Yuuki Guilherme

Paul Koga

Blandine Comperat

Virginie Albert

Ricardo Roche
email: ricardo.nogueira@ku.ac.ae

Nogueira Pereira

Guilherme Yuuki Koga
email: guilherme.koga@dema.ufscar.br

Paul Comperat

Blandine Albert

Virginie Roche

Ricardo Pereira Nogueira

Effect of endogenous chloride contamination on the electrochemical and hydration responses of reinforced Belite-Ye'elimite-Ferrite (BYF) cement mortars

Keywords: Mild steel, Steel reinforced concrete, Sulfoaluminate cement, Corrosion, Chloride

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Greenhouse gases emissions have become a highly important subject that directly concerns the cement industry, which is responsible for about 7% of the global anthropogenic CO2 emissions [START_REF]Technology roadmap Low-carbon transition in the cement industry[END_REF]. The pressure to reduce the CO2 release during Portland cement production has been one of the factors contributing to the development of lower-carbon binders [START_REF] Scrivener | Eco-efficient cements: Potential, economically viable solutions for a low-CO2, cement-based materials industry[END_REF][START_REF] Damtoft | Sustainable development and climate change initiatives[END_REF]. A relatively new type of clinker, the Belite-Ye'elimite-Ferrite (BYF), is under increasing interest due to lower CO2 emissions compared to the ordinary Portland cement (OPC) manufacturing.

The BYF family has been initially developed for a wide range of concrete applications, including those requiring steel bars reinforcement, which makes the ability to protect inexpensive steels against corrosion an essential attribute. Recently, Koga et al. [START_REF] Koga | A comparative study of mild steel passivation embedded in Belite-Ye'elimite-Ferrite and Porland cement mortars[END_REF][START_REF] Koga | On the intrinsic passivating ability of Belite-Ye'elimite-Ferrite towards carbon steel: A straightforward comparison with ordinary Portland cement[END_REF] have shown that, in the absence of chlorides and carbonation, steel rebars effectively passivate in some BYF cement matrices, but studies on the chloride-induced corrosion of steel embedded in BYF matrices are still limited. In the present paper we were interested in the impact of endogenous chloride contaminations. The term endogenous here refers to chloride ions incorporated to the structure during the different steps of its elaboration before it is commissioned and delivered to service.

Potential sources of endogenous chloride contamination are the cement itself, but also supplementary cementitious materials, mixing water, and unwashed constituents such as seadredged aggregates [START_REF] Bertolini | Corrosion of steel in concrete[END_REF][START_REF] Broomfield | Corrosion of steel in concrete[END_REF]. On the other hand, exogenous contaminations are related to the exposition of the structure to external chloride-rich media, mainly marine environments or structural elements exposed to de-icing salts [START_REF] Thomas | Concrete Durability[END_REF][START_REF] Angst | Chloride induced reinforcement corrosion: Electrochemical monitoring of initiation stage and chloride threshold values[END_REF].

The intrinsic passivating ability of most concretes is related to the highly alkaline media they provide. Hydroxyl ions on the interstitial pore solution around the steel rebar thus play a major role in assuring and maintaining its passivation, while chlorides have deleterious effects, leading to a competition between the alkaline-driven passivation and chloride-induced corrosion [START_REF] Hausmann | Steel corrosion in concrete -How does it occur?[END_REF]. These counteracting effects make the pore solution's [Cl -]/[OH -] ratio at the vicinity of the metal surface a major parameter controlling passivation, or, on the contrary, corrosion attacks. It is also worth noticing that only free chlorides in the interstitial pore solution directly affects the passivation. This means that systems that bind higher proportions of chlorides in the form of complexed compounds or by physical uptake should offer greater corrosion protection to the embedded steel rebar. The amount of chemical and/or physical Cl binding upon hydration depends on a great extent on the composition of the cement [START_REF] Arya | Factors influencing chloride-binding in concrete[END_REF]. Therefore, it is possible that the concentration of free chlorides in the pore solution of different concrete varies upon hydration even if their initial endogenous chloride content is the same.

Chloride contaminations present since the first stages of the cement/water mixing are particularly dangerous and, in concentrations high enough, can prevent effective passivation, leading to premature corrosion of steel rebars. It is thus a common practice to limit the endogenous chloride content of reinforced concrete as a percentage by cement mass. The most common upper limit for reinforced Portland concrete is 0.4% of chlorides by cement mass proposed by the European Concrete Standard EN-206 [START_REF]European Committee for Standarization, EN 206-1. Concrete -Part 1: Specification, performance, production and conformity[END_REF], this limit being reduced to 0.2 % for prestressed concrete elements. However, for a given chloride to cement ratio in concrete, the pore solution [Cl - ]/[OH -] ratio is expected to be affected by a wide range of parameters, specially the cement composition. As a result, the maximum permissible chloride content is likely to vary between concretes based-on different types of cements.

In the present work, the impact of different levels of endogenous chloride contamination on reinforced BYF and OPC mortars was evaluated by means of electrochemical measurements.

Potential readings, linear polarization resistance (LPR), and electrochemical impedance spectroscopy (EIS) were performed to study the corrosion behaviour of the steel reinforcement over 365 days. Physicochemical measurements were performed to bring elements for a better understanding of the overall effect of endogenous chlorides on reinforced BYF and OPC cement mortars.

Experimental

Materials

The experiments were carried out using a BYF cement from a clinker resulting from an industrial pilot project [START_REF] Gartner | Project Aether[END_REF] as well as a reference CEM I 52.5 CE CP2 NF OPC cement, according to the standard NF EN 197-1 [START_REF]Cement -Part 1: Composition, specifications and conformity criteria for common cements[END_REF]. Ribbed steel rebars measuring 130 mm long and 14 mm in diameter were used as mortar reinforcement. The relevant characteristics of the cements and steel are detailed in Fig. 1a,b, andc, respectively.

As specified in Table 1, three mortar mixes were considered: one based on OPC cement with a water to cement (w/c) ratio equal to 0.5, and two made with BYF cement with two different w/c ratios (0.5 and 0.67). High purity (> 99.5%) sodium chloride, NaCl, was dissolved in the mix water prior to mortar mixing to simulate endogenous chloride contamination. The amounts correspond to 0.2, 0.4 and 1.0% of chlorides. Whenever mentioned through the text, these % values correspond to the mass of chloride relative to cement mass. These levels were chosen based on the EN 206-1 standard, which suggests that the risk of corrosion in reinforced OPC concrete is generally low for endogenous chloride contents below 0.4%, and high for 1.0% [START_REF]European Committee for Standarization, EN 206-1. Concrete -Part 1: Specification, performance, production and conformity[END_REF]. The detailed procedure to produce reinforced samples to be used in electrochemical analyses is described in reference [START_REF] Koga | A comparative study of mild steel passivation embedded in Belite-Ye'elimite-Ferrite and Porland cement mortars[END_REF].

Standard mortar prisms were prepared with different endogenous chloride contents and their compressive strengths measured according to the NF EN 196-1 [START_REF]Methods of testing cement-Part 1: Determination of strength[END_REF]. Equivalent neat pastes were made for physicochemical characterizations.

Electrochemical techniques

Electrochemical measurements were performed according to the configuration detailed in Fig. 2, where the steel reinforcement was the working-electrode and the electrolyte the interstitial pore solution of the mortar. A co-axial carbon sheet was used as counter-electrode (CE) placed against the reinforced mortar surface surrounded by a soaked sponge band to guarantee proper electrical continuity. A 15 mm diameter window in the CE allowed to place the tip of a saturated calomel electrode (SCE) in contact with the soaked sponge band and perpendicularly to the reinforcing steel bar.

Corrosion measurements were performed regularly from 1 day to 365 days using a BioLogic SP-300 potentiostat. A combination of non-destructive electrochemical techniquespotential readings, LPR, and EIS -was used to assess the active corrosion and passivation of reinforced mortars over 365 days. After being analysed with those non-destructive techniques, the samples returned to the controlled chamber till subsequent measurements. Specimens tested with destructive large range anodic polarization curves were discarded after each test. For each technique, four samples were analysed to ensure repeatability. Potential readings were recorded versus time at room temperature (20 ± 1 °C). The data acquisition and interpretation were based on the ASTM C876-09 [START_REF]Standard test method for half-cell potentials of uncoated reinforcing steel in concrete[END_REF] and RILEM TC 154-EMC [START_REF] Elsener | Half-cell potential measurements-Potential mapping on reinforced concrete structures[END_REF]; the validity of the ASTM C876 was recently verified and the standard has shown to be a reliable tool to evaluate the corrosion risk of reinforced OPC and BYF matrices [START_REF] Koga | Revisiting the ASTM C876 standard for corrosion of reinforcing steel: On the correlation between corrosion potential and polarization resistance during the curing of different cement mortars[END_REF]. LPR measurements were performed with a 20 mV amplitude, 10 mV min -1 scan rate, being the ohmic drop compensation considered as per [START_REF] Koga | A comparative study of mild steel passivation embedded in Belite-Ye'elimite-Ferrite and Porland cement mortars[END_REF]. EIS measurements were performed at open circuit with a 1 MHz to 10 mHz, 10 mVrms amplitude sinusoidal perturbation, with 13 points per decade. Large anodic potentiodynamic polarizations were performed on reinforced mortars aged of 365 days, from -30 mV to +1000 mV vs. the open circuit potential at a scan rate of 10 mV min -1 . Visual examinations were conducted by splitting the reinforced mortars after 365 days to confirm the electrochemical results.

The passivation efficiency depends not only on the film's chemical composition but also on its morphology, structure and homogeneity, which can induce different levels of protection. In this sense, throughout the Results section, effective passivation is associated to:

• Ecorr > -126 mVSCE, low corrosion risk [START_REF]Standard test method for half-cell potentials of uncoated reinforcing steel in concrete[END_REF].

• icorr < 0.1 µA/cm², negligible corrosion rate [START_REF] Andrade | Test methods for on-site corrosion rate measurement of steel reinforcement in concrete by means of the polarization resistance method[END_REF].

• Low-frequency EIS responses similar to those obtained with Cl-free sample.

Physicochemical characterization

Physicochemical analyses were carried out on samples of neat paste equivalent to the mortar series. Isothermal calorimetry measurements were performed on a Thermal Activity Monitor (TAM) AIR 3115 isothermal micro-calorimeter at 20.0 ± 0.1 °C. 10 g of anhydrous cement (OPC or BYF) were mixed with water containing NaCl. The heat released by hydration of about 4 g of cement paste was collected up to 300 h. Chloride-contaminated neat pastes were cast into plastic containers, which were sealed and stored at 20 ± 1 °C without drying. Acid-and water-soluble chloride concentrations in the contaminated pastes were measured as per [START_REF] Chaussadent | AFREM test procedures concerning chlorides in concrete: Extraction and titration methods[END_REF] by potentiometric titration with silver nitrate solution using a chloride ion selective electrode in a Mettler Toledo® G20 compact titrator. Pore solution of the neat pastes was extracted, its chemical composition identified by ICP-AES (Varian 720-ES ICP Optical Emission Spectrometer), and the pH measured using a PH2010 STANDARD pHmeter MaterLab® from Radiometer Analytical. Before the setting time, it was possible to extract the pore solution by centrifugation (performed on a Sigma 3-16 centrifuge with 3000 rpm during 5 min). The pore size distribution of paste specimens at curing ages of 1, 7, and 28 days was determined by using an Autopore™ IV 9500 automated mercury porosimeter from Micromeritics.

The hydration of the solid phases of the cement pastes was stopped after different hydration times. To stop hydration, the crushed cement-paste samples (particle diameter inferior to 100 µm) was double washed in a large volume of acetone then with diethyl ether. X-ray diffraction and thermal analyses were used to determine the mineralogical composition of the solid phases.

Thermogravimetric analyses (TGA) were carried out under N2 atmosphere at a heating rate of 10 °C min -1 up to 1000 °C. The so-called "bound water content" (i.e., the water constitution of all hydrates precipitated at that time) was deduced and corresponds to the weight loss between 50 and 550 °C. The apparatus used was a SETARAM TG-DTA 92-16. X-ray powder diffraction (XRD) measurements were performed using Cu-Kα radiation on a PANalytical X'Pert Pro diffractometer in a θ-2θ configuration with an angular scan of 5° -65° (2θ), the step size of 0.016° / 60 s, and an X'Celerator detector. Rietveld analysis was performed with the HighScore Plus (version 3.0d) PANalytical software. A dedicated Rietveld refinement file was developed in order to quantify the phase assemblage formed during hydration as detailed by Morin et al. [START_REF] Lothenbach | Hydration of Belite-Ye'elimite-Ferrite cements: Thermodynamic modeling[END_REF].

Results and Discussion

Electrochemical survey over 365 days

Portland samples

Fig. 3 shows the impact of the endogenous chlorides on the evolution of Ecorr and icorr as mean values and standard deviations from measurements taken from four different specimens over 365 days. Samples containing 0.2 and 0.4% of Cl exhibited potential values slightly lower compared to the Cl-free samples (Fig. 3a). Nevertheless, after 150 days, the Ecorr of these Cl-contaminated samples were higher than the -126 mVSCE threshold corresponding to a low corrosion risk confirmed by the decrease of icorr values, Fig. 3b. Indeed, the increase of Ecorr during the first stages of the curing (ca 28 days) was accompanied by the decrease and subsequent stabilization of icorr values below 0.1 µA/cm² assimilated to negligible corrosion rate [START_REF] Andrade | Test methods for on-site corrosion rate measurement of steel reinforcement in concrete by means of the polarization resistance method[END_REF]. Results in Fig. 3 clearly indicate that endogenous chloride contaminations of OPC matrices up to 0.4% still allow the rapid formation of a highly protective passive layer. However, samples containing 1.0% of endogenous Cl showed a different evolution of Ecorr and icorr. Ecorr values (Fig. 3a) remained below -276 mVSCE during the first 210 days, suggesting high corrosion risk, and then between -126 and -276 mVSCE, indicating intermediate/unknown risk of corrosion [START_REF]Standard test method for half-cell potentials of uncoated reinforcing steel in concrete[END_REF]. Regarding icorr, the initiation of corrosion is assimilated to sustained values higher than 0.2 µA/cm 2 . As seen in Fig. 3b, in the case of 1.0% Cl contamination, the icorr values did remain above 0.2 µA/cm² throughout the test, thus indicating non-negligible corrosion rates, consistently with the Ecorr evolution. It hence clearly appears that the contamination of OPC matrices with 1.0% Cl prevents effective passivation, at least during the 365 days of testing. Fig. 4 shows the anodic polarization curves obtained for the different levels of chloride contamination. Up to 0.4%, similar curves were obtained regardless of the chloride content with only slight differences on the Ecorr and icorr values. Results indicate low corrosion rates and a satisfying level of protection thanks to the effective steel passivation even for specimens containing 0.4% Cl. However, higher levels of chloride contamination, 1.0%, showed considerable higher icorr values related to a less protective layer. 5d. A sharp analysis of the diagrams seems to indicate a tendency of the impedance modulus to slightly decrease with the chloride contamination but without major incidence on the system overall behaviour. Conversely, the diagrams obtained for the 1% Cl-contaminated systems show a much lower impedance modulus since the very first stages of curing. With respect to the high-frequency domain, related to the mortar response, chlorides up to 0.4% have no significant impact on the impedance responses. For 1.0% Cl, the first semi-circle shifts towards higher impedance values (cf diamond plot in Fig. 5d,inset). This apparently surprising result (it could be expected that increasing chloride contents would induce a decrease in the electrolyte resistivity and then in the high-frequency limit of the EIS), is to be ascribed to a non-negligible impact of 1.0% Cl on the microstructure development of the OPC mortar. We shall come back to this point later in Section 3.2.

After one year, the samples were split for visual inspection. Up to 0.4% Cl the samples do not exhibit any corrosion signs (Fig. 6 a to c), as expected thanks to negligible icorr values as seen in Fig. 3b. However, some corrosion spots are visible for the specimens with 1.0% Cl, Fig. 6d, consistently with the non-negligible -even if smallicorr (cf Fig. 3b). The spots of corrosion products coincide, as it usually happens, with the ribs of the reinforcing bars, regions with higher densities of metallurgical defects and residual stress where corrosion is more likely to initiate.

In summary, as expected for the OPC series taken as reference, results showed that the steel can effectively passivate in the presence of up to 0.4% of chlorides, while the presence of 1% must be precluded. Indeed, the current densities measured by LPR for OPC series with 0.2 and 0.4% chlorides were roughly the same as without chlorides, around 0.1 μA/cm², which demonstrates effective protection.

BYF samples

BYF samples produced with w/c = 0.50

Fig. 7a andb show the evolutions of Ecorr and icorr for BYF samples over 365 days. In the presence of chlorides, Ecorr values measured during the first day were lower than those measured for the Cl-free samples, the difference increasing with the chloride content. After one day, the corrosion potential of the samples made with 0.2 and 0.4% of Cl continuously increased with time, reaching values greater than -276 mVSCE after 90 days, and superior to -126 mVSCE (associated to passivation) after 365 days. The values for these two series were nevertheless about 100 mVSCE lower than those measured for the Cl-free samples. In the case of 1.0% Cl, the initial potential values were remarkably low, about -500 mVSCE, and remained at this level throughout the testing period, which, according to the usual standards, should indicate a lack of effective passivation since the first stages of the test.

icorr values for all reinforced BYF mortars were higher than 0.2 μA/cm² within the first days after steel embedding (Fig. 7b). The initial values were different according to the chloride content.

For the reference Cl-free specimen, the current density values fell below the limit of 0.1 μA/cm² after 14 days while for the samples with initial chloride contents up to 0.4% the icorr values remained above 0.2 μA/cm² for 60 days. After this initial period where slight active corrosion could take place, the icorr values of the samples made with initial 0.2 and 0.4% of Cl decreased continuously with time towards fairly stable minima of 0.1 μA/cm² after 210 days, which suggest a tendency for effective steel passivation. On the other hand, the icorr values for samples with 1.0% of Cl remained somewhat high (between 0.5 and 1 µA/cm²) with strong differences between the different samples (large standard deviation). This behaviour is in good agreement with the low Ecorr values depicted in Fig. 7a, corroborating the evidence of active corrosion taking place all over the test period.

Potentiodynamic curves presented in Fig. 8 clearly show the influence of the increase in the endogenous chloride contents on the Ecorr and icorr values of steel embedded in BYF mortars (w/c = 0.50) after 365 days of curing. Compared to the polarization curves for the Cl-free specimen, the curves corresponding to the samples with chlorides are shifted to lower Ecorr and higher icorr values. This shift is small for samples containing 0.2 and 0.4% Cl, with icorr < 0.2 μA/cm 2 , corresponding to a satisfying level of protection assured by the formation of a protective film. The anodic curve related to the sample with an initial 1.0% Cl, in turn, is strongly shifted towards lower Ecorr values and icorr up to 1.44 µA/cm² (one order of magnitude when compared to reinforced samples without chloride), indicating that, in these conditions, the reinforcing steel is undergoing sustained corrosive attacks. It is important to stress that, if the increase of corrosion current with chloride contamination is an expected behaviour, results presented here bring a systematic and robust insight pointing out the good corrosion resistance of BYF samples exposed to endogenous chloride contents up to 0.4%. This trend has been confirmed by the EIS analysis discussed below. EIS data at 1, 28, 90 and 365 days are represented by Nyquist and Bode diagrams in Fig.

9.

As for the OPC series, Fig. 5, the Nyquist diagrams for the BYF samples are composed of a truncated capacitive loop in the middle-low frequencies domain, preceded by another capacitive high-frequency loop, more clearly visible after 28 days of curing. At the first day, the curves of the samples contaminated with 0.2 and 0.4% Cl are similar to that of the specimens containing 1.0% Cl, possibly indicating some corrosion at the first stages of embedding (Fig. 9a).

Nevertheless, in the case of low chloride contamination levels, this behaviour drastically changed with curing time. Indeed, during the first 28 days, an increase of the low-frequency capacitive loop was already observed for samples containing up to 0.4% Cl (Fig. 9b) and after 90 days, the low-frequency response of these low Cl samples seemed to approach that exhibited by the Cl-free specimens (Fig. 9c). This positive trend of increasing impedance modulus spread with time so that after 365 days (Fig. 9d) their resistance against corrosion were almost identical to those of Cl-free samples. The results hence show a clear improvement of the corrosion properties over time, despite the initial high aggressiveness induced by the presence of endogenous 0.2 and 0.4% of chlorides. The same improvement was nevertheless not achieved for samples contaminated with initial 1.0% Cl. As well as for the OPC mortars, results indicate that the presence of endogenous chloride contamination must be avoided, or at least limited to levels below 0.4%.

Mortars with 1.0% Cl must be mandatorily prohibited because of its deleterious impact on the development of the passivating film throughout the experiments.

A deep interpretation of the high-frequency loop evolution is beyond the aims of the present study. Results from Figs. 5 and9, however, allow some interesting points to be discussed in a preliminary approach. The high-frequency region of the Nyquist plots has been associated mainly with the effects of hydration process, which controls the formed phases and the resulting microstructure [START_REF] Xie | impedance model for microstructural characterization of hydrationg cement systems[END_REF][START_REF] Keddam | Impedance measurements on cement paste[END_REF][START_REF] Cabeza | Impedance spectroscopy to characterize the pore structure during the hardening process of Portland cement paste[END_REF], thus determining the pore network and the resistance against ion movement. The high-frequency loops in Fig. 5 and 9 increase over time for all series. Nevertheless, this growth is less pronounced as the chloride concentration increases, which is clearly seen for the 1.0% Cl-contaminated sample. This behavior can be ascribed to the counteracting effects of, in one hand, the hampered ion movement due to hydration (increase of the electrolyte resistivity) and, in the other hand, the increasing chloride contents (decrease of the electrolyte resistivity), that makes the 1.0% Cl sample less susceptible to the hydration effects.

Visual examinations performed after 365 days, Fig. 10, confirmed the results discussed above. Indeed, no corrosion products are observed for specimen without Cl, indicating effective passivation. Specimens containing initial 0.2 and 0.4% of chloride show some small spots of brown rust much probably formed during the initial months when the current density values were higher than 0.2 μA/cm². Finally, large areas of corrosion products can be seen for the sample containing 1.0% Cl, consistently with the high corrosion current densities sustained over one year of testing.

The spots are mainly localized at the ribs, as for the OPC specimens, Fig. 6.

It then clearly appears that steel embedded in BYF mortars made with a w/c ratio of 0.5 passivates despite the presence of 0.2% and 0.4% of chlorides. Effective passivation, however, only took place after a longer curing time compared to the free chloride samples. 60 days were required for the current densities to be less than 0.2 μA/cm², and 210 days to stabilize at 0.1 μA/cm², corresponding to an effective protection as for OPC cement.

BYF samples produced with w/c = 0.67

Similar measurements were carried out on reinforced BYF mortars with a higher water to cement ratio (0.67 instead of 0.50). The aim was to evaluate the effect of the w/c ratio on the susceptibility to corrosion induced by chlorides contamination. For all the different techniques employed in the present study, results are qualitatively very similar to those obtained for the w/c = 0.50 BYF system, the main difference being the impairment of the corrosion resistance at 0.4% of chloride contamination, that showed values around 0.3 to 0.4 μA/cm² even after 365 days, which means that the steel did not completely passivate during 365 days in those higher w/c ratio mortars.

For samples cast with initial 1.0% of Cl, the potential values remained low (about -500 mVSCE) and coupled to icorr values markedly high (around 2 to 3 μA/cm²), indicating sustained high corrosion rates that induced premature cracking of the samples after just 210 days of the survey.

Physicochemical investigations

Impact of sodium chloride addition on the kinetics of hydration

Salt addition is well known for several decades to potentially accelerate the hydration of alite (C3S), the main component of OPC. CaCl2 is the most efficient but NaCl is also expected to have a significant accelerating effect [START_REF] Abalaka | Effects of sodium chloride solutions on compressive 29 strength development of concrete containing rice husk ash[END_REF][START_REF] Andreeva | The solubility of tricalcium silicate in solutions of calcium chloride[END_REF]. On the other hand, the effect of NaCl on the kinetics of hydration of BYF or CSA cement is still poorly known. Calorimetric measurements were thus performed to evaluate the influence of the various NaCl levels added on the early-age hydration of the OPC and BYF used in the present study. Fig. 11a gives the heat flow released during the hydration of the OPC cement mixed at a w/c ratio of 0.5, in the presence of NaCl corresponding to 0, 0.2, 0.4 and 1% Cl. The shape of the curve is typical of OPC calorimetry curve as described by Bullard et al. [START_REF] Bullard | Mechanisms of cement hydration[END_REF], with a brief heat release due to the wetting of the grains, then a period of "slow-reaction" of the alite up to two hours.

During this period, C3A and calcium sulphate react to form ettringite, but this is not clearly visible by calorimetry. After 2 hours, there is a large exothermic peak due to the heat of dissolution of the alite whose hydration is accelerated for two hours, then slowed down after 10 hours. The shoulder observed around 14 hours corresponds to the heat of dissolution of residual C3A after calcium sulphate depletion. The last peak visible in the inset and observed after 50 hours corresponds to the formation of calcium monosulfoaluminate.

The results show that the addition of NaCl has a significant impact only on the peak of heat release relative to alite. This peak became larger and occurred earlier, which could correspond to a period of more intense acceleration of hydration of alite in the presence of NaCl. Samples of neat paste were cast without and with NaCl (1% Cl, content for which the effect is most visible), and their hydration was stopped at 1, 7, 14 and 28 days according to the protocol previously described.

The bound water contents measured by TGA are given in Fig. 11b. It can be seen that the bound water is higher after 1 day of curing for samples with 1.0% Cl compared to samples without Cl but similar values are observed from 14 days of hydration.

Concerning BYF cement, heat release curves of BYF pastes are given in Fig. 11c. All curves have the same shape. According to Morin et al [START_REF] Morin | Hydration of a belite-calcium sulfoaluminate-ferrite cement : Aether TM[END_REF], the peak occurring around 2 hours after mixing corresponds to the heat of dissolution of ye'elimite (C4A3$) whose hydration in the presence of calcium sulphate is accelerated, after a less active period. The addition of NaCl significantly amplifies this peak and accelerates its occurrence. The next peak observed after 4 hours corresponds to a renewed dissolution of ye'elimite after sulphate depletion. Interestingly, the addition of NaCl seems to slightly diminish this second peak. At last, the peak observable after 100 hours corresponds to the dissolution of the belite and announces the start of the contribution of this phase to the hydration process. The addition of NaCl seems to significantly shift the occurrence of this peak, indicating an important delay of belite hydration. For the series 1% Cl, the peak is even not detected during the measurement period.

As previously done for OPC, BYF neat paste samples without and with 1% NaCl were analysed. The bound water content measured at one day is the same for both series, but, at larger times, is clearly lower for the 1% chloride series (Fig. 11d). This indicates that the Cl-induced acceleration of ye'elimite hydration does not affect the water content at short times (1 day). This can be explained by the intrinsic hydration of ye'elimite itself, that is well known to be already fast and almost complete to a day in BYF cement [START_REF] Morin | Hydration of a belite-calcium sulfoaluminate-ferrite cement : Aether TM[END_REF]. On the contrary, the addition of NaCl seems to significantly delay the contribution of belite (C2S) and, possibly, even that of ferrite (C4AF). This is confirmed by results given in Fig. 12, showing the hydration levels of the main BYF cement compounds determined by phase quantification using the Rietveld method. It clearly appears that the degree of hydration of belite and ferrite was lower, while the degree of hydration of ye'elimite was not affected. This ensemble of results for BYF shows that in the presence of NaCl, the hydration of ye'elimite, that is already fast, is even more accelerated during the first hours of hydration. The contribution of belite and ferrite, in turn, is delayed by several weeks.

Effect of the sodium chloride addition on the pore size distribution

The delayed contribution of belite and ferrite to hydration is likely to negatively impact the refinement of the microstructure during the first weeks of hydration of BYF cement. To better address this issue, mercury porosimetry analyses were performed on samples of neat paste made with a w/c = 0.5, with and without the addition of NaCl (0.4% and 1%).

Derivative and cumulative pore volume vs. pore diameter of OPC cement pastes aged of 1, 7, and 28 days are presented in Fig. 13. At 24 hours, the curve of the chloride-free sample has a bimodal distribution with the largest population of pore entries of 0.2 to 0.3 μm, which corresponds to the remaining space initially occupied by the mixing water. After 7 and 28 days, this population of coarse pores disappears, and the curves display a unimodal population with a threshold entry pore diameter between 0.2 and 0.1 μm. The hydration was sufficiently advanced after 7 days to partially fill the empty pores and to refine the microstructure. The intruded mercury was able to enter only pores with an entry diameter smaller than 0.2 μm. In parallel, the total porosity of the chloride-free samples decreased over time with the hydration progress.

With the addition of NaCl, the mercury accessible pore distribution curve has a unimodal distribution by 24 hours, with a threshold diameter smaller than 0.2 μm. The addition of NaCl clearly accelerated pore filling during the first 24 hours of hydration, as indicated by the absence of the higher-diameters related peak appearing for the chloride-free samples. There is still a slight positive effect on the total porosity at 7 and 28 days, especially for the 1% Cl samples (Fig. 13d andf).

Regarding BYF neat paste samples, Fig. 14a andb show, respectively, the derivative and cumulative mercury volume for BYF pastes cured at 24 hours. There are two populations of entry pore sizes, the coarsest with 0.2-0.3 μm mean diameter, and the thinnest with 0.03-0.04 μm mean diameter. The addition of NaCl does not seem to have had an impact at 24 hours as the curves for the three series are similar. At 7 and 28 days, for the chloride-free series, the coarser population became less important compared to the population of thinner pores (Fig. 14c ande). This is also visible for the 0.4% Cl series. In this case, the coarser pores seemed to be no longer reachable by mercury at 28 days. On the other hand, the population of the coarser pores remained the majority for the samples with 1% Cl. The total porosity decreased with time, but less rapidly for the samples with 1% Cl (Fig. 14b,d,f). There is also a delay at 7 days for the samples with 0.4% Cl compared to the chloride free series, but this delay was resorbed after 28 days.

These results clearly show that the addition of NaCl has a negative effect on the refinement of the microstructure beyond a day of hydration of BYF cement pastes. This can be explained by the delay in the contribution of belite and ferrite to the hydration process in the presence of NaCl, which has been previously shown in Figs. 11 and12. The impact on the delay of the microstructure development is pronounced for the equivalent of 1% of chlorides, and more attenuated in the case of 0.4% of chlorides.

Impact of sodium chloride addition on the strength development

Given the effect of the addition of NaCl on the porosity accessible by mercury intrusion, its impact on the development of mechanical strengths has been evaluated through compressive strength measurements performed up to 56 days of mortar cure. Fig. 15a andb give the values for OPC and BYF mortars with w/c = 0.50, respectively. The addition of 1.0% Cl promotes a slight improvement of the compressive strength development of OPC mortars as expected from the calorimetry and MIP results that highlighted the accelerating effect of the early hydration of alite, and the resulting reduced porosity. Conversely, chloride addition of 1.0% from NaCl added in mixing water has a deleterious effect on the compressive strength development of BYF mortar between 7 and 28 days. This can be explained by the delay of the contribution of belite and ferrite to the hydration process of the BYF cement and the resulting higher total porosity, as discussed before. However, as far as the final strength gain is concerned, the effect due to the addition of NaCl does not considerably impair the compressive strength at 56 days.

Final Remarks

From a practical point of view, it appears that Cl contamination levels up to 0.4% in mass could be tolerated for both OPC and BYF matrices with w/c = 0.5, without inducing significant corrosion during the first stages of curing, that means, prior to the development of a highly protective passive layer (icorr < 0.1 µA/cm²). For higher w/c ratios, the allowable chloride content should be reduced to 0.2%. This reveals the importance of the porous network characteristics to hamper ion mobility and to allow efficient passivation. Higher levels of contamination, ca 1.0% Cl, clearly impair passivation and must thus be avoided. With respect to steel corrosion and cement hydration, the BYF cement of the study seems to be more sensitive than the OPC to the presence of chloride.

In comparison to the results obtained with the OPC series, BYF samples needed a longer curing period to obtain an efficient passivation, which can explain the presence of some spots of rust, always located at the ribs of the rebar that can be assumed to have developed in the first stages of curing. It has also been found that the addition of NaCl has not only an effect on the steel passivation but also on the development of the microstructure. Koga et al. [START_REF] Koga | A comparative study of mild steel passivation embedded in Belite-Ye'elimite-Ferrite and Porland cement mortars[END_REF] already observed that a longer time was needed to passivate steel with BYF cement even without chloride addition, due to the relatively low initial pH at short times after samples casting, when the steel is vulnerable to corrosion initiation. This trend was reinforced in the presence of chlorides with even higher current density values during the first day and an even longer times to achieve effective passivation (several weeks instead of several days).

During the first hour of hydration of BYF cements, the pH of the solution remains constant around 10.6, Fig. 16. The sulfur (in the form of sulphates) and calcium concentrations are constant as well. Potassium is also present, resulting from the soluble alkali of the cement. Between 1 and 4 hours, the concentration of sulphates starts to decrease. In the meantime, the pH begins to increase from 10.5 to about 12.5, probably due to the increasing concentration of hydroxyl ions to compensate the decrease of sulphate anions. Calcium content diminishes whereas the concentration of aluminum increase. The increase of aluminum in solution could be explained by the pH increase that favors a higher solubility of AH3 in solution, as suggested by Winnefeld and Lothenbach [START_REF] Winnefeld | Hydration of calcium sulfoaluminate cements -Experimental findings and thermodynamic modelling[END_REF].

After 8 h, the concentration of calcium is too low to be detected and the concentration of aluminum and sulphates remain stable, whereas the alkali concentrations increase, leading to an increase of the pH. As hydration proceeds, water is consumed by different hydration products and alkalis trapped in slowly-hydration phases are released progressively. As a result, alkali ion concentration increases, and the pH after 24 hours is around 13.

When the steel rebar is embedded in chloride-contaminated fresh mortar, the metallic surface is brought into contact with an aggressive Cl-rich medium before an effective passive film could be formed. Fig. 17a andb show the evolution of pH and of free chlorides for OPC and BYF cement pastes containing initial 0.4% of chlorides. The initial low alkalinity (pH ~ 10.5) of BYF pore solution exposes the steel rebar to higher corrosion risk compared to the OPC pore solution.

Indeed, for the same chlorides content in solution, the BYF pH was 2 units lower than the OPC pH, which means a [Cl -]/[OH -] ratio 100-times higher, thus likely to yield higher current values at early stages after mixing. This low alkalinity period is however limited to the first hours after mixing, only. Indeed, as the hydration progressed, the pH values of the BYF pore solutions reached higher values, comparable to those exhibited by OPC cement pastes, Fig. 17a.

Beyond the first 24 hours of hydration of the BYF samples, several other phenomena may explain why the current density takes time to decrease, mainly in the case of chloride contamination (cf. Fig 11). Firstly the delay in the contribution of belite and ferrite to the hydration process of the BYF cement with 0.4% and, mainly, 1% chloride. Also important, matrices with 0.4 and 1% chloride remained more porous than the chloride-free samples for several weeks as seen in Fig. 14.

Another major point concerns the free chloride availability in the different systems. As shown in Fig 17b , unless for the very first stages of curing, the concentration of free chlorides is lower for OPC than for BYF samples. Even if the free chloride contents in BYF matrix decreased with time, they remained systematically higher than those measured for OPC. This indicates that BYF cement pastes are less efficient to uptake free chlorides. Indeed, in the case of OPC matrices, chloride binding is mostly due to their physical interaction with the C-S-H phase [START_REF] Luping | Chloride binding capacity and binding isotherms of OPC pastes and mortars[END_REF]. Chlorides can also be incorporated into calcium monosulfoaluminate from the partial substitution of sulphates leading to the formation of Friedel's salt. On the other hand, the hydration of BYF cement leads to the formation of ettringite, alumina hydroxide and calcium monosulfoaluminate at early age and then etttringite, straetlingite, calcium monosulfoaluminate, siliceous hydrogarnet and C-S-H (present in very few amounts as show by Lothenbach et al [START_REF] Lothenbach | Hydration of Belite-Ye'elimite-Ferrite cements: Thermodynamic modeling[END_REF]). Probably, the only phase playing an efficient role in chloride binding in BYF matrices is calcium monosulfoaluminate, as shown by Paul et al. [36]. This can explain the limitation of the BYF matrix to bind chloride in comparison to OPC.

The evolution of porosity and free chloride concentrations in the BYF matrices for several weeks bring us elements for understanding the high frequency answer of the EIS measurements.

The electrolyte resistance (Re) -assimilated to the diameter of the high-frequency loop -is the result of a counterbalancing effect of the hydration processes (overall tendency to Re increase) and the availability of free-chlorides in the pore/channels network solution (overall tendency to Re decrease). By trapping free ions and by creating a complex or discontinuous network of pores and microchannels that can hamper ion mobility, hydration is expected to induce an increase in the electrolyte resistance Re, which effectively happened, mainly in the case of BYF samples, as seen in Fig. 9.

Re values are also influenced by the moisture content of the specimens (environment) and the composition (formulation). In addition, it depends on the individual properties of each phase (unhydrated cement, hydrated products, aggregates, and pore solution), and how they intervene in the system. The direct comparison between the absolute Re values from OPC and BYF specimens is discouraged, since their mineralogy and phase assemblage are different. However, it is possible to qualitatively compare reinforced samples based-on the same cement type. Since the samples were stored in a 95% R.H. chamber, it is observed that resistance increases as the hydration progresses for both OPC and BYF reinforced mortars, Fig. 18. However, increasing the amount of Cl induced a decrease of the Re values in BYF but resulted in their increase in OPC equivalents. This means that no only reinforced BYF mortars are more likely to be impaired by the Cl presence in terms of steel corrosion but also regarding the hydrations progress and the related microstructural densification. As expected, the EIS measurements could reveal the double effect of the chloride addition during mixing on both the corrosion of steel and the development of the bulk microstructure of the cement mortar.

Conclusion

• Steel reinforcement embedded in BYF chloride-contaminated mortars are more prone to corrode than in OPC equivalent, specially at early ages. Chloride contaminations up to 0.4% by mass of cement are admissible, regarding corrosion, to both reinforced BYF and OPC cement mortar with water to cement ratio equals to 0.50. Higher w/c ratio decrease the tolerable endogenous chloride content from 0.4%, BYF (w/c = 0.50), to 0.2%, BYF (w/c = 0.67).

• Due to both pH and chloride binding during hydration, especially in BYF specimens, the electrochemical responses of the steel rebar evolve considerably during 1 year of survey.

• Sodium chloride contaminations have an acceleration effect on the hydration of OPC cement pastes, resulting in a reduction of the porosity and, therefore, increasing the compressive strength development.

• Sodium chloride contamination accelerates the hydration of ye'elimite but retard the hydration of belite and ferrite. The overall effect is delaying the hydration up to 56 days.

As a result, the increasing of the endogenous NaCl content from 0.4 to 1.0% by mass of cement leads to the increase of the porosity, which impairs the compressive strength gain during the first 56 days.

List of Captions

Table 1 Mortar mixtures used in the production of different reinforced mortars. following the empirical criteria for probability of corrosion suggested by ASTM C876 [START_REF]Standard test method for half-cell potentials of uncoated reinforcing steel in concrete[END_REF] and by RILEM recommendations [START_REF] Andrade | Test methods for on-site corrosion rate measurement of steel reinforcement in concrete by means of the polarization resistance method[END_REF] included. Data for Cl-free samples taken from Koga et. al work [START_REF] Koga | A comparative study of mild steel passivation embedded in Belite-Ye'elimite-Ferrite and Porland cement mortars[END_REF]. 34 The reference data without chloride addition are from [START_REF] Schneider | Sustainable cement production -Present and future[END_REF] performed in the same conditions. following the empirical criteria for probability of corrosion suggested by ASTM C876 [START_REF]Standard test method for half-cell potentials of uncoated reinforcing steel in concrete[END_REF] and by RILEM recommendations [START_REF] Andrade | Test methods for on-site corrosion rate measurement of steel reinforcement in concrete by means of the polarization resistance method[END_REF] included. Data for Cl-free samples taken from Koga et. al work [START_REF] Koga | A comparative study of mild steel passivation embedded in Belite-Ye'elimite-Ferrite and Porland cement mortars[END_REF].

5 .

 5 EIS measurements were performed to go further in the analysis of the time-varying impact of the presence of endogenous chlorides on the electrochemical behaviour of reinforced mortar.Results in what followsconcern 1, 28, 90, and 365 days of curing to represent fresh-, middle-, and long-term curing samples. The Nyquist plots of reinforced Portland mortars at different curing times and chloride contents are displayed in Fig. As a general trend, diagrams corresponding to Cl contamination up to 0.4% show a very similar behaviour, close to the results from the Cl-free samples for all the different curing periods, from 1 to 365 days, as clearly seen in the Bode plots insets in Figs 5a and

Fig. 5

 5 Fig. 5 also points out that the curing time has a positive effect on the corrosion resistance.

Fig. 1

 1 Fig. 1 Rietveld quantitative phase analysis of the amount of the unhydrated phases of a) OPC, and

Fig. 2

 2 Fig. 2 Schematic view of the test set-up used to perform electrochemical measurements on lollipop-

Fig. 3 a

 3 Fig. 3 a) Time evolution of the corrosion potential of reinforced OPC (w/c = 0.50) mortars with

Fig. 4

 4 Fig. 4 Anodic polarization curves, after 365 days, for reinforced OPC (w/c = 0.50) mortars with

Fig. 5

 5 Fig. 5 Nyquist plots of Cl-contaminated OPC mortars measured after a) 1 day, b) 28 days, c) 90

Fig. 6

 6 Fig. 6 Visual inspections, after 1 year, of steel reinforcements embedded in OPC mortars with

Fig. 7

 7 Fig. 7 Electrochemical measurements, over time, for reinforced BYF (w/c = 0.50) with different

Fig. 8 Fig. 9

 89 Fig. 8 Anodic polarization curves, after 365 days, for BYF (w/c = 0.50) mortars with different

Fig. 9

 9 Fig. 9 Visual inspections, after 1 year, of steel reinforcements embedded in BYF (w/c = 0.50)

Fig. 10

 10 Fig. 10 Visual inspections, after 1 year, of steel reinforcements embedded in BYF (w/c = 0.50)

Fig. 11

 11 Fig. 11 Effect of initial chloride contamination level on the calorimetry curve and on the percentage

Fig. 12

 12 Fig.[START_REF] Broomfield | Corrosion of steel in concrete[END_REF] Impact of the initial addition of 1.0% Cl by cement mass, from NaCl addition in the mixing

Fig. 13

 13 Fig.[START_REF] Thomas | Concrete Durability[END_REF] Relationship between the derivative Hg volume and pore radius for 1-, 7-, and 28-day old

Fig. 14

 14 Fig.[START_REF] Angst | Chloride induced reinforcement corrosion: Electrochemical monitoring of initiation stage and chloride threshold values[END_REF] Relationship between the derivative Hg volume and pore radius for 1-, 7-, and 28-day old

Fig. 15 Fig. 16

 1516 Fig.[START_REF] Hausmann | Steel corrosion in concrete -How does it occur?[END_REF] Impact of chloride addition (% by cement mass, from the addition of NaCl to the mixing

Fig. 17 a

 17 Fig. 17 a) Evolution of the pH of OPC and BYF cement pastes over 1440 min (24 h) and b)

Fig. 18

 18 Fig.[START_REF] Gartner | Project Aether[END_REF] Evolution of the OPC and BYF electrolyte resistance over time and for different amount

Fig. 1

 1 Fig. 1 Rietveld quantitative phase analysis of the amount of the unhydrated phases of a) OPC, and

Fig. 3 a

 3 Fig. 3 a) Time evolution of the corrosion potential of reinforced OPC (w/c = 0.50) mortars with

Fig. 4

 4 Fig. 4 Anodic polarization curves, after 365 days, for reinforced OPC (w/c = 0.50) mortars with

Fig. 5 Fig. 6

 56 Fig. 5 Nyquist plots of Cl-contaminated OPC mortars measured after a) 1 day, b) 28 days, c) 90

Fig. 7

 7 Fig. 7 Electrochemical measurements, over time, for reinforced BYF (w/c = 0.50) with different

Fig. 8

 8 Fig. 8 Anodic polarization curves, after 365 days, for BYF (w/c = 0.50) mortars with different

Fig. 9 Fig. 10

 910 Fig. 9 Nyquist plots of Cl-contaminated BYF (w/c = 0.50) mortars measured after a) 1 day, b) 28

Fig. 12

 12 Fig. 12 Impact of the initial addition of 1.0% Cl by cement mass, from NaCl addition in the mixing

Fig. 13

 13 Fig.[START_REF] Thomas | Concrete Durability[END_REF] Relationship between the derivative Hg volume and pore radius for 1-, 7-, and 28-day old

Fig. 14

 14 Fig. 14 Relationship between the derivative Hg volume and pore radius for 1-, 7-, and 28-day old

Fig. 15

 15 Fig. 15 Impact of chloride addition (% by cement mass, from the addition of NaCl to the mixing

Fig. 16

 16 Fig. 16 Concentration evolution of potassium (K), aluminum (Al), sulphates (S), calcium (Ca), and

Fig. 17 a

 17 Fig. 17 a) Evolution of the pH of OPC and BYF cement pastes over 1440 min (24h) and b)

Fig. 18

 18 Fig. 18 Evolution of the OPC and BYF electrolyte resistance over time and for different amount

  .

Table 1

 1 Mortar mixtures used in the production of different reinforced mortars. Sodium Chloride AnalaR NORMAPUR® ACS (purity > 99.5).

		Mix label
	OPC (0.50)	BYF (0.50)	BYF (0.67)

a CHRYSO® Fluid

Premia 196. b

Acknowledgements

The authors gratefully acknowledge the financial support of the Brazilian institution CNPq (grant number 229886/2013-2), of the French institution ANRT (grant number 2013/1440), and of the LafargeHolcim group.

Fig. 11 Effect of initial chloride contamination level on the calorimetry curve and on the percentage of bound water (calculated from TGA); (a) and (b), OPC (w/c = 0.50) cement pastes and (c) and (d), BYF (w/c = 0.50) cement pastes.


domain: chim spi sde

text:
Siteng Tieng

Zixian Jia
email: zixian.jia@lspm.cnrs.fr

Sana Labidi

Ana Paola Diaz-Gomez

Pierre Trevino

Eric M Eloy

Khay Gaigneaux

Andreï Chhor

Kanaev

Ana Paola Diaz-Gomez Trevino

Pierre Eloy

Eric Gaigneaux

Khay Chhor

Andrei Kanaev

Major non-volatile intermediate products of photo-catalytic decomposition of ethylene

Keywords: Photocatalysis, process kinetics, Fe-doped TiO2 nanoparticles, ethylene, non-volatile products, polyfuran, polyfurfural

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

I. Introduction

Photocatalysis in the gas phase has attracted much attention during recent years because of its wide potential for air purification. The most commonly used photocatalyst is titanium dioxide (TiO2) known for its photocorrosion resistance and low cost. The photocatalytic activity of this material is due to the generation of the electron-hole pair under UV light illumination. However, the major problem that restraints its industrial implementation is related to low quantum yield of the photooxidation reactions [1]. Cationic and anionic doping of TiO2 is one of the typical approaches to enhance photocatalytic efficiency under UV light illumination. Researchers have investigated the effects of transition metal such as iron [2][3][4], which is much cheaper than noble metal such as Ag [5] and Au [6] on reducing band gaps, decreasing electron and hole recombination rate, and using visible light.

The heterogeneous photocatalysis generally involves intermediate reaction products, which desorption and reactivity affect the overall process efficiency. Their knowledge is of particular importance for the process optimization and rigorous comparison of the material activities. Between different pollutants, ethylene is often considered as the model one because of its seemingly simple decomposition scheme involving few intermediate reaction products. In the same time, the ethylene abatement remains one of important environmental tasks. The photocatalytic oxidation of ethylene has been largely reported in literature [7][8][9][10][11]. Several studies have addressed intermediate products of the ethylene photodegradation. Bhattacharyya et al. [12] have observed formic acid as the main byproduct, while Wang et al. [13] have reported on the formation of aldehydes and alcohols. Einaga and Teraoka [14] have analyzed the ethylene photooxidationon TiO2 doped with platinum. In the absence of water vapor, absorption bands in the range of 1200-1800cm -1 have been observed and attributed to oxygenated compounds containing C-O, COO -and C=O groups. The concentration of these intermediate species decreases with the irradiation time in presence of water vapors, leading to their complete mineralization into CO2. Several studies [15][16][17] have reported on the formation of ethane C2H6, while oxygenated hydrocarbons CH3CHO, CH3COCH3 and CH3COC2H5 were present in a small quantity. In the same time, non-volatile reaction products, capable remaining on the catalyst surface and affecting the process kinetics and reaction yield, have not been investigated in previous studies, probably because of their not easy detection.

Previously, we have reported on the effective ethylene decomposition using nanoparticulate TiO2 coatings homogeneously doped with Fe(III) [18,19]. A net difference in the photocatalytic kinetics between the first and subsequent reactor runs has been observed on the initial process stage, which could be related to a participation of unknown reaction products. In the present communication we report on the kinetics analysis of gaseous ethylene photocatalytic decomposition on nanoparticulate Fe(III) doped anatase titania. We assign major non-volatile reaction products and provide a theoretical model explaining their influence on the reaction kinetics.

II. Experimental methods

Sample preparation

The Fe(III)-doped TiO2 anatase photocatalyst with the optimal composition was used in this work because of its higher reaction efficiency compared with a pure titania. The preparation procedure has been described in our previous works [18,19]. In short, the material was obtained in the two-stage process including (1) fabrication, doping and immobilization on a glass support of titanium-oxoalkoxy (TOA) nanoparticles, followed by (2) thermal treatment of the nanoparticulate coating. The doping of TOA nanoparticles with Fe 3+ ions was achieved at the nucleation stage. The homogeneity of the coating doping has been evidenced in the previous studies. The colloids have been prepared using hydrolysis ratio H= 2.0, which corresponds to one of the stability domains of the TOA sol-gel process and enables metastable chemically active monodispersed nanoparticles of size 2R=5.2 nm. The concentration of iron in TiO2 was fixed to 0.005 mol%, which corresponds to the best photocatalytic activity of the coating [19]. The reactive colloid has been set in a short contact with the supporting material, preliminary treated with 95 mol% sulfuric acid at 70 °C for 4 hours and washed in ultrasound bath. The beads with the deposited photocatalyst were heat treated at 500 °C for 4 hours to obtain anatase crystalline phase.

For the synthesis, analytical grade reagents and distilled water were used. Iron (III) acetylacetonate of 99.9% purity was supplied by Sigma-Aldrich. Titanium tetraisoproxide (TTIP) and 2-propanol with respectively 98% and 99.5% purity were purchased from Acros Organics.

Sample characterization

The Fe(III)-doped TiO2 anatase photocatalyst preparation was previously reported by our group [18,19]. The characterization of catalyst was summarized in supporting information. From Fig. S1, it is observed that the surface with mean roughness of ~0.5 nm and composed of agglomerates formed by particles of average size ~6 ± 3 nm which was also evidenced by TEM image (Fig. S1). The XRD (Fig. S3) indicates that Fe(III)-TiO2 powder well crystallizes in the anatase phase in the considered temperature range. Fig. S4 displays XPS spectra of the doped samples in two relevant energy ranges of 450-470eVand530-540eV. In agreement with studies of pure TiO2,photoemission bands appearing at 464.5 ± 0.2 eV and 458.7 ± 0.2 eV can be attributed to Ti 2p 1/2 and Ti 2p 3/2 states of Ti 4+ and the signal associated with singlet oxygen was also observed at 530.0 ± 0.4 eV with a shoulder at 531.5 ± 0.3 eV.

Fig. S5 shows typical XPS spectrum of our Fe(III)-TiO2 coatings in the energy range 700-745eV.Because of the low Fe concentration, two weak signals were observed at 709.9 eV and 723.5

eV as well as the satellites of a very low intensity.

Photocatalytic experiments

The experimental conditions are listed in Table 1. The photocatalytic experiments were performed with a laboratory gas-phase continuous flow fixed-bed reactor depicted in Fig. 1. In short, air gas flow containing pollutant and water vapour passes through the reactor tube made of transparent glass in the UVA spectral region of λ≥320 nm. The reactor tube of 6 mm diameter is surrounded by six 8-W lamps emitting at 362 nm (∆λhwfm= 22 nm).

Ethylene concentrations before (Cin) and after (Cout) the reactor tube were analysed by a gas chromatography device (Varian CP 3800) equipped with two injection loops allowing measurements in a continuous sampling mode. The temperature of the photocatalytic bed was continuously monitored. The pollutant conversion (or reactor yield) is calculated according to η(%)=100⋅(1-Cout/Cin).

III. Results and discussion

Intermediate reaction products

The ethylene conversion kinetics over the freshly prepared photocatalyst shows three main stages as depicted in The slow intermediate stage 2 was only observed with the freshly prepared photocatalyst. This stage was absent with the reused material where the pollutant conversion rapidly attained the maximum level. The strong increase (almost twice) of the process efficiency during stage 2 suggests significant modification of the reactive interface that can be related to the accumulation of reaction products. Apparently, the known acetaldehyde [20] or CO2-CO [21] cannot be candidates since do not accelerate the decomposition kinetics and may only slow it down by closing the active surface sites. In the same time, the gas chromatography analysis at the reactor exit did not evidence any significant traces of new products except for ethylene, CO/CO2 and water. It is assumed therefore that the major kinetics modification at this stage is due to an unknown non-volatile product accumulated at the photocatalyst surface.

In order to confirm the presence of non-volatile intermediate products, accumulated during the transitory stage 2, photocatalyst already used in the process (which don't manifest the transitory stage and rapidly attains stationary performance) was exposed to a secondary thermal treatment. Such heat treatment under ambient air is expected to sublimate or burn the deposited species that converts the reactive interface to its original state. The photocatalytic test was performed after the secondary heat treatment in order to check if the reaction kinetics recovers. The results are shown in Fig. 3 the photocatalysts heat treated at temperatures (a) 60 °C for 3 hours under UV illumination (this reference series reproduces the reactor conditions), (b) 100 °C for 3 hours, (c) 200 °C for 3 hours and (d) 300 °C for 1 hour. One can see that the used photocatalysts exposed to temperatures T≤200 °C (Fig. 3a,b,c) do not regain the original kinetics, while that exposed to T=300 °C (Fig. 3d) restores kinetics of the freshly prepared material. These results indicate that the intermediates product involved in the photocatalytic process of ethylene decomposition has relatively high stability against desorption at the photocatalyst surface. Previous DSC-TGA measurements have evidenced the departure of the adsorbed alcohol and water at temperatures ~80 °C [22], which indicates that, the surface species are much heavier in our case. Consequently, it was suggested that the intermediate reaction products of the ethylene decomposition are strongly bound to the surface. Commonly, the temperature range 200 °C < T ≤ 300 °C corresponds to the burning of surface organics [23]. It is concluded therefore that photocatalytic decomposition of ethylene includes as an intermediate step the formation of stable nonvolatile products at the photocatalyst surface. disappear after being exposed to the temperature of 300 °C (d). This observation is consistent with the burning of organics chemically bound to the photocatalyst surface. The following assignment for the observed IR bands can be proposed. The large band around 3300 cm -1 belongs to O-H stretching mode of physisorbed water, surface hydroxyl groups and/or hydroxyl groups of by-product and the band at 1630 cm -1 is associated with O-H deformation mode. The weak band at about 2980 cm -1 is due to stretching vibration ν(CH) and the band at 1480 cm -1 is due to deformation vibration δ(CH2). The band located around 1360 cm -1 can be attributed to the deformation vibration modes δ(OH)+δ(CH2) [24].

The relatively weak bands appearing at 1150 and 1070 cm -1 were tentatively assigned to the deformation vibration δ(CH2) and antisymmetric stretching ν(C-O) [25] of alcohols. As mentioned above, several authors have reported the presence of carbonyl intermediates such as CH3CHO, CH3COCH3 or CH3COC2H5 [24,26,27]. In this work, the absence of the absorption band at 1700cm -1 allows to reject the presence of carbonyl derivatives as intermediates. Several studies have been reported in the past on organic photopolymerization in presence of semiconducting solids. Popovićet al. [28] have observed polymerization of methacrylic acid on CdS, CdS/HgS and CdS/TiO2 triggered by free holes photogenerated in valence band. Damm et al. [29] have compared UV photopolymerization of methyl tri-acrylate with P25-TiO2 and anatase TiO2 prepared by spray-hydrolysis. Strandwitz et al. [30] have examined the polymerization of pyrrole impregnated into mesoporous TiO2 with the goal to obtain organic-inorganic hybrids. Li et al. [31] have analyzed photopolymerization of diacetylene in the presence of TiO2 thin films. Ni et al. [32] and Dong and Ni [33] have performed the UVA assisted polymerization of methyl methacrylate in the presence of anatase TiO2 and P25 TiO2 particles dispersed in the aqueous monomer solutions. As these studies show, intermediates with high-molecular weight can be formed in photocatalytic experiments.

The formation of high molar weight intermediates, as non-volatile polymeric hydrocarbons, apparently took place under UVA illumination of ethylene as in our experiments. Raman spectra of the black (a) and white (b,c) solid products collected after the photocatalytic process followed by the photocatalyst heating at 400 °C.

The direct analysis of the non-volatile reaction product was complicated by its small surface mass density. In order to collect this product from the large photocatalyst surface, the heat treatment at temperature ~400 °C under dry air flow was used. As a result, two kinds of compact solid products were collected in the reactor output area: black particles and white semi-transparent filaments, which masses ~0.1-1 mg were sufficient for the Raman analysis. Raman spectra of these products are shown in Fig. 6. The black product (Fig. 6a) was identified with glassy carbon, which Raman spectrum shows two characteristic resolved disorder-induced D and first-order allowed G (E2g) bands [34]. The Raman spectrum of the white product in Fig. 6b shows a complex structure containing multiple vibrational bands; its assignment is less straightforward. Our analysis of available Raman spectra of ethylene and its oxidized and polymerized derivatives brought us to a conclusion about the formation of furan derivatives in the photocatalytic process [35]. In particular, the six observed bands between 900 and 1600 cm -1 shown in Fig. 6b fit the calculated Raman spectra of oligomers composed of 6 and 8 furan molecules [36]. Moreover, the band at 1730 cm -1 identified with the C=O vibration can be observed in polyfurane [37] and indicates the presence of the oxidized derivatives, the best candidate of which is furfural [35]. Because of this, we tentatively assign the deposited product to the furan and/or furfural oligomers (OlN). More evidence to the polyfurfural assignment can provide Raman analysis of the C-H vibration mode in the frequency range of ~3000 cm -1 presented in Fig. 6c. The observed spectral band has partially resolved structure, which belongs to CH groups of the molecule in different geometric positions. Given the structure of the furfural molecule shown in inset of Fig. 6c and tendencies of the frequency evolution [35], the subbands at 2875 cm -1 , 2915 cm -1 , 2937 cm -1 and 2970 cm -1 can be respectively assigned to the aldehyde CH stretching (4), antisymmetric and symmetricβ-CH stretching

(2 / 3) and α-CH symmetricstretching (1) modes. Somewhat lower frequencies of the bands compared to those of the single furfural molecule can be attributed to a partial polymerization leading to the formation of furfural oligomers.

As observed in our experiments, the deposited solid products can be removed from the photocatalyst surface by a heat treatment at temperatures ~300 °C, as show the experimental results of Fig. 2 and Fig. 3. The photocatalyst regeneration could be explained by the oligomers desorption and/or thermal decomposition. In the same time, polyfuran is known to resist temperatures up to 400 °C [37,38], which may explain the successful collection of a small part of this product. Apart of these bands, we observed several narrow peaks at 1041, 1580, 1600 and 1614 cm -1 which may be related to C-C vibrations of minor carbonaceous species.

According to the above assignment, the photocatalytic oxidation of ethylene results in the formation of short polymeric chains of furan and furfural molecules on the photocatalyst surface. An increase of temperature results in the desorption of larger oligomers and thermal decomposition of smaller ones. The glassy carbon is formed as results of the thermal decomposition. In contrast, the larger polyfurans and polyfurfurals are stable and survive after the thermal desorption. Although we were not optimised the desorption temperature permitting analysis of a full scope of non-damaged surface products, the collected oligomers OlN (N=6÷8) evidences their formation at the photocatalyst surface as one of major non-volatile products. Since the small Fe(III) doping 0.005 mol% does not provide any significant modification of the titania band gap energy, surface area, etc. and only improves the electron-hole separation efficiency [19], we expect the discussed mechanism of the ethylene decomposition is the general feature of TiO2-based photocatalysts.

Modeling

In this chapter, a kinetic model is proposed in order to suppose partial polymerisation of ethylene and formation of oligomers, which do not desorb from the photocatalyst surface. The physical significations of all introduced parameters were summarized in table 2: Where Eg/Eads, pE and p stand for gaseous/adsorbed ethylene, adsorbed polymeric products and gaseous reaction products (CO2), which are correspondingly defined as N/n, m and p in the following equations. The reaction order is 1. It is supposed that the adsorption/desorption process (1) is in equilibrium while photodecomposition reactions (2) and (3) are not.

The ethylene is known to be weakly adsorbed at the TiO2 surface (Ea~3 kCal/mol or ~5kT at room temperature), while the heavy ethylene oligomers are totally adsorbed. Therefore,

m k mN n m k nN k dt dm 3 0 ' ' 1 ' 1 ) / 1 ( - - + = (5) m k n k dt dp 3 2 + = (6)
The second term of above Eq. ( 5) is completed by a factor (1-m/n0), which takes into account the catalyst surface coverage by polymeric products. The equations ( 4)-( 5) can be reduced to

0 2 ' ' 1 3 ' 1 ' ' 1 0 ' 1 ) ( n m N k m k N k N k N n k dt dm - - - + = γ γ (7)
where

N k N k eq eq + = 1 γ .
Eq. ( 7) is solved in approximation of a small coverage of TiO2 by both monomers and oligomers:

m n n , 0 >>
(we remark that this corresponds to γ<<1). For the sake of simplicity, it is supposed that the associations "monomer-monomer" and "polymer-monomer" propagate with the same constants 12) is numerically solved in order to fit the experimental data with four free parameters α, β, k1 and keq. The best fit with α=0.023, β=0.9, k1=0.00055 and keq=0.012 is shown in Fig. 7 by dotted line. The agreement between the model and experiment is good except of the early reaction times.

m k k k v n m k n k v dz dN ) ( 2 2 1 2 3 2 0 3 2 γ γ - + - = + - = (11
Since the lamp attains a stationary luminosity within three minutes, the transient period t≤50 min is attributed to the activation of photocatalytic reactions. Indeed as the measurements show (Fig. 2), the reactor temperature increases at this stage.

Since the ethylene polymerisation progresses slowly, it is assumed that the ethylene decomposition is affected by the temperature increase: ). The photocatalytic process is generally found not very sensitive to temperature with activation energies in the range between 5 and 16 kJ/mol [39]. The apparent activation energy of ethylene degradation over TiO2 of 13.9-16.0 kJ/mol has been reported by Yamazaki et al. [39]. Our value of Ea fits the upper limit of the earlier reported values. It is concluded that the proposed theoretical model is quite satisfactory.

The proposed theoretical model successfully explained the experiment, suggesting polymerisation of ethylene initiated by UVA photons at the photocatalyst surface. In agreement with our previous studies of light-induced processes at the organic-TiO2 interface, the photogenerated hole can efficiently escape the inorganic solids into surrounding organics [40,41]. Its localisation on a C=C double bond of ethylene molecule will provoke its opening that initiates polymerisation process.

IV. Conclusion

In this work the decomposition kinetics of gaseous ethylene onto the optimized Fe(III)-TiO2 anatase photocatalyst with Fe/Ti ratio of 0.005 mol% was studied in a continuous-flow fixed-bed reactor. Based on FTIR, Raman and thermal post-treatment analyses, it was concluded about the formation of a major non-volatile intermediate reaction product in course of the ethylene decomposition, assigned to the oxidized polymeric hydrocarbons: furan and furfural oligomers OlN with N=6÷8. The analysis of the photocatalytic process kinetics showed that this product covers the photocatalyst surface and delays the stationary reaction kinetics.

A theoretical model is proposed explaining the process kinetics. The overall photo-assisted process therefore includes two main stages: (i) on the first one, the high-molecular mass product is synthetized enhancing contact with the photocatalyst surface (in contrast to weakly adsorbing ethylene molecules) and (ii) on the second one, the heterogeneous decomposition of the non-volatile species takes place. A weaker rate constant of the oligomers decomposition (as compared with ethylene gas) is compensated by their much stronger adsorption, which enhances the overall reactor yield. The oligomers decomposition can be thermally activated with Ea=19±2 kJ/mol.

Figure 1 .

 1 Figure 1. (a) Cut section and (b) cross section of the photocatalytic reactor and (c) Emission spectrum of the UV light.

Figure 2 .

 2 photocatalyst keeps over week without any signs of deactivation.

Figure 3 .

 3 Figure 3. Ethylene conversion of used Fe(III)-TiO2 photocatalyst subsequently heat treated in a dry air flow: (a) at 60°C during 3hours under UVA illumination, (b) at 100°C during 3 hours,(c) at 200°C during 3 hours and d) at 300°C during 1 hour. (Each photocatalyst has been studied in two repeated cycles).

Figure 4 .Figure 5 .

 45 Figure 4. FTIR spectra of Fe(III)-TiO2 photocatalyst (a) freshly prepared, (b) freshly prepared after being exposed to gaseous ethylene flow in dark for 3 hours and(c) after UVA illumination in presence of pollutant for 3 hours.

  Figure 6.Raman spectra of the black (a) and white (b,c) solid products collected after the

  represents the density per unitary volume of the active TiO2 surface sites available for adsorption). However, m slowly increases until 0 describing (2)-(3) are:

.

  Moreover in order to explain the long-lasted accumulation of new species, it is assumed order to obtain the output ethylene concentration, it should consider the pollutants propagation through the reactor bed. Then the reduction of N(z) is related to the surface sites liberation due to the decomposition of both ethylene molecules and polymerised chains and it is relate to the rate of the products exit: the pollutant flow rate through the reactor bed. It is assumed here that dm/dt is a slow function of time (characteristic time of the polymerisation is ~1 hour) while dp/dt is a quick function of time (characteristic residence time of gas molecules in the reactor bed is ~1 s). One can rewrite Eq.

Figure 7 .

 7 Figure 7.Ethylene concentration at the photocatalytic reactor exit versus time (experiment and modelling).

(

  mol is shown in Fig. 7 by solid line.The validity of the model assumptions was check in the following section. From the fit, the equilibrium constant is small indeed. The fit also results in the reaction constants ratio which signifies that the photocatalytic decomposition of ethylene is easier than that of oligomers. One can see that the assumption 1 gas speed: v=30cm/5.7s=5.3 cm/s=318 cm/min,

Table 1 :

 1 Experimental conditions of photocatalytic experiments

	Gas mixture flow rate	90 ml/min
	Reactor length	30 cm
	Photocatalyst mass	6,6 mg
	Ethylene concentration	34 ppm
	Relative humidity RH	13 %
	Pollutant residence time in the reactor tube	1,4 s

Table 2 :

 2 The physical signification of all introduced parameters

	Symbol									Physical signification
	Eg/Eads Gaseous/adsorbed ethylene
	pE	Polymeric products
	p	Gaseous reaction products (CO2)
	k	Reaction rate constant
	n0	Density per unitary volume of the active TiO2 surface sites available for adsorption
	n	Density per unitary volume of the active TiO2 surface sites available for ethylene
		adsorption
	m	Density per unitary volume of the active TiO2 surface sites available for polymeric
		side products adsorption
	y	Ratio between m et n0
	v	Pollutant flow rate through the reactor bed
	L	Reactor length (30 cm)
	Ea	activation energy
	This process can be triggered by the photogenerated hole transfer to C=C double bonds. Both
	adsorbed monomers and oligomers contribute to this process. The relevant reactions are:
								E	g	k  eq  → ←	E	ads	(1)
		E	ads	+	E	g	+	hv	k →  ' 1	pE
		pE	+	E	g		+	hv	k →  ' ' 1	pE	(2)
						E	ads	+	hv	k →  2	p

Acknowledgments

This work is supported by French C'Nano (IdF) network.

that the polymerisation reaction dominate over the decomposition products formation:

Then Eq. ( 7) can be reduced to:

( )

. Because γ<<1, Eq. ( 8) can be rewritten as:

The solution of Eq. ( 9) is