Microstructural and photocatalytic characterization of cement-paste sol-gel synthesized titanium dioxide

Abstract

A route for the in paste synthesis of TiO₂ loaded cement is described. TiO₂ sols are blended with fresh cement paste as an alternative process to add photocatalytic functionality to cement. The modification of cement paste structure after the addition of TiO₂ sols is analyzed by XRD, SEM and TGA. As a particular microstructural feature, TiO₂ containing calcium silicate hydrate (C-H-S) particles are identified as networking centers of a C-S-H gel fiber matrix. The increase of the TiO₂ sol concentration induces a decrease of pore size and an increase in the specific surface area in the cement composites. The photocatalytic activity of the TiO₂/cement system is evaluated from the degradation of Methylene Blue (MB) under UV irradiation, monitored through the absorbance at 665 nm. The results show that, although TiO₂ phases reveal no long range order structure, the cement paste exothermal treatment in presence of hydrate products and alkaline conditions leads to a photocatalytic composite. Such new cement matrix may be twofold advantageous since it additionally promotes the formation of C-S-H gel, main determinant of cement mechanical properties.

Keywords

cement composites / photocatalytic TiO₂ / sol-gel / C-S-H gel / microstructure

Introduction

Hardened cement is essentially formed by a quasi amorphous calcium silicate hydrate (C-S-H gel) formed upon hydration of Ca₃SiO₅ and Ca₂SiO₄ [ 1]. In the classification proposed by Taylor [ 2] C-S-H gel is morphologically made up of two varieties: the inner C-S-H, developed as hydration shells around the original boundaries of the cement grains, and the outer C-S-H, formed in the originally water-filled space. Another hydration product that extensively deposits between the cement grains is crystalline calcium hydroxide (Ca(OH)₂, portlandite). These two hydrates of hardened cement paste determine the mechanical performance of the cement system at the macroscale.

Nanoscale modification is being extensively considered to enhance the performance of cementitious materials [ 3- 8]. Concrete is the second most widely used commodity in the world after water. Therefore, the utilization of technologies reducing the environmental impact of concrete is greatly justified. New trends in cement systems are focused on providing the cementitious materials with photocatalytic functionality in order to reduce the levels of urban pollution. One of the most investigated photocatalytic additives is TiO₂ [ 9, 10] and its addition in the cement paste is known to accelerate the rate of hydration and increase the degree of hydration of the blended cement [ 11, 12].

The first applications of TiO₂ as photocatalysts in the construction industry were intended to obtain a self-cleaning process for building facades based on white cement [ 13]. Despite the proven functional characteristics, technical aspects are still a limitation for the large scale application of this light-driven technology [ 14]. The large band gap of TiO₂, the electron-hole recombination process and deactivation of TiO₂ by intermediate blocks that occupy active catalytic sites are of concern. Additionally, it is still not clear which is the most efficient way to incorporate TiO₂ photocatalysts in the cementitious materials. In this sense, the choice of TiO₂ synthesis technique can be a key factor in determining the effectiveness of the photocatalyst.

This research focuses on the incorporation of non-condensed TiO₂ sols into the cement paste. In this approach, not only the hydration and hardening of cement are influenced by the addition of TiO₂ sols, but also the final microstructure of TiO₂ is influenced by the thermodynamics and the chemical species in the cement paste. Several previous works emphasize the interest of particular microstructures of amorphous TiO₂ for photocatalytic purposes obtained through interaction of ion species [ 15] or silica based environments [ 16]. In fact, those could modify not only the band structure of the TiO₂ formed, but also the degree of hydration of the structures. Relevantly, hydrated amorphous forms present larger band gap than anatase particles and are able to create Ti3+ species on their surface upon UV excitation, which may explain the photocatalytic performance of these amorphous TiO₂ materials [ 17].

These aspects point to the interest of performing a synthesis of TiO₂ materials in the cement paste in view of the richness of this environment in free ions and hydrated species. This work has two aims: to set forth a suitable addition of TiO₂ precursors in the search of photocatalytic activity, and to analyze the influence of embedded TiO₂ precursors on the structure of C-S-H gel. The experimental results are presented in a way that will stress the close relationship between these two objectives.

Materials and methods

Specimen preparation

CEM I 42,5 ordinary Portland cement (Type I according to ASTM C150) was supplied by Portland Valderrivas, Madrid, Spain. The chemical composition was SiO₂ (20.80), Al₂O₃ (4.40), Fe₂O₃ (2.90), CaO (62.30), MgO (2.70), SO₃ (3.14) and the fineness was 1800 cm₂/g. The cement paste was prepared by mixing with de-ionized water, using a mass water/cement ratio of 0.5.

The TiO₂ sols were prepared according to the following procedure: titanium isopropoxide (TTIP) (97%) was purchased from Sigma-Aldrich. TTIP solution was then prepared at 0.4 M in ethanol, with a precise TTIP/water molar ratio of 0.82 and pH of 1.27 controlled with HCl. The produced sol was stored at room temperature for 24 h for the completion of hydrolysis reactions. No calcination process was performed. In a previous work, the TTIP solution described herein was modified with Polyethylene-glycol (PEG). Results showed a distribution of dispersed nanoparticles displaying yellow color which, could be attributed to colloidal particle nucleation. The size distribution was characterized resulting a mean diameter of 14±7 nm [ 18].

Five series of TiO₂ precursor-cement paste were prepared. The first series (CONTROL) consisted of pure cement paste stored at 20°C in air, with no TiO₂ addition. The second to fifth series corresponded to cement paste mixed with TiO₂ sols. Four different TiO₂/cement paste content ratios were used, labeled TiO₂/CEM (1:10), (1:100), (1:1000) and (1:10000) according with their TiO₂/cement weight ratio. All samples were 1cm × 1cm × 0.3cm in size. The conditions for TiO₂ loading are summarized in Table 1, together with the sample code.

Characterization methods

Crystalline phases in the samples were identified by XRD diffractometry, by using the diffratometrer X’Pert PRO of Panalytical, with Q/2Q geometry, primary K_α1, and ultra-fast detector X’Celerator multichannel, with 0.02° scan step and 6 s integration time. The phase identification was first obtained by using the Match! Software version 1.10 together with the JCPDS database.

Scanning electron micrographs were acquired using a ZEISS EVO 50 SEM operating at 20 KeV accelerating voltage, and equipped with an energy dispersive X-ray spectrometer, OXFORD Instruments, INCA Pentafet X3. Samples were Au/Pd sputter coated with an Emitech K550. The morphology of the samples was analyzed at two different hydration times: 7 and 28 days.

The degree of hydration and the C-S-H content was estimated by TGA experiments conducted using TA Instruments, Q500 and Q100. Prior to testing, a specimen was immersed in isopropyl alcohol for 1 day to stop hydration at a desired hydration period and it was then dried to air for 1 day to remove the isopropyl alcohol from the system. The temperature was ramped from room temperature to 900°C at 107°C/min with a 100 ml/min nitrogen gas flow. The analysis was performed by the TA Instruments Universal Analysis V4.2E.

Photocatalysis experiments

The photocatalytic activity of the materials was investigated by studying the photodegradation of Methylene Blue (MB) in aqueous solution via UV/vis spectrophotometery, with slight modifications to ISO 10678; 2010 [ 19]. MB was supplied by Sigma-Aldrich at 0.04% solution in water. After the curing time, the specimens were inserted in 3 cm diameter quartz cuvettes filled with an aqueous dilution of Methylene blue previously prepared with de-ionised water with an initial concentration of 18 × 10^-6 M. The MB solution was kept in the dark before using it. The UV-irradiation was provided by a high pressure UV 22 125 W mercury lamp (Optical Engineering) which emits radiation predominantly at 351 nm. To measure the absorption spectra of MB as a function of UV irradiation time, monochrome (SpectraPro 150) transmitted light was measured with an SPD-M10Avp photodiode connected to a DSP dual lock-in amplifier (7225 Signal Recovery).

Samples were irradiated with the UV lamp for different selected times, and then introduced in the spectrophotometer for measurement. The spectrophotometer is provided with a top optical window whose section area was ca. 11 cm², through which the MB dilution was illuminated during the absorbance measurement. The visible absorption peaks of the analyzed samples were also recorded in the 200–800 nm range.

Results and discusion

X-ray diffraction

The XRD diffractograms for the specimens after 7 days of hydration are shown in Fig. 1. All samples present the most characteristic peaks of portlandite Ca(OH)₂ (012) (JCPDS 01-076-0571) and calcite CaCO₃ (JCPDS 04-012-8072), displaying a diffraction peak intensity distribution that appears slightly modified as a function of the TiO₂ concentration. The formation of calcite within the samples comes either from the CaO content of the anhydrous material or from the reaction between Ca(OH)₂ and atmospheric CO₂.

As a matter of interest, at 1:10 TiO₂/Cement weight ratio, Ca(OH)₂ content is lower than in any other sample, whereas the intensity of the peak corresponding to crystalline forms of C-S-H, according to Lachowski and co-authors [ 20], is the highest in this sample. In addition, lower intensity in the calcite peaks can be observed in this sample. As reported by Taylor [ 2] C-S-H reacts with CO₂ forming CaCO₃, which might be especially important in laboratory studies when dealing with mixes of high surface to volume ratio. This consideration suggests that the high content of TiO₂ sol in the 1:10 samples hinders the reaction between C-S-H and CO₂, which therefore decreases the carbonation processes.

The presence of C-S-H, especially in 1:10 samples, confirms that high addition of TiO₂ reduces the carbonatation process. Furthermore, the low intensity of the portlandite peak suggests that portlandite is the main source for the formation of carbonate products. Gypsum is found in all samples and Ettringite (a minor hydration product commonly present in early stages of hydration) is better defined in samples 1:10 and 1:1000, although their peaks are in any case weak. No indication of TiO₂ crystalline phases was found in the XRD patterns, possibly due to the absence of high temperature annealing of the sol-gel synthesized TiO₂ [ 21]. Therefore, in the mild exothermal conditions induced upon formation of the TiO₂ sol- cement paste, we might only expect short range order in the TiO₂ structures.

In conclusion, XRD results suggest that the mineralogical composition of hydrated phases in the cement paste has not been altered by the addition of TiO₂ sols. This is an indication that the addition of TiO₂ sols to the cement matrix does not deprive the system from progressing normally in every hydration stage.

SEM characterization

To obtain detailed information on the morphology of the TiO₂/Cement system, SEM images were taken after 7 and 28 days of hydration. Significant contents of microspherulites were detected in all cement containing TiO₂ (Fig. 2) as compared to the CONTROL sample. All TiO₂ loaded samples presented two distinctive microstructural features. Microspherulite particles appear interconnected by fibrillar structures. The EDX technique was used in selected areas, in order to identify the composition of the different microstructural features (Table 2). It was patent from these measurements that the particle features concentrate dominantly the TiO₂ content of the sample, though a trace composition of TiO₂ in the fibrillar structures cannot be discarded. These observations are consistent with the geometries proposed by Richardson [ 22], where a distinction between high density inner product and low density outer product is established.

At the lowest loading, 1:10000 TiO₂/CEM (Fig. 2(a)), it can be seen in that the TiO₂ containing particles were already covered with C-S-H gel fibers. Increasing the TiO₂ loading seems to increase the density of spherulites within the cement paste. In parallel the density of C-S-H gel fibers is also increased with a notable decrease in the size of the fibers (Figs. 2(b), (c) and (d)).

The observation of such particular controlled microporous structure in cement is consistent with a certain decrease in the degree of hydration until the highest loading process (Fig. 2(d)). As previously reported [ 23], TiO₂ could accelerate C-S-H gel formation and improve the microstructure of cementitious materials by shifting the distributed pores to finer ones. Identical trends were observed in samples observed after 28 days hydration. Figure 2(e) and (f) show higher magnification images of the presence of portlandite crystals and the fibrillar structure of the bridging C-S-H gel after this longer hydration period in CONTROL sample.

SEM characterization of the TiO₂/Cement composite samples can conclude that the preferred location of TiO₂ is within the microspherulites of the cement paste, providing the system with additional nucleation sites for the formation of C-S-H gel. The specific surface area of this TiO₂/Cement system is likely to enhance the photocatalytic activity, as reported in literature regarding the importance of surface microstructure on the photocatalytic activity of TiO₂ phases [ 21].

Thermal analysis

The thermogravimetric charts presented in Fig. 3 (TG (A) and DTA (B), respectively) allowed the identification of a series of peaks: an endothermic peak at 100°C due to the decomposition of ettringite and C-S-H, an intense peak of calcium hydroxide decomposition occurring at 410°C and a significant peak at 650°C, due to the CO₂ released during calcium carbonate decomposition. The respective weight losses can be obtained from the corresponding TG curves, which in turn allow the quantification of the respective phases. The thermogravimetric analysis of the TiO₂/Cement samples shows no significant differences from the curve obtained for the CONTROL sample. Therefore, it can be concluded that the presence of TiO₂ containing particles embedded in cement paste, at the studied concentrations, has no influence on the characteristic temperatures of the thermal degradation of cement hydrated phases.

Further analysis of the TG/TGA curves allows the determination of C-S-H content and degree of hydration of the samples. From C-S-H gel and ettringite dehydration (100°C–135°C) there are also dehydration reactions of other minor hydrated compounds, which result in continuously decreasing TG curves of the hydrated pastes, (up to 400–500°C) and the subsequent calcium hydroxide decomposition (which ends by 500°C). Therefore, the total weight loss from ambient temperature to 500°C were used to evaluate the degree of hydration of the paste. For a better evaluation of the hydration process, all the estimated contents and the water released from other hydrates were calculated with reference to the final ignited mass of the pastes at 1000°C [ 2].

Figure 4(a) depicts the calculated degree of hydration for each specimen. It is observed that the degree of hydration progressively decreases as the concentration of TiO₂ sols in the cement paste increases, until reaching the highest concentration at 1 part of TiO₂ sol: 10 parts of cement, where the degree of hydration sharply increases, reaching even a larger value than that corresponding to the CONTROL sample. This fact suggests that there was a significant effect of the dosage of TiO₂ on the degree of hydration. Specifically, relative to the CONTROL sample, at the TiO₂ concentration 1:10000, 1:1000 and 1:100 a gradual decrease in the degree of hydration is observed (by -0.6%, -2.8% and -5.8% TiO₂). In contrast, the highest doping of TiO₂ sols in the cement paste, resulted in a significant increase in the degree of hydration, by+ 2.3%. These findings suggest that loading of TiO₂ sols has an effect on the hydration process of cement, being only high concentration the one promoting the formation of hydration products by consumption of water within the cement paste microstructure.

The inversion in the trend by increasing the content of TiO₂ sols can be explained in terms of a competition between surfactant and thermodynamic effects of the ethanol present in the sols. At low addition levels, the adsorption of ethanol to solid nucleating particles reduces their hydration not influencing much the temperature of the paste. At high addition levels (TiO₂/CEM 1:10) the total addition of ethanol has a non negligible influence in the total temperature of the curing paste. Evaporated ethanol cools down the paste, reducing the speed of the hydration process allowing an increase in the content of hydrated phases.

This can also explain the effects on the microstructure as discussed in previous section. The pore size of the cement with a high addition level of TiO₂ sols has to be necessarily lower than for cements with lower TiO₂ concentrations. In the 1:10 sample, the displacement of capillary water from the micropores induces the formation of hydration products that gradually fill up the interspace.

Figure 4(b) displays the C-S-H content calculated from the weight loss occurring in the temperature range 100°C–300°C, at which gel C-S-H loses its bound water and descomposes completely [ 22]. The evolution of the content of C-S-H displays a similar behavior to the previously analyzed degree of hydration. It is observed that the cement paste with the highest TiO₂ doping level increases its content in C-S-H by 78%. These results are in agreement with reported work on effects of dispersed TiO₂ in cementitious matrix [ 23]. The present study confirms that TiO₂ sols, in a high enough concentration, induce the formation of cement hydrates: the degree of hydration as well as the content of C-S-H in TiO₂/CEM 1:10 sample is significantly higher than that in the CONTROL specimen. These results are in agreement with the microstructural trends derived from the analysis of the XRD patterns and SEM images.

Photocatalytic activity

The absorption spectrum of MB aqueous solution, measured from 400 to 1000 nm, is shown in the inset of Fig. 5. The evolution of the MB absorption at 665 nm upon intervals of 100 s of UV irradiation to the CONTROL and TiO₂/CEM samples was used to build the absorption kinetics. The variation of MB absorption in the presence of cement paste modified with different concentrations of TiO₂ sols under UV irradiation, is represented in Fig. 5. It can be seen that there is a clear decrease of MB concentration for all TiO₂/Cement samples. The process is considered as intrinsically photo-induced, since no diminution of concentration of MB was observed during pre-irradiation (samples were in contact with the different cement paste for 1h prior to irradiation with no evidence of MB concentration decay), neither during the irradiation time for the sample containing 100% cement. The photoactivity appears to be less significant in sample 1:10000, although it still shows certain photocatalytic activity revealed by a decrease of ≈30% in the MB concentration. Sample 1:10 shows the highest photocatalytic activity, whereas samples 1:100 and 1:1000 display similar MB concentration, with no remarkable differences.

When discussing the dye degradation mechanism, it is reminded that the loading of high TiO₂ content leads to the reduction of pore size. In view of the size of MB molecules (MB molecule has a minimum molecular cross-section of about 0.8 nm) [ 24], it is expected that they diffuse less through TiO₂/CEM micropores when high concentrations of TiO₂ sols are involved. Accordingly, for TiO₂/CEM 1:10 samples, MB molecules can find more photoactive TiO₂ sites and suffer from photoinduced degradation as previously suggested [ 25]. In fact, the MB concentration for 1:10 samples decreases from 18 to 8.2 mM after 300 min UV irradiation, exhibiting a ≈52% degradation. With regards to parallel non photoinduced mechanisms, such as adsorption and pH degradation, they remain as negligible mechanisms since both should be active during pre-irradiation period. In particular, raising pH should not be considered as a key factor determining photodegradation since large amounts of TiO₂ containing particles result in less available volume for the portlandite crystals, main soluble phase inducing alkaline environments [ 26].

The photocatalytic efficiency can be established from the following first-order kinetic equation:

\ln (c/c_{\mathrm{0}})=-Kt

where c₀ is the initial concentration of MB, c is the MB concentration at a given time t from which a decay constant K was calculated. This allows to estimate a degradation rate calculated from fitting the kinetics of MB absorption to exponential decays, as shown in Fig. 5. The values of the rate of degradation (mM min^-1) were calculated from the kinetic data and plotted versus TiO₂ doping level, as shown in Fig. 6 (a). For the concentrations tested, it is observed that the degradation rate does not reach saturation for TiO₂ sol loading, suggesting that there are still MB molecules available for degradation if the concentration of TiO₂ sols was increased.

To examine the controlling mechanism of photodegradation process, the experimental data were also analyzed using the pseudo-first and pseudo-second-order kinetic models, and kinetic constants were calculated in all samples. The pseudo-first-order kinetic model described by Lagergren [ 27] provided only moderate increases of constant rates for increasing concentration of TiO₂ sols, which did not reproduce the experimental data. Thus, no applicability of the pseudo-first-order model in predicting the kinetics of MB degradation in the TiO₂ doped cement paste was observed.

The pseudo second-order model predicts that intra-particle diffusion/transport process is the rate controlling step, which may involve valency forces through sharing or exchange of electrons between dye anions and photoreactants. The kinetic data were further analyzed using Ho’s pseudo-second-order kinetics [ 28],

t/q=\mathrm{1}/\left(k_{\mathrm{2}}{q}_e^{\mathrm{2}}\right)+(\mathrm{1}/q_e)t,

where q_e and q are the amounts of photodegraded MB by the TiO₂/Cement system at equilibrium and at time t, respectively and k₂ is the rate constant of second-order (1/min).

By applying the second-order kinetics, the plot of t/q against t gives a linear relationship, from which q_e and k₂ can be determined from the slope and intercept of the plot. The fitted plots are given in Fig. 6(b) and the calculated q_e, k₂, and the corresponding linear regression correlation coefficient values are summarized in Table 3. ORIGIN software (Microcal Software, Northampton, MA) was used for curve-fitting and posterior study. The smallest correlation coefficient in this case was 0.98977, which corresponds to 1:10000 TiO₂/Cement weight ratio. The rest of the samples display correlation coefficients for the second-order kinetics model greater than 0.99, indicating the applicability of this kinetics equation and the second-order nature of the photodegradation process of MB in the TiO₂/Cement system. This fact indicates that the rate controlling step is intraparticle diffusion, which is in fact the kinetic mechanism followed by the hydration of anhydrous particles in the cement paste at the hydration period under examination [ 29]. In general, the results confirm that the in C-S-H gel aggregation nature of the TiO₂ prepared contributes to its photoreactivity [ 30].

Conclusions

TiO₂ containing C-S-H inner particles within a C-S-H gel fiber matrix were successfully obtained by the addition of Ti alkoxide sols to the cement paste along with the hydration water. In this way, the TiO₂ precursors were part of the cement microstructure from the pre-induction period. SEM observations revealed that TiO₂ containing particles were preferentially forming nucleation centers for the C-S-H gel development. In fact, an increase of C-S-H content was also disclosed from the XRD analyses and thermogravimetric studies. The TiO₂ containing particles embedded in the cement matrix decrease the pore size of the cement and promote the formation of hydration products by consumption of capillary water displaced out of the pores. An appreciable induced formation of C-S-H gel is observed after the presence of a high concentration of TiO₂ sols in the cement paste.

The photocatalytic activity of the TiO₂/cement system was measured as the relative change in the absorption of a methylene blue immersion solution. Our results clearly indicate that the sol-gel synthesized TiO₂/cement system is photoactive, with the highest photocatalytic activity obtained under the mass proportion: 1 part of TiO₂ sol over 10 parts of cement paste. Though TiO₂ appears in a poorly condensed phase, cement may provide for the formation of highly surface effective microstructures in view of the cement exothermal reactions and loaded free ions. The results suggest that the concomitant increase of TiO₂ dose and the related increase of surface area are the main enhancing parameters of the photocatalytic activity. The rate controlling step for the photocatalytic processes is intraparticle diffusion, which is in fact the kinetic mechanism followed by the hydration of anhydrous particles in the cement paste. Further insight into the final microstructure of TiO₂ within the C-S-H gel particles is desirable in order to further illustrate their microstructural role in the photocatalytic properties of the composites.

Thus, the TiO₂ based cement presented in this article is particularly suitable for outdoor application due to its photoactive properties. Furthermore, TiO₂ sols added to the cement matrix not only supply the system with photocatalytic properties, but also promote the formation of C-S-H gel, the main hydration product, which controls the mechanical performance of the overall material. The mechanical properties of the described TiO₂-cement composites are under investigation after appropriate upscaling of samples.

References

[1] Richardson I. The nature of C-S-H in hardened cements. Cement and Concrete Research, 1999, 29: 1131–1147

[2] Taylor H F. Cement Chemistry. Thomas Telford, 1997

[3] Korpa A, Kowald T, Trettin R. Hydration behaviour, structure and morphology of hydration phases in advanced cement-based systems containing micro and nanoscale pozzolanic additives. Cement and Concrete Research, 2008, 38: 955–962

[4] Li G. Properties of high-volume fly ash concrete incorporating nano-sio₂. Cement and Concrete Research, 2004, 34: 1043–1049

[5] Sobolev K, Gutiérrez M F. How nanotechnology can change the concrete world. American Ceramic Society Bulletin, 2005, 84: 14

[6] Ge Z, Gao Z. Applications of nanotechnology and nanomaterials in construction. In: Proceedings of First International Conference of Construction in Developed Countries. 2008, 235–240

[7] Vera-Agullo J, Chozas-Ligero V, Portillo-Rico D, García-Casas M, Gutiérrez-Martínez A, Mieres-Royo J, Grávalos-Moreno J. Mortar and concrete reinforced with nanomaterials Nanotechnology in Construction 3. Springer, 2009, 383–388

[8] Makar J, Margeson J, Luh J. Carbon nanotube cement composites. Early results and potential applications. In: the 3rd International Conference on Construction Materials: Performance, Innovations and Structural Implications. Vancouver, B C, Aug 22–24, 2005, 1–10

[9] Cassar L, Pepe C, Tognon G, Guerrini G L, Amadelli R. White cement for architectural concrete, possessing photocatalytic properties. In: Proceedings of the 11th International Congress on the Chemistry of Cement. 2003, 2012–2021

[10] Nazari A, Sh R, Sh R, Shamekhi S F, Khademno A. Assessment of the effects of the cement paste composite in presence Tio₂ nanoparticles. Journal of American Science. 2010, 6: 43–46

[11] Lee B Y, Thomas J J. Influence of TiO₂ Nanoparticles on Early C3S Hydration. Nanotechnology of Concrete: The Next Big Thing is Small. In ACI Convention, New Orleans, LA, USA, 8–12 November 2009, 35–44

[12] Lee B Y, Kurtis K E. Proposed acceleratory effect of TiO₂ nanoparticles on belite hydration: Preliminary results. Journal of the American Ceramic Society, 2012, 95: 365–368

[13] Cassar L. Photocatalysis of Cementitious Materials: Clean Buildings and Clean Air. MRS Bulletin, 2004, 29: 328–331

[14] Maury-Ramirez A, Demeestere K, De Belie N. Photocatalytic activity of titanium dioxide nanoparticle coatings applied on autoclaved aerated concrete: Effect of weathering on coating physical characteristics and gaseous toluene removal. Journal of Hazardous Materials, 2012, 211: 218–225

[15] Zang L, Macyk W, Lange C, Maier W F, Antonius C, Meissner D, Kisch H. Visible-light detoxification and charge generation by transition metal chloride modified titania. Chemistry Chemistry—A European Journal (Weinheim an der Bergstrasse, Germany), 2000, 6: 379–384

[16] Kisch H, Zang L, Lange C, Maier W F, Antonius C, Meissner D. Modified, amorphous titania—a hybrid semiconductor for detoxification and current generation by visible light. Angewandte Chemie International Edition, 1998, 37: 3034–3036

[17] Zhang Z, Maggard P A. Investigation of photocatalytically-active hydrated forms of amorphous titania, TiO₂·nH₂O. Journal of Photochemistry and Photobiology A Chemistry, 2007, 186: 8–13

[18] Vaquero V S, Noval A M, Sánchez N T, Pérez Roldán M J, Valsesia A, Ceccone G, García Ruiz J P, Manso Silván M, Rossi F. Preparation, modification and cellular evaluation of peg-pegd supports with titania nanoparticle loads. Surface and Interface Analysis, 2010, 42: 481–485

[19] ISO 10678: 2010. Fine ceramics, advanced technical ceramics–determination of photocatalytic activity of surfaces in an aqueous medium by degradation of methylene blue. ISO, Geneva, 2010

[20] Lachowski E E, Mohan K, Taylor H, Lawrence C, Moore A. Analytical electron microscopy of cement pastes: III, pastes hydrated for long times. Journal of the American Ceramic Society, 1981, 64: 319–321

[21] Su C, Hong B Y, Tseng C M. Sol–gel preparation and photocatalysis of titanium dioxide. Catalysis Today, 2004, 96: 119–126

[22] Richardson I. The calcium silicate hydrates. Cement and Concrete Research, 2008, 38: 137–158

[23] Jalal M, Fathi M, Farzad M. Assessment of nano-TiO₂ and class F fly ash effects on flexural fracture and microstructure of binary blended concrete. Mechanics of Materials, 2013, 61: 11–27

[24] Valdés H, Sánchez-Polo M, Rivera-Utrilla J, Zaror C A. Effect of ozone treatment on surface properties of activated carbon. Langmuir, 2002, 18: 2111–2116

[25] Zhang Y, Deng S, Sun B, Xiao H, Li L, Yang G, Hui Q, Wu J, Zheng J. Preparation of TiO₂-loaded activated carbon fiber hybrids. An application in a pulsed discharge reactor for decomposition of methyl orange. Journal of Colloid and Interface Science, 2010, 347: 260–266

[26] Li H, Zhang M h, Ou J P. Abrasion resistance of concrete containing nano-particles for pavement. Wear, 2006, 260: 1262–1266

[27] Lagergren S. About the theory of so-called adsorption of soluble substances. Kungl Svenska Vetenskapsakademiens Handlingar, 1898, 24: 1–39

[28] Ho Y S. Removal of copper ions from aqueous solution by tree fern. Water Research, 2003, 37: 2323–2330

[29] Van Breugel K. Dissertation for the Doctoral Degree. Delft: Delft University of Technology, 1991–11–18, 1991

[30] Kumaresan L, Prabhu A, Palanichamy M, Murugesan V. Synthesis of mesoporous TiO₂ in aqueous alcoholic medium and evaluation of its photocatalytic activity. Materials Chemistry and Physics, 2011, 126: 445–452

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