Field of the invention

[0001] The present invention relates to a method to manufacture a 3D-shaped object of a cementitious material while supporting the obtained 3D-shaped object at least partially by a liquid or semi-liquid fluid.

Background art

[0002] Additive manufacturing of cementitious materials attracted a lot of attention during recent years. By additive manufacturing successive layers of material are deposited to form a 3D-shaped object. Although several challenges are still to be addressed, additive manufacturing of cementitious material has already proven to have a high number of advantages. Additive manufacturing offers a quick and cost-efficient way to build complex structures. Furthermore, it disposes the need to use conventional formwork structures requiring a labor-intensive and thus expensive process.

[0003] Although additive manufacturing of cementitious materials allows for the creation of unique and complex shapes that are unattainable through conventional fabrication processes, this technique still reveals constraints limiting the geometric complexity of 3D-shaped objects. Firstly, the number of layers that can be built on top of each other is limited. Furthermore, this technique does not allow forthe manufacturing of objects with a significant overhang. The maximum overhang of 3D-shaped object of cementitious material obtained by additive manufacturing is usually limited to 5 to 30 degrees. The maximum overhang is amongst others depending on the material, the meteorological conditions such as the ambient temperature, the printing speed and the layer height. Additionally, bridging, i.e. extrusion of material to connect two points without support below, is impossible or difficult to achieve. Furthermore, for many (fast-setting) 3D ‘printable’ material mixtures, early-age (plastic) shrinkage is a problem.

[0004] The early-age print material performance (for example stiffness and strength evolution) can be improved by using mixtures comprising accelerators and/or other additives or by using two component solutions. However, even with fast-setting materials, the buildability, the maximum overhang and bridging is limited. Additionally, the use of fast-setting products has the disadvantage that the CO2 footprint of the printed structure is increased considerably and that the plastic and autogenous shrinkage is increased.

[0005] To avoid collapsing of a 3D-shaped object, for example having a high overhang or the occurrence of bridging, the use of temporary scaffolding structures (for example using Eggshell technology) or the use of granular support materials has been proposed. Such techniques are however slow, labor intensive and not scalable. Furthermore, the scaffolding material is usually not reusable.

[0006] Others have tried to print cementitious materials by injecting cementitious materials in a liquid suspension. One can distinguish several sub-classes as given below concrete in suspension (CiS) whereby concrete is injected in a non-hardening suspension; suspension in concrete (SiC) whereby a non-hardening suspension is injected into a vessel filled with fresh concrete; concrete in concrete (CiC) whereby concrete is used for both the extrusion and the supporting material, whereby the two concrete materials have different properties. One possibility is to inject a high-performance concrete with high strength into a concrete with low strength.

[0007] All these technologies enable 3D printing of complex designs with cementitious materials. However, node connectivity and layer adhesion are not guaranteed due to the presence of the support material prior to printing. In addition, the more material is injected into the other, the more the volume of the supporting material will be displaced. The mechanisms of the rising material level and the associated positional changes of the already injected material are problematic.

Summary of the invention

[0008] It is an object of the present invention to provide a method to manufacture 3D-shaped objects of a cementitious material avoiding the drawbacks of the prior art.

[0009] It is another object of the present invention to provide a method to manufacture 3D-shaped objects having a complex shape, for example 3D-shaped objects having large overhangs or which include bridging.

[0010] It is also an object of the present invention to provide a method to manufacture 3D-shaped objects of a cementitious material thereby preventing or reducing early-age and autogenous shrinkage.

[0011] Furthermore, it is an object of the present invention to provide a method to manufacture 3D-shaped objects of a cementitious material using a more sustainable concrete mix, for example a concrete mix not requiring accelerators.

[0012] It is a further object of the present invention to provide a method to manufacture 3D-shaped objects not requiring support structures (scaffolding) or granular support materials as for example sand during manufacturing.

[0013] Furthermore, it is an object of the present invention to provide a method to manufacture 3D-shaped objects whereby perfect bonding between layers is ensured.

[0014] It is still a further object of the present invention to provide a method to temporarily support a 3D-shaped object during its manufacturing by layer-by-layer deposition of cementitious material and optionally also during the self-setting and/or self-hardening of this cementitious material.

[0015] According to a first aspect of the present invention, a method to manufacture a 3D-shaped object of a cementitious material is provided. The method comprises layer-by-layer deposition of n consecutive layers of cementitious material while surrounding the obtained 3D-shaped object (at least) partially with a supporting liquid or semi-liquid fluid. The number of layers n of the 3D-shaped object is at least 2, for example at least 5, at least 10 or at least 20. The number of layers n may be higher than 50, higher than 500 or higher than 1000. The deposition of a layer i, with i ranging from 1 to n, comprises extrusion of a cementitious material from a nozzle and deposition of the cementitious material on a printing area of a substrate for layer i=1 or on a printing area of layer i-1 , for i ranging from 2 to n, while the nozzle and the printing area are moving along a (predetermined) printing path.

The cementitious material extruded from the nozzle comprises a self-setting and/or self-hardening material. Preferably, the cementitious material comprises a self-setting and self-hardening material. The cementitious material comprises cement particles, aggregate particles and water or an aqueous solution. The cement particles are capable of binding the aggregate particles in the presence of water or in the presence of the aqueous solution. The cement particles have a particle size of at least 1 pm and the aggregate particles have a particle size larger than the particle size of the cement particles.

The method according to the present invention is characterized in that during the deposition of the cementitious material to form layer i, with i ranging from 2 to n, the printing area of layer i-1 is not covered with the liquid or semi-liquid fluid.

[0016] For the purpose of this invention the term cementitious material refers to materials comprising cement as for example concrete or mortar and includes fresh cementitious material, partially or fully hardened cementitious material. For a person skilled in the art, it is clear that cementitious material also comprises water or an aqueous solution and aggregates. Cementitious material may further comprise other additional components and/or additives, such as additional components and/or additives known in the art, in particular mineral additional components and/or additives, for example mineral additional components and/or additives in powder form and/or chemical admixtures, for example plasticizers, viscosity modifying agents and/or chemical activators to control/or accelerate setting and/or hardening.

[0017] The terms “cement” and “cement particles” refer to materials, in particular particles capable of binding aggregate particles together and includes hydraulic cement as well as supplementary cementitious materials (SCMs). The terms ’’cement” and “cement particles’ include amongst others Portland cement, calcium aluminate cement, lime, gypsum, geopolymer cement and other inorganic binders such as hydraulic, latent hydraulic or alkali-activated binders. Cement particles have an average particle size of at least 1 pm, for example an average particle size of at least 10 pm. Preferably cement particles have an average particle size below 100 pm. Preferably, cement particles have an average particle size ranging between 1 pm and 50 pm, for example 10 pm, 20 pm, 30 pm or 40 pm. The particle size of cement particles can be measured by laser scattering (laser sizing or laser granulometry). For the purpose the terms cement and cement particles are used interchangeably.

[0018] The terms “aggregates” and “aggregate particles” referto granular materials and comprises for example sand, gravel, crushed stones and iron blast-furnace slag. The granular material has preferably an average particle size that is several times larger than the average particle size of the cement particles. Preferably, aggregate particles have an average particle size of at least 100 pm. Fine aggregate particles have an average particle size ranging from 100 pm to 4 mm, preferably an average particle size ranging from 250 pm to 2 mm, as for example 500 pm or 1 mm. Coarse aggregate particles have an average particle size larger than the particle size of fine aggregate particles and have for example an average particle size ranging from 4 mm to 40 mm. Preferably, coarse aggregate particles have an average particle size ranging up to 20 mm. For the purpose of this invention the terms aggregates and aggregate particles are used interchangeably.

[0019] When mixed with water or with an aqueous solution, cement acts as a binder, allowing the cementitious material to set and harden. The setting and hardening of the cementitious material is due to the hydration of the constituent compounds of cement.

[0020] The term “aqueous solution” refers to a solution having water as solvent and includes pH neutral aqueous solutions as well as acidic and alkaline aqueous solutions.

[0021] The term “setting” refers to stiffening of a cementitious material (cement paste) comprising cement particles and water or an aqueous solution and optionally comprising aggregate particles to retain its shape and thus refers to the change from the fluid state to the rigid state.

[0022] The term “hardening” refers to the gain of strength of a set cementitious material (cement paste). It is clear that during the setting of the cementitious material some strength may be obtained. [0023] The term “self-setting” refers to setting of a cementitious material (cement paste) whereby the cementitious material (cement paste) stiffens just by mixing cement particles with water or with an aqueous solution and whereby no other trigger, for example no external trigger, is required. In particular the self-setting process is an intrinsic process not requiring the presence of a further chemical compound, not requiring a chemical reaction, not requiring a temperature change, for example heating or cooling (for example rapid cooling or freezing) and not requiring a pH change (increase or decrease).

[0024] Although application of heat is not required to allow the self-setting and/or self-hardening of the material some moderate heating may accelerate the self-setting and/or self-hardening. Similarly or additionally, chemical admixtures for example plasticizers, viscosity modifying agents and/or chemical activators can be added to the cementitious material, for example to accelerate setting and/or hardening.

[0025] It is clear that as the nozzle moves during the deposition of cementitious material along a path, called the printing path, the printing area moves along the same path.

[0026] The printing area is defined as the area of a layer or substrate that receives extruded material from the nozzle, i.e. the area of a substrate or layer that receives the fresh material extruded from the nozzle.

[0027] The substrate can be any type of substrate suitable to hold or carry the obtained 3D-shaped object. The substrate is preferably a planar substrate. The substrate can be movable or nonmovable. In particular examples the substrate can be lowered, for example in the surrounding liquid or semi-liquid fluid. The size of the printing area depends amongst others on the size of the flow of the extruded cementitious material from the nozzle, on the printing speed and on the properties of the cementitious material.

[0028] According to the method of the present invention it is important that during the deposition of the cementitious material to form layer i, with i ranging from 2 to n, the printing area of layer i-1 is not covered with the liquid or semi-liquid fluid so that the cementitious material extruded from the nozzle to form layer i is making direct contact with the cementitious material of layer i-1. Consequently, the cementitious material that is extruded from the nozzle to deposit layer i, is in the printing area of layer i-1 making direct contact with the material of layer i-1 and is in the printing area of layer i-1 not contacting the liquid or semi-liquid fluid surrounding the obtained 3D-shaped object. During deposition, the cementitious material extruded from the nozzle is not contacting the liquid or semi-liquid fluid surrounding the obtained object, except when for example bridging or large overhang occur. The direct contact of the material of layer i and the material of layer i-1 ensures good adhesion between the material of layer i and the material of layer i-1 .

[0029] One way to obtain that the printing area of layer i-1 is not covered with the liquid or semiliquid fluid while the liquid or semi-liquid is supporting the obtained 3D-shaped object, is by adding the liquid or semi-liquid fluid either continuously or discontinuously during deposition of the cementitious material in a controlled way so that the liquid or semi-liquid fluid is supporting the obtained 3D-shaped object but is leaving the printing area of layer i-1 uncovered (i.e. not covered with liquid or semi-liquid fluid) during the deposition of layer i, with i ranging from 2 to n. The liquid or semi-liquid fluid is for example continuously added either with a constant or a variable flow rate. Alternatively, the liquid or semi-liquid fluid is discontinuously added, for example during certain time intervals.

[0030] An alternative way to obtain that the printing area of layer i-1 is not covered with the liquid or semi-liquid while the liquid or semi-liquid is supporting the obtained 3D-shaped object fluid, is by lowering the substrate on which the 3D-shaped object is deposited in the liquid or semi-liquid fluid surrounding the 3D-shaped object in a controlled way so that the liquid or semi-liquid fluid is supporting the obtained 3D-shaped object but is leaving the printing area of layer i-1 uncovered (i.e. not covered with liquid or semi-liquid fluid) during the deposition of layer i, with i ranging from 2 to n. The substrate (with the obtained 3D-shaped object) is for example lowered in a controlled way in a vessel comprising a liquid or semi-liquid fluid so that the liquid or semi-liquid fluid is supporting the obtained 3D-shaped object but leaving the printing area of layer i-1 uncovered.

[0031] It is clear that it is also possible to obtain that the printing area of layer i-1 is not covered with the liquid or semi-liquid fluid by adding the liquid or semi-liquid fluid either continuously or discontinuously during deposition of the cementitious material in a controlled way and by lowering the substrate on which the 3D-shaped object is deposited in the liquid or semi-liquid fluid surrounding the 3D-shaped object in a controlled way so that the liquid or semi-liquid fluid is supporting the obtained 3D-shaped object but is leaving the printing area of layer i-1 uncovered (i.e. not covered with liquid or semi-liquid fluid) during the deposition of layer i, with i ranging from 2 to n.

[0032] The function of the liquid or semi-liquid fluid that is at least partially surrounding the obtained 3D-shaped object is to at least temporarily support the obtained 3D-shaped object during the layer- by-layer deposition of the cementitious material. The liquid or semi-liquid fluid is preferably supporting the obtained 3D-shaped object during the layer-by-layer deposition of the cementitious material and during the self-setting and/or self-hardening of the cementitious material. Preferably, the liquid or semi-liquid fluid is removed once support of the 3D-shaped object is no longer required, for example once the cementitious material is set and/or hardened.

[0033] Preferably, the liquid or semi-liquid fluid allows to fill internal voids of the obtained 3D- shaped object. The liquid or semi-liquid fluid is preferably supporting the obtained 3D-shaped object on both sides of the printed layers, ensuring a horizontal force equilibrium. Preferably, the liquid or semi-liquid fluid surrounding the obtained 3D-shaped object exerts an upward buoyancy force on the object. Consequently, the apparent density of the printed cementitious material is reduced and the buildability is increased.

[0034] As the cementitious material comprises a self-setting and/or self-hardening material, the presence of the supporting liquid or semi-fluid liquid is not required to initiate the setting and/or hardening of the cementitious material. It is thus not a necessity for the supporting liquid or semiliquid fluid to act as an activator and/or trigger for the self-hardening and/or self-setting of the cementitious material, since the self-hardening and/or self-setting of the cementitious material is an intrinsic process. Preferably, the supporting liquid or semi-liquid is not triggering and/or not influencing the self-setting and/or self-hardening of the cementitious material.

[0035] Preferably, the supporting liquid or semi-liquid is not triggering and/or not influencing the self-setting and/or self-hardening of the cementitious material and is chemically not reacting with the cementitious material.

[0036] Preferably, the liquid or semi-liquid fluid is chemically not reacting with the cementitious material.

[0037] More preferably, the supporting liquid or semi-liquid is not triggering and/or not influencing the self-setting and/or self-hardening of the cementitious material and is chemically not reacting with the cementitious material and the liquid or semi-liquid fluid is chemically not reacting with the cementitious material.

[0038] According to the method of the present invention, it is important to control the level of liquid or semi-liquid fluid that is surrounding the obtained 3D-shaped object so that the area of layer i-1 that receives cementitious material from the nozzle is not covered with the liquid or semi-liquid fluid during deposition of layer i. The level of the liquid or semi-liquid fluid surrounding the obtained 3D- shaped object is also referred to as the height of the liquid or semi-liquid fluid surrounding the obtained 3D-shaped object. Depending on the liquid or semi-liquid fluid, the fluid has a planar free area surface or a non-planar free area surface.

[0039] As mentioned above, the level of the liquid or semi-liquid fluid can amongst others be controlled by controlling the addition of the liquid or semi-liquid fluid and/or by controlling the level of the substrate (carrying the obtained 3D-shaped object).

[0040] During deposition of each layer i, with i ranging from 2 to n, the level of the liquid of semiliquid fluid surrounding the 3D-shaped object should at least be controlled in the area of layer i-1 receiving cementitious material from the nozzle, i.e. in the printing area of layer i-1. Before deposition of layer i the obtained 3D-shaped object has a height HM, after deposition of layer i the obtained 3D-shaped object has a height Hi. During deposition of each layer i, with i ranging from 2 to n, at least in the printing area of layer i-1 the level of the liquid or semi-liquid fluid surrounding the obtained 3D-shaped object is lower than the height HM of the obtained 3D-shaped object.

[0041] The difference between the height HM and the height H, corresponds with the layer height of layer i (i.e. the thickness of layer i). The layer height of layer i ranges preferably between 0.2 and 20 cm, for example between 0.5 and 10 cm or between 0.5 and 5 cm. The different layers of the 3D-shaped object may have the same height. Alternatively, the consecutive layers do not have a constant height.

[0042] On the one hand, the level of the liquid or semi-liquid fluid surrounding the obtained 30- shaped object should be low enough so that the area of layer i-1 that receives cementitious material from the nozzle is not covered with the fluid during deposition of layer i to ensure a good adhesion between consecutive layers of cementitious material. On the other hand, the level of the fluid surrounding the 3D-shaped object should be high enough to provide the required support to the 3D-shaped object.

[0043] It is clear that for 3D-shaped objects having a complex shape, for example objects having high overhangs and/or bridging, it is more critical that the level of the surrounding liquid or semiliquid fluid is sufficiently high. Also for objects having a high number of layers (for example 30- shaped objects having more than 50 layers), it is important that the level of the surrounding liquid or semi-liquid fluid is sufficiently high.

[0044] Preferably, when depositing layer i, with i ranging from 2 to n, the level of the liquid or semiliquid fluid surrounding the obtained 3D-shaped object is at least 20 % of the height HM, i.e. the height of the obtained 3D-shaped object after deposition of layer i-1 .

[0045] In particular embodiments, when depositing layer i, with i ranging from 2 to n, the level of the liquid or semi-liquid fluid surrounding the obtained 3D-shaped object is at least 40 %, at least 50 %, at least 60 %, at least 70 %, at least 80 % or at least 90% of the height HM. In particular embodiments, when depositing layer i, with i ranging from 2 to n, the level of the liquid or semi-liquid fluid surrounding the obtained obtained 3D-shaped object is at least in the printing area of layer i-1 just below or (substantially) equal to the height HM, leaving the printing area of layer i-1 free of liquid or semi-liquid fluid.

[0046] Preferably, when depositing layer i, with i being higher than 10, the level of the liquid or semi-liquid fluid surrounding the obtained 3D-shaped object is at least 40% of the height HM. [0047] More preferably, when depositing layer i, with i being larger than 10, the level of the liquid or semi-liquid fluid surrounding the obtained 3D-shaped object is at least 50%, at least 60 %, at least 70 %, at least 80 % or at least 90% of the height HM. In particular embodiments, when depositing layer i, with i being larger than 10, the level of the liquid or semi-liquid fluid surrounding the obtained obtained 3D-shaped object is at least in the printing area of layer i-1 just below or (substantially) equal to the height HM, leaving the printing area of layer i-1 free of liquid or semiliquid fluid.

[0048] Preferably, when depositing layer i, with i being higher than 50, the level of the liquid or semi-liquid fluid surrounding the obtained 3D-shaped object is at least 60% of the height HM. [0049] More preferably, when depositing layer i, with i being higher than 50, the level of the liquid or semi-liquid fluid surrounding the obtained 3D-shaped object is at least 70 %, at least 80 % or at least 90% of the height HM. In particular embodiments, when depositing layer i, with i being larger than 50, the level of the liquid or semi-liquid fluid surrounding the obtained 3D-shaped object is at least in the printing area of layer i-1 just below or (substantially) equal to the height HM, leaving the printing area of layer i-1 free of liquid or semi-liquid fluid.

[0050] For 3D-shaped objects having a complex shape, for example objects having an overhang of at least 20°, at least 30° or at least 40° or for 3D-shaped objects including bridging, the level of the liquid or semi-liquid fluid surrounding the obtained 3D-shaped object is at least 50 %, at least 60 %, at least 70 %, at least 80 % or at least 90% of the height HM . In particular embodiments, for 3D-shaped objects having such complex shapes, the level of the liquid or semi-liquid fluid surrounding the obtained 3D-shaped object is at least in the printing area of layer i-1 just below or (substantially) equal to the height HM , leaving the printing area of layer i-1 free of liquid or semiliquid fluid.

[0051] Preferably, the 3D-shaped object is formed in a container whereby the container is holding the liquid or semi-liquid fluid. When the 3D-shaped object is formed in a container, layer i=1 is for example printed on the bottom of the container. In such case the bottom of the container may function as the substrate on which layer i=1 is deposited.

[0052] In alternative embodiments, a substrate for holding the 3D-shaped object other than the bottom of the container can be used. This substrate can for example be lowered in the container and in the liquid or semi-liquid fluid that the container is holding.

[0053] As liquid or semi-liquid fluid, any type of fluid that is giving support to the 3D-shaped object once the fluid is surrounding the 3D-shaped object, can be considered. Preferably, the liquid or semi-liquid fluid is flowable, for example when added during the layer-by layer deposition of the cementitious material. Preferably, the liquid or semi-liquid fluid allows to fill the voids of the obtained 3D-shaped object, for example the voids of the obtained 3D-shaped object created during the layer- by-layer deposition of the cementitious material.

[0054] A liquid refers to a nearly incompressible fluid that conforms the shape of its container but retains a (nearly) constant volume independent of pressure. Examples of liquids comprise water, cation-rich liquids, anion-rich liquids and oils.

[0055] A semi-liquid fluid refers to a fluid having properties between a solid and a liquid. Examples of semi-liquid fluids comprise gels and pastes. Examples comprise fresh concrete, mortars or cement pastes, hydrogels or mixtures comprising a polymer, for example a (saturated) absorbent or superabsorbent polymer, suspensions such as thixotropic clay suspensions or inert filler suspension.

[0056] Preferably, the liquid or semi-liquid fluid can be removed easily once the 3D-shaped object is set and/or hardened. Using a liquid or semi-liquid fluid that can be removed easily, simplifies the method and allows for the reuse of the liquid or semi-liquid fluid. [0057] The density of the liquid or semi-liquid fluid is preferably ranging between 900 kg/m ³ and 2100 kg/m ³ , for example ranging between 1100 and 2000 kg/m ³ The density of the supporting material is for example 1000 kg/m ³ , 1200 kg/m ³ , 1500 kg/m ³ or 2000 kg/m ³ .

[0058] Preferably, the density of the liquid or semi-liquid fluid is lower than the density of the cementitious material. Preferably, the density of the liquid or semi-liquid fluid is on the one hand low enough to avoid buoyancy and on the other hand high enough to reduce the apparent self-weight of the printed object. In preferred embodiments, the density of the liquid or semi-liquid fluid is 5 %, 10 %, 20 % or 30 % lower than the density of the cementitious material.

[0059] The viscosity of the liquid or semi-liquid fluid is preferably lower than 150 Pa.s, for example lower than 100 Pa.s. Preferably, the viscosity of the liquid or semi-liquid fluid ranges between 0.0001 Pa.s and 150 Pa.s, for example between 0.001 Pa.s and 100 Pa.s or between 0.1 Pa.s and 2 Pa.s.

[0060] In preferred embodiments the liquid or semi-liquid fluid has a density ranging between 900 kg/m ³ and 2100 kg/m ³ and a viscosity lower than 150 Pa.s, for example a density of 1000 kg/m ³ , 1200 kg/m ³ , 1500 kg/m ³ or 2000 kg/m ³ and a viscosity between 0.001 Pa.s and 100 Pa.s for example between 0.1 Pa.s and 2 Pa.s.

[0061] In preferred embodiments the liquid or semi-liquid fluid has a density between 900 kg/m ³ and 2100 kg/m ³ being 5 %, 10 %, 20 % or 30 % lower than the density of the cementitious material and a viscosity ranging between 0.001 Pa.s and 100 Pa.s, for example between 0.1 Pa.s and 2 Pa.s.

[0062] In preferred embodiments, the liquid or semi-liquid fluid has a time-dependent viscosity having an initial viscosity vo and increasing with time to a maximum viscosity v _m . The initial viscosity vo is the viscosity of the liquid or semi-liquid fluid when added, for example pumped, to surround the 3D-shaped object. The maximum viscosity v _m is the viscosity of the liquid or semi-liquid fluid when providing support to the 3D-shaped object. The maximum viscosity v _m is for example reached at a time of for example 5, 10, 15, 20 or more minutes after the addition of the fluid.

[0063] The initial viscosity vo of the liquid or semi-liquid fluid is preferably ranging between 0.0001 Pa.s and 10 Pa.s, for example 0.001 Pa.s, 0.01 Pa.s or 1 Pa.s. The maximum viscosity v _m of the liquid or semi-liquid fluid is preferably ranging between 0.001 Pa.s and 100 Pa.s, the maximum viscosity v _m is for example 0.01 Pa.s or 1 Pa.s.

[0064] Preferred liquids or semi-liquid fluids that have a time-dependent viscosity comprise liquids or fluids comprising a liquid absorbent or superabsorbent material, for example comprising liquid absorbent or superabsorbent polymers. Preferably, the liquid absorbent or superabsorbent material comprises water absorbent or water superabsorbent materials, such as absorbent or superabsorbent polymers. Absorbent or superabsorbent polymers are preferably cross-linked or are in situ cross-linked. [0065] Absorbent and superabsorbent materials are defined as materials that can absorb and retain large amounts of liquid compared to their own weight. Absorbent materials may absorb an amount of liquid at least equal to its own weight, i.e. having a liquid absorbing capacity of at least 1 g/g, for example between 5 g/g and 10 g/g. Superabsorbent materials may absorb an amount of liquid at least equal to 10 times their own weight, i.e. a liquid absorbing capacity of at least 10 g/g, for example between 30 g/g and 1000 g/g.

[0066] The absorbent and superabsorbent materials have for example a swelling time ranging between a few seconds, for example 5 seconds, resulting in immediate gel formation, and hours, for example 24 hours, allowing for gradual swelling and gradual viscosity increase to guarantee uniform levelling. Preferably, the swelling time ranges between a few seconds, for example 5 seconds and a few minutes, for example 2 minutes. The swelling time of a material is defined as the time needed for the material to absorb 90% if its total capacity in liquids, either based on volume or mass calculations. The swelling time can be measured by means of a vortex test.

[0067] Examples of liquid absorbent or superabsorbent materials, for example water absorbent or superabsorbent materials, comprise polymers such polyacrylamide polymers or copolymers, (cross-linked) polyacrylates polymers or copolymers, (cross-linked) polyethylene oxides based polymers or copolymers, polyvinyl alcohol based polymers or copolymers, alginates, gelatin, agarose and combinations thereof. Such polymers can for example be obtained by bulk polymerization, solution polymerization, inverse emulsion polymerization, inverse suspension polymerization, extrusion.

[0068] A particular example of a liquid absorbing material comprises polyacrylate based polymers comprising acrylic acid and acrylamide as monomers and methylbismethacrylate as cross-linker. Such polyacrylate based polymers have for example an absorption capacity ranging between 10 g/g and 1000 g/g in demineralized water and/or a simulated pore solution and have a swelling time for example ranging between 10 s and 300 s.

[0069] Another example comprises cross-linked calcium alginate. The cross-linked calcium alginate is for example obtained, starting from sodium alginate and using a binder filtrate solution as in-situ cross-linker. The cross-linked calcium alginate has for example an absorption capacity between 10 g/g and 400 g/g.

[0070] In preferred examples the liquid or semi-liquid fluid comprises the same or similar cations as the printed material to guarantee the overall stability of the surrounding over time. The liquid or semi-liquid fluid comprises for example mono-, di- and/or trivalent cations. In a particular embodiment, the liquid or semi-liquid fluid comprises a Ca ²⁺ rich solution. The viscosity of the fluid is for example changing with maximum 10 % from addition of the fluid till removal of the fluid once the 3D-shaped object is hardened sufficiently.

[0071] Preferably, minimal changes in osmotic pressure and/or a constant cation content is ensured to maintain the viscosity of the liquid or semi-liquid fluid surrounding the obtained 3D- shaped object over time, i.e. during the hardening of the cementitious material of the 3D-shaped object. [0072] In preferred embodiments the liquid or semi-liquid fluid has a density ranging between 900 kg/m ³ and 2100 kg/m ³ and a time dependent viscosity having an initial viscosity vo ranging between 0.0001 Pa.s and 10 Pa.s and a maximum viscosity v _m ranging between 0.001 Pa.s and 150 Pa.s. The liquid or semi-liquid fluid has for example a density of 1000 kg/m ³ , 1200 kg/m ³ , 1500 kg/m ³ or 2000 kg/m ³ , an initial viscosity of 0.001 Pa.s, 0.01 Pa.s or 1 Pa.s and a maximum viscosity v _m of 0.01 Pa.s or 1 Pa.s. Preferably, the density of the liquid or semi-liquid fluid is 5 %, 10 %, 20 % or 30 % lower than the density of the cementitious material.

[0073] The yield stress of the liquid or semi-liquid fluid is preferably lower than 10 kPa. The yield stress of the liquid or semi-liquid fluid is preferably lower than 1 kPa, for example ranging between 0.01 kPa and 0.1 kPa or between 10 Pa and 100 Pa.

[0074] In preferred embodiments, the liquid or semi-liquid fluid has a time-dependent yield stress having an initial yield stress so and increasing with time to a maximum yield stress a _m

The initial yield stress so is the yield stress of the fluid when added, for example pumped, to surround the 3D-shaped object. The maximum yield stress a _m is the yield stress a _m when providing support to the 3D-shaped object. The maximum yield stress a _m is for example reached at a time of for example 5, 10, 15, 20 or more minutes after the addition of the fluid. Using a liquid or semi-liquid fluid that is flowable at the start allows for self-levelling around the obtained 3D-shaped object without adding too much lateral pressure due to uneven filling. Using a liquid or semi-liquid fluid having a time-dependent yield stress increasing with time has the advantage that motion is counteracted thus allowing the structuration (hardening overtime) of the cementitious material. [0075] The initial yield stress sq of the liquid or semi-liquid fluid is preferably ranging between 0.01 Pa and 10 Pa. The initial yield stress sq of the liquid or semi-liquid fluid is for example 0.1 Pa, 1 Pa or 10 Pa. The maximum stress am of the liquid or semi-liquid fluid is preferably ranging between 10 Pa and 1000 Pa. The maximum yield stress am is for example 100 Pa or 1000 Pa. The yield stress increases for example up to 1 , 10 or 100 Pa/min.

[0076] Preferred liquids or semi-liquid fluids that have a time-dependent yield stress comprise liquids or fluids having thixotropic properties. Examples of thixotropic materials comprise clay, clay suspensions (preferably nano-clay suspensions), coatings and paint products, inks, food industry products such as ketchup and dairy products, waxes, oil or oil suspensions, creams, pharmaceutical products such as gel or creams.

[0077] A particular example of a thixotropic material comprises a nano-clay suspension comprising water, limestone filler, nano-clay and a superplasticizer.

[0078] The thixotropic behaviour of the liquid or fluid is preferably reversible after long time spans offor example 1 , 2 or3 days or even after longer time spans so that the fluid is reusable. To maintain liquidity and thereby reversibility of the liquid or semi-liquid fluid, (excessive) evaporation of the liquid or semi-liquid fluid (e.g. dehydration) surrounding the 3D-shaped object is preferably avoided. Therefore, the container in which the 3D-shaped object is deposited is preferably covered during the deposition and/or during the hardening of the 3D-shaped object. [0079] In preferred embodiments the liquid or semi-liquid fluid has a time-dependent viscosity and a time-dependent yield stress, whereby both the viscosity and the yield stress increase with time. [0080] In preferred embodiments the liquid or semi-liquid has a density ranging between 900 kg/m ³ and 2100 kg/m ³ , a viscosity (possibly a time-dependent viscosity) lower than 150 Pa.s and a yield stress lower than 10 kPa. The density is for example 1000 kg/m ³ , 1200 kg/m ³ , 1500 kg/m ³ or 2000 kg/m ³ , the viscosity is for example ranging between 0.001 Pa.s and 100 Pa.s, for example between 0.1 Pa.s and 2 Pa.s and the yield stress is for example ranging between 0.01 kPa and 0.1 kPa or between 10 Pa and 100 Pa. In case the viscosity is time-dependent the initial viscosity vo ranges preferably between 0.0001 Pa.s and 10 Pa.s and the maximum viscosity v _m ranges ranges preferably between 0.001 Pa.s and 150 Pa.s.

[0081] In preferred embodiments the liquid or semi-liquid has a density ranging between 900 kg/m ³ and 2100 kg/m ³ , a viscosity (possibly a time-dependent viscosity) lower than 150 Pa.s and a time dependent yields stress with an intitial yield stress sq ranging between 0.01 Pa and 10 Pa, for example 0.1 Pa, 1 Pa or 10 Pa and a maximum yield stress am ranging between 10 Pa and 1000 Pa.

[0082] In preferred embodiments the flow rate of the liquid or semi-liquid fluid that is added is controlled. This offers a better distribution of surrounding liquid or semi-liquid fluid by spreading the liquid or semi-liquid fluid more evenly from the start. In such case, liquids or semi-liquid fluids having a higher initial viscosity and/or higher initial yield stress can be used.

[0083] Preferably, the method according to the present invention further comprises the step of removing the liquid or semi-liquid fluid once the 3D-shaped object is at least partially hardened.

[0084] According to a second aspect of the present invention a method to temporarily support a 3D-shaped object made of a self-setting and/or self-hardening cementitious material during its manufacturing or during its manufacturing and during the self-setting and/or self-hardening of the cementitious material is provided. The cementitious material comprises cement particles, aggregate particles and water or an aqueous solution. The cement particles are able to bind the aggregate particles in the presence of water or in the presence of the aqueous solution. The cement particles have an average particle size of at least 1 pm and the aggregate particles have an average particle size larger than the cement particles. The method comprises layer-by-layer deposition of n consecutive layers of the cementitious material, with n being at least 2, while surrounding the obtained 3D-shaped object (at least) partially during the deposition of the n consecutive layers with a supporting liquid or semi-liquid fluid, wherein the deposition of a layer i, with i ranging from 1 to n, comprises extrusion of a cementitious material from a nozzle and deposition of the cementitious material on a printing area of a substrate for layer i=1 or on a printing area of layer i-1 , for i ranging from 2 to n, while the nozzle and the printing area are moving along a printing path. The method is characterized in that during the deposition of the cementitious material to form layer i, with i ranging from 2 to n, the printing area of layer i-1 is not covered with the liquid or semi-liquid fluid. The number of layers n of the 3D-shaped object is at least 2, for example at least 5, at least 10 or at least 20. The number of layers n may be higher than 50, higher than 500 or higher than 1000.

[0085] The method to support a 3D-shaped object according to the present invention allows to manufacture 3D-shaped objects having a complex shape, for example 3D-shaped objects having an overhang of at least 20°, at least 30° or at least 40° or for 3D-shaped objects including bridging without requiring additional precautions such as providing scaffolding elements.

[0086] The cementitious material used in the method according to the second aspect of the present invention corresponds to the cementitious material used in the method according to the first aspect of the present invention.

[0087] Similar to the method according to the first aspect of the present invention, it is important to control the level of liquid or semi-liquid fluid that is surrounding the obtained 3D-shaped object so that the area of layer i-1 that receives cementitious material from the nozzle is not covered with the liquid or semi-liquid fluid during deposition of layer i.

[0088] The liquids or semi-liquid fluids suitable to be used in the method according to the second aspect corresponds to the liquids or semi-liquid fluids used in the method according to the first aspect of the present invention.

[0089] The function of the liquid or semi-liquid fluid that is at least partially surrounding the obtained 3D-shaped object is to at least temporarily support the obtained 3D-shaped object during the layer- by-layer deposition of the cementitious material. The liquid or semi-liquid fluid is preferably supporting the obtained 3D-shaped object during the layer-by-layer deposition of the cementitious material and during the self-setting and/or self-hardening of the cementitious material. Preferably, the liquid or semi-liquid fluid is removed once support of the 3D-shaped object is no longer required, for example once the cementitious material is set and/or hardened.

[0090] As the cementitious material comprises a self-setting and/or self-hardening material, the presence of the supporting liquid or semi-fluid liquid is not required to initiate the setting and/or hardening of the cementitious material. Preferably, the supporting liquid or semi-liquid is not triggering and/or not influencing the self-setting and/or self-hardening of the cementitious material. [0091] Preferably, the supporting liquid or semi-liquid is not triggering and/or not influencing the self-setting and/or self-hardening of the cementitious material and is chemically not reacting with the cementitious material.

[0092] Preferably, the liquid or semi-liquid fluid is chemically not reacting with the cementitious material.

[0093] More preferably, the supporting liquid or semi-liquid is not triggering and/or not influencing the self-setting and/or self-hardening of the cementitious material and is chemically not reacting with the cementitious material and the liquid or semi-liquid fluid is chemically not reacting with the cementitious material.

[0094] The substrate used in the method according to the second aspect corresponds to the substrate used in the method according to the first aspect of the present invention.

[0095] The container used in the method according to the present invention corresponds to the container used in the method according to the first aspect of the present invention. [0096] As mentioned above, the level of the liquid or semi-liquid fluid can amongst others be controlled by controlling the addition of the liquid or semi-liquid fluid and/or by controlling the level of the substrate (carrying the obtained 3D-shaped object). Addition of the liquid or semi-liquid fluid and/or the lowering of the substrate can be realized as described above.

[0097] During deposition of each layer i, with i ranging from 2 to n, the level of the liquid of semiliquid fluid surrounding the 3D-shaped object should at least be controlled in the area of layer i-1 receiving cementitious material from the nozzle, i.e. in the printing area of layer i-1. Before deposition of layer i the obtained 3D-shaped object has a height HM, after deposition of layer i the obtained 3D-shaped object has a height Hi. During deposition of each layer i, with i ranging from 2 to n, at least in the printing area of layer i-1 the level of the liquid or semi-liquid fluid surrounding the obtained 3D-shaped object is lower than the height HM of the obtained 3D-shaped object.

[0098] The difference between the height HM and the height H, corresponds with the layer height of layer i (i.e. the thickness of layer i). The layer height of layer i ranges preferably between 0.2 and 20 cm, for example between 0.5 and 10 cm or between 0.5 and 5 cm. The different layers of the 3D-shaped object may have the same height. Alternatively, the consecutive layers do not have a constant height.

[0099] On the one hand, the level of the liquid or semi-liquid fluid surrounding the obtained 30- shaped object should be low enough so that the area of layer i-1 that receives cementitious material from the nozzle is not covered with the fluid during deposition of layer i to ensure a good adhesion between consecutive layers of cementitious material. On the other hand, the level of the fluid surrounding the 3D-shaped object should be high enough to provide the required support to the 3D-shaped object.

[00100] It is clear that for 3D-shaped objects having a complex shape, for example objects having high overhangs and/or bridging, it is more critical that the level of the surrounding liquid or semiliquid fluid is sufficiently high. Also for objects having a high number of layers (for example 30- shaped objects having more than 50 layers), it is important that the level of the surrounding liquid or semi-liquid fluid is sufficiently high.

[00101] Preferably, when depositing layer i, with i ranging from 2 to n, the level of the liquid or semiliquid fluid surrounding the obtained 3D-shaped object is at least 20 % of the height HM, i.e. the height of the obtained 3D-shaped object after deposition of layer i-1 .

[00102] In particular embodiments, when depositing layer i, with i ranging from 2 to n, the level of the liquid or semi-liquid fluid surrounding the obtained 3D-shaped object is at least 40 %, at least 50 %, at least 60 %, at least 70 %, at least 80 % or at least 90% of the height HM. In particular embodiments, when depositing layer i, with i ranging from 2 to n, the level of the liquid or semi-liquid fluid surrounding the obtained obtained 3D-shaped object is at least in the printing area of layer i-1 just below or (substantially) equal to the height HM, leaving the printing area of layer i-1 free of liquid or semi-liquid fluid.

[00103] Preferably, when depositing layer i, with i being higher than 10, the level of the liquid or semi-liquid fluid surrounding the obtained 3D-shaped object is at least 40% of the height HM. [00104] More preferably, when depositing layer i, with i being larger than 10, the level of the liquid or semi-liquid fluid surrounding the obtained 3D-shaped object is at least 50%, at least 60 %, at least 70 %, at least 80 % or at least 90% of the height HM. In particular embodiments, when depositing layer i, with i being larger than 10, the level of the liquid or semi-liquid fluid surrounding the obtained obtained 3D-shaped object is at least in the printing area of layer i-1 just below or (substantially) equal to the height HM, leaving the printing area of layer i-1 free of liquid or semiliquid fluid.

[00105] Preferably, when depositing layer i, with i being higher than 50, the level of the liquid or semi-liquid fluid surrounding the obtained 3D-shaped object is at least 60% of the height HM.

[00106] More preferably, when depositing layer i, with i being higher than 50, the level of the liquid or semi-liquid fluid surrounding the obtained 3D-shaped object is at least 70 %, at least 80 % or at least 90% of the height HM. In particular embodiments, when depositing layer i, with i being larger than 50, the level of the liquid or semi-liquid fluid surrounding the obtained 3D-shaped object is at least in the printing area of layer i-1 just below or (substantially) equal to the height HM, leaving the printing area of layer i-1 free of liquid or semi-liquid fluid.

[00107] For 3D-shaped objects having a complex shape, for example objects having an overhang of at least 20°, at least 30° or at least 40° or for 3D-shaped objects including bridging, the level of the liquid or semi-liquid fluid surrounding the obtained 3D-shaped object is at least 50 %, at least 60 %, at least 70 %, at least 80 % or at least 90% of the height HM. In particular embodiments, for 3D-shaped objects having such complex shapes, the level of the liquid or semi-liquid fluid surrounding the obtained 3D-shaped object is at least in the printing area of layer i-1 just below or (substantially) equal to the height HM, leaving the printing area of layer i-1 free of liquid or semiliquid fluid.

[00108] Preferably, the method according to the present invention further comprises the step of removing the liquid or semi-liquid fluid once the 3D-shaped object is at least partially hardened.

Brief description of the drawings

[00109] The present invention will be discussed in more detail below, with reference to the attached drawings, in which:

Figure 1 shows a system to manufacture a 3D-shaped cementitious object according to the present invention;

Figure 2 shows the manufacturing of a 3D-shaped cementitious object deposited along a spiralized printing path;

Figure 3 shows the deposition of cementitious material that is supported by a thixotropic fluid functioning as supporting material; and

Figure 4 shows the manufacturing of a 3D-shaped cementitious object having a complex shape. Description of embodiments

[00110] The present invention will be described with respect to particular embodiments and with reference to certain drawings; but the invention is not limited thereto but only by the claims. The drawings are only schematic and are non-limiting. The size of some of the elements in the drawing may be exaggerated and not drawn to scale for illustrative purposes. The dimensions and the relative dimensions do not correspond to actual reductions to practice of the invention.

[00111] When referring to the endpoints of a range, the endpoint values of the range are included. [00112] When describing the invention, the terms used are construed in accordance with the following definitions, unless indicated otherwise. [00113] The term ‘and/or’ when listing two or more items, means that any one of the listed items can by employed by itself or that any combination of two or more of the listed items can be employed.

[00114] Figure 1 is an illustration of a system to manufacture a cementitious 3D-shaped object according to the present invention. The system comprises a robot having a robotic arm A, for example a 6-axis robotic arm. The robotic arm A is provided with a nozzle C. Cementitious material is extruded from the nozzle C in a container B. The nozzle C has preferably a length equal to or larger than the depth of the container B. Cementitious material is introduced from supply unit D to the entrance of the nozzle C. Preferably, the cementitious material is mixed before being introduced in the nozzle. Liquid or semi-liquid fluid is added from supply unit E to the container B. The deposition of the cementitious material and the addition of liquid or semi-liquid fluid is controlled by means of control unit F.

[00115] Figure 2 shows the manufacturing of a 3D-shaped object 20. Consecutive layers 21 of cementitious material 22 are deposited following a spiralized printing path. The cementitious material 22 is extruded from nozzle 24. During the deposition process the obtained 3D-shaped object 20 is at least partially surrounded by a liquid or semi-liquid fluid 23. The liquid or semi-liquid fluid is thereby functioning as supporting material. The addition of liquid or semi-liquid fluid is controlled in such a way that the area 26 of layer i-1 that receives cementitious material from the nozzle 24, also referred to as the printing area, is not covered with the liquid or semi-liquid fluid so that the cementitious material of layer i makes direct contact with the cementitious material of layer i-1.

[00116] Figure 3 shows the deposition of cementitious material 31 that is for example supported by a thixotropic fluid 33 functioning as supporting material. As indicated in Figure 3, the free area surface of the thixotropic fluid is non-planar. To form layer i, in Figure 3 indicated with 34, cementitious material is deposited on layer i-1 , in Figure 3 indicated with 36. The area 38 of layer i- 1 that receives fresh cementitious material from the nozzle to form layer i is during the deposition of layer i not covered with thixotropic material. In this way, direct contact between the cementitious material of layer i and cementitious material of layer i-1 during the deposition is guaranteed. [00117] Figure 4 shows the construction of a 3D-shaped object 40 having a complex shape. Cementitious material 41 is extruded from the nozzle 44. During the deposition process the obtained 3D-shaped object 40 is supported by a liquid or semi-liquid fluid 43.

Experiment 1

[00118] A straight wall having a length of 400 mm and a width of 20 mm is built in a container by a layer-by-layer deposition process. Cementitious material having the composition given in Table 1 is extruded with a print speed of 40 mm/s. The fresh cementitious material has a density of 2000 kg/m ³ . The printing path has a width of 20 mm. The deposited layers have a height of 10 mm.

[00119] The layer-by-layer deposition is continued until failure occurred

(i) without using a supporting fluid surrounding the obtained wall;

(ii) using water (density of 1000 kg/m ³ ) as supporting fluid;

(iii) using a mixture comprising saturated polyacrylate as super absorbent polymer (SAP) and water (density 1000 kg/m ³ ) as supporting fluid; and

(iv) using a thixotropic suspension comprising a nano-clay suspension comprising water, limestone filler, nano-clay and a superplasticizer (density 1700 kg/m ³ ) as supporting fluid.

[00120] For (ii), (iii) and (iv) the fluid is added to the container to surround the obtained wall in such a way that the level of the fluid is during the deposition of layer i equal to the height of the wall obtained after deposition of layer i-1 , i.e. height HM.

Table 1

[00121] The results, in function of the height reached at failure are given in Table 2.

Table 2

Experiment 2 (theoretical experiment)

[00122] A circular shaped hollow column having a radius of 20 mm and a height of 2 m is built by layer-by-layer deposition of a cementitious material comprising a low percentage of accelerator. The fresh cementitious material has a density of 2000 kg/m ³ . The printing path has a width of 50 mm. The deposited layers have a height of 10 mm.

[00123] The hardening overtime in function of time is estimated as c (t) = 0.5 · t + 3 [kPa], with c = cohesion/strength of the material [00124] The layer-by-layer deposition is repeated (i) without using a supporting fluid surrounding the obtained wall;

(ii) using water (density of 1000 kg/m ³ ) as supporting fluid; and

(iii) using a clay suspension (density 1700 kg/m ³ ) as supporting fluid.

[00125] For (ii) and (iii) the supporting fluid is added to the container to surround the wall in such a way that the level of the supporting fluid is during the deposition of layer i equal to the height of the wall obtained after deposition of layer i-1 , i.e. height HM.

[00126] The results, in function of maximum print speed in order to avoid failure due to plastic collapse of the bottom layer are given in Table 3. Table 3

Experiment 3 (theoretical experiment)

[00127] A 3D-shaped object having a complex shape with an overhang of 40° is built by a layer-by- layer deposition process. Cementitious material comprising an accelerator was used. The fresh cementitious material has a density of 2000 g/m ³ . The printing path has a length of 40 mm. The different layers have a height of 10 mm. [00128] The hardening overtime in function of time is estimated as c (t) = 20 · t + 10 [kPa], with c = cohesion/strength of the material [00129] The 3D-shaped object is built

(i) without using a supporting fluid surrounding the obtained wall, and (ii) using a supporting fluid comprising a thixotropic suspension (details?) according to the present invention.

[00130] For the layer-by-layer deposition without using a supporting fluid early failure occurred because of the sagging of the complex 3D-shaped object having excessive overhang.

[00131] For the layer-by-layer deposition using the supporting fluid comprising the thixotropic suspension, no failure occurred. Care should be taken to evenly distribute the thixotropic suspension.