obtained from the literature considering the scope of this study. It is determined that the results of the study are similar to the results found in the literature.

3.9 Cost analysis

In this part of the study, the effects of the materials used in the experimental studies on the m³ price of concrete were analyzed. The price increase resulting from the use of powdered materials such as TiO₂ and Ag, which were used in all the experimental studies, is shown in Tab.15.

4 Conclusions

1) In the grain size analysis carried out after modification, the 5T15Ag sample reached the highest grain size values. The formation of new compounds during the modification phase of TiO₂ with Ag increased the grain size. This revealed that the use of planetary ball mill is a very efficient method for alloying.

2) By FTIR analysis, after Ag loading on TiO₂, it was observed that the Ag atom had a chemical interaction with TiO₂, as the Ti-Ag vibration band as well as the main peaks of TiO₂ were observed.

3) In the antibacterial activity tests performed with S. aureus bacteria, it was observed that samples containing Ag-modified TiO₂ destroyed 100% of the bacteria in 10 min. Considering how important it is to destroy bacteria in the early periods in terms of preventing contagion, the importance of the modification process emerges once again. In the repetitions made after 3 months, a decrease in the performance of the 5T5Ag samples was observed in the 1 h measurements, while the 5T15Ag samples again showed a 100% effect.

4) In antibacterial activity tests performed with E. coli bacteria, samples containing Ag-modified TiO₂ reached 100% performance level in 1 h. In the repeats made after 3 months, it was seen that almost all samples showed performance decreases in 1 h measurement, while 5T15Ag samples gave effective performance after 6 h. As a result of antibacterial tests, it was revealed that TiO₂ should be modified for early and effective performance.

As a result of the analysis and experiments carried out on concrete samples, it was seen that TiO₂ should be modified with Ag to increase the effect of antibacterial performance in short and long periods. For this reason, it is recommended to use TiO₂ modified with Ag on concretes that may be exposed to the effects of bacteria and viruses. For further studies, it is recommended to compare the results using different modification methods.

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