Cementitious mixture for a 3D printer and relative use in said printer

Abstract

A cementitious mixture for a 3D printer and its relative use are described, more specifically for the production of finished products having a complex geometry using a 3D printing apparatus.

Claims

1. A cementitious mixture for a 3D printer which comprises: a) from 10% to 70% by weight of a cement or hydraulic binder, b) from 0.5% to 25% by weight with respect to the total weight of the cementitious mixture of a natural or artificial latent hydraulic addition having a specific surface ranging from 3,500 cm2/g to 6,500 cm2/g, determined according to the Blaine method according to EN 196-6:2010, c) from 10% to 50% by weight of a filler, d) aggregates, e) additives, f) water, wherein: component c) conforms with standard UNI EN 12620-1: 2008 and is selected from calcareous, silica or silico-calcareous fillers, and mixtures thereof having a particle size wherein at least 90% by weight of the filler passes through an 0.063 mm sieve; component d) is present in a quantity ranging from 10% to 80% by weight with respect to the total weight of the cementitious mixture, and comprises calcareous, silica or silico-calcareous aggregates, in accordance with standard UNI EN 206: 2014, alone or mixed with each other, having a particle size with a maximum diameter lower than 1 mm, said component d) comprising an aggregate fraction having a particle size with a diameter greater than 0.2 mm and an aggregate fraction having a particle size with a diameter less than or equal to 0.2 mm and such that less than 2% by weight passes through a sieve of 0.063 mm; component e) comprises from 0.01% to 1.5% by weight of a superfluidifying additive selected from superfluidifying acrylic-based polycarboxylates, liposulfonates, naphthalene sulfonates, melamine or vinyl compounds from 0.01% to 5.0% by weight of a rheology modifying additive; from 0.01% to 2.0% by weight of modified starch; from 0.0% to 1.0% by weight of a shrinkage reducing agent: from 0.05% to 0.5% of a hydrophobic additive selected from silicone or silane derivatives and mixtures thereof, said cementitious mixture having a viscosity value ranging from 80,000 Pa.Math.s to 150,000 Pa.Math.s, measured at a shear rate of 0.01 s.sup.1 and at a temperature of 20 C., wherein the weight ratio between the binder and the aggregate ranges from 0.5 to 2.0, the binder comprising components a) and b) of the cementitious mixture.

2. The cementitious mixture according to claim 1, wherein a weight ratio between water and the binder ranges from 0.25 to 0.8, the binder comprising components a) and b) of the cementitious mixture.

3. The cementitious mixture according to claim 1, wherein a weight ratio between water and the total cementitious mixture in powder form is within the range of 17% to 20%.

4. The cementitious mixture according to claim 1, wherein component a) of the mixture is selected from the group consisting of CEM I 52.5 R and CEM I 52.5 N.

5. The cementitious mixture according to claim 1, wherein component b) of the mixture is granulated blast-furnace slag, having a specific surface ranging from 3,500 cm.sup.2/g to 6,500 cm.sup.2/g, determined according to the Blaine method according to EN 196-6:2010.

6. The cementitious mixture according to claim 1, wherein: component a) is selected from the group consisting of CEM I 52.5 R and CEM I 52.5 N; component b) is present in a quantity by weight from 0.5% to 20% and is granulated blast-furnace slag, having a specific surface ranging from 4,000 cm2/g to 5,000 cm2/g, determined according to the Blaine method according to EN 196-6:2010; component c) is calcareous filler and is present in a quantity by weight from 15% to 40%; component d) is present in a quantity by weight from 25% to 50%; component e) comprises from 0.2% to 1.0% by weight of the superfluidifying additive based on polycarboxylic ether; from 0.10% to 0.50% by weight of a theology modifying additive which is hydroxymethylethyl cellulose; from 0.1% to 1.0% by weight of modified starch; from 0.3% to 0.6% by weight of a shrinkage reducing agent, from 0.1% to 0.30% of a hydrophobic additive selected from silicone or silane derivatives and mixtures thereof; wherein the binder/aggregate weight ratio ranges from 0.62 to 1.36, wherein the binder components a) and b), and said cementitious mixture has a viscosity value ranging from 80,000 Pa.Math.s to 150,000 Pa.Math.s, measured at a shear rate of 0.01 s.sup.1 and at a temperature of 20 C.

7. A 3D printing process comprising the following steps: preparing the cementitious mixture according to claim 1; feeding the cementitious mixture to a 3D printing apparatus; extruding the cementitious mixture from the 3D printer by means of a single-screw extruder; printing a 3D model by the deposition of consecutive layers of the cementitious mixture; wherein the ratio between a maximum diameter of the aggregates of the cementitious mixture and the distance between a screw and an internal wall of the screw extruder ranges from 0.02 to 0.8.

8. An apparatus suitable for implementing the printing process of a 3D object fed with the cementitious mixture according to claim 1, said apparatus comprising a cylindrical gas-pressurized supply tank, a screw extruder, a flexible pipe which connects the tank to the screw extruder and a pumping system, wherein the screw extruder is a single-screw extruder provided with a screw, an extrusion chamber and a circular nozzle, the difference between an internal diameter of the extrusion chamber and a diameter of the screw ranging from 1.25 mm to 3.33 mm.

9. The apparatus according to claim 8, wherein the screw extruder is characterized by the screw having a height ranging from 35 to 140 mm, a pitch ranging from 7 to 30 mm, a helix angle ranging from 12 to 43, and the circular nozzle having a diameter ranging from 2 to 30 mm and a height ranging from 5 to 50 mm.

10. The cementitious mixture according to claim 1 wherein component (c) comprises a calcareous filler.

11. The cementitious mixture according to claim 1 wherein component (d) is present in a quantity ranging from 25% to 50% by weight with respect to the total weight of the cementitious mixture having a fraction having a particle size with a diameter greater than 0.6 mm.

12. The cementitious mixture according to claim 1 wherein the hydraulic binder or cement is selected from the group consisting of Portland cement, sulfoaluminate cement, aluminous cement, quick-setting natural cement, and mixtures thereof.

13. The cementitious mixture according to claim 1 wherein the natural or artificial hydraulic addition is from 0.5% to 20% by weight of the cementitious mixture and comprises a granulated blast-furnace slag, having a specific surface ranging from 4,000 cm2/g to 5,000 cm2/g determined according to the Blaine method according to EN 196-6:2010.

14. The cementitious mixture according to claim 1 wherein the filler is from 15% to 40% by weight of the cementitious mixture, said filler comprising calcareous fillers.

15. The cementitious mixture according to claim 1 wherein the calcareous, silica or silico-calcareous aggregates, alone or mixed with each other, are from 25% to 50% by weight of the cementitious mixture, having a particle size with a maximum diameter lower than 1 mm, said component, d) being composed of one or more fractions having a particle size greater than 0.6 mm, and a fraction having a particle size with a diameter less than or equal to 0.2 mm and such that less than 2% by weight passes through a sieve of 0.063 mm.

16. The cementitious mixture according to claim 1 wherein the superfluidifying additive is from 0.2% to 1.0% by weight and comprises polycarboxylic ethers; the rheology modifying additive is from 0.10% to 0.50% by weight and comprises hydroxymethylethyl cellulose; the modified starch is from 0.1% to 1.0% by weight; the shrinkage reducing agent is from 0.3% to 0.6% by weight; the hydrophobic additive is from 0.10% to 0.30% and comprises an alkyloxysilane.

17. The cementitious mixture according to claim 1 wherein the binder/aggregate weight ratio ranges from 0.62 to 1.36.

18. The cementitious mixture according to claim 2, wherein the water/binder weight ratio ranges from 0.4 to 0.6.

Description

(1) In the attached figures

(2) FIG. 1 is a schematic representation of an extruder for extruding the cementitious mixture according to the present invention;

(3) FIG. 2 is a photographic reproduction of the pressurized tank, empty and full of cementitious mixture according to the present invention;

(4) FIG. 3 is a photographic reproduction of the finished product having a complex geometry obtained according to example 1;

(5) FIGS. 4A and 4B are a photographic reproduction of the main components that make up the apparatus for carrying out the 3D printing process.

(6) As previously indicated, the main components of the apparatus for carrying out the 3D printing process, to which the cementitious mixture according to the present invention is fed, to be subsequently extruded and deposited, are the following: 1) cylindrical supply tank, gas pressurized; 2) flexible tube connecting the tank to the extruder; 3) screw extruder; 4) nozzle with circular outlet.

(7) The extrusion device can be mounted on any type of machine or robot that can receive it, so as to combine the extrusion process with the specific advantages relating to the kinematics of the machine/robot.

(8) More specifically:

(9) FIG. 4 shows the cylindrical gas-pressurized supply tank (1) which contains a piston which pushes the fluid, i.e. the cementitious mixture. The pressure is supplied by pressurized air, directly connected to the tank and regulated by a pressure gauge.

(10) The flexible plastic tube (2) that connects the pump-tank system (1) to the extruder (3) is characterized by a circular section, with an internal diameter of 20 mm and a length ranging from 1.5 to 3 m.

(11) The single-screw extruder (3) has been optimized for application with the cementitious mixture according to the present invention and is schematically shown in FIG. 1.

(12) All the parts of the extruder are made of ABS (acrylonitrile-butadiene-styrene) and are in turn printed using a 3D printer capable of processing polymeric materials. The only exception with respect to the plastic parts of the extruder is the metal shaft.

(13) The screw was printed with a hole in which the shaft was glued and a bearing was mounted on the screw to limit friction. The screw was printed in a vertical position to ensure adherence to the surface of the printing area during the process; supporting structures (in the shape of a triangle) are provided so that the screw having the desired geometry can be correctly printed. The rotation rates supported by the screw range from 90 to 180 rpm.

(14) The diameter of the nozzle is 10 mm and the nozzle is designed to be interchangeable, cone-shaped to reduce the friction of the cementitious mixture.

(15) The diameter of the nozzle can vary depending on the formulation, for this reason it was therefore conceived as interchangeable.

(16) The printing parameters can be controlled with various types of software. This software allows the object designed to be divided into sections governed by the printing resolution to be obtained. In particular, the object to be printed is designed by creating a 3D digital model using a CAD application, and is then divided into layers using the above-mentioned software, subsequently providing the machine with instructions and establishing the path (layer by layer) that the nozzle must follow in order to build the object. The software for dividing the object into layers has generally been created to manage materials such as plastic or metal and therefore it does not allow some important parameters, such as for example the screw rate, to be controlled directly.

(17) In order to control the screw rate (and therefore the flow-rate of the extruded material), an approach was followed similar to the control model of plastic extrusion. The first step is to calculate the flow-rate required for printing the object. This is the product of the height of the extruded layer, the diameter of the nozzle and the speed of the print head. Therefore, once the value of the flow is known, the rotation rate of the screw can be calculated using the following equations of a model of a single-screw extruder:

(18) $\{\begin{array}{c}Q=A*N+B*\frac{P}{}\\ Q=k*\frac{P}{}\end{array}$

(19) wherein N is the rotation rate of the screw in rpm, P is the increase in pressure inside the extrusion chamber, is the viscosity of the cementitious mixture (assuming that, under conditions of high flow stress, it behaves as a Newtonian fluid), A and B are functions of the geometry of the extruder and k is a function of the geometry of the nozzle.

(20) The screw is moved by the same motor that pushes the polymer thread into the extruder for the polymeric materials. The rotation of the motor must ensure a sufficient flow-rate for supplying the polymer to the extruder and therefore its rate depends on the diameter of the thread. By supplying the software with the correct value of this diameter, the speed of the motor of the screw extruder can be defined.

(21) In the case of the cementitious mixtures object of the present invention, a much higher value of the thread diameter than that of a plastic thread must be imposed on the software, to correctly use the nozzle with the desired diameter and obtain the right flow-rate necessary for printing materials of this type. This expedient is necessary for imposing the right rotation rate (rpm) on the extruder screw. It is also possible to change the diameter of the thread to increase the flow-rate and therefore the printing rate.

(22) The examples provided hereunder aim at demonstrating the efficiency or otherwise of cementitious compositions according to the present invention, when processed by means of a 3D printing apparatus.

EXAMPLE 1

(23) A formulation of a cementitious mixture having the composition shown in the following Table 1 was prepared using a Hobart mixer, according to the following procedure: the solid components were mixed for 1 minute and 30 seconds at a speed of 140 rpm; water was then added for 1 minute at a speed of 140 rpm; all the components were then further mixed for 2 minutes at a speed of 285 rpm and subsequently for 1 minute at a speed of 322 rpm; the mixing was interrupted for 45 seconds to collect any material remaining on the walls of the container: all the components were then mixed for 1 minute at a speed of 322 rpm and then for 1 minute at a speed of 240 rpm.

(24) TABLE-US-00001 TABLE 1 Formulation extruded according to Example 1. Composition Component (weight %) Cement I 52.5 R 18.13% GGBS 17.50% Calcareous Filler 32.88% Silico-calcareous sand (0.00-0.200 mm) 20.0% Silico-calcareous sand (0.600-1.000 mm) 10.0% Superfluidifying additive 0.49% Rheology modifier 1 0.3% Rheology modifier 2 0.2% Shrinkage reducing agent 0.35% Hydrophobic agent 0.15% Water/binder 0.52 Water/Total powder cementitious mixture 18.55% Binder/aggregate 1.19

(25) The cement is a cement of the type I 52.5 R cement from the Rezzato plant. The GGBS included in the formulation constitutes the latent hydraulic addition and is a granular blast furnace slag (GGBS: ground grain ground slag), having a specific surface equal to 4,450 cm.sup.2/g (determined according to the Blaine method according to the standard EN 196-6: 2010), supplied by the company Ecocem with the trade-name of Loppa di altoforno granulata macinata (Ground granular blast furnace slag).

(26) The calcareous filler is a high-purity filler, marketed by Omya Spa under the trade-name of Omyacarb 2-AV. The silico-calcareous aggregates were added in two fractions, a first fraction with a particle-size distribution ranging from 0.00 to 0.200 mm and a second fraction with a particle-size distribution ranging from 0.60000 to 1.000 mm.

(27) The superfluidifying additive is based on polycarboxylic ether, called Melflux 2641 F, and marketed by BASF. The rheology modifier 1 is a hydroxymethylethylcellulose called Tylose MH 60004 P6 marketed by ShinEtsu. The rheology modifier 2 is a starch modified with ether groups, marketed under the name Aqualon ST2000 from Ashland.

(28) The shrinkage reducing agent (SRA), called SRA04, is marketed by Neuvendis; this is a mixture of glycols and special surfactants.

(29) The hydrophobing agent is a silane-based additive, more precisely an alkyl oxysilane, called SEAL 200, marketed by Elotex.

(30) These five additives are in solid form.

(31) The water/binder ratio is equal to 0.52, the percentage referring to the weight ratio water/total cementitious mixture ratio in powder form is 18.55%, whereas the binder/aggregate ratio is equal to 1.19 (where the binder is composed of cement and the latent hydraulic addition GGBS).

(32) At the end of the mixing, the cementitious mixture having the composition indicated in Table 1 was characterized by means of a Haake RotoVisco RV1 rheometer, with coaxial cylinders, at a temperature of 20 C. The test allowed the viscosity of the material to be characterized within a share rate range of 0.01 to 10 s.sup.1, using a step method. Each step was maintained for 30 seconds and the total duration of the test was 30 minutes. The viscosity of the cementitious mixture measured at a shear stress value of 0.01.sup.s1 is equal to 100,000 Pa.Math.s.

(33) At the end of the mixing, the mortar was inserted into the cylindrical gas-pressurized supply tank (as shown in FIG. 2) with the help of a spatula and arranged so as to completely fill the container reducing the air trapped in the material as much as possible. The cylindrical gas-pressurized supply tank was thus prepared for being connected to the extruder mounted on the printing machine, using the tube previously described. The tank pressure was set at 5.0 bars.

(34) The mixture prepared as previously indicated was extruded using a triple-layered spiral path having a geometry deriving from an octagon. The 3D model to be printed was an octagonal element, inscribed in a circumference having a diameter of 23.6 cm and 25 cm high. The model was successfully printed (as shown in FIG. 3) in a single printing session, applying the following printing parameters: Pressure at the cylindrical gas-pressurized supply tank: 5.0 bar; Thread diameter imposed: 10 mm; Height of layer: 7.0 mm; Printing speed: 25 mm/s; Screw rotation rate: 38.4 rpm; Flight clearance: 1.5 mm Ratio between maximum diameter of the aggregate and distance between the screw and the inner wall of the extruder: 0.67.

(35) The mechanical resistance to compression at 24 hours was equal to 5.88 MPa, according to the loading ramp as described in EM 196-1-2016.

EXAMPLE 2

(36) A formulation of a cementitious mixture having the composition shown in the following Table 2 was prepared using a Hobart mixer, according to the following procedure: the solid components were mixed for 1 minute and 30 seconds at a speed of 140 rpm; water was then added for 1 minute at a speed of 140 rpm; all the components were then further mixed for 2 minutes at a speed of 285 rpm and subsequently for 1 minute at a speed of 322 rpm; the mixing was interrupted for 45 seconds to collect any material remaining on the walls of the container all the components were then mixed for 1 minute at a speed of 322 rpm and then for 1 minute at a speed of 240 rpm.

(37) TABLE-US-00002 TABLE 2 Formulation extruded according to Example 2. Composition Component (weight %) Cement I 52.5 R 18.13% GGBS 17.50% Calcareous filler 32.88% Silico-calcareous sand (0.00-0.200 mm) 10.0% Silico-calcareous sand (0.20-0.350 mm) 10.0% Silico-calcareous sand (0.600-1.000 mm) 10.0% Superfluidifying additive 0.49% Rheology modifier 1 0.3% Rheology modifier 2 0.2% Shrinkage reducing agent 0.35% Hydrophobic agent 0.15% Water/binder 0.52 Water/Total powder cementitious mixture 18.55% Binder/aggregate 1.19

(38) The cement is a cement of the type I 52.5 R cement from the Rezzato plant. The GGBS included in the formulation constitutes the latent hydraulic addition and is a granular blast furnace slag (GGBS: ground grain ground slag), having a specific surface equal to 4,450 cm.sup.2/g (determined according to the Blaine method according to the standard EN 196-6: 2010), supplied by the company Ecocem with the trade-name of Loppa di altoforno granulata macinata (Ground granular blast furnace slag).

(39) The calcareous filler is a high-purity filler, marketed by Omya Spa under the trade-name of Omyacarb 2-AV.

(40) The silico-calcareous aggregates were added in two fractions, a first fraction with a particle-size distribution ranging from 0.00 to 0.200 mm and a second fraction with a particle-size distribution ranging from 0.60000 to 1.000 mm.

(41) The superfluidifying additive is based on polycarboxylic ether, called Melflux 2641 F, and marketed by BASF. The rheology modifier 1 is a hydroxymethylethylcellulose called Tylose MH 60004 P6 marketed by ShinEtsu. The rheology modifier 2 is a starch modified with ether groups, marketed under the name Aqualon ST2000 from Ashland.

(42) The shrinkage reducing agent (SRA), called SRA04, is marketed by Neuvendis; this is a mixture of glycols and special surfactants.

(43) The hydrophobic agent is a silane-based additive, more precisely an alkyl oxysilane, called SEAL 200, marketed by Elotex.

(44) These five additives are in solid form.

(45) The water/binder ratio is equal to 0.52, the percentage referring to the weight ratio water/total cementitious mixture ratio in powder form is 18.55%, whereas the binder/aggregate ratio is equal to 1.19 (where the binder is composed of cement and the latent hydraulic addition GGBS).

(46) At the end of the mixing, the cementitious mixture having the composition indicated in Table 2 was characterized by means of a Haake RotoVisco RV1 rheometer, with coaxial cylinders, at a temperature of 20 C. The test allowed the viscosity of the material to be characterized within a share rate range of 0.01 to 10 s.sup.1, using a step method. Each step was maintained for 30 seconds and the total duration of the test was 30 minutes. The viscosity of the cementitious mixture measured at a shear stress value of 0.01.sup.s1 is equal to 100,000 Pa.Math.s.

(47) At the end of the mixing, the mortar was inserted into the cylindrical gas-pressurized supply tank (as shown in FIG. 2) with the help of a spatula and arranged so as to completely fill the container reducing the air trapped in the material as much as possible. The cylindrical gas-pressurized supply tank was thus prepared for being connected to the extruder mounted on the printing machine, using the tube previously described. The tank pressure was set at 5.0 bars.

(48) The mixture prepared as previously indicated was extruded using a triple-layered spiral path having a geometry deriving from an octagon. The 3D model to be printed was an octagonal element, inscribed in a circumference having a diameter of 23.6 cm and 25 cm high. The model was successfully printed in a single printing session, applying the following printing parameters: Pressure at the cylindrical gas-pressurized supply tank: 5.0 bar; Thread diameter imposed: 10 mm; Height of layer: 7.0 mm; Printing speed: 25 mm/s; Screw rotation rate: 38.4 rpm; Flight clearance: 1.5 mm Ratio between maximum diameter of the aggregate and distance between the screw and the inner wall of the extruder: 0.67.

(49) The mechanical resistance to compression at 24 hours was equal to 5.01 MPa, according to the loading ramp as described in EM 196-1-2016.

COMPARATIVE EXAMPLE 3

(50) A formulation of a cementitious mixture having the composition shown in the following Table 3 was prepared using a Hobart mixer, according to the procedure described in Example 1.

(51) TABLE-US-00003 TABLE 3 Formulation extruded according to Example 3. Composition Component (weight %) Cement I 52.5 R 15.0% GGBS 15.0% Calcareous filler 27.9% Silico-calcareous sand (0.200-0.350 mm) 8.0% Silico-calcareous sand (0.600-1.000 mm) 18.1% Silico-calcareous sand (1.000-1.500 mm) 15.0% Superfluidifying additive 0.55% Rheology modifier 1 0.22% Rheology modifier 2 0.23% Water/binder 0.54 Water/Total powder cementitious mixture 16.20% Binder/aggregate 0.73

(52) The cement is a cement of the type I 52.5 R cement from the Rezzato plant. The GGBS included in the formulation constitutes the latent hydraulic addition and is a granular blast furnace slag (GGBS: ground grain ground slag), having a specific surface equal to 4,450 cm.sup.2/g (determined according to the Blaine method according to the standard EN 196-6: 2010), supplied by the company Ecocem with the trade-name of Loppa di altoforno granulata macinata (Ground granular blast furnace slag).

(53) The calcareous filler is a high-purity filler, marketed by Omya Spa under the trade-name of Omyacarb 2-AV.

(54) The silico-calcareous aggregates were added in three fractions, a first fraction with a particle-size distribution ranging from 0.200 to 0.0.350 mm, a second fraction with a particle-size distribution ranging from 0.600 to 1,000 mm and a third fraction with a particle-size distribution ranging from 1,000 to 1,500 mm.

(55) The superfluidifying additive is based on polycarboxylic ether, called Melflux 2641 F, and marketed by BASF. The rheology modifier 1 is a hydroxymethylethylcellulose called Tylose MH 60004 P6 marketed by ShinEtsu. The rheology modifier 2 is a starch modified with ether groups, marketed under the name Aqualon ST2000 from Ashland. These three additives are all in solid form.

(56) The water/binder ratio is equal to 0.54, the percentage referring to the weight ratio water/total cementitious mixture ratio in powder form is 16.20%, whereas the binder/aggregate ratio is equal to 0.73 (wherein the binder is composed of cement and the latent hydraulic addition GGBS).

(57) At the end of the mixing, the cementitious mixture having the composition indicated in Table 3 was characterized by means of a Haake RotoVisco RV1 rheometer, by means of the method already described in Example 1. The viscosity of the cementitious mixture measured at a shear stress value of 0.01.sup.s1 is equal to 100,000 Pa.Math.s.

(58) At the end of the mixing, the mortar was inserted into the cylindrical gas-pressurized supply tank which was connected to the extruder mounted on the printing machine, as described in Example 1. The tank pressure was set at 5.0 bars.

(59) The mixture prepared as previously indicated was extruded using a triple-layered spiral path having a geometry deriving from an octagon. The 3D model to be printed was an octagonal element, inscribed in a circumference having a diameter of 23.6 cm and 250 cm high.

(60) The model was printed applying the same printing parameters indicated for Example 1, in which, however, the value of the ratio between the maximum diameter of the aggregate and the distance between the screw and the inner wall of the extruder is equal to 1.0.

(61) The mixture prepared as indicated above was not processable. Although having a viscosity value included within the range provided by the present invention, the formulation, in fact, provides a maximum diameter of the aggregates outside the maximum limit of the range provided by the same and also a ratio between the maximum diameter of the aggregate and the distance between the screw and internal wall of the extruder not included within the range provided by the invention.

(62) The value of the mechanical resistance to compression at 24 hours was equal to 3 MPa, according to the loading ramp as described in EM 196-1-2016.

BIBLIOGRAPHY

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