IT201800009566A1 - 3D printing process of a cement mixture and related apparatus - Google Patents

Description

"3D printing process of a cement mixture and related apparatus"

The present invention relates to a 3D printing process of a cement mixture and the related apparatus for manufacturing finished products with complex geometry.

The present invention is part of the sector of cement mixtures or compositions to be used, by means of 3D printing technologies, for the realization of three-dimensional products, in particular by means of extrusion 3D printing.

Mechatronics has reached a high level of pervasion of various industrial sectors, where robotic production has now been a consolidated process for several years. Additive technology (or Additive Manufacturing, AM, in English) is gaining more and more importance in the rapid prototyping industry. There are examples of the use of this technology for the production of complex pieces, especially when it comes to objects for which a production in a high number of copies is not necessary, not only for example for dental implants or jewelry, but also for the realization of cobalt-chrome fuel nozzles, printed by General Electric for the new A320 LEAP jet engines of the Airbus group [1].

This technology is particularly advantageous when the products can be obtained directly from the digital model, with an absolutely reduced use of additional support material that will inevitably be wasted after finishing the object.

Different techniques in the field of additive technologies allow the use of different materials, such as thermoplastic resins that can melt / harden in a narrow range of temperatures, photo-crosslinkable resins that are hardened by means of a laser beam or metal powders that melt using a laser beam and harden immediately after passing the laser.

The ASTM F42 International Technical Committee on Additive Technologies defines additive technology as the "process of joining materials to create objects from 3D model data, usually layer by layer, as opposed to subtractive manufacturing methodologies" (this definition is subject to ISO harmonization by ISO 17296 -1) [2].

Cement-based materials have also been introduced in the additive technology sector. These are materials that have completely different behaviors than the other materials mentioned above and normally used in this type of technology. The characteristics required for a cement mix or composition to be used as material for AM must clearly take into account the typicality of the printing process.

Additive technologies in the cement sector can be used in various fields, including architecture, construction, art and design. Recently these technologies have attracted a growing interest in the construction sector, interest that mainly derives from the possibility of offering more freedom in the design of complex shapes, with potential aesthetic and functional advantages, reducing production times and costs [3]. However, before printing any object, it is necessary to make a 3D model, using appropriate software. The 3D model is divided into a certain number of layers which then correspond to the different deposition layers envisaged by the AM process. These steps require specific skills, not common in industrial construction, and an error in the 3D modeling phase inevitably leads to an error in production as well.

Among the existing techniques that apply additive technology, extrusion 3D printing appears to be the one with the greatest potential for development in the construction sector. This technique generally provides at least one print head to which a generally pressurized nozzle is mounted. The print head is fed with a cement mixture and driven by motors at precise points in space, following a 3D model of the object to be printed.

The speed with which the material is extruded through the nozzle and the speed with which the print head moves in space are some of the design parameters that determine the final resolution of the print. The nozzle is piloted to trace the paths in space that allow the digitally represented object to be reproduced. As the material exits the nozzle, it is placed on the surface under construction of the object and the construction of the object itself then proceeds in the form of a succession of overlapping layers, in a vertical direction, until the entire object has been constructed.

Conceptually, the entire printing process could be divided into five stages:

- Realization of the model of the objects in 3D CAD;

- Section of the model in layers;

- Conversion of the map of each layer into instructions for the machine;

- Realization of the object by depositing successive layers of cement material;

- Recovery of the object.

The object, designed as a 3D CAD model, is converted into an STL format file and cut into layers of the desired thickness. The print path of each layer is then generated to create a G-Code print file. The preparation of the cementitious material involves mixing and placing the material itself in a suitable container. Once the fresh material has been placed in the container, it can be transported through a pump-pipe-nozzle system to print cement filaments, which can thus build, layer by layer, the desired object. This procedure has the advantage of allowing the deposition of material only in the spaces foreseen by the 3D model, unlike traditional technologies in the building field, and the possibility of creating multi-material objects. But, on the other hand, the disadvantage of this method could be the need to identify a suitable support technique for the creation of complex objects.

3D printing of cementitious materials, using the extrusion technique, appears for the first time in 2007, thanks to the research team of the University of Loughborough (UK) [4]. This research group presented for the first time the potential of using cementitious materials in AM, focusing attention on some critical aspects, such as the production of large objects, the complexity of the formulations, the need to identify the correct properties. rheological and mechanical properties of the same during printing and curing, the need to ensure sufficient adhesion of the intermediate layers. The result of these studies led to the creation of a 3D printer for cementitious materials, which extrudes a high-performance mixture under computer control. This 3D printer makes it possible to produce objects such as complex structural components, curvilinear cladding panels and particular architectural elements.

The main characteristics to evaluate whether a cementitious material is suitable as a material for extrusion 3D printing, now widely identified and defined, are the following [5]:

- Extrudability: that is the characteristic that allows the material to flow easily through the nozzle. This characteristic is controlled by the correct balance between pumping power, extrusion flow rate and geometry of the nozzle;

- Workability time of the material (in English open time): that is the time that passes from the preparation of the material to when it is too viscous to be extruded correctly in the 3D printing process; - Self-supporting capacity (in English buildability): that is, the ability of the fresh material to support the weight of the upper layers, which is a property that depends on the rheology of the material, but also on the adhesion between the layers.

It is necessary to find the right balance to get the right formulation as these characteristics are antithetical. For this reason, it is essential to identify suitable additives, as well as the correct dispersion of the aggregates in the cement matrix, in order to optimize the formulation.

Other significant examples in the AM extrusion applied to the cement sector are the following:

- University of Southern California: has developed a layered fabrication technology called Contour Crafting (CC, which uses computer control to create smooth and precise surfaces both planar and any shape [6]. Although the technique is based on the 'extrusion of AM materials, is a hybrid method that combines an extrusion process for forming the surfaces of the object and a filling process (pouring or injection) to build the core of the object, also using industry standard materials [7] . The extrusion process constructs only the outer edges (circles) of each layer of the object. After the complete extrusion of each closed section of a given layer, if necessary, the fill material can be poured to fill the defined area from extruded edges. The application of CC in the construction of buildings is performed by a trestle structure that carries the nozzle and moves it on two parallel lanes installed on l constructive site. [8];

- WinSun: is a company that uses large 3D printers that extrude a mixture of fast-drying concrete and recycled materials [9]. The technology is based on the AM extrusion technique and uses a CAD drawing as a model. A computer controls a mechanical extruder arm to deposit the cement material, which is treated with hardeners so that each layer is solid enough to support the next, making one wall at a time. The pieces are then subsequently joined to each other, directly in the construction site;

- University of Technology of Eindhoven: this research group has studied a new model of 3D concrete printing technology, which similarly to other machinery (such as the Contour Crafting printer), has a similar appearance to that of a crane. It is therefore a non-portable machine, with a swiveling printer head, with concrete mixing, a pump and a print volume of 11x5x4 m <3>.

Over the years, specific cementitious formulations have therefore been developed to be printed by suitable 3D printers and some of these have also been patented. In this regard, as regards the cement-based formulations, the documents CN104310918, CN201510838044A, WO2017 / 050421A1, US2014 / 0252672A1 are cited. As far as the extrusion technology applied to this sector is concerned, the most significant patents / patent applications are the result of the activity of the research centers mentioned above, and we cite, by way of example, US7641461B2, US7837378B2, US7878789B2 and US 7753642B2.

Although specific cementitious formulations / mixtures have been developed to be printed by 3D printers, the need is particularly felt to identify cementitious compositions that solve the problems related to the following specificities:

- the cement mixture to be 3D printed by extrusion must be extrudable and self-supporting in the fresh state;

- the 3D printer for cement mixtures must have peculiar characteristics that are not found in printers currently on the market. In order to solve the technical problems considered above, the purposes of the present invention are:

- identify specific cement mixtures, optimized in terms of simultaneous extrudability and self-supporting capacity in the fresh state, in order to accurately reproduce a 3D model;

- re-design and print, with a plastic filament, some parts of a 3D printer to adapt it to process / print the desired cement mixtures.

Therefore, the subject of the present invention is a 3D printing process comprising the following steps:

- preparation of a cement mixture which includes a) cement or hydraulic binder, b) latent hydraulic addition, c) filler, d) aggregates, e) additives, f) water, said mixture being characterized by the fact that component d) is present in an amount from 10% to 80% by weight, preferably from 25 to 50% by weight with respect to the total weight of the cement mixture, and consists of calcareous, siliceous or silico-calcareous aggregates, alone or in a mixture with each other, having a particle size with a maximum diameter of less than 1 mm, component e) includes super-plasticising additives, rheology modifiers, shrinkage reducing agents and related mixtures, said cement mixture having a viscosity value ranging from 4000 Pa s to 35000 Pa s, measured at a shear rate of 0.01 s <-1>; - feeding the cement mixture to a 3D printing apparatus;

- extrusion of the cement mixture from the 3D printing apparatus using a single screw extruder;

- 3D model printing by depositing successive layers of cement mixture;

the ratio between the maximum diameter of the aggregates of the cement mixture and the distance between the screw and the internal wall of the extruder being between 0.02 and 0.8.

Viscosity is measured by a rheological method, with a controlled shear rate rheometer model Haake Rotovisco RV1 with coaxial cylinders, using a cylinder and a vane (with 4 blades), having a diameter of 41 and 22 mm respectively. The materials of the present invention were characterized using a step method by varying the shear rate from a minimum value equal to 0.01 s <-1> to a maximum value of 10 s <-1>. The total duration of the test is 30 minutes in which the precise data are collected at the desired speeds.

Preferably, the cementitious mixture for 3D printers for the process according to the invention comprises

a) from 10% to 70% by weight of hydraulic binder or cement, preferably selected from Portland cement, sulfoaluminous cement and / or aluminous cement and / or quick-setting natural cement, alone or in mixture;

b) from 0.0% to 25% by weight, preferably from 0.5 to 20% by weight, of a natural or artificial hydraulic addition, preferably granular slag from blast furnaces, having a specific surface ranging from 3500 cm <2 > / g to 6500 cm <2> / g, preferably from 4000 cm <2> / g to 5000 cm <2> / g;

c) from 10% to 50% by weight, preferably from 15% to 40% by weight, of a filler, selected from calcareous, siliceous or silico-calcareous fillers, preferably calcareous, alone or in a mixture;

d) from 10% to 80% by weight, preferably from 25% to 50% by weight of aggregates selected from calcareous, siliceous or silico-calcareous aggregates, alone or in mixture, having a particle size with a maximum diameter of less than 1 mm;

e) from 0.01% to 1.5% by weight, preferably from 0.2% to 1.0% by weight of a superplasticizing additive selected from acrylic-based polycarboxylated superplasticizers, lignosulfonates, naphthalenesulfonates, melamines or vinyl compounds , more preferably polycarboxylic ethers; 0.01% to 5.0% by weight, preferably 0.10% to 0.50% by weight, of a rheology modifier additive, preferably cellulose, more preferably hydroxymethylethylcellulose; 0.01% to 2.0% by weight, preferably 0.1% to 1.0% by weight of modified starch; 0.0% to 1.0% by weight, preferably 0.3% to 0.6% by weight of a shrinkage reducing agent;

where the binder / aggregate weight ratio is in the range from 0.5 to 2.0, preferably from 0.62 to 1.36 and said mixture has a viscosity value ranging from 4000 Pa s to 35000 Pa s , measured at a shear rate of 0.01 s <-1>.

The percentages indicated above are percentages by weight with respect to the total weight of the powdered cementitious mixture, i.e. excluding water.

In the cement mixture of the process according to the present invention, the water / binder weight ratio is in the range from 0.25 to 0.8, preferably from 0.4 to 0.6.

The cement mixture of the process according to the present invention is surprisingly characterized by an optimal balance of the properties of interest: in fact it guarantees at the same time a good self-supporting capacity, extrudability and workability time, thus being particularly suitable for deposition by extrusion 3D printing.

This optimization was surprisingly obtained thanks to the specific combination of suitable additives, a precise dispersion of the aggregates with specific dimensions in the binder matrix and a specific viscosity range.

In fact, it should be remembered that from a rheological point of view the relevant parameters go in exactly opposite directions: it is necessary that the material in the fresh state has a viscosity that guarantees it is correctly extruded, but at the same time allows it to sustain itself during the process. printing, in order to guarantee the realization of the designed 3D object.

Therefore, extrudability and self-supporting capacity, in order to coexist, require a correct compromise in terms of rheological properties, since they have an opposing effect on these two parameters.

The concept of self-supporting capacity is not to be confused with green strength, defined as the strength of the uncured concrete material in order to maintain its original shape until the material begins to set and the hydration products provide sufficient mechanical strength [10].

Once deposited, the described cement mixture must be able to sustain itself (concept of self-supporting capacity) during the entire molding process layer by layer. This property, as mentioned, mainly depends on the rheological behavior of the material and, at the same time, on the adhesion between the layers.

The even more preferred cementitious mixture for 3D printing to be used in the process according to the present invention consists of:

a) from 10% to 70% by weight of hydraulic binder or cement selected from CEM I 52.5 R or CEM I 52.5 N, preferably CEM I 52.5R;

b) from 0.5 to 20% by weight of granular slag from blast furnaces, having a specific surface ranging from 4000 cm <2> / g to 5000 cm <2> / g;

c) from 15% to 40% by weight of a calcareous filler, alone or in mixture;

d) from 25 to 50% by weight of calcareous, siliceous or silico-calcareous aggregates, alone or in mixture, having a particle size with a maximum diameter of less than 1 mm; e) from 0.2 to 1.0% by weight of superplasticizing additive based on polycarboxylic ether; 0.10 to 0.50% by weight of a rheology modifier additive which is hydroxymethylethylcellulose; 0.1% to 1.0% by weight of modified starch; from 0.3% to 0.6% by weight of a shrinkage reducing agent, where the binder / aggregate weight ratio varies in the range from 0.62 to 1.36 and said cement mixture has a viscosity value ranging from 4000 Pa s at 35000 Pa s, measured at a shear rate of 0.01 s <-1>.

In the present description, the term "cement or hydraulic binder" refers to a powder material which, when mixed with water, forms a paste which hardens by hydration and which, after hardening, maintains its strength and stability even under water. The hydraulic binder or cement of the cement mixture according to the present invention is preferably selected from Portland cement, sulfoaluminous cement and / or aluminous cement and / or quick-setting natural cement. These cements can also be used in mixture with each other. The Portland cement according to the present invention is Portland cement I 42.5 or 52.5 resistance, with ordinary (N) or high (R) initial resistance class, according to the EN 197-1: 2011 standard. The preferred cement is CEM I 52.5 R or CEM I 52.5 N, even more preferred is CEM I 52.5R.

In the present description, the term "latent hydraulic addition" means a natural or artificial hydraulic addition, preferably granular slag from blast furnace (GGBS: "ground slag to ground grain"), having a specific surface ranging from 3500 cm <2> / g to 6500 cm <2> / g, preferably from 4000 cm <2> / g to 5000 cm <2> / g, determined according to the Blaine method according to EN 196-6: 2010.

Latent hydraulic addition is added to the formulation to improve the machinability of the material. When present, this type of addition forms part of the binder, therefore the binder in the binder / aggregate and water / binder ratio is given by the sum of the cement or hydraulic binder and the latent hydraulic addition (or GGBS).

In this description, the term "filler" is defined in accordance with the UNI EN 12620-1: 2008 standard as an aggregate, characterized by having a particle size such that approximately 90% of the filler passes through a 0.063 mm sieve. It can be added to building materials to impart different properties. The filler according to the present invention is selected from calcareous, siliceous or silico-calcareous fillers, preferably calcareous, alone or in a mixture.

In the present description, the term "aggregate" means calcareous, siliceous or silico-calcareous aggregates which are known and commonly available products. Aggregates for use in cementitious compositions are defined in the UNI EN 206: 2014 standard as a natural, artificial, recovered or recycled granular mineral constituent suitable for use in concrete.

Aggregates are normally used to obtain greater strength, less porosity and a decrease in efflorescence. In the present invention, the aggregates preferably have a particle size with a maximum diameter of less than 1 mm.

The aggregates in the cement mixture according to the present invention also comprise a fraction having a particle size with a diameter ranging from 0.00 mm to 0.20 mm. This range of particle size can be seen from the technical data sheet of this fraction, which is commercially called "Impalpable".

In the present description, the term "additives" refers to different types of additives which, in the cementitious mixture according to the present invention, allow to obtain an optimized cementitious mixture for 3D printing. In fact, in combination with the specific dispersion and size of the aggregates, they guarantee a synergistic effect of good construction speed, extrudability, workability time, workability and development of mechanical properties. The superplasticizer is an additive that is added to improve the workability of the product without increasing the water content. Among these, an acrylic-based polycarboxylate superplasticizer is preferred, dosed according to the temperature of the mixture, the ambient temperature and the degree of fluidity required in the formulation. Other possible superplasticizers are lignosulfonates, naphthalenesulfonates, melamine or vinyl compounds, the most preferred are the polycarboxylic ethers. A further additive in the cement mixture according to the present invention is the "rheology modifying agent", ie a substance which, if present in a cementitious composition, is capable of modifying the rheological properties in the fresh state and the adhesion to the substrate. This additive is added to this type of formulations to increase the viscosity of the product in order to avoid segregation. Cellulose derivatives such as cellulose, more preferably hydroxymethylethylcellulose, hydroxyethylcellulose, hydroxymethylpropylcellulose, carboxymethylcellulose are preferred rheology modifiers according to the present invention. The cement mixture according to the present invention can also comprise, as additives, starch derivatives used to influence the consistency of the mortars and improve the workability of the formulation. These compounds are chemical starches modified with ether groups that find application in the construction sector, in particular in plaster based on gypsum, cement and lime. Another preferred additive to add is the shrinkage reducing agent, also known as SRA (Shrinkage Reducing Agent), which includes a wide variety of glycols and polyols and is responsible for reducing shrinkage deformation throughout the service life of the product. hardened.

The use of the cement mixtures used in the process according to the present invention as extrusion material in a 3D printer is also described.

The object of the present invention is also an apparatus suitable for carrying out the printing process of a 3D object according to the present invention, said apparatus comprising a cylindrical pressurized gas supply tank, a screw extruder, a flexible tube that connects the tank to the extruder and a pumping system, where the extruder is a single screw extruder, equipped with a circular nozzle, the ratio between the maximum diameter of the aggregates of the cementitious composition and the distance between the screw and the internal wall of the extruder being between 0.02 and 0, 8.

More precisely, the aforementioned apparatus is part of a 3D printer, with which an object, previously designed by means of dedicated software, is made using the cement mixture according to the present invention. Said apparatus comprises a cylindrical pressurized gas supply tank, a screw extruder and a flexible tube which connects the tank to the extruder. The pumping system can be any pumping system known in the art, but preferably in the present apparatus a piston has been used to push the cement mixture, contained inside the supply tank. The cement mixture through a flexible tube is thus fed to the single screw extruder mounted on the print head. The extruder is equipped with a circular nozzle.

More specifically, in the 3D printing process according to the present invention, the cement mixture is fed by means of a flexible tube to an extruder of a 3D printer that allows for the creation of an extrusion positioned in the printing area of the same.

This extruder is characterized by a screw having a height that varies from 35 to 140 mm, preferably from 40 to 80 mm, a pitch that varies from 7 to 30 mm, preferably from 8 to 15 mm, and a helix angle that varies from 12 ° to 43 °, preferably from 14 ° to 26 °, a nozzle with a diameter ranging from 2 to 30 mm, preferably from 2.5 to 7 mm, and a height ranging from 5 to 50 mm, preferably from 10 to 40 mm. The aforesaid extruder allows to deposit cement mixtures according to the present invention and more precisely mixtures which include aggregates having a particle size with a maximum diameter of less than 1 mm and a viscosity value ranging from 4000 Pa s to 35000 Pa s, measured at a shear rate of 0.01 s <-1>.

Furthermore, the ratio between the maximum diameter of the aggregates and the distance between the screw and the internal wall of the extruder (to be understood as the difference between the internal diameter of the extrusion chamber and the diameter of the screw, in English flight clearance) must be between 0.02 and 0.8.

This ratio discriminates the ability to extrude aggregates with a certain value of the maximum diameter inside the extruder itself.

A further object of the present invention is a finished product with complex geometry obtained by 3D printing with an apparatus suitable for carrying out the printing process of a 3D object according to the present invention.

In the attached figures

Figure 1 is a schematic representation of an extruder for extruding the cement mixture according to the present invention;

Figure 2 is a representation of the support structure for printing the screw of the extruder;

- figure 3 is a photographic reproduction of the pressurized tank, empty and full of cement mixture, of the apparatus according to the present invention; - figure 4 is a photographic reproduction of the finished product with complex geometry obtained according to example 1.

- figure 5 is a photographic reproduction of the main components that make up the apparatus for carrying out the 3D printing process according to the present invention.

As previously highlighted, the main components of the apparatus for carrying out the 3D printing process according to the present invention, to which the cement mixture is fed to be subsequently extruded and deposited, are the following:

1) cylindrical supply tank, pressurized gas;

2) flexible tube that connects the tank to the extruder;

3) screw extruder;

4) circular outlet nozzle.

The extrusion device can be mounted on any type of machine or robot that can receive it, in order to combine the extrusion process with the specific advantages related to the kinematics of the machine / robot.

In details:

Figure 5 shows the cylindrical gas pressurized supply tank (1) which contains a piston which pushes the fluid, ie the cement mixture. The pressure is supplied by pressurized air, directly connected to the tank and regulated by a pressure gauge.

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 between 1.5 and 3 m.

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

All 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 regarding the plastic parts of the extruder is the metal shaft.

The screw was stamped 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; support structures (in the shape of a triangle) have been provided (shown in figure 2) so that the screw of desired geometry could be correctly molded. The rotation speeds supported by the screw are between 90 and 180 rpm.

The diameter of the nozzle is equal to 4.8 mm and the nozzle is designed in one piece, cone-shaped to reduce the friction of the cement mixture.

The printing parameters can be controlled with various software. These are software that allow you to divide the designed object into sections dictated by the print resolution you want to obtain. In particular, the object to be printed is designed by creating a 3D digital model using a CAD application, and then divided into layers using the aforementioned software, thus providing the machine with instructions and establishing the path (layer by layer) that the nozzle must follow to build the object. The software for dividing the object into layers was generally created to manage materials such as plastic or metal and therefore do not allow you to directly control some important parameters, such as the speed of the screw. To control the speed of the screw (and therefore the flow rate of the extruded material) an approach similar to the plastic extrusion control model was followed. The first step is to calculate the range needed to print the object. It is the product of the height of the extruded layer, the diameter of the nozzle and the speed of the print head. Therefore, knowing the flow rate value, it is possible to calculate the rotation speed of the screw, using the following equations of a single screw extruder model:

where N is the rotation speed of the screw in rpm, ΔP is the increase in pressure inside the extrusion chamber, μ is the viscosity of the cement mixture (assuming that, in conditions of high flow stress, it behave 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.

The screw is moved by the same motor that pushes the polymeric wire into the extruder for polymeric materials. The rotation of the motor must ensure a sufficient feed rate of the polymer to the extruder and therefore its speed depends on the diameter of the wire. By providing the software with the correct value of this diameter, it is possible to define the speed of the screw extruder motor.

In the case of the cementitious mixtures object of the present invention, it is necessary to impose on the software a value of the diameter of the wire much higher than that of a plastic wire, in order to correctly use the nozzle with the desired diameter and to obtain the right flow rate necessary for printing of this type of materials. This is necessary to impose the right rotation speed (rpm) on the extruder screw. It is also possible to change the wire diameter to increase the range and therefore the printing speed.

The examples reported below are aimed 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

A cement mixture formulation 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 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 the speed of 285 rpm and subsequently for 1 minute at the speed of 322 rpm;

- mixing was interrupted for 45 seconds to collect any material remaining on the container walls;

- all the components were then mixed for 1 minute at the speed of 322 rpm and then for 1 minute at the speed of 240 rpm.

Table 1: Extruded formulation according to example 1.

The cement is a type I 52.5 R cement from the Rezzato plant. The limestone 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 between 0.00 and 0.200 mm and a second fraction with a particle size distribution between 0.200 and 0.35 mm. Both materials are marketed by Sabbie Sataf s.r.l. with the trade names, respectively, of Impalpabile and 113.

The superplasticizer is based on polycarboxylic ether, called Melflux 1641 F, and marketed by BASF. The rheology modifier 1 is a hydroxymethyl-ethylcellulose called "Tylose MH 60004 P6" marketed by ShinEtsu. The rheology modifier 2 is a modified starch with ether groups, marketed under the name Aqualon ST2000 by Ashland. These three additives are all in solid form.

The water / binder ratio is equal to 0.56, while the binder / aggregate ratio is equal to 1.03.

At the end of the mixing, the cement mixture having the composition indicated in table 1 was characterized by means of a Haake RotoVisco RV1 rheometer, with coaxial cylinders. The test allowed to characterize the viscosity of the material in a range of share rates between 0.01 and 10 s <-1>, by means of a step method. Each step was held for 30 seconds and the overall duration of the test was 8 minutes. The viscosity of the cement mixture measured at the shear stress value of 0.01 s <-1> is equal to 20000 Pa · s.

At the end of the mixing, the mortar was inserted into the cylindrical gas pressurized supply tank (as shown in Figure 3) with the help of a spatula and arranged so as to completely fill the container, reducing as much as possible the air trapped in the material. The gas pressurized cylindrical feed tank was thus prepared to be connected to the extruder mounted on the printing machine, using the previously described tube. The pressure at the tank has been set to a value of 4.0 bar.

The mixture prepared as mentioned above was extruded using a single-layer spiral print path. The 3D model to be printed was a cylindrical element characterized by an internal diameter of 20 cm and a height of 20 cm. The model was successfully printed (as evident from Figure 4) in a single print session, applying the following print parameters:

Gas pressurized cylindrical supply tank pressure: 4.0 bar; Imposed wire diameter: 1.68 mm;

Layer height: 3.2mm;

Print speed: 10mm / s;

Screw rotation speed: 180 rpm;

Flight clearance: 0.5

Ratio between the maximum diameter of the aggregate and the distance between the screw and the internal wall of the extruder: 0.7.

Example 2

A cement mixture formulation having the composition reported in the following table 2 was prepared using a Hobart mixer, according to the procedure described in example 1.

Table 2: Extruded formulation according to example 2.

The cement is a type I 52.5 R coming from the Rezzato plant. The GGBS included in the formulation constitute the latent hydraulic addition and are a granular slag from blast furnace (GGBS: "ground slag with ground grain"), having a specific surface equal to 4000 cm <2> / g (according to EN 196- 6: 2010), provided by the ILVA company. The limestone filler is a high purity filler, marketed by Omya Spa under the trade name of Omyacarb 2-AV. The silico-calcareous aggregate is a fraction with a particle size distribution between 0.00 and 0.200 mm marketed by Sabbie Sataf s.r.l. with the trade name Impalpabile.

The superplasticizer is based on polycarboxylic ether, called Melflux 1641 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 modified starch with ether groups, marketed under the name Aqualon ST2000 by Ashland. These three additives are all in solid form.

The water / binder ratio is equal to 0.59, while the binder / aggregate ratio is equal to 1.17.

At the end of the mixing, the cement mixture having the composition indicated in table 2 was characterized by means of a Haake RotoVisco RV1 rheometer, using the method already described in example 1. The viscosity of the cement mixture measured at the shear stress value of 0.01 s <-1> is equal to 11620 Pa s. 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 printer, as described in example 1. The pressure at the tank was set to the value of 4.0 bar .

The mixture prepared as mentioned above was extruded using a single-layer spiral print path. The 3D model to be printed was a cylindrical element characterized by an internal diameter of 20 cm and a height of 20 cm. The model was printed by 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 internal wall of the extruder is equal to 0.4.

The model was printed correctly up to the height of 15 cm.

Even if the specific object envisaged by the 3D model has not been completely printed with the formulation indicated in Table 2, the cement mixture of Table 2 is still suitable to be printed with this extrusion device in multiple print sessions, rather than in one session. unique, because it is intrinsically characterized by an excellent compromise between rheological properties and self-supporting capacity.

Example 3 (comparative example)

A formulation containing the composition shown in Table 3 was prepared using a Hobart mixer according to the procedure described in Example 1.

Table 3: Extruded formulation according to example 3.

The cement is a type I 52.5 R coming from the Rezzato plant. The GGBS included in the formulation constitute the latent hydraulic addition and are a granular slag from blast furnace (GGBS) having a specific surface equal to 4000 cm <2> / g (according to the EN 196-6: 2010 standard), supplied by the company ILVA . The limestone 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 between 0.00 and 0.200 mm and a second fraction with a particle size distribution between 0.600 and 1.00 mm. Both materials are marketed by Sabbie Sataf s.r.l. with the trade names, respectively, of Impalpabile and 103.

The superplasticizer is based on polycarboxylic ether, called Melflux 1641 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 modified starch with heterecovered groups under the name Aqualon ST2000 by Ashland. The Shrinkage Reducing Agent (SRA), called SRA04, is marketed by Neuvendis; it is a mixture of glycols and special surfactants. The four additives are all solid state.

The water / binder ratio is equal to 0.57, while the binder / aggregate ratio is equal to 0.85.

At the end of the mixing, the formulation indicated in table 3 was characterized by a Haake RotoVisco RV1 rheometer, using the method already described in example 1.

The viscosity value measured at the shear stress value of 0.01 s <-1> is equal to 21000 Pa · s.

At the end of the mixing, the mortar, characterized by a value of the ratio between the maximum diameter of the aggregate and the distance between the screw and the internal wall of the extruder equal to 2, was inserted into the cylindrical gas-pressurized supply tank which was connected to the 'extruder mounted on the printer, as described in example 1. The pressure at the tank has been set at 4.0 bar.

The mixture prepared as previously mentioned was not processable. Example 4 (comparative example)

A formulation containing the composition shown in Table 4 was prepared using a Hobart mixer according to the procedure described in Example 1.

Table 4: Extruded formulation according to example 4.

The cement is a type I 52.5 R coming from the Rezzato plant. The limestone filler is a high purity filler, marketed by Omya Spa under the trade name of Omyacarb 2-AV. The silico-calcareous aggregate is a fraction with a particle size distribution between 0.200 and 0.350 mm marketed by Sabbie Sataf s.r.l. under the trade name 113.

The superplasticizer is based on polycarboxylic ether, called Melflux 1641 F, and marketed by BASF. The rheology modifier 1 is a hydroxymethylethylcellulose called "Tylose MH 60004 P6" marketed by ShinEtsu. Rheology modifier 2 is a modified starch with ether groups, marketed under the name Aqualon ST2000 by Ashland. These three additives are all in solid form.

The water / binder ratio is equal to 0.58, while the binder / aggregate ratio is equal to 0.95.

At the end of the mixing, the formulation indicated in table 3 was characterized by a Haake RotoVisco RV1 rheometer, using the method already described in example 1.

The viscosity value measured at the shear stress value of 0.01 s <-1> is equal to 813 Pa · s.

At the end of the mixing, the mortar, characterized by a value of the ratio between the maximum diameter of the aggregate and the distance between the screw and the internal wall of the extruder equal to 0.7, was inserted into the cylindrical gas-pressurized supply tank which was connected to the extruder mounted on the printer, as described in example 1. The pressure at the tank has been set at 2.0 bar.

The mortar was processed in a 3D printing session, but the expected number of layers could not be achieved.

The previous examples show that both the viscosity value and the maximum diameter of the aggregates are essential for the cementitious mixture for 3D printing according to the present invention.

In fact, example 1, where the viscosity and the maximum diameter of the aggregates fall within the ranges envisaged by the present invention, allows to obtain a cementitious mixture for 3D printing, extrudable and with an optimal self-supporting capacity. This mixture therefore allows you to print the desired product in a single printing session.

The cement mixture of example 2 has a lower viscosity than the cement mixture of example 1, however included in the range envisaged by the present invention, as well as the maximum diameter of the aggregates. The resulting cement mixture for 3D printing is in any case extrudable. and it has a self-supporting capacity that still allows you to print the desired product, but with a multiplicity of printing sessions.

The cement mixture of comparative example 3, while having a viscosity value included in the range envisaged by the present invention, provides for a maximum diameter of the aggregates outside the maximum limit of the range envisaged by the present invention. For this reason, the mixture is not processable. The cement mixture of comparative example 4, although presenting an aggregate having a maximum diameter included in the range envisaged by the present invention, is characterized by a viscosity value outside the minimum viscosity limit. envisaged by the present invention. Therefore, it was not able to support the number of layers predicted by the model.

Bibliography:

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[5] T.T. Le, S.A. Austin, S. Lim, R.A. Buswell, A.G.F. Gibb, T. Thorpe, “Mix design and fresh properties for high-performance printing concrete”, 45, 2012, Materials and Structures, p. 1221-1232.

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Claims (9)

CLAIMS 1. 3D printing process comprising the following steps: - preparation of a cement mixture which includes a) cement or hydraulic binder, b) latent hydraulic addition, c) filler, d) aggregates, e) additives, f) water, said mixture being characterized by the fact that component d) is present in an amount from 10% to 80% by weight, preferably from 25 to 50% by weight with respect to the total weight of the cement mixture, and consists of calcareous, siliceous or silico-calcareous aggregates, alone or in a mixture with each other, having a particle size with a maximum diameter of less than 1 mm, component e) includes superplasticizing additives, rheology modifiers, shrinkage reducing agents and related mixtures, said cement mixture having a viscosity value ranging from 4000 Pa s to 35000 Pa s, measured at a shear rate of 0.01 s <-1>; - feeding the cement mixture to a 3D printing apparatus; - extrusion of the cement mixture from the 3D printing apparatus using a single screw extruder; - 3D model printing by depositing successive layers of cement mixture; the ratio between the maximum diameter of the aggregates of the cement mixture and the distance between the screw and the internal wall of the extruder being between 0.02 and 0.8. 2. Process according to claim 1, where the cement mixture comprises: a) from 10% to 70% by weight of hydraulic binder or cement, preferably selected from Portland cement, sulfoaluminous cement and / or aluminous cement and / or quick-setting natural cement, alone or in mixture; b) 0.0% to 25% by weight, preferably 0.5 to 20% by weight, of a natural or artificial hydraulic addition, preferably granular slag from blast furnaces, having a specific surface ranging from 3500 cm <2 > / g to 6500 cm <2> / g, preferably from 4000 cm <2> / g to 5000 cm <2> / g; c) from 10% to 50% by weight, preferably from 15% to 40% by weight, of a filler, selected from calcareous, siliceous or silico-calcareous fillers, preferably calcareous, alone or in a mixture; d) from 10% to 80% by weight, preferably from 25% to 50% by weight of aggregates selected from calcareous, siliceous or silico-calcareous aggregates, alone or in mixture, having a particle size with a maximum diameter of less than 1 mm; e) from 0.01% to 1.5% by weight, preferably from 0.2% to 1.0% by weight of a superplasticizing additive selected from acrylic-based polycarboxylate superplasticizers, lignosulfonates, naphthalenesulphonates, melamines or vinyl compounds , more preferably polycarboxylic ethers; 0.01% to 5.0% by weight, preferably 0.10% to 0.50% by weight, of a rheology modifier additive, preferably cellulose, more preferably hydroxymethylethylcellulose; 0.01% to 2.0% by weight, preferably 0.1% to 1.0% by weight of modified starch; 0.0% to 1.0% by weight, preferably 0.3% to 0.6% by weight of a shrinkage reducing agent; where the binder / aggregate weight ratio is in the range from 0.5 to 2.0, preferably from 0.62 to 1.36 and said mixture has a viscosity value ranging from 4000 Pa s to 35000 Pa s , measured at a shear rate of 0.01 s <-1>. 3. Process according to one or more of the preceding claims, where the water / binder weight ratio is in the range from 0.25 to 0.8, preferably from 0.4 to 0.6. 4. Process according to one or more of the preceding claims, where the mixture consists of: a) from 10% to 70% by weight of hydraulic binder or cement selected from CEM I 52.5 R or CEM I 52.5 N, preferably CEM I 52.5R; b) from 0.5 to 20% by weight of granular slag from blast furnaces, having a specific surface ranging from 4000 cm <2> / g to 5000 cm <2> / g; c) from 15% to 40% by weight of a calcareous filler, alone or in mixture; d) from 25 to 50% by weight of calcareous, siliceous or silico-calcareous aggregates, alone or in mixture, having a particle size with a maximum diameter of less than 1 mm; e) from 0.2 to 1.0% by weight of superplasticizing additive based on polycarboxylic ether; 0.10 to 0.50% by weight of a rheology modifier additive which is hydroxymethylethylcellulose; 0.1% to 1.0% by weight of modified starch; from 0.3% to 0.6% by weight of a shrinkage reducing agent, where the binder / aggregate weight ratio varies in the range from 0.62 to 1.36 and said cement mixture has a viscosity value ranging from 4000 Pa s at 35000 Pa s, measured at a shear rate of 0.01 s <-1>. 5. Process according to one or more of the preceding claims, where component a) of the mixture is selected from CEM I 52.5 R or CEM I 52. 5 N, preferably CEM I 52.5R. 6. Process according to one or more of the preceding claims, where component b) of the mixture is a granular slag from blast furnace having a specific surface ranging from 3500 cm <2> / g to 6500 cm <2> / g, preferably from 4000 cm <2> / g to 5000 cm <2> / g. 7. Apparatus suitable for carrying out the process of printing a 3D object according to one or more of claims 1 to 6, said apparatus comprising a cylindrical pressurized gas supply tank, a screw extruder, a flexible tube which connects the tank to the extruder and a pumping system, where the extruder is a single-screw extruder, equipped with a circular nozzle, the ratio between the maximum diameter of the aggregates of the cementitious composition and the distance between the screw and the internal wall of the extruder being between 0.02 and 0 , 8. 8. Apparatus according to claim 7, where the extruder is characterized by a screw having a height ranging from 35 to 140 mm, preferably from 40 to 80 mm, a pitch ranging from 7 to 30 mm, preferably from 8 to 15 mm, and a helix angle ranging from 12 ° to 43 °, preferably from 14 ° to 26 °, a nozzle with a diameter ranging from 2 to 30 mm, preferably from 2.5 to 7 mm, and a height which varies from 5 to 50 mm, preferably from 10 to 40 mm. 9. Finished product with complex geometry obtained by 3D printing with an apparatus suitable for carrying out the printing process according to any one of claims 1 to 6.