Henri Berthiaux
email: henri.berthiaux@mines-albi.fr

Léonard Legoix

Cendrine Gatumel

Mathieu Milhé

Analysis of powder flow and in-system rheology in a horizontal convective mixer with reclining blades

Keywords: Cohesive powders, convective blender, flow regimes, powder mixing, rheology none defined

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Analysis of powder flow and in-system rheology in a horizontal convective mixer with reclining blades

Léonard Legoix, Cendrine Gatumel, Mathieu Milhé,

Introduction

Powder blending is a key operation for many industries, such as pharmaceutical, agro-food industries, construction, or metallurgy. Technologies to mix powders can be divided into two groups: tumbling blenders for which the rotation of the vessel provokes the motion and subsequent mixing of the powder bed, and convective blenders for which powder is put into motion by the action of an impeller inside a fixed tank. For free-flowing powders of different particle sizes, the avalanching created by most of tumbling blenders enhances segregation [START_REF] Moakher | Experimentally validated computations of flow, mixing and segregation of non-cohesive grains in 3D tumbling blenders[END_REF], as well as it favors the aggregation of the particles for cohesive systems. Despite some difficulties that may arise in their cleaning procedures, the use of convective blenders is now generalized in the industry, because of their polyvalence to process any kind of systems. In the case of cohesive systems, they are the most recognized alternative for their ability to break agglomerates and create a better mixture at the smallest scale.

Convective blenders can be operated either in continuous or batch regimes, the first one gaining more ground every day, as a way to enhance productivity in factories. However, batch mixing processes are still the rule, because of their reliability to assess batches of products. For years, batch convective mixers have been designed through a simple principle: impellers mounted on a horizontal shaft rotating at a fixed speed, the main technology being the Lödige 1 mixer. This blender has been studied through experiments and simulations with powders of different shapes [START_REF] Laurent | Comparative study by PEPT and DEM for flow and mixing in a ploughshare mixer[END_REF][START_REF] Cleary | Particulate mixing in a plough share mixer using DEM with realistic shaped particles[END_REF]. Its impellers are designed like a ploughshare that penetrates into the powder bed progressively and creates side flows. Other complex designs of impellers exist like ribbons or screws, but blades are simpler and processed in different technologies of powder blending, like high-shear mixers [START_REF] Knight | Prediction of impeller torque in high shear powder mixers[END_REF] or the Triaxe 1 planetary blender [START_REF] Demeyre | Caractérisation de l'homogénéité de mélange de poudres et de l'agitation en mélangeur Triaxe 1[END_REF].

In the scientific literature, several papers studied the action of various blade systems on the powder with a physical approach, so as to get a better understanding of powder flow at the smallest scale. The action of a single flat rectangular blade over a granular bed has been studied since the 1960s in various works, through camera observations and torque measurements [START_REF] Bagster | The flow of granular material over a moving blade[END_REF][START_REF] Bagster | The measurement of the force needed to move blades through a bed of cohesionless granules[END_REF]. More recently, it has been investigated with the help of high-velocity camera acquisition, and analysis by Particle Image Velocimetry [START_REF] Radl | Flow and mixing of granular material over a single blade[END_REF], as well as modeling by discrete element method [START_REF] Chandratilleke | A DEM study of the mixing of particles induced by a flat blade[END_REF][START_REF] Siraj | Single-blade convective powder mixing: The effect of the blade shape and angle[END_REF]. These studies were focused on the influence of different parameters on rheology, such as blade angle or shape, blade speed, or blade position inside the powder bed. They proved that a recirculation zone on the surface in front of the blade exists and its size depends on blade position, angle, and velocity. This recirculation is a helpful mechanism in the general blending process. These fundamental studies that are essential to understand how powder moves around a moving obstacle are of limited interest since they concern the flow over a blade in two dimensions surrounded by transparent lateral walls along the flow direction. Indeed, this is not the case in a blending process that uses the motion of numerous impellers that can cover the vessel in the three dimensions of space. This is the reason why mixing or rheological studies performed during processing can add valuable information that can be applied directly in the industry.

At the level of the process, research works have been concerned with horizontal convective blenders [START_REF] Malhotra | Fundamental particle mixing studies in an agitated bed of granular materials in a cylindrical vessel[END_REF]Malhotra and Mujumdar 1990;Malhotra, Mujumdar, and Miyahara 1990a;Malhotra, Mujumdar, and Okazaki 1990b;[START_REF] Laurent | Influence of agitator design on powder flow[END_REF] and also vertical convective blenders [START_REF] Makishima | Experimental study on the power requirements for agitating beds of solid particles, and proposal of a new model[END_REF][START_REF] Zhou | Microdynamic analysis of the particle flow in a cylindrical bladed mixer[END_REF][START_REF] Conway | Granular flow and segregation in a four-bladed mixer[END_REF][START_REF] Chandratilleke | Effects of blade rake angle and gap on particle mixing in a cylindrical mixer[END_REF][START_REF] Remy | Experiments and simulations of cohesionless particles with varying roughness in a bladed mixer[END_REF][START_REF] Remy | Polydisperse granular flows in a bladed mixer: Experiments and simulations of cohesionless spheres[END_REF][START_REF] Halidan | Prediction of the mixing behaviour of binary mixtures of particles in a bladed mixer[END_REF][START_REF] Havlica | The effect of rotational speed on granular flow in a vertical bladed mixer[END_REF]. These papers provide valuable information about powder flow, rheology, and mixing kinetics. However, they are usually confined to free-flowing systems and mixers operating in single flow regimes.

The existence of different flow regimes and their impact on the prevalence of certain mixing mechanisms and mixing kinetics has been studied for tumbling blenders [START_REF] Mellmann | The transverse motion of solids in rotating cylindersforms of motion and transition behavior[END_REF]Mayer-Laigle 2012). It has been found that their knowledge is essential in the scaling-up procedure of mixers, as dynamics or kinematics similarities are never considered in the conception of powder mixers. The present work aims at identifying and studying flow regimes in convective blenders and can be placed in the continuity of these previous studies. For this, a transparent polyvalent prototype equipped with a torque measurement system has been designed. It aims at identifying the impact of different geometrical parameters, such as the number of blades, their inclination, the vessel's volume, the positioning of the blade system inside the vessel, and the inclination of the tank itself. Different kind of powders can be studied according to their rheological characterization (free-flowing or cohesive) that can be verified in situ. This work is restricted to the horizontal position of the mixer with four reclining blades mounted on the shaft and is divided into three main parts. First, powder motion and flow will be characterized through simple observation in order to identify the flow regimes in the blender. Then, a focus will be made on the mechanical power required to stir powders of different types, with different blade angles and different filling ratios through process rheometry. Finally, a general method to express the rheological measurements will be suggested for scale-up calculations.

Materials and methods

Powders considered

Two powders have been studied: a free-flowing one and a cohesive one. The free-flowing powder is fine semolina (Le Renard) and the cohesive powder chosen is lactose (Granulac140). Both particles are of polygonal shape, as shown by SEM pictures in Figure 1.

Table 1 shows the main properties of these two powders, d 50 being the median diameter measured by LASER diffraction with a Mastersizer3000 (Malvern) under an air pressure of 3.5 bars. The particle true density q p has been measured thanks to an Accumulator 1 Pyc 1330 (Micromeritics) with the 10 cm 3 cell. The bulk and tapped densities q b and q t have been measured with a volumenometer (Erweka 1 ) for a powder mass of 110 g. The tapped density has been measured after 1000 taps, which is enough to be sure that the powder cannot be more packed by the apparatus since the volume stabilizes around 300 taps. The Carr Index for semolina is smaller than 15%, which makes it an easy flowing powder. For lactose, the Carr Index is greater than 15% which is characteristic for powders that do not flow easily [START_REF] Leturia | Characterization of flow properties of cohesive powders: A comparative study of traditional and new testing methods[END_REF]. This attests that semolina can be considered as a free-flowing powder, whereas lactose is a cohesive system.

Convective blender prototype

A prototypal convective mixer has been designed and built in order to get a better understanding of powder flow regimes and governing mechanisms under very different geometrical configurations. The cylindrical tank, delimited by two lateral walls, is made of PMMA because of its mechanical resistance and so as to observe the progression of the mixing process (Figure 2). The vessel diameter is 24 cm, blade-to-wall distance is 1 mm, and blade width is 6 cm. The rotational speed can be varied between 2 and 20 rad.s 1 thanks to a gear-motor.

A torquemeter allows recording the motor torque between 0 and 15 Nm. Rotational speed and torque are acquired thanks to a Labview 1 program that records the torque at 1 Hz. The powder can be loaded thanks to a rectangular aperture in the cylinder.

The mixer has been conceived as a multifunctional tool, so as to study the influence of four parameters: the number of blades, the blade angle, the vessel angle, and the position of the walls with respect to the stirring system.

Blades

Between 1 and 4 blades can be mounted on the shaft, in any of the four locations available as shown in Figure 3. The distance between each blade tip and the vessel is about 1 mm. The blade angle a with respect to the shaft can also be adjusted blade by blade, between 0° and 180° (Figure 3). In the present study, the four blades will be mounted on the shaft, all at the same angle. Two angles will be studied: 0° and 33°. For a à 0°, the blades will be qualified as "straight blades" and for a à 33°, the blades will be qualified as "reclined blades".

Vessel

The vessel angle can be adjusted with the spinning table on which the vessel, the torquemeter, and the motor are fixed (Figure 4). The vessel angle can be chosen between 90° and 90°, with the angle orientation as shown in Figure 4. In this study, the vessel angle will be fixed at 0°, placing therefore the mixer in the horizontal position.

Walls

The powder remains inside the cylindrical vessel thanks to two disk-shaped lateral walls. Each wall is equipped with O-rings, giving a good sealing of the vessel to fine powders. They both can be placed everywhere on the shaft on each parts of the blades, so that the agitation device is not necessarily centered between these walls (Figure 5). For this work, the blades are centered at a wall-blade distance of 10 cm.

Experimental procedure

As stated above, the torquemeter allows performing in-process rheometry by measuring the torque exerted on the shaft during stirring. Each experiment begins by running the blades 5 min at a speed of 8 rad.s 1 in the empty vessel. Torques are then measured at a speed increasing from 3 to 18 rad.s 1 with a 3 rad.s 1 step. For each speed, the agitation is maintained during 5-10 min to be sure that the torque is stabilized, after which its mean value is calculated over 1 min and will be considered as the recorded empty torque. Then, the vessel is filled with the powder studied and after a conditioning step that lasts 5 min at 8 rad.s 1 , the same torque measurement procedure is repeated. For all studies, three filling ratios f are investigated: 0.42, 0.63, and 0.83. These filling ratios are calculated as follows:

f à m p q b V tank ; Ö1Ü
where q b (kg.m 3 ) is the bulk density of the powder, m p (kg) is the mass of powder, and V tank (m 3 ) is the volume of the blender, which is about 8.85 l. The power consumption can be calculated as P à x T f T 0 ; Ö2Ü where x (rad.s 1 ) is the rotational speed of the blade, T 0 (N.m) is the empty torque, and T f (N.m) is the torque with filled vessel. Dimensional analysis of this process leads to the definition of different numbers, such as the Froude number F r and the power number N p . These two numbers can be calculated as

F r à R:x 2 g ; Ö3Ü N p à P q b x 3 R 5 ; Ö4Ü
where g (m.s 2 ) is the standard gravity, x (rad.s 1 ) is the rotational speed of the blade, R (m) is the radius of the blade, q b (kg.m 3 ) is the bulk density of the powder, and P (W) is the power consumption. This approach is useful for scale-up calculations, or to predict power consumption at industrial scale, thanks to lab-scale or pilot-scale experiments that require less powder and consume less energy. With the rotational speeds considered in this work, the Froude numbers involved are 0.1, 0.4, 1.0, 1.8, 2.8, and 4.0. The different process parameters of each experiment are provided in Table 2. For these 12 combinations of powder cohesion, blade inclination, and filling ratios, the six rotational speeds listed above, ranging from 3 to 18 rad.s 1 , will be studied. Flow observations have been performed using a Canon 650D camera with an 18-55 mm lens. The same rotational speeds and filling ratios are studied as for rheology measurements. An exposure time of the digital sensor of 5 ms is used, which is the greatest duration available with the photoflash for this device. This is fast enough to strap up the powder flow, even for the greatest rotational speeds, which is useful to observe flow regimes. All the pictures are taken in the front view of the blender, meaning that the vector of the angular speed goes from the left to the right. The direction of the rotation is reminded on each figure with a flat white arrow, while black arrows highlight powder flows.

Attrition and temperature

Attrition and temperature elevation have been measured to ensure that powder properties do not change during and after the mixing operation. A size distribution analysis has been made after one typical series of measures with various rotational speeds (3, 6, 9, 12, 15, and 18 rad.s 1 ) at 10 min of agitation for each speed. The powder was sampled inside the zone agitated by the blades, the motor being turned off. The particle size distributions described by the three characteristic diameters d 10 , d 50 , and d 90 are almost the same.

The temperature was measured near the blades just after the stirring operation described before. The ambient temperature of the laboratory was about 22°C and the powder temperature reached 27°C. This observable temperature increase is not enough to influence significantly the torque exerted on blades.

Identification of powder flow regimes

Free-flowing material flow

Qualitative observations of different flow regimes in the agitated zone of the blender were made thanks to the pictures taken with the camera. Two main flow regimes have been distinguished for the free-flowing particulate system: the rolling regime and the cataracting regime. These names are given in analogy with those employed for tumbling blenders operating with free-flowing material, as in the classification given by [START_REF] Mellmann | The transverse motion of solids in rotating cylindersforms of motion and transition behavior[END_REF]. This can be adapted for the convective blender, since the powder flow has some similarities. Concerning the rolling regime, the powder follows the walls or the blades until it reaches a vertical position, and then rolls down by avalanches. At higher speeds, a cataracting regime can be observed. Once the particles are not in contact with the walls or the blades, they follow the same trajectory and do not roll by avalanching. This is characterized by projections of particles in the air.

The Froude number that compares the centrifugal force with gravity helps characterizing the flow regime that takes place in the blender. Knowing the rotational speed x (rad.s 1 ) and the distance R from the center of the shaft to the blade tip, the maximal centrifugal force provided by blades can be calculated and divided by the standard gravity g (Equation 3). Pictures taken while semolina is stirred show that when the Froude number is smaller than 1, the flow regime is rolling. This can be observed especially when the blade goes from a horizontal position to a vertical position, and is off the powder bed (Figure 5). The powder taken by a blade rolls by avalanches to the next blade, since the centrifugal force is less influent than gravity. Conversely, when the Froude number is greater than 1, the powder flow regime is cataracting. This is easy to observe since the powder is projected by the blade instead of rolling in front of it (Figure 6). For this flow regime determination, observations are characteristic of the flow in front of the blades, at the freesurface level. Indeed, for a filling ratio of 0.42, 0.63, and 0.83, a Froude number of 1 is always the border of a flow regime change (Table 3).

The influence of blade angle has also been observed. The same conclusions as for straight blades can be drawn: for a Froude number smaller than 1, the flow regime is rolling and when it is greater than 1, the flow regime is cataracting. However, if the blade angle chosen here does not change the flow regime, the powder flow is obviously different in other ways. For instance, an asymmetric axial flow has been identified. This can be observed in the front views of Figure 7, in which the powder moves axially from the right to the left of the vessel: blades throw the powder along a preferential direction, depending on the blade inclination. The pictures taken with reclined blades show that when the Froude number is too small, the powder can fall on the next blade, and part of it rolls down to the right. For higher blade speeds, with a Froude number greater than 1, the lateral projection of powder is more efficient since all the powder moved by the blades goes to the same lateral way, on the left. These qualitative observations highlight the central role of F r in the transport mechanisms involved in the stirring of free-flowing powders.

Cohesive powder flow

Experiments carried out with lactose show the same behavior to what concerns Froude numbers. A rolling regime and a cataracting regime can be identified when the Froude number is smaller or greater than 1, respectively. As for semolina, the filling ratio has no influence on these regimes.

However, a difference between semolina and lactose flow can be detected in the rolling regime. For semolina, all the powder carried by blades rolls down, but for lactose, the powder avalanching occurs by blocks that fall one after the other. This is due to cohesion forces between particles that do not allow them to fall separately (Figure 8). Concerning reclined blades, as for semolina, the same flow regimes of rolling and cataracting are observed, with an axial asymmetry of the flow (Figure 9). Concerning the rolling regime, the avalanche on the next blade of a part of the powder swept by the previous blade is also observed.

In-mixer rheology

Power consumption has been recorded for the six rotational speeds x, the three filling ratios f studied, and the two particulate systems. Each experiment has been repeated twice to evaluate measurements repeatability. The deviation of the power P 1 and P 2 between two identical experiments can be calculated as

DP à P 1 P 2 j j P 1
⇥ 100: Ö5Ü

The data are reproducible for semolina with straight blades, with ΔP values smaller than 8% (Table 4). When blades are reclined, data are less reproducible for semolina with values comprised between 0.4% and 20.1%. Concerning lactose, values of ΔP can reach 45.1% for straight blades and 57.0% for reclined blades.

Influence of blade inclination

Figure 10(a-c) reports the power consumption of the system for semolina, as a function of the rotational speed of the impeller for the three filling ratios investigated. For all the cases studied, P increases with x, as expected. The values of the power are close to each other whether the position of the blades is, especially at small rotational speeds (rolling regime). While promoted by the rotation of the blades, the flow of such particulate systems is still guided by the individual behavior of the particles rather than by the process conditions themselves. As a result, the effect of the blade angle is negligible in this regime. For the two highest speeds considered (cataracting regime), it seems that the power is slightly greater for straight blades than for reclined ones. This may indicate that the more vigorous action of the impeller in the cataracting regime has a deeper impact on powder flow, and as a result the conditions under which the process runs, such as blade angle, are about to be denoted. The agitation of lactose has been investigated in the same way, the results being given in Figure 11(a-c). The power consumption is approximately three times smaller for such a cohesive system than for a free-flowing powder. Here, particle-particle interaction helps counteracting particle weight and facilitates the motion of the blades that displaces blocks of powder. Power is also greater for straight blades than for reclined blades, especially for medium-to-high rotational speeds. The presence of a 33° blade angle induces shear of the powder bed in the same direction as that of the blade motion itself. This is more efficient to displace a cohesive system than a flat angle that offers its whole surface to the bulk. With a greater powder mass or filling ratio (i.e., f à 0.83), the difference is all the more marked and favors the use of reclined blades to stir cohesive systems in the present horizontal equipment configuration. In the cataracting regime, i.e., for rotational speeds above 15 rad.s 1 , the material being pushed as blocks of powder, less energy will be required for reclined than for straight blades.

Influence of filling ratio

The effect of the filling ratio f on the power P can be seen in Figure (12a-b), only for the case of the reclined blades, for both particulate systems.

For the free-flowing powder, the power increases with the filling ratio, whatever the rotational speed considered. As no compaction of such a system is expected, the bulk density does not change and the filling ratio is proportional to the powder weight (Equation 1). Figure 12(a) therefore reflects directly the fact that moving a greater powder mass requires a greater power consumption, in particular if this has to be done at a high speed.

For the cohesive powder, Figure 12(b) reflects a notably different tendency. Indeed, if the power increases between the first two filling ratios (0.42 and 0.63), it decreases for the highest value of f in the majority of the speeds considered. This can be attributed to a local decrease of bulk density of the powder conveyed by the blades or in their vicinity. This is more likely to be noted when the blender is filled and blades are immerged in the system, as the level of stress in the bulk is greater. It can also be noted that blade inclination has a lesser impact on the power than the filling ratio has.

The role of packing state in cohesive powders rheology

It has been observed that both the rotational speed and the filling ratio have an influence on power requirement to stir powders. For semolina, an increase in filling ratio and an increase of rotational speed imply an increase of power consumption. This has also been observed for a 180-250 µm sand in a three-bladed high-shear mixer in the past [START_REF] Knight | Prediction of impeller torque in high shear powder mixers[END_REF]. The motion of these systems is mainly driven by friction and their packing state is not influenced significantly by strains applied on the bulk. This means that the average contact number between particles in the bulk does not change enough to reduce or increase the influence of friction forces on rheology.

On the contrary, dry cohesive system packing state is more sensitive to the external forces that are applied on the bulk. Two packing states could be observed under small and strong external strength: 1. Under small forces applied, the cohesion force creates a network between particles, in turn creating a porous material demonstrating few inter-particle contacts. 2. On the contrary, when external forces are significant, the air inside these inter-particle pores gets out of the network, increasing the contact numbers between particles. This is the reason why cohesive powders are well known to show a great compressibility [START_REF] Carr | Evaluating flow properties of solids[END_REF]. So like free-flowing powders, the number of contact between particles is important, but the presence of cohesion forces between particles will create a more consolidated particle bed. This sensibility of the packing state of cohesive powders leads to a different rheological behavior in bladed mixers.

For instance, Cavinato et al. observed an increase of impeller torque with filling ratio during the high-shear mixing of a cohesive powder [START_REF] Cavinato | Scale-up effects on flow patterns in the high shear mixing of cohesive powders[END_REF]). On the contrary, in our study the value of the torque decreases for most of greater values of filling ratios. This behavior could be explained by a more tied packing of particles for greater values of f (higher than 0.63 in our study), creating more consolidated walls on each lateral side of the blades. As a consequence, the powder forming these vertical walls cannot be in contact with blades by slipping or shearing. Along with this phenomenon, a moving of blocs of particles in front of blades may take place, impeding the creation of a wider contact network of particles, which could exist in free-flowing systems interacting with impeller blades [START_REF] Boonkanokwong | The effect of the number of impeller blades on granular flow in a bladed mixer[END_REF]. Therefore, our measurements are in agreement with Cavinato et al., since their range of filling ratio remains relatively small (less than 0.4), and in this range, we also observed an increase of power consumption with filling ratio, which is a behavior similar to that of free-flowing material.

Dimensionless representation

The previous results of power measurements can be represented thanks to dimensionless numbers and correlations. The power is expressed as a power number N p and the agitation speed as a Froude number F r . As reported for powder stirring experiments in other studies of convective powder blenders [START_REF] Knight | Prediction of impeller torque in high shear powder mixers[END_REF][START_REF] André | Dimensional analysis of a planetary mixer for homogenizing of free flowing powders: Mixing time and power consumption[END_REF], the typical correlation that links those numbers is of the following form: N p à a:F b r : Ö6Ü

Establishment of a general correlation

The dimensionless correlation for semolina, with straight blades and a filling of 0.42, is given as an example in Figure 13. In the figure, the logarithms of dimensionless numbers are plotted and linear fits are performed in accordance with Equation ( 6). This representation shows that for Froude numbers smaller than 1 and for Froude numbers greater than 1, separated correlations should be considered.

The slope represents the b coefficient, the intercept being ln (a). The squared coefficients of variation, both greater than 0.99, demonstrate that the power correlation between N p and F r is verified. Concerning Froude numbers smaller than 1, meaning that the flow regime is rolling, the coefficient b is close to 1 and the coefficient a has a value of about 0.65. For the cataracting regime, b is close to 0.7 and a value is about 0.63. The coefficient b is therefore significantly different between the two regimes, while the coefficient a can be said to be the same. Similar correlations have been investigated for the other filling ratios and blade angle, still for the free-flowing material (Table 5). The coefficient b can be said to be independent of the filling ratio, and practically independent of the blade angle in the rolling regime for which its value is close to 1. It is quite sensitive to the flow regime in general, its value being close to 0.7 in the cataracting regime for straight blades and 0.8 for reclined blades, therefore marking a light impact of the blade angle that may be the transcription of the flow asymmetry that has been qualitatively evidenced. To what concerns coefficient a, it is particularly insensitive to the flow regime and to a smaller extent to the blade inclination. Its main acting parameter is undoubtedly the filling ratio that can triplicate its value while f is only doubled.

Table 5 also shows similar results for the cohesive material. The coefficient b is globally always equal to 1 in the rolling regime, except for the greatest filling ratios and when the blender is equipped with reclined blades (equals to 1.4). In the cataracting regime, while the correlation is of a regular quality, b is still close to 0.7 for straight blades and 0.8 for reclined blades. Once again, a filling ratio of 0.83 for reclined blades apparently drives to a different situation. As for semolina, the coefficient a is remarkably independent on the flow regime and the blade inclination, its value being the fact of the filling ratio, and the type of material: approximately three times less important for the cohesive powder than for the free-flowing powder.

Globally, it can be said that the coefficient a depends on the powder (kind, mass), whereas the coefficient b is more sensitive to a change in flow regime.

Comparison with a planetary blender

The planetary blender Triaxe 1 has been used in previous work on powder blending and rheology by our research team [START_REF] Demeyre | Caractérisation de l'homogénéité de mélange de poudres et de l'agitation en mélangeur Triaxe 1[END_REF][START_REF] André | Derivation of dimensionless relationships for the agitation of powders of different flow behaviours in a planetary mixer[END_REF][START_REF] Legoix | Rheology of cohesive powders in a pilot scale planetary blender[END_REF]). This blender is characterized by a dual movement of rotation and gyration that moves the blade support around the vessel. The suppression of the gyration leads to a movement of the four blades close to the rotation provided by the polyvalent blender studied in this paper. The ratio between the blade width L 0 b and the blade-wall distance d 0 w is the same as that of the polyvalent blender (Figure 14):

L b L 0 b à d w d 0 w : Ö7Ü
As for the horizontal blender, dimensional analysis can be performed for the Triaxe 1 blender, thanks to the introduction of a power number N p and a Froude number F r . These two numbers can be calculated as follows:

F r à D:x 2 ra 2:g ; Ö8Ü N p à P q b x 3 ra D 2 5 ; Ö9Ü
where x ra (rad.s 1 ) is the rotational speed of the blades, D à 0.448 m is the distance between two opposite blade tips, q b (kg.m 3 ) is the bulk density of the powder, and P (W) is the power consumption. N p is plotted against F r for both equipment in the different operating conditions already studied here ). It must be said that the data are not in the same range of Froude numbers, the Triaxe 1 covering smaller values while the polyvalent blender covers greater ones. However, there is a partial overlap particularly around F r à 1. Whatever the particulate system considered and the filling ratio, the log-log representation considered in the previous section leads to similar correlations for both technologies. This shows that the polyvalent convective blender designed for this work may be helpful for scale up or power prediction in blenders that cannot be equipped with torquemeters.

Concluding remarks

The polyvalent convective blender, built for this work, allows to investigate in-situ powder rheology induced by blades, and may be helpful in scale-up procedures and understanding of powder flow in different systems. This device has been studied for two powders of different kinds: fine semolina that is free-flowing and lactose, which is cohesive. Flow regimes have been identified for these two powders, three filling ratios (0.42-0.63-0.83) and two blade angles (0-33°). For Froude numbers smaller than 1, a rolling regime is evidenced and characterized by avalanches of powders at the free-surface. For Froude numbers greater than 1, a cataracting regime is observed: the powder is projected when it reaches the surface of the powder bed. However, the great differences in power consumption for the two powders highlight that the flow mechanisms in the powder beds might be completely different.

There is practically no effect of the blade angle on the power consumption for the free-flowing system considered, while it is significant for cohesive powders, especially for high filling ratios in the cataracting regime. This may be confirmed and investigated more into details in future works involving powders of different cohesions. When investigating the effect of the filling ratio on power consumption, a hypothesis on a possible variation of bulk density due to powder compaction under stressing process conditions has been given and may also need confirmation. Correlations between a power number and a Froude number through a simple power law have been established for free-flowing and cohesive powders. These relations involve two coefficients depending on the powder type (coefficient a) and on the flow regime considered (coefficient b) and are in good agreement with previous work.

In future work, we will focus on the influence of different configurations of blades, vessel and wall position, or vessel inclination, as well as combinations of these. This will give more insight into the flow mechanisms at play, especially for cohesive systems.

Figure 1 .

 1 Figure 1. SEM pictures of semolina (a) and lactose (b).

Figure 2 .

 2 Figure 2. Blending prototype showing the four rectangular blades and the adjustable lateral walls.

Figure 3 .

 3 Figure 3. The four blade positions in the polyvalent bladed blender, side view (a) and blade angle definition (b).

Figure 4 .

 4 Figure 4. Vessel inclination (M: motor, T: torquemeter).

Figure 5 .

 5 Figure 5. Observation of the rolling flow regime for semolina inside the convective blender equipped with straight blades, at F r à 0.1 and f à 0.63. The white and black arrows show, respectively, blade rotation and powder motion.

Figure 6 .

 6 Figure 6. Centrifuging flow regime for semolina inside the convective blender equipped with straight blades, at F r à 1.0 and f à 0.63 (left) and at F r à 1.8 f à 0.83 (right).

Figure 7 .

 7 Figure 7. Rolling flow regime (left) and cataracting flow regime (right) for semolina inside the convective blender equipped with reclined blades.

Figure 8 .

 8 Figure 8. Consecutive falling of lactose powder by blocks, at F r à 0.1 and f à 0.42, straight blades.

Figure 10 .

 10 Figure 10. Power requirements to stir semolina in the convective blender according to blade angles, for f à 0.42 (a), f à 0.63 (b), and f à 0.83 (c).

Figure 9 .

 9 Figure 9. Rolling flow regime (left) and cataracting flow regime (right) of lactose inside the convective blender equipped with reclined blades.

Figure 11 .

 11 Figure 11. Power requirements to stir lactose in the convective blender according to blade angles, for f à 0.42 (a), f à 0.63 (b), and f à 0.83 (c).

Figure 12 .

 12 Figure 12. Evolution of power consumption with filling ratio, according to different rotational speeds in the reclined blades case for either semolina (a) or lactose (b).

Figure 14 .

 14 Figure 14. Schematic view of the Triaxe 1 mixer showing some characteristic lengths.

Figure 13 .

 13 Figure 13. Representation of the correlation between F r and N p in the prototype equipped with straight blades, f à 0.42, semolina case.

Figure 15 .

 15 Figure 15. Comparison of the power-law correlations obtained in the Triaxe 1 blender and the actual prototype: semolina and f à 0.42 (a); semolina and f à 0.83; lactose and f à 0.42; lactose and f à 0.83.

Table 1 .

 1 Powder properties measured.

	Powder	d 50 (µm)	d 10 (µm)	d 90 (µm)	q p (kg.m 3 )	q b (kg.m 3 )	q t (kg.m 3 )	CI (%)
	Semolina 328 330 208 207 496 492 1463	679	720	5.8
	Lactose	62 62 17 17 144 144 1533	661	795	16.9
	In bold: Particle size values after agitation showing the absence of attrition.

Table 2 .

 2 Powder and process parameters investigated.

	Powder	Semolina-lactose
	Blade angle (°)	0-33
	f x (rad.s 1 )	0.42-0.63-0.83 3-6-9-12-15-18

Table 3 .

 3 Map of flow regimes for semolina agitation with the convective blender.

	f \ F r	0.1	0.4	1.0	1.8	2.8	4.0

0.42 Rolling Rolling Cataracting Cataracting Cataracting Cataracting 0.63 Rolling Rolling Cataracting Cataracting Cataracting Cataracting 0.83 Rolling Rolling Cataracting Cataracting Cataracting Cataracting

Table 4 .

 4 Deviations of the power measurements (%).

	Powder	Blades	f	3 rad.s 1	6 rad.s 1	9 rad.s 1	12 rad.s 1	15 rad.s 1	18 rad.s 1
	Semolina Straight 0.42	6.5	7.2	7.4	5.1	1.1	1.8
			0.63	3.2	3.7	0.3	1.1	1.3	1.6
			0.83	0.4	4.2	2.5	2.0	0.6	0.6
		Reclined 0.42 13.5	16.2	20.1	2.3	4.9	4.8
			0.63	7.4	4.9	3.7	0.4	1.5	1.7
			0.83	7.1	7.0	3.6	15.4	14.3	0.1
	Lactose Straight 0.42	7.7	8.4	8.3	0.8	10.9	11.2
			0.63 20.9	27.6	27.2	19.7	19.6	4.4
			0.83 17.9	45.1	42.0	18.4	15.8	8.6
		Reclined 0.42 11.1	7.7	57.0	28.2	30.4	11.8
			0.63 10.0	3.8	3.1	2.1	3.6	3.9
			0.83	4.4	5.6	20.3	12.8	11.2	6.8

Table 5 .

 5 Correlation N p à a:F b r coefficients for semolina and lactose agitation with the convective blender.

		Parameters		b		a		R 2
	Powder	Blades	f	Rolling	Cataracting	Rolling	Cataracting	Rolling	Cataracting
	Semolina	Straight	0.42	0.98	0.71	0.65	0.63	0.999	0.991
			0.63	0.95	0.67	1.15	1.18	0.999	0.999
			0.83	0.89	0.75	1.84	1.79	0.999	0.978
		Reclined	0.42	0.92	0.84	0.64	0.67	0.991	0.992
			0.63	0.93	0.79	1.30	1.32	0.998	0.998
			0.83	0.86	0.81	2.12	1.99	0.997	0.942
	Lactose	Straight	0.42	0.99	0.51	0.22	0.22	0.999	0.986
			0.63	0.96	0.72	0.50	0.51	0.998	0.991
			0.83	1.18	0.58	0.41	0.44	0.985	0.952
		Reclined	0.42	1.08	0.69	0.24	0.24	0.999	0.995
			0.63	1.07	0.84	0.47	0.49	0.998	0.999
			0.83	1.40	0.49	0.33	0.33	0.999	0.935

Acknowledgments

We would like to thank Asma Toualoua and Luana Grillet, internship students in our laboratory, for their help during rheology measurements that are presented in this work. Many thanks to Pierre Bertorelle for his technical ideas and dedication, both necessary for the elaboration of the blending prototype.


Fatima-Zohra Mezahi
email: mezahif@yahoo.fr

Anita Lucas-Girot

Hassane Oudadesse

Abdelhamid Harabi

Reactivity kinetics of sol-gel derived 52S4 glass versus the treatment temperature

Keywords: Sol-Gel process, Temperature, Simulated Body Fluid, Kinetics reactivity, Bone like-apatite. 1

This work is devoted to study the reactivity of the quaternary glass 52S4 (52 % SiO 2 -30 % CaO -Na 2 O -14% 4 % P 2 O 5 (wt. %)), synthesized by sol-gel process, versus the treatment temperature.

The dried gel was heat-treated at 600 and 650 °C and soaked in Simulated Body Fluid (SBF). XRD results confirm the amorphous character of glass treated at 600 °C even though the heat treatment at 650 °C induces Na 2 Ca 2 Si 3 O 9 formation. After soaking in SBF, SEM and EDS results show the formation of carbonated hydroxyapatite (HA) at the glass surface for both temperatures. For the glasses treated at 600 and 650 °C, two phenomena were observed: the glass dissolution in SBF and the carbonated HA precipitation but the reactivity kinetics of glass was different when temperature changes. For SGDG600, the carbonated HA began to crystallize after 16 h and became well crystallized after 15 days. For SGDG650, a glass ceramic made of a glassy matrix and of Na 2 Ca 2 Si 3 O 9 , the crystallized carbonated HA was observed after 2 h. In addition a new crystallization at the glass surface of Na 2 Ca 2 Si 3 O 9 was observed after 15 days.

Introduction

Bone defect filling or reconstruction required a wide research and development of biocompatible and bioactive materials. The bioactive material, in contact with physiological solutions, elicits a specific biological response at the interface of the material which results in the formation of a direct biochemical link (carbonated hydroxyapatite) when it is implanted in human body [START_REF] Hench | Bonding mechanism at the interface of ceramics prosthetic materials[END_REF]. Since Hench has launched the concept of bioactive glass, many new materials and products have been prepared from variations on bioactive glasses [START_REF] Hench | The story of bioglass[END_REF], glass-ceramics [START_REF] Kokubo | Bioactive glass ceramics: properties and applications[END_REF] and ceramics such as synthetic and natural hydroxyapatite (HA) [START_REF] Legeros | Properties of osteoconductive biomaterials: Calcium phosphates[END_REF][START_REF] Mezahi | Effect of stabilised ZrO 2 on sintering of hydroxyapatite[END_REF][START_REF] Mezahi | Effect of stabilised ZrO 2 , Al 2 O 3 and TiO 2 on sintering of hydroxyapatite[END_REF][START_REF] Mezahi | Dissolution kinetic and structural behaviour of natural hydroxyapatite vs. thermal treatment : Comparison to synthetic hydroxyapatite[END_REF][START_REF] Harabi | Sintering of bioceramics using a modified domestic microwave oven[END_REF][START_REF] Mezahi | Effect of ZrO 2 , TiO 2 , and Al 2 O 3 additions on process and kinetics of bonelike apatite formation on sintered natural hydroxyapatite surfaces[END_REF][START_REF] Harabi | Effect of B 2 O 3 on mechanical properties of porous natural hydroxyapatite derived from cortical bovine bones sintered at 1050°C[END_REF][START_REF] Harabi | Effect of P 2 O 5 on mechanical properties of porous natural hydroxyapatite derived from cortical bovine bones sintered at 1050°C[END_REF][START_REF] Harabi | Grain growth in sintered natural hydroxyapatite[END_REF][START_REF] Harabi | A modified milling system, using a bimodal distribution of highly resistant ceramics. Part1: A natural hydroxyapatite study[END_REF], wollastonite [START_REF] Harabi | Preparation process of a highly resistant wollastonite bioceramics using local raw materials[END_REF][START_REF] Chehlatt | Biological properties study of bioactive wollastonite containing 5 wt% B 2 O 3 prepared from local raw materials[END_REF][START_REF] Chehlatt | In vitro bioactivity study of pure wollastonite prepared from local raw materials[END_REF] and diopside [START_REF] Zouai | A process for sintering of diopside prepared from dolomite[END_REF][START_REF] Harabi | A New and economic approach to synthesize and fabricate bioactive diopside ceramics using a modified domestic microwave oven. Part 1: Study of sintering and bioactivity[END_REF]. Hench et al. have demonstrated that glass with the molar composition: 46.1 % SiO 2 , 24.4 Na 2 O, 26.9 CaO and 2.6 % P 2 O 5 , termed Bioglass ® 45S5, is able to develop, at the interface with body fluids, an apatite like layer similar to bone mineral. This bond is so strong that it could not be removed without breaking the bone [START_REF] Hench | Bonding mechanism at the interface of ceramics prosthetic materials[END_REF]. However, the poor mechanical strength and toughness of bioactive glass have restricted their use in several clinical applications. To resolve this problem and consequently to broad the range of clinical applications, the glasses were transformed into glass-ceramics [START_REF] Kokubo | Bioceramics and Their Clinical Applications[END_REF][START_REF] Hench | Biomaterials: a forecast for the future[END_REF][START_REF] Aza | Bioactive glasses and glass ceramics[END_REF].

Despite these benefits, contradictory results were published on the reactivity and biological interest of these glass ceramics (crystalline or semi-crystalline). They may be less soluble in body fluid and the mineral formation rate and bone integration at the tissue-material interface may be affected [START_REF] Greenlee | Glass ceramic bone implants[END_REF][START_REF] Peitl | Effect of crystallization on apatite layer formation on bioactive glass 45S5[END_REF][START_REF] Peitl | Highly bioactive P 2 O 5 -Na 2 O-CaO-SiO 2 glass-ceramics[END_REF]. At the same time, the crystallization of some glassy systems significantly decreases their bioactivity in comparison with the same amorphous glassy systems [START_REF] Kokubo | Bioceramics and Their Clinical Applications[END_REF][START_REF] Hench | Biomaterials: a forecast for the future[END_REF][START_REF] Aza | Bioactive glasses and glass ceramics[END_REF]. Li et al. [START_REF] Li | The effect of residual glassy phase in a bioactive glassceramic on the formation of its surface apatite layer in vitro[END_REF] showed that a bioactive glass can be transformed into an inert glass-ceramic. In vitro test showed that the carbonated HA layer was formed only if the glass ceramic contain over 90% of a residual glassy phase. Whereas, Peitl et al. [START_REF] Peitl | Effect of crystallization on apatite layer formation on bioactive glass 45S5[END_REF][START_REF] Peitl | Highly bioactive P 2 O 5 -Na 2 O-CaO-SiO 2 glass-ceramics[END_REF] have developed the first bioactive glass-ceramic in the SiO 2 -CaO-Na 2 O-P 2 O 5 system with both good mechanical properties and high bioactivity. It contains about 30 to 65% crystalline phase, and the main phase is Na 2 Ca 2 Si 3 O 9 . After that, Ravagnani et al.

developed a highly bioactive fully crystalline glass-ceramic in SiO 2 -CaO-Na 2 O-P 2 O 5 system (Biosilicate ® glass-ceramic) [START_REF] Zanotto | Process and compositions for preparing particulate, bioactive or resorbable biosilicates for use in the treatment of oral ailments[END_REF]. Almost all the works have shown that Na 2 Ca 2 Si 3 O 9 formation, observed in some glass-ceramics, enhances the mechanical properties of the starting glass and maintains the high bioactivity of particular compositions in the SiO 2 -CaO-Na 2 O-P 2 O 5 system.

Peitl et al. have shown that glass crystallisation decreased development kinetics of a carbonated HA layer but did not inhibit its formation, even in fully crystallized ceramics [START_REF] Peitl | Highly bioactive P 2 O 5 -Na 2 O-CaO-SiO 2 glass-ceramics[END_REF]. In 2011, Siqueira et al. have shown that Na 2 Ca 2 Si 3 O 9 , formed in SiO 2 -CaO-Na 2 O-P 2 O 5 system, has bioactivity behaviour similar to that of Biosilicate® glass-ceramic [START_REF] Siqueira | Gel-derived SiO 2 -CaO-Na 2 O-P 2 O 5 bioactive powders: synthesis and in vitro bioactivity[END_REF].

In the our previous works, a glass with the new composition (wt%) 52% SiO 2 -30% CaO-14% Na 2 O-4% P 2 O 5 (mol%: 52.3% SiO 2 -32.3%CaO-13.7%Na 2 O-1.7% P 2 O 5 ) (named 52S4) was synthesized both by melting and an original sol-gel route [START_REF] Lucas-Girot | Sol-gel synthesis of a new composition of bioactive glass in the quaternary system SiO 2 -CaO-Na 2 O-P 2 O 5 : Comparison with melting method[END_REF]. The effect of synthesis mode on glass bioactivity after soaking in simulated body fluid (SBF) was investigated [START_REF] Mezahi | Reactivity kinetics of 52S4 glass in the quaternary system SiO 2 -CaO-Na 2 O-P 2 O 5 : Influence of the synthesis process: Melting versus sol-gel[END_REF]. The dried gel was then heat treated at 550°C (below the crystallization temperature). The obtained results showed that the main differences between the Sol-Gel Derived Glass (SGDG) and Melting Derived Glass (MDG), from the structural and textural point of view, are induced by the synthesis mode. The obtained results have led to conclude that 52S4 is a bioactive glass if it is prepared by the two routes: melt and sol-gel. However, this glass was more reactive when it was prepared by sol-gel method. The sol-gel route enhanced fast and continuous glass resorption associated to the early formation of the bone like-apatite at the glass surface. However, the HA did not crystallized even after 30 days. This result was attributed to a continuous formation of silica gel up to the amorphous apatite layer.

The aim of the present study is to investigate a possible evolution in reactivity behaviour of the solgel derived glass by increasing the thermal treatment temperature previously used [START_REF] Lucas-Girot | Sol-gel synthesis of a new composition of bioactive glass in the quaternary system SiO 2 -CaO-Na 2 O-P 2 O 5 : Comparison with melting method[END_REF][START_REF] Mezahi | Reactivity kinetics of 52S4 glass in the quaternary system SiO 2 -CaO-Na 2 O-P 2 O 5 : Influence of the synthesis process: Melting versus sol-gel[END_REF].

Experimental section

Material preparation

The sol-gel process was used to prepare the following glass composition 52S4: (wt.%) 52% SiO 2 -30% CaO-14% Na 2 O-4% P 2 O 5 . The glass synthesis was described in detail previously [START_REF] Lucas-Girot | Sol-gel synthesis of a new composition of bioactive glass in the quaternary system SiO 2 -CaO-Na 2 O-P 2 O 5 : Comparison with melting method[END_REF]. The glass was prepared in a hermetically cylindrical Teflon container from high-purity raw materials as tetraethyl orthosilicate TEOS: Si (OC 2 H 5 ) 4 (>98%, Fluka), triethylphosphate TEP: OP (C 2 H 5 ) 3 (>99.8 %, Sigma-Aldrich), calcium carbonate CaCO 3 (>98.5%, Merck) and sodium carbonate Na 2 CO 3 (>99.5%, Merck). Accurately weighed amounts of these reactants were added, one by one each hour under magnetic stirring, to a 2 M aqueous acetic acidic solution. The resultant sol was kept for hydrolysis and polycondensation at room temperature for 6 days. After that, it was kept at 70°C for 3 days and dried at 150°C for 52 h.

In this study, 52S4 glass is studied in two different microstructural forms (amorphous and partially crystalline). The obtained dried-gel was thermally treated at 600 and 650°C (noted SGDG600 and SGDG650) during 3 h under vacuum with a heating rate of 0.5°C.min -1 . Let's remember that the domain of thermal stabilization of the dried gel is between the transition temperature T G (T G = 596°C) and the crystallization temperature T C (T C = 631°C) as determined by Thermo-Gravimetric associated with Differential Scanning Calorimetry (TG/DSC) analyses [START_REF] Lucas-Girot | Sol-gel synthesis of a new composition of bioactive glass in the quaternary system SiO 2 -CaO-Na 2 O-P 2 O 5 : Comparison with melting method[END_REF].

In vitro assays

To evaluate the kinetic reactivity of the prepared glasses, SBF (SBF-K9 type) was used as soaking solution which has the ion concentrations similar to those in human blood plasma. According to Kokubo's protocol [START_REF] Kokubo | Solutions able to reproduce in vivo surface-structure changes in bioactive glass-ceramic A-W[END_REF], the SBF was prepared by dissolving reagent-grade CaCl 2 , NaCl, KCl, MgCl 2 .6H 2 O, K 2 HPO 4 .3H 2 O and NaHCO 3 in distilled water. It was buffered at pH 7.4 with (hydroxymethyl)-aminomethane [(CH 2 OH) 3 CNH 2 ] and hydrochloric acid (HCl). The glasses were soaked in SBF in the form of disks. 0.5 g of the dried gel in powder, with particle sizes ranged from 40 to 63 μm, were compacted at 675 MPa under cold uniaxial pressing. After that, the compacted dried gel disks were heat treated under the conditions mentioned before. The ratio between the geometric areas of disks and the volumes of SBF was adjusted to 0.075 cm -1 . The glasses were kept in SBF at 37°C for time periods ranging from 2 h to 30 days (SBF immersion tests were carried out in triplicate). After soaking, the disks were emerged from SBF, washed with ethanol and dried in air.

Materials Characterization

Different techniques and apparatus were used to study the physical and chemical properties of prepared glasses before and after soaking in SBF.

The specific surface area and porosity measurements were performed using multi-point BET, ASAP 2010 Micromeritics). In addition, the phase compositions were identified using X-ray diffraction (XRD) data collected in a BRUKER, D8 ADVANCE with a CuKα radiation (λ = 0.154 nm, filter: Ni, voltage= 30 kV and current= 20 mA, step = 0.05°, scan step time= 10 s). To improve the structural analysis, Fourier Transformed Infra-Red spectroscopy (FTIR) (BRUKER EQUINOX 55) was used. 1 mg of material scraped from the glass surface was mixed with dried KBr to perform the FTIR studies. The microstructure of glass surfaces was observed using a Scanning Electron Microscope (SEM) (HITACHI, JSM-6301 F) working at a 7 kV as an accelerating voltage. Before observation and EDS analyses (Oxford Link INCA), the surfaces were gold-coated (Au-20 wt% Pd) in order to allow surface conduction. For the in vitro tests in SBF, Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) (SPECTRO Ciros Vision) technique was used to evaluate the variations of silicon, calcium and phosphorous concentrations versus soaking time in SBF.

Results

Characteristics of 52S4SGDG before soaking in SBF

Specific surface area and porosity

The BET specific surface area and pore volume of SGDG decreased when temperature increased from 600 to 650°C. Their values are in order: 18.26 m 2 .g -1 and 0.058 cm 3 .g -1 (for SGDG600) and 6.00 m 2 .g -1 and 0.013 cm 3 .g -1 (for SGDG650), respectively. Also, the median pore diameter decreased from 127 Å (for SGDG600) to 88.34 Å (for SGDG650).

X-ray diffraction

Figure 1 shows the XRD data for SGDG600 and SGDG650. The XRD of SGDG600 showed that the final product is mainly amorphous. However, the data reveal that the glass exhibited a very low crystallization of hydroxyapatite (HA) before any immersion in SBF, as for the glass treated at 550°C [START_REF] Lucas-Girot | Sol-gel synthesis of a new composition of bioactive glass in the quaternary system SiO 2 -CaO-Na 2 O-P 2 O 5 : Comparison with melting method[END_REF]. In addition, two diffusion halos are observed in the range 27°-34° and 15°-23° (2θ).

Infrared studies

Figure 2 shows the FTIR spectra of all glasses. They exhibit the same IR bands. As discussed in early work [START_REF] Lucas-Girot | Sol-gel synthesis of a new composition of bioactive glass in the quaternary system SiO 2 -CaO-Na 2 O-P 2 O 5 : Comparison with melting method[END_REF], the observed bands at 500, 762 and 1035 cm -1 are assigned to silicate network vibrations Si-O-Si. The band observed at 606 cm -1 is related to P-O bending in crystalline HA [START_REF] Sepulveda | In vitro dissolution of melt derived 45S5 and sol-gel derived 58S bioactive glasses[END_REF][START_REF] Elbatal | Characterization of some bioglass-ceramics[END_REF]. This band was reinforced with temperature mainly at 650°C, which confirms the crystallisation of HA during heat treatment. In the other hand, the band located at 500 cm -1 become more intense when the temperature increases. This band may be attributed to the crystal phase Na 2 Ca 2 Si 3 O 9 [START_REF] Peitl | Highly bioactive P 2 O 5 -Na 2 O-CaO-SiO 2 glass-ceramics[END_REF][START_REF] Chang | FT-IR study for hydroxyapatite/collagen nanocomposite cross-linked by glutaraldehyde[END_REF]. The bands at 933 and 762 cm -1 , related to Si-O-Si stretching of non-bridging oxygen atoms [START_REF] Elbatal | Characterization of some bioglass-ceramics[END_REF], are better defined in all spectra. The bands at 870, 1419 and 1496 cm -1 are related to the presence of CO 3 2-groups [START_REF] Moura | In vitro osteogenesis on a highly bioactive glass-ceramic (Biosilicate®)[END_REF]. The intensity of these bands is weak.

Fig. 2. FTIR spectra of SGDG heated at 600°C and 650°C.

In vitro assays results

In order to evaluate the formation kinetics of bone like apatite at surface of soaking glass in SBF, the pH and variations in Si, Ca and P concentrations in SBF were measured. The study was accomplished by analyzing the formed apatite layer at the glasses surface by XRD, FTIR and SEM-EDS techniques.

ICP results and pH measurements

Figure 3 presents ICP results and pH measurements.

The Ca concentration evolution followed a similar tendency for the two materials (Fig. 3-a). For the both materials, it decreased slowly at the first soaking hours. After that, for SGDG600, this concentration increased slowly until 1 day where it increased dramatically and reached a maximum value. The maximum calcium release from glass surface is achieved after 7 days. After that a continuous decrease in Ca concentration was observed up to 30 days. For SGDG650, the maximum value for the Ca concentration in SBF was reached after 3 days, then decreased strongly and remained constant in the interval time 7-30 days.

During the experiment, P concentrations in SBF decreased continually (Fig. 3-b) for the both materials. This indicates that the amount of P released from glass or glass-ceramic surfaces did not compensate its consumption due to the apatite formation. It can be observed that P concentrations were nil in SBF where SGDG600 was soaked after 7 days and after 15 days for SGDG650. All P ions were consumed from SBF.

The increase in Ca concentration in SBF is a result of Ca ions leaching from SGDG surface to SBF.

However, the decrease in the Ca and P concentrations in SBF is attributed to the rapid growth of the apatite at the glass surface that overcame the release rate of Ca and P ions to SBF [START_REF] Martinez | Bioactivity of CaO-SiO 2 binary glasses system[END_REF].

As SBF is initially silicon -free, the presence of Si in SBF is consistent with a release of this element from the glass surface to the soaking solution. The Si concentration in SBF increased with soaking time for both materials (Fig. 3-c). The Si concentration increased strongly in the first hours for SGDG600 and SGDG650. However, there was a fluctuation in Si concentration during soaking time from 16 h to 15 days for SGDG600. For SGDG650, the increase in Si concentration was continuous.

The pH evolution is relevant to the dissolution process of glasses soaked in physiological solutions.

The increase in pH of SBF is the result of ion exchanges between the protons H + in SBF and the glass modifying cations. The pH value of SBF increased strongly after 2 h soaking time for both glasses (Fig. 3-d). For SGDG600, the pH was stable around 8.3 between 4 h and 1 day. The increase in pH values was not continuous: a fluctuation occurred in the interval 3-15 days. Between 3 and 7 days, the increase in pH value was followed by a slight decrease between 7 and 15 days. A more noticeable increase in pH value occurred after 30 days. It achieved 9.9 units. For SGDG650, pH evolution follows the same trend than that of SGDG600. However, the fluctuation in pH values occurred in the 1-7 days interval. After that, an important increase occurred in pH value which achieved 10 units after 30 days. In general, the pH values for SGDG650 were lower than that of SGDG600. versus soaking time of SGDG heated at 600°C and 650°C.

Structural modifications after soaking in SBF

XRD results

According to XRD data for SGDG600 surface, after soaking in SBF (Fig. 4), crystalline HA peaks appeared only after 15 days (natural HA, prepared from calcined bovine bone, was used as a reference [START_REF] Mezahi | Dissolution kinetic and structural behaviour of natural hydroxyapatite vs. thermal treatment : Comparison to synthetic hydroxyapatite[END_REF]). Before this time, it is difficult to distinguish this phase. Noted that after 7 days, the XRD data revealed the formation of calcite, justified by the apparition of its most intense peak at 29.4° (2θ) ( ICDD powder diffraction: file n o 86.2334). The intensity of this peak increased with time and it became a major visible phase on XRD diagram after 30 days. As a consequence, the intensity of crystallized HA peaks decreased between 15 and 30 days.

For SGDG650, the crystalline HA peaks appeared on the XRD pattern after 2 h. The peaks intensity of HA increased after 4 h. Additionally, XRD highlighted the presence of Na 2 Ca 2 Si 3 O 9 phase at the same time. This latter phase became the dominant crystallized phase after 16 h, as the peaks intensity of HA became lower. Between 4 h and 1 day, XRD also revealed the presence of an amorphous phase at the glass-ceramic surface. A broad halo was appeared around 20°C which can be attributed to silica gel [START_REF] Aina | On the dissolution/reaction of small-grain Bioglass ® 45S5 and F-modified bioactive glasses in artificial saliva (AS)[END_REF]. After that, the peaks intensity of HA increased and those of

FTIR results

In SGDG600 spectra (Fig. 5-a), between 2 and 4 h, the characteristic peaks of amorphous HA appeared at 464 cm -1 and a broadening band situated around 1056 cm -1 [START_REF] Elbatal | Characterization of some bioglass-ceramics[END_REF]. The presence of crystallized HA was confirmed after 16 h by the apparition of a weak new peak at 560 cm -1 and reinforcement of band situated at 605 cm -1 . These bands are distinguishable until 15 days.

Additionally, the characteristic bands of C-O vibrations, mainly attributable to calcite, appeared after 7 days and are clearly visible after 15 days. Also, these bands can be attributed to C-O in carbonated HA. As indicated by XRD, after 30 days, the calcite was present simultaneously with crystallized HA. In IR spectra, the characteristic bands of glass network progressively disappeared with time which led to conclude that carbonated HA layer is dense.

A new weak band at 823 cm -1 , characteristic of Si-O symmetric stretching vibrations [START_REF] Elbatal | Characterization of some bioglass-ceramics[END_REF][START_REF] Vallet-Regi | XRD, SEM-EDS, and FTIR studies of in vitro growth of an apatite-like layer on sol-gel glasses[END_REF],

appeared after 1 day. This band was also present until 15 days. This result confirms the formation of a Si-rich layer at the glass surface.

For SGDG650 (Fig. 5-b), no difference appeared between spectra before soaking and after 2 h in SBF. After 4 h, the characteristic bands of P-O appeared at 454, 533 cm -1 and a weak band at about 610 cm -1 . These bands confirmed the formation of crystallized HA. These bands were reinforced until 3 days. After 7 days, the intensity of the band situated at 533 cm -1 decreased and another band, characteristic of P-O vibration in crystallised HA, appeared at 562 cm -1 . No change occurred after 15 days. After 30 days, the band situated at 533 cm -1 merged in the band situated at 562 cm -1 . In addition, the intensity of the band situated at 610 cm -1 decreased. It can be observed that the intensity of band at 933 cm -1 , related to Si-O-Si vibration, decreased with soaking time. Also, the band around 1040 cm -1 becomes sharp and two other bands characteristic of P-O in HA appeared at 1045 and 1085 cm -1 . The characteristic bands of C-O situated at 870, 1420 and 1496 cm -1 were clearly visible after 3 days which led to conclude that the formed apatite is carbonated HA. These bands were also well noticeable after 30 days. According to XRD results, these bands are attributable to CaCO 3 phase and carbonated HA. A weak band appeared at 800 cm -1 [START_REF] Elbatal | Characterization of some bioglass-ceramics[END_REF][START_REF] Vallet-Regi | XRD, SEM-EDS, and FTIR studies of in vitro growth of an apatite-like layer on sol-gel glasses[END_REF], which is characteristic band of Si-O symmetric stretching vibrations, after 4 h and 1 day. This band reappeared strongly after 7 day.

Hence, the FTIR results are in a good agreement with XRD results. as it was confirmed by XRD and ICP results. After 4 h, the SEM images showed that there were two phases one up to another with different morphologies. The EDS analyses showed a significant increase in P concentration at the glass surface. This result, associated to the ICP analyses ones, is consistent with the formation of the apatite layer the glass surface up to the silica gel layer. After 1 day, this phase becomes compact as confirmed by EDS analysis. After 3 days, it is very difficult to distinguish the grain boundaries. A formed gel has covered the totality of the last layer. The IR analysis showed the formation of silica gel justified by apparition of Si-O-Si band at 800 cm -1 . After 7 days, a very dense layer was formed. The entire glass surface was covered with compacted needle-like crystallites, very rich in Ca and P elements, consistent with aggregates of carbonated HA. In addition, the ICP results showed that Ca and P ion concentrations decreased mainly in SBF

Discussion

The heat treatment at 650°C, above the temperature of crystallization outset [START_REF] Lucas-Girot | Sol-gel synthesis of a new composition of bioactive glass in the quaternary system SiO 2 -CaO-Na 2 O-P 2 O 5 : Comparison with melting method[END_REF], induced the crystallization of Na 2 Ca 2 Si 3 O 9 in a mainly amorphous matrix. Compared with the two others materials SGDG550 [START_REF] Mezahi | Reactivity kinetics of 52S4 glass in the quaternary system SiO 2 -CaO-Na 2 O-P 2 O 5 : Influence of the synthesis process: Melting versus sol-gel[END_REF] and SGDG600, SGDG650 is a glass-ceramic.

Results indicate that each material, synthesized with a same initial composition 52S4SGDG exhibited, after a different heat treatment, special reactivity features.

The bioactivity characteristic is associated to the occurrence of successive steps. As described by Hench for bioactive glasses [START_REF] Hench | Bioceramics-From Concept to Clinic[END_REF] and Kokubo [START_REF] Kokubo | Novel Bioactive Materials[END_REF] for bioactive materials containing CaO-SiO 2 , a hydrated silica layer is formed at the surface prior to the deposition of HA. After rapid exchange of glass modifier cations (Na + and /or Ca 2+ ) with H + in solution, the soluble silica is loosed as Si(OH) 4 by breaking of Si-O-Si bridges and subsequent silanol Si-OH groups are formed at the material surface. After that, silanol groups condense and their polymerization forms a SiO 2 rich surface layer. Silica gel thickness increases with glass surface leaching [START_REF] Ebisawa | Bioactivity of Fe 2 O 3 containing CaOSiO 2 glasses: in vitro evaluation[END_REF]. Up to this layer, CaO -P 2 O 5 rich layer forms and further crystallized as carbonated HA layer [START_REF] Hench | Bioceramics-From Concept to Clinic[END_REF][START_REF] Kokubo | Handbook of Bioactive Ceramics[END_REF][START_REF] Li | Process of Formation of Bone-Like Apatite Layer on Silica Gel[END_REF][START_REF] Li | In vivo calcium phosphate formation induced by sol-gel-prepared silica[END_REF]. The nuclei thus-formed later grow at the expense of the ions in the solution that was saturated with respect to apatite. Kim et al. [START_REF] Kim | Compositional dependence of calcium-phosphate layer formation in fluoride bioglasses[END_REF] reported that glasses with <50 mol% SiO 2 form a Ca-P-rich layer and a Si layer simultaneously while glasses with >50 mol% SiO 2 form a Ca-P layer on the top of a Si-rich layer.

These differences are related to the glass structure in relation with composition.

Dissolution rates and SiO 2 rich layer formation are mainly associated to the evolution of silicon concentration and pH of SBF. Also, the formation of carbonated HA layer is associated to the evolution of calcium and phosphorous concentrations in SBF. These results indicate that the three different materials: SGDG550, SGDG600 and SGDG650 presented different features.

By comparing to the previous ICP results [START_REF] Mezahi | Reactivity kinetics of 52S4 glass in the quaternary system SiO 2 -CaO-Na 2 O-P 2 O 5 : Influence of the synthesis process: Melting versus sol-gel[END_REF], SGDG600 have similar behavior than SGDG550 up to 4 h soaking: an important release of silicon and an increase of pH above 8. During this time, calcium and phosphorous concentrations decrease in SBF for the two glasses. As confirmed by ICP, RX, IR and EDS results, the formation of carbonated HA layer occurred simultaneously to the glasses dissolution. Our results are in accordance with those of Kim et al. [START_REF] Kim | Compositional dependence of calcium-phosphate layer formation in fluoride bioglasses[END_REF] who reported that glasses with 50 mol % SiO 2 form a Ca-P-rich layer and a Si layer simultaneously. Our glass contains 52.3% SiO 2 close to 50% mol.

In our previous work for SGDG550 [START_REF] Mezahi | Reactivity kinetics of 52S4 glass in the quaternary system SiO 2 -CaO-Na 2 O-P 2 O 5 : Influence of the synthesis process: Melting versus sol-gel[END_REF], Si concentration in SBF increased up to 3 days before a decrease which was consistent with the formation of the SiO 2 layer. The successive silica layers were formed during soaking time simultaneously with carbonated HA layers. XRD and FTIR analyses showed that no crystallized apatite phase was formed at the SGDG550 surface even after 30 days. But, after 2 h, the calcite was formed at the glass surface. This phase was previously observed for a glass with a molar composition 60%SiO 2 -35%CaO -5% P 2 O 5 [START_REF] Mami | Investigation of the surface reactivity of a sol-gel derived glass in the ternary system SiO2-CaO-P 2 O 5[END_REF]. The presence of this phase is not related to the glass composition but due to the high concentration of HCO 3 -in SBF that induced, in the presence of high calcium concentrations near the glass surface, the calcite CaCO 3 precipitation instead of the surface controlled crystallization of carbonated HA.

For SGDG600, after 4 h, a decrease in Si concentration in SBF was observed. This is consistent with the formation of the SiO 2 layer at the material surface after 1 day. This silica layer was already detected until 30 days. Between 4 h and 7 days, the glass dissolution process is the dominant feature compared to the carbonated HA precipitation process. After 7 days, the HA precipitation predominated the dissolution process as indicated by the decrease of Ca and P concentrations in SBF. IR analysis highlighted the presence of a carbonated apatitic phase between 1 and 7 days. This phase was well crystallized after 15 days as confirmed by XRD and IR results.

The secondary precipitation of calcite CaCO 3 is observed after 30 days. This phenomenon is amplified by the total consumption of SBF phosphorous ions after 7 days. As consequence, there were no P ions to allow the continuous apatite formation.

For SGDG650 during the period 0 -4 h, the silicon release is slower than the two other materials and the pH stays under 8. The pH value stayed under 8 and slowed down the Si dissolution [START_REF] Iler | The Chemistry of Silica: Solubility, polymerization, colloid and surface properties and biochemistry[END_REF] which inhibited the formation of silanol groups Si-OH at the glass surface and consequently their release as Si(OH) 4 . These results may be related to a lower reactivity of glass-ceramic due to the significant reduction of porosity for SGDG650 comparatively to the other materials associated to a difference in the reaction mechanism related to the presence of the crystallized phase associated to the glass matrix, which induced an inhomogeneity in the reactivity of the material surface.

During the two first hours the glass-ceramic dissolved weakly and a calcium and phosphorous ions uptake by the material surface occurred. The silica gel layer was detected only by SEM analysis, the thickness of this layer was weak and consequently not detectable by IR analysis. Up to this silica layer, the carbonated HA spherical particles was formed as justified by XRD, IR, MEB-EDS results. However, from 2 h to 7 days, a more complex process set up. An important release of calcium from the material occurred. This release was not associated to a proportional release of silicon in SBF. The temperature treatment has a marked effect on kinetics of carbonated HA formation at the glass surface. One of the important factors associated to bioactivity is ionic exchanges between material and surrounding medium. Porosity and material specific surface area greatly influence these exchanges. According to our results, when the heat temperature increases, the surface area and the pore volume decrease, then the precipitation process became less dominant that the dissolution process which has observed only for glasses treated at 600 °C and 650°C. According to obtained results, when the temperature increases from 600°C to 650°, the glass density increases and consequently the corresponding time of dissolution onset decreases. In addition, the carbonated HA was formed after 2 h at glass surface for all treatment temperature. However, the corresponding time to carbonated HA crystallization decreases with heat treatment. Moreover, the corresponding time for apparition of characteristic bands of silica gel increases with treatment temperature. It was suggested, for SGDG550, that the continuous formation of silica gel at the glass surface during the soaking time inhibited the HA crystallization. For SGDG600 and SGDG650, the thickness of silica layers was thinner and their formation were limited comparatively to SGDG550, which allowed the HA crystallization.

In this work, a special behaviour was observed for SGDG650: Na 2 Ca 2 Si 3 O 9 phase , initially present in the material, was reprecipitated at the material surface during soaking time. It can be suggested that the dissolution of this phase occurred and the diffusion of ions promoted the formation of this phase at the glass surface.

So, the heat treatment and the formation of the crystallized phases such as Na 2 Ca 2 Si 3 O 9 , maintained the high bioactivity of 52S4 as others particular compositions of the SiO 2 -CaO-Na 2 O-P 2 O 5 system in other works [START_REF] Peitl | Effect of crystallization on apatite layer formation on bioactive glass 45S5[END_REF][START_REF] Peitl | Highly bioactive P 2 O 5 -Na 2 O-CaO-SiO 2 glass-ceramics[END_REF][START_REF] Siqueira | Gel-derived SiO 2 -CaO-Na 2 O-P 2 O 5 bioactive powders: synthesis and in vitro bioactivity[END_REF][START_REF] Chen | 45S5 Bioglass®-derived glass-ceramic scaffolds for bone tissue engineering[END_REF][START_REF] Boccaccini | Sintering, crystallisation and biodegradation behaviour of Bioglass ® -derived glass-ceramics[END_REF][START_REF] Huang | A two-scale model for simultaneous sintering and crystallization of glass-ceramic scaffolds for tissue engineering[END_REF][START_REF] Bretcanu | Simple methods to fabricate Bioglass -derived glass-ceramic scaffolds exhibiting porosity gradient[END_REF]. Moreover, the heat treatment at 600°C and 650°C allowed the HA crystallization.

Conclusion

The obtained results lead to conclude that the bioactivity of 52S4SGDG glass was not affected by the heat treatment modification. For all temperatures, carbonated HA was formed at the material surface after soaking in SBF. For SGDG550, the carbonated HA remained amorphous even after 30 days and it was formed up to the silica layer which reformed at any time. For SGDG600, carbonated HA began to crystallize after 16 h and became well crystallized after 15 days. However, the heat treatment at this temperature did not prevent the reformation of a silicon oxide layer up to the carbonated HA one. The silica layer was detected later than the carbonated HA formation, accordingly to IR results. Otherwise, for SGDG650, the crystallized carbonated HA was observed after 2 h. In addition, the crystallized phase Na 2 Ca 2 Si 3 O 9 in the initial material before soaking was reprecipitated at the glass surface after 15 days.

This study highlighted the original behavior of this new composition and the possible adjustment of reactivity by modifying the heat treatment temperature, without changing the materials bioactive features.

  Analogous maxima were obtained byMartinez et al. in their study of binary glasses SiO 2 -CaO.They attributed the diffraction maximum between 28 and 34° 2θ to the incipient presence of different crystalline phases of calcium silicates, whereas the maximum at 22.5°2θ to a formation of SiO 2 content in the glass[START_REF] Martinez | Bioactivity of CaO-SiO 2 binary glasses system[END_REF]. Otherwise, DRX of SGDG650 confirmed the Na 2 Ca 2 Si 3 O 9 crystallization (ICDD powder diffraction: file n o 22.1455), in a glassy matrix. The HA phase is also present as a very minor phase.

Fig. 1 .

 1 Fig. 1. X-ray diffraction patterns of SGDG heated at 600°C and 650°C.

Fig. 3 .

 3 Fig. 3. Evolutions of Ca (a), P (b) and Si (c) concentrations in SBF, and evolution of SBF pH (d),

Na 2

 2 Ca 2 Si 3 O 9 decreased up to 7 days when the peaks intensity of the two phases were similar. After 15 days, Na 2 Ca 2 Si 3 O 9 appeared again as the mainly formed phase. After 30 days, the calcite peak was present. Consequently, it can be observed that the peaks intensities of previous existent phases HA and Na 2 Ca 2 Si 3 O 9 decreased strongly.

Fig. 4 .

 4 Fig. 4. XRD patterns of SGDG600 and SGDG650 surfaces before and after soaking in SBF solution

Fig. 5 .

 5 Fig. 5. FTIR spectra of SGDG600 and SGDG650 before and after soaking in SBF.

Fig. 7 .

 7 Fig. 7. SEM micrographs and EDS spectra of SGDG650 surfaces before and after soaking in SBF.

  The material dissolution is associated to the simultaneous formation of a new amorphous phase and the reformation of Na 2 Ca 2 Si 3 O 9 at the material surface as shown in XRD diagrams. In parallel, between 4 h and 3 days, the Ca and P concentrations at the glass surface increased as demonstrated by EDS analysis which confirm the formation of carbonated HA layer, its densification and grain size enlargement with time. The IR and XRD analysis confirmed this result, the intensity of P-O bands increased with time and the intensity of characteristic XRD peaks of Na 2 Ca 2 Si 3 O 9 decreased with the continuous formation of carbonated HA layer. This particular behavior may be related to the initial presence of Na 2 Ca 2 Si 3 O 9 which had a different dissolution rate from the glass matrix. The consistency between the obtained results shows that the dissolution of Na 2 Ca 2 Si 3 O 9 was followed by a reprecipitation of this phase at the glass surface, associated to a rapid formation of the calcium free amorphous silicon rich phase and a consecutive formation of the carbonated HA. After 7 days, the Ca and P concentrations in SBF decrease as those at the glass surface achieved their maximum values which is consistent with the densification of carbonated HA layer and its formation as compacted needle-like crystallites aggregates. After 15 days, all P ions in SBF were consumed, and then after 15 days, there were no P ions available to form HA. Calcium ions concentration increased slightly and stabilized between 7 and 30 days as silicon concentration increased continuously during soaking time. These results indicate that the material dissolution went out. The formation of carbonated HA was stopped by lack of phosphorous in SBF and up to 15 days, the reprecipitation of Na 2 Ca 2 Si 3 O 9 is favoured. After 30 days, the calcite CaCO 3 was formed, up to Na 2 Ca 2 Si 3 O 9 , by the combination of calcium ions from glass and HCO 3

	-from SBF
	[45].

Acknowledgements

The authors would like to thank the authorities responsible for the financial support of this work (M'sila University and Algerian minister of superior education and research). Many thanks to Bernard Lebeuvre (Equipe Chimie du Solide et Matériaux, UMR 6226 Rennes), F. Gouttefangeas (CMEBA centre of UMR-CNRS 6226-Rennes) and D. Laillé (INSA-Rennes) for their fruitful collaborations.

SEM-EDS results

SEM images and EDS analyses (Fig. 6) depicted the morphological modifications and composition evolutions of SGDG600 surface with soaking time. After 2 h, a new layer, formed by spherical particles, dispersed at the glass surface. The EDS analysis, with the significant increase in phosphorous concentration, confirmed the formation of an apatite layer at the glass surface after 2h in SBF as suggested by FTIR and ICP results. This layer becomes denser with time. However, after 30 days, the Ca and P concentrations decreased. According to ICP result, the P concentration was nil between 7 and 30 days. So, there were not P ions to lead to continuous apatite formation. In addition, SBF is saturated by carbonate ions which led to calcite formation up to crystallized carbonated HA (The characteristic rhombohedral particles of CaCO 3 as calcite were not observed at the glass surface giving evidence of the presence of this phase as localized precipitate and not as surface controlled phase).

The Si concentration decreased at the glass surface after 15 days. This result is consistent with the formation of a dense apatite layer at the glass surface. However, the Si concentration increased after 30 days. This result may be related to the presence of Si in the apatite layer due to the continuous Si release between 7 and 30 days revealed by ICP analyses or to a new visibility of the silica gel formed at 7 days (as evidenced by IR analysis) due to the impossibility of the apatite layer formation to progress without phosphorous ions in SBF. The morphological modifications of SGDG650 surface with soaking time were depicted by SEM images and EDS analyses (Fig. 7). After 2 h, some spherical particles dispersed on a uniform gel formed at the glass surface. The EDS analysis showed that there was a slight increase in both Si and P at the glass surface. This layer is attributed to silica gel and the spherical particles to apatite layer