DESC:FIELD OF INVENTION
This invention relates to the field of construction materials, more specifically to the field of gypsum related materials for making 3D printed elements for building construction, insulation, decorative items and other purposes.
BACKGROUND OF THE INVENTION
Gypsum is one of the important materials in commercial construction, aesthetic businesses, being used for drywalling, hollow walls, and insulation and decorative purposes. The chemical composition of Gypsum is hydrated calcium sulfate (CaSO4·2H2O). Crude gypsum is used in making fertilizer, in paper and textiles, and also in portland cement, plaster of paris, Keene’s cement, board products, and tiles and blocks and also in Gypsum plaster. The drawbacks of gypsum plaster boards are that it has to be premade in factories and installed later at sites, and filling of corners or adjusting on site is a bit difficult when using gypsum plaster boards.
There are many variations in gypsum-based construction materials depending on the constituents used. These can be suitable for 3D printing or other conventional production techniques.
In the patent US5643510A (Producing foamed gypsum board using a foaming agent blend), a method for developing light-weight gypsum boards developed with foaming agents is described. The foaming agents are used to create voids in the board. The foam is generated by mixing a liquid foaming agent, air and water in a suitable foam generating apparatus. However, in our subject invention, foaming agents are not used. Instead, a chemical admixture is used to change the density by generating voids through chemical reaction during printing. The generation of the voids are controlled using the dosage concentration and ratio of the chemical.
The patent US7790302B2 (Lightweight compositions and articles containing such) explains the use of cementitious materials for the constructing lightweight building components. In the stated patent, expanded polymer particles are used to reduce the weight of the panel. Whereas, in our present subject invention, no expanded polymers are used for making the panel light weight.
The patent CN104744000A (Gypsum material for 3D printing and preparation method thereof) relates to a gypsum mix for 3D printing which consists of hemihydrate gypsum, filling material, fibres, water, a flexible adhering material, and a coagulating material. The materials and the method of preparation of the stated invention is different from our present invention.
The patent CN105731968A (Glass fiber reinforced gypsum capable of being used for 3D printing) relates to a glass fiber reinforced gypsum capable of being used for 3D printing. The glass fiber reinforced gypsum is prepared from phosphorus gypsum, slag powder, cement, glass fibers, a water-reducing agent and an early strength agent. Our present invention does not use cement or glass fibers and is composed of materials different to the stated invention.
OBJECTIVE OF THE INVENTION
The main objective of our invention is to enable 3D printing of lightweight gypsum wall panels for insulation purposes using an extrusion-based 3D printing system. Additionally, our present invention allows variable density and aeration of the mix based on demand, that can be scaled as per need at both on-site or off site.
Another objective of our present invention is to develop a gypsum-based lightweight mix which can be used to fill the hollow interior dry walls.
SUMMARY OF THE INVENTION
The following summary is provided to facilitate a clear understanding of the new features in the disclosed embodiment and it is not intended to be a full, detailed description. A detailed description of all the aspects of the disclosed invention can be understood by reviewing the full specification, the drawing and the claims and the abstract, as a whole.
The lightweight 3D printable Gypsum mix of our present invention consists of: a calcined gypsum with chemical formula of CaSO4 .0.5H2O (also known as plaster of Paris or Stucco and turns to CaSO4 .2H2O, when added to water), citric acid, water and metakaolin. The 3D printable gypsum mix is transferred to the pump. The mix is pumped to the nozzle head/ auger and extruded in the defined shape. Aluminium sulphate solution is pumped using a dosing pump to the nozzle head, where it is mixed with the gypsum-mix using a mixing blade.
The density of the mix can be varied between 1 g/cc and 2 g/cc which can be done even during the printing. The mix is cement-free and has a minimum initial buildability of 20 to 40 mm at room temperature. The volume of the printed or deposited material increases post deposition, allowing a volumetric filling of gaps. A 30% to 80% increase in volume is obtained.
BRIEF DESCRIPTION OF THE DRAWINGS
The manner in which the present invention, briefly summarized above, is formulated in a more particular description below and may be had by reference to the components, some of which is illustrated in the appended drawing. It is to be noted; however, that the appended drawing illustrates only typical embodiments of this invention and therefore should not be considered limiting of its scope, for the system may admit to other equally effective embodiments.
Throughout the drawings, the same drawing reference numerals will be understood to refer to the same elements and features.
The features and advantages of the present invention will become more apparent from the following detailed description along with the accompanying figures, which forms a part of this application and in which:
Fig 1: Block Diagram describing the mixing process components of our 3D printable lightweight gypsum mix in accordance with our present invention;
Fig 2: Block Diagram describing the filling of hollow walls in accordance with our present invention;
Fig 3: 3D printed gypsum element (plaster of Paris) without any expansion agent in accordance with our present invention;
Fig 4: Expansion with addition of aluminium sulphate solution during 3D printing of gypsum element in accordance with our present invention;
Fig 5: Expansion for different dosages of aluminium sulphate solution in cube moulds in accordance with our present invention;
Fig 6: Expansion for cube after addition of aluminium sulphate solution with time from addition in accordance with our present invention;
Fig 7: Cross section of the printed elements (chunks) with different dosages of aluminium sulphate solution in accordance with our present invention;
REFERENCE NUMERALS
100 - Mixer
101 - Pump for gypsum mix
102 - Water
103 - Aluminium sulphate
104 - Auger for extrusion
200 - Gypsum mix
201 - Cavities in wall
202 - Gypsum mix filling
DETAILED DESCRIPTION OF THE INVENTION
The principles of operation, design configurations and evaluation values in these non-limiting examples can be varied and are merely cited to illustrate at least one embodiment of the invention, without limiting the scope thereof.
The embodiments disclosed herein can be expressed in different forms and should not be considered as limited to the listed embodiments in the disclosed invention. The various embodiments outlined in the subsequent sections are construed such that it provides a complete and a thorough understanding of the disclosed invention, by clearly describing the scope of the invention, for those skilled in the art.
The gypsum is the primary binder in the subject invention. The gypsum-based mix of one embodiment of our present invention consists of a calcined gypsum with chemical formula of CaSO4 .0.5H2O (also known as plaster of Paris or Stucco and turns to CaSO4 .2H2O, when added to water), citric acid, water, and metakaolin. The 3D printable gypsum mix is transferred to the pump. The mix is pumped to the nozzle head/ auger and extruded in the defined shape. Aluminium sulphate solution is pumped using a dosing pump to the nozzle head, where it is mixed with the gypsum-mix using a mixing blade.
The gypsum hemihydrate used in this present invention as the raw material is commercially available plaster of Paris. Any grade of gypsum hemihydrate can be used to manufacture the 3D printable gypsum mix in another embodiment of our present invention.
Mixing can be done using commonly available concrete mixers, mortar mixers and paint mixers. The mixing protocol of one embodiment of our present invention is given below, as shown in Figure 1:
The citric acid is mixed with the water in the mixer (100). The gypsum is added to the solution and mixed until a proper gypsum mix is obtained. The metakaolin is added to the gypsum mix. The 3D printable gypsum mix is transferred to the pump, as shown in Figure 1. The mix is pumped to the nozzle head/ auger (104) and extruded in the defined shape. Aluminium sulphate solution (103) is pumped using a dosing pump (101) to the nozzle head, where it is mixed with the gypsum-mix using a mixing blade. The density of the layers is altered on-demand by differing the concentration and dosage of the accelerator solution. A single layer can be printed with different densities.
The dosing pump flow rate of the present invention is altered manually or by an automated control system. The path of the movement of the nozzle is controlled by the computer software. The extruder is also used to fill the cavities of the hollow walls, as shown in Figure 2. The gypsum-mix expands volumetrically once deposited inside (202), depending on the concentration and dosage of the aluminium sulphate solution.
The other requirements needed for our method of preparation of our subject invention are listed below:
Potable water satisfying any building code or structural code (like IS 456) can be used to manufacture the product. The expected pH is around 7. The water to gypsum ratio (by weight) is maintained around 0.5 to 1.25.
Retarders such as citric acid, sodium polyphosphate, calcium acetate, etc. are generally used to delay the setting of the gypsum. This invention uses citric acid and its salts as the retarder. Anhydrous citric acid from qualigen chemicals with molecular weight of 192.19 is used in this research. However, any commercial and lab grade citric acid can be used to develop this product. The citric acid dosage is 0.5% to 1% of gypsum weight.
Metakaolin from Kaolin industries with purity of 90% to 100% is used. Metakaolin is a form of calcined clay. The metakaolin is used to improve the rheology of the gypsum mix. The dosage of metakaolin ranges from 0-13%.
Aluminium sulphate solution is used to reduce the density of the gypsum mix by generating air in the deposited layers. The aluminium sulphate solution can have a solid content ranging from 20% to 65%. The dosage for aeration can vary from 0 to 5%. Commercially available alkali-free aluminium sulphate-based shotcrete accelerators can also be used for the same purpose.
Positive displacement pumps - screw and piston-based - are able to pump the material. The mix has an open time of 120 to 150 minutes and can be pumped and extruded till the end of open time. High concentration and dosage of aluminium sulphate solution can be used to have on-demand setting post deposition.
Mesh reinforcement with glass textiles in one embodiment of our present invention can be put between two consequent layers during printing to increase the flexural capacity of the printed elements.
For wall panels for dry walls, most buildable reinforced lightweight gypsum blocks are printed in vertical direction, using one embodiment of the present invention. Extra reinforcements are provided between the panels to increase the flexural capacity. The printing can be continuous for the height of the wall on-site.
Post processing for drywall wall-panels:
1. Few post processing activities, such as cutting of extra material or surface grinding, is required as per the finish and dimensional requirements of panels.
2. Coating of water repellent paints can be done on the surface of the printed elements to increase the durability.
For Hollow drywall filling, in one embodiment of our present invention, the process of filling the cavities as shown in Figure 2 is given as:
1. The extruder is moved to the cavities (201) using the robotic system (202).
2. The gypsum mix (200) of our present invention is pumped to the nozzle and the solution is mixed.
3. The material is filled in the cavities.
Results of Tests
The 3D printed panel with gypsum is shown in Figure R1. The mix is printed without addition of any accelerating admixture. The addition of aluminium sulphate solution (1:1 ratio with water) in different dosage leads to an increase in the volume due to expansion as shown in Figure R2. The expansion is further observed as the increase in the height of the centre point of the cube as shown in Figure R3 and Figure R4. The density of the material for each specimen is measured by dipping the material chunk into a measuring cylinder with kerosene of known density. The change in specific gravity is shown as a measure of the expansion during printing as shown in Table 1. The cross section of the chunks are shown in Figure R5.
Table 1 Change in specific gravity due to expansion is provided in the complete specification.
The chunks are further coated with varnish as a protective element. The absorption of the varnish by the specimens is a measure of the pores present in the expanded systems, as shown in Table 2.
Table 2 Absorption of varnish by different specimens showing expansion is provided in the complete specification.
The benefits of our present invention are listed below:
? The mix containing plaster of Paris as primary binder can be used to 3D print elements.
? The mix can be used to fill wall cavities on-site with provision of variation in expansion, i.e., the volume expansion can be varied from 10% to 80% depending on the requirement of density of the material as a filling material.
? The specific gravity can vary from 2.55 to 1.54.
? The printable mix consists of gypsum, water, citric acid and metakaolin. The mix is pumpable for 120 to 150 minutes from mixing water to raw materials. A solution consisting of aluminium sulphate is added to the mix at nozzle, before deposition. The density of the printed layer can be varied by mixing the solution at different dosages and solid concentration during printing or cavity filling.
? The elements can be printed in any desired shape and design.
? The deposited layers expand to form the lightweight layers. The density and air entrainment of the deposited layers can be controlled while printing and the designs can be changed by changing the dosage rate.
? The expansion can happen within 30 seconds from addition of aluminium sulphate solution.

,CLAIMS:I/We Claims:
1. A method of manufacturing expandable lightweight gypsum elements, comprising:
providing a gypsum-based mixture comprising:
i) calcined gypsum;
ii) water;
ii) a retarder;
iv) a filler;
extruding said gypsum-based mixture through a 3D printing nozzle;
simultaneously or sequentially introducing an accelerating agent to said gypsum-based mixture;
depositing the mixture to form a shape; and
allowing the deposited mixture to expand
2. A method of manufacturing expandable lightweight gypsum elements as claimed in claim 1 wherein the retarder is citric acid.
3. A method of manufacturing expandable lightweight gypsum elements as claimed in claim 1 wherein the filler is metakaolin.
4. A method of manufacturing expandable lightweight gypsum elements as claimed in claim 1 wherein the accelerating agent is aluminum sulfate.
5. A method of manufacturing expandable lightweight gypsum elements as claimed in claim 1 wherein the expansion of the deposited mixture is controlled by varying the concentration and dosage of the accelerating agent.
6. A method of manufacturing expandable lightweight gypsum elements as claimed in claim 1 wherein the gypsum-based mixture is used to fill cavities in a wall.
7. A method of manufacturing expandable lightweight gypsum elements as claimed in claim 1 wherein the lightweight gypsum element has a density of between 1.5 g/cc and 2.5 g/cc.
8. A method of manufacturing expandable lightweight gypsum elements as claimed in claim 1 wherein the element has a porosity of between 30% and 80%.