3D Print with Salt

One of the new promising materials is salt (sodium chloride), which has been a valuable material for many centuries. During the past decades it has attracted wide attention as a residue of desalination and potash production. Due to increasing demand for fresh water and its limited supply, almost 47% of the world population does not have enough fresh water for at least one month each year [11]. In some coastal areas seawater has been used as a resource for producing fresh water. The process of extracting fresh water from seawater and discharging the residue (hypersaline brine) into the sea is called desalination. Around the world, some 15,906 operational desalination plants (see Fig. 1) produce a total of 95 million cubic metres of desalinated water per day and an estimated 142 million cubic metres of hypersaline brine, of which approximately 8.45 million cubic metres of salt per day are disposed into the sea [12]. In terms of its origin, 70.3% of global brine disposal comes from the Middle East and North Africa, 10.5% from East Asia and Pacific, 5.9% from Western Europe, 3.9% from North America, 3.9% from Latin America and Caribbean, 2.6% from Southern Asia, 1.8% from Eastern Europe and Central Asia and 1.0% from Sub-Saharan Africa. The disposal of hypersaline brine into the sea and its negative environmental consequences have already been acknowledged in scientific research. The observed negative effects include; change of water quality (increase of salinity, temperature), less light passing through the water (due to the growth of algae), and a reduction of biodiversity (fish, corals) [13‐15]. At the moment there is a lack of regulation for brine disposal into nature, although measures such as brine discharge system configurations or the reuse of the brine are proposed in different research projects [14, 16].

Another source of salt waste (NaCl) is the potash industry [18]. Due to extensive agriculture, many fertilizers are used to enhance crop growth. One common fertilizer is potash as found in sylvite and carnalite minerals [18]. The remainder of the minerals consists of salt (NaCl) and small amounts of clay, silt, sand, and dolomite [19]. Taking into account that annual global production presents around 68.1 million tons of potash [20], salt waste (83.5 wt% of the mineral) can be calculated at around 335 million tons a year. Salt waste from mining (see Fig. 2) is also disposed into nature as salt tailing on the surface [18], as brine injection/backfilling underground or as direct discharge to rivers and seas [21]. One of the best known salt tailings in Germany is the roughly 250 m high “Monte Kali” composed of 236 million tons of mining salt waste [22]. It has been discovered that the salt contamination to nearby waters are clearly caused by the salt load and prevents “a permanent recolonization by typical sensitive freshwater species” [23]. In Spain and Canada researchers discovered various negative environmental effects caused by potash mining activities: a decrease of water conductivity and the number of living communities in the Llobregat Basin [24] and a devastating impact of high salt concentration in the rivers near Saskatchewan’s potash mines [19]. If the world’s annual salt waste were spread on the surface of the city of Munich (310.43 m²) a 54 m-high layer of salt would be created within 5 years.

Sodium chloride or table salt is one of the best-known substances on the Earth. It consists of two different elements: Na (Sodium) and Cl (Chloride), which are linked by an ionic bond [25, 26]. Salt crystals come in different sizes, are white and without smell [27]. Microscopically observed, salt does not have many open spaces in its structure and is known for its low permeability and porosity [28]. Literature records sodium chloride’s specific heat as between 0.853 J/(gK) [29] and 0.859 J/(gK) [30, 31]. The thermal conductivity of salt varies from 6 to 6.5 WmK at room temperature [29, 32‐35] and 3.3 W/mK at higher temperatures (approx. 300 °C) [35]. Salt melts at 801 °C [36, 37] and is used for heat storage in solar power plants [37].

In addition to the properties noted above, salt can dissolve in water, methanol or formic acid [29]. At a relative humidity above 75.3% [29] salt crystals start to dissolve and change to brine (salt with water). However, in the drying process the water evaporates and crystals start to grow again. During the past decades, salt rooms have been developed and used for drug free treatment of respiratory illness, skin diseases [38] and lung cancer cells [39]. The treatment consists of sitting in salt rooms, salt caves or at the seaside and breathing air saturated with salt particles (aerosols) [40, 41]. Several studies focus on the positive effect of treatment with a saline environment [39, 41‐43], but unfortunately no research has been conducted about how salt as construction material impacts the quantity of aerosols in indoor environments and health.

In historic building construction, salt materials were mostly used in dry-hot weather conditions and in locations where conventional resources were not available. In some cases, salt materials were cut from nearby salt seas or mines and used directly as a building block for exterior walls [44‐46]. One of these buildings is the hotel “Palacio de Sal” in Salar de Uyuni in Bolivia (Material District 2016), which is built out of about a million salt blocks. Other salt-based construction materials are mixtures of salt and different binders (clay, volcanic ash, cement or starch) [44, 47].

Karshif and Roman maritime materials are among the oldest existing mixed salt materials. Karshif, from the Siwa Oasis in Egypt, is a material for load bearing walls made of small salt rocks (up to 95% of the material) and salt-clay mortar [44]. Due to its hygrothermal and chemical properties, Karshif stone can be formed to “a sort of monolith” [48] under repeated cycles of absorbing humidity from the environment and emitting it again. The Romans mixed volcanic ash, lime and seawater [49] to build highly durable protection walls in the Mediterranean sea. Different studies on the long-term compressive strength of concrete made with seawater discovered that there is almost no difference to that of concrete mixed with freshwater [50‐52]. Other mixtures of concrete and salt were developed for the protection of radioactive waste in abandoned salt caves [53‐57], here salt comprised around 50% of the whole mass [53].

Further salt materials may be produced by pressing, melting, additive technology or natural salt crystallization. Salt lick blocks [58] are made of compressed salt as an additive to animal nutrition and dissolve with animal saliva [59‐62] if they do not contain special additives [63] or coatings [62]. Salt crystallization processes are used in contemporary art and architectural structures by soaking different materials in very salty water or leaving materials outside to dry. Over time, salt crystals grow and create salt crusts which are also used as artworks or as shading systems [64‐67].

New salt mixtures (salt and starch) have gained attention in the last decades as a result of a recent building technology made possible by advances in data processing—additive manufacturing [68‐70] (see Fig. 3). The first 3D printing prototypes were developed by a research team at the Solheim Additive Manufacturing Laboratory of the Mechanical Engineering Department of the University of Washington [68, 69] (see Fig. 3). Their 3D printed elements were composed of salt (8 parts by weight), maltodextrin (1 part by weight) and water. The same recipe was later adapted and slightly modified by researchers from Emerging Objects to create a 3D-printed salt pavilion [71] (see Fig. 3) and by TU Delft to examine mechanical properties of the salt-starch mixture [69]. Researchers from Japan mixed salt-starch with wheat flour and dextrin to create 3D moulds [72].

As a porous material salt has a potential in additive manufacturing processes if it can be bound. There are various methods in additive manufacturing such as binder jetting, fused deposition modelling and paste extrusion. However, the most common processes that have been used with salt are binder jetting [68, 69, 71, 72] and paste extrusion. Binder jetting was invented almost 30 years ago at the Massachusetts Institute of Technology [69] and generally includes spreading a thin layer of powder, deposing a binder on the powder, repeating the first two steps multiple times and if needed applying a heat treatment [73]. Paste extrusion includes mixing solids with a liquid binder, pushing the paste through a nozzle, thus creating an object layer by layer from the bottom up [74] (Fig. 4).

Dr. Mark Ganter and his team from University of Washington [69] were the first to use binder jetting for salt and succeeded in creating the first, very small 3D printed objects (up to 5.2 cm). Only some years later a research team from “Emerging Objects” used the same 3D printing process and powder mixtures (salt and maltodextrin) but a different binder (rice wine). They developed 3D printed salt elements which were light, translucent and waterproof due to a coating with wax. 330 salt printed elements were connected to create a salt pavilion called Saltygloo [71]. This recipe was also used by a student from UC Berkeley to print his 3D master-thesis model [75].

Paste extrusion processes with salt were examined by the researcher and designer Karlijn Sibbel who managed to print small 3D printed salt objects with a low height (less than 10 cm) and a low quality [76]. Unfortunately, we were unable to find any scientifically relevant information about the mixtures and the process. Japanese [72] and Swiss researchers [77] have been investigating the creation of salt scaffolds by paste extrusion which can later be removed by soaking the object in water (salt leaching).

Most previous research in this area has focused on salt materials’ properties themselves rather than whether the salt materials are appropriate for 3D printing and which additive manufacturing process is the most appropriate for salt. As a consequence of the increase of building with 3D printing devices on construction sites, a better understanding of the interaction between 3D printing materials and additive manufacturing is growing. Nevertheless, the challenge of using more resource efficient materials in 3D printing persists. In this paper, we address these research gaps by using salt waste as a means to improve the resource efficiency of printing materials and to evaluate advantages and disadvantages of using it in paste extrusion processes.

To accompany this research, our chair (EBB) also organized two elective master courses in winter semester 2020 and summer semester 2021, whose focus was to extend the knowledge of salt-mixtures, the historical significance of salt as well as potential applications of the 3D printed salt-mixtures. Research of already established applications of salt in everyday life was categorized to understand future fields of application for 3D printing with salt (see Fig. 9).

Using salt as a local and environmentally friendly resource (geographical usage): This approach is based on the concept of building with local materials in dry and hot areas where little rain falls and salt waste is available from desalination plants or potash mining. Materials found on site can be 3D printed and used for buildings. To find suitable locations for this approach, weather data need to be overlaid onto maps showing areas with salt waste. Once defined the use of salt as a building material can be trialled in the identified regions (see Fig. 9, Left above).

Modular prefabricated elements: 3D printed building-elements fabricated off site from salt-mixtures are installed on sites with the right weather conditions. If the humidity is below 75% but high enough to fuse the salt-crystals without dissolving, the separate building elements will join and work as a unified element (see Fig. 9, Middle below)

Health/respiratory applications: This approach focuses on maximizing salt-rich surface area assuming these particles are released over time due to air circulation in indoor spaces. This concept focuses on the medical application of salt for respiratory ailments (see Fig. 9, Right above).

Salt as air filter: Salt particles in the air are renowned for clearing the lungs and helping asthmatic patients. This principle was the inspiration for the design of a breathing shelter, where the chimney effect was used to intensify the airflow within the space and to strengthen the therapeutic working (see Fig. 9, Left above).

3D printed salt-mixtures for antiseptic applications: A differentiation of surfaces for interior spaces can be utilized in a more meaningful way: The usage of 3D printed salt-containing surfaces can be used for antiseptic purposes; reducing germs on surfaces but also controlling the growth of specific plants. This project focused on green walls, allowing only salt resistant plants to grow (see Fig. 9, Right below).

All experiments were performed with the Potter Bot Micro 10, which is designed for 3D printing with clay, principally used for the printing of ceramics. The ideal material for 3D printing is, by default and as suggested by the manufacturer, a fine grain clay: “Maccabees cone 5 stoneware” clay or similar is suggested by the company; the printer is not designed to print concrete or other tough materials by default. A block of clay (10 kg) was separated into smaller pieces with a micro fiber towel placed in between to spread the approximately 350 ml of added water equally in the block of clay. The preparation and soaking process took two to three days.

The chair purchased the Potter Bot Micro 10 for the first test series due to its simple technology and construction. The main advantage for the material-tests is the direct extrusion. A threaded rod, controlled by a stepper motor, presses the material directly through a plexiglass cylinder out through the nozzle onto the printing bed: Other printers use compressed air systems with their elements connected with long tubes to process the material, which could cause clogging and corrosion. The maximum material capacity of the printer’s cylinder is 1000 ml and the maximum printing envelope is X-280 mm, Y-265 mm and Z-305 mm. While the cylinder with the material and nozzle only moves vertically, the base/print-bed (266 mm × 266 mm) moves in the X and Y directions to enable full 3D movement. The printer is WIFI controlled with a built-in web interface and can read and print common G-code files. All prints were set up with CURA as slicing software. The speed, Z position and material output can be changed during the print process and adjusted to the material’s behaviour.

The printer properties of the Potter Bot Micro 10 and the material-mixtures are highly dependent on each other. The test results derive from the behaviour and capacities of this printer and are likely to change with bigger and more powerful printers. Since the printer is designed to print clay, our salt mixtures (salt-cement, salt-gypsum etc.) showed less convincing results.