by Manuel De LucaPublished 15 April 2021In recent years, projects relating to man's stay on the moon have seen a notable evolution.Fifty years after Neil Armstrong's first walk on the lunar surface and after the countless advances in the astronautics field in recent decades, man is preparing to return to our satellite.The purpose, however, is quite different: we will return to the moon to "stay", in total safety.Long-term human exploration projects place great importance on the use of the Moon as the first space “port”: a place of arrival and stay, of study and experimentation, a point of observation and passage.Long-term stay poses numerous problems.First of all, it is necessary to create “habitats” capable of providing adequate protection from radiation, large temperature variations and the impacts of micro-meteorites.However, the actual construction, carried out using “traditional methods” such as the use of prefabricated elements transported on site, is economically unsustainable.Notice the ingredients, all that remains is to find a solution.To date, numerous studies have been conducted in this regard.The main lines of research concern the use of light modular elements (obviously, light takes on a different meaning on the Moon), composed in sequence, rather than semi-rigid pneumatic structures.The latter have the great advantage of being relatively light, which would facilitate their transport.Some solutions see the use of the two systems in a combined form.Think of the well-known TransHab, designed by NASA as an additional module of the ISS and as a future home in the lunar environment.The module consists of a rigid central core, cylindrical in shape, surrounded by a semi-rigid pneumatic shell (i.e. partially equipped with a rigid structure).However, these prototypes lack an essential component capable of guaranteeing mechanical resistance against shocks, thermal and radiation insulation, and adequate behavior in the short and long term.By long-term behavior we mean the complex of phenomena that characterize a given viscous material when subjected to prolonged stress and / or deformation states.Fortunately for us, the Moon, whose surface layer is made up of a heterogeneous set of sediments, dust and fragments of materials commonly known as regolith (paraphrasing Neil Armstrong's "very fine grained"), is a source of raw materials suitable for this scope.In this regard, research in the field of lunar (but also terrestrial) constructions has shown particular interest in geopolymeric compounds, and in particular for their 3D printing, with the aim of protecting the pneumatic structures mentioned above.But let's go in order.The term “geopolymer” identifies a wide range of inorganic compounds with a polymeric structure, both natural and synthetic, generated by the reaction of alumino-phosphate, silico-aluminate or ferro-silico-aluminate powders in alkaline solutions.The latter, also defined as hardeners, can be generated through different methodologies, i.e. cooking processes of sodium carbonate (Na₂CO₃) or potassium carbonate (K₂CO₃) then dissolved in distilled water, or hydrothermal processes through which siliceous materials are dissolved directly. in sodium hydroxide (NaOH) or potassium hydroxide (KOH).The Moon is rich in silicate and aluminate compounds and this solves an essential problem, namely the procurement of raw materials.This aspect is of vital importance as the transport of a large quantity of raw material would be economically unsustainable.What distinguishes geopolymers from other cementitious materials generated by hydration reaction (among these the classic Portland cement and the well-known Lunarcrete or Mooncrete) or from alkaline activated cements is the geopolymerization reaction, or geosynthesis.In this case, it is a reaction that chemically reconstitutes the materials, consisting of six distinct phases that lead to the solidification of the geopolymer into a highly resistant three-dimensional structure.They therefore appear as strong three-dimensional networks consisting of tetrahedral groups based on Silicon (SiO₄) and Aluminum (AlO₄), arranged in chains or rings and linked together by sharing an oxygen atom.Furthermore, in geopolymeric cements, the molar ratio (i.e. the ratio between the quantities in moles of two compounds involved in a chemical reaction) Silicon: Aluminum is equal to 2. The following figure clarifies the concept better:The prefix "geo" is due to the fact that these compounds chemically mimic (in the molecules they form) natural rocks.The main characteristics obtained are therefore hardness, chemical stability and a longevity comparable to the geological one.We also note:The geopolymerization reaction, of the exothermic type (as well as hydration), occurs at relatively low temperatures, generally not higher than 120 ° C.In any case, this figure is strongly influenced by the type of base material.Aggregates (mainly responsible for compressive strength), such as fragments of feldspar rock, granite, basalt, and additives to increase the ductility characteristics, such as basalt fibers, polyvinyl acetate and microsilica.Some additives, on the other hand, are used to provide the compound with “self-healing” capacity, literally self-healing, ie the ability to self-repair itself after the formation of shrinkage and / or viscosity cracks.The main geopolymeric compounds tested to date or in use in the "terrestrial" environment see the use of Metakaolin 750 (deriving from the cooking at 750 ° C of kaolinite, a typical mineral present in kaolin).More specifically of interest in the astronautical field are the so-called natural volcanic slags (Ground Volcanic Scoria), as they are chemically comparable to the lunar regolith.In addition to the many advantages described and the possible arrangement of 3D printing systems, the geopolymers are particularly durable.They show particular resistance to corrosion and are less vulnerable to freeze and thaw cycles.Experiments conducted on geopolymers of different nature, which included 50 freeze-thaw cycles, showed a lower loss of compressive strength (between 11% and 22%) compared to common cement materials.Geopolymers tend to show excellent mechanical resistance, due to the high degree of polycondensation.Compressive strength is commonly used as a yardstick for assessing whether geopolymerization has taken place.However, the chemical interactions involved in the geosynthesis process greatly affect the mechanical properties.For example, the increase in alkaline hydroxide (NaOH or KOH) or the decrease in the quantity of silicate (raw material) induces an increase in the mechanical strength of the geopolymer as an excess of silicate inhibits the evaporation of water (a fundamental process in geopolymerization) and therefore the formation of the geopolymer structure.Another factor having a key role in obtaining a material with high compressive strength is the consolidation temperature, also called curing temperature.At room temperature, the reaction between the reactive powder and the activating solution is very slow, while an initial treatment at high temperature "catalyzes" the formation of the necessary chemical systems.Maximum values ​​of compressive strength are generally observed when the material is cured in a temperature range between 30 and 90 ° C.If the geopolymerization takes place in a different temperature range (below 30 ° C or above 90 ° C) the mechanical performances tend to be significantly lower.Let us now analyze the mechanical behavior of the geopolymers most similar to those achievable on the lunar soil.In a recently published study, conducted at the Chinese Society of Aeronautics and Astronautics in collaboration with Beihang University, the chemical affinity between lunar soil samples collected during the Apollo missions and natural volcanic waste (GVS Ground Volcanic Scoria), making and testing geopolymer compounds.Also in this case the alkaline solution used and the curing temperature were the main factors capable of influencing the mechanical properties (compressive and flexural strength).Centered compression and bending tests were carried out on the samples, obtained by varying the ratio of the main "ingredients": sodium hydroxide (mSH) and sodium silicate (mSS), constituents of the alkaline solution, and GVS (mVA), raw material in dust.The results show considerable variance, demonstrating the importance of the relationship between the different constituents and the geopolymerization temperature.In any case, some mix-designs allow excellent mechanical resistance to be achieved.This demonstrates the good predisposition of the regolith to form geopolymeric compounds for the future lunar "colonies".Rheology is the science of materials that studies the equilibrium and deformation characteristics of matter when subjected to external actions.In a generic fluid (any fluid) viscosity is defined as the friction force generated between two fluid planes when they slide with respect to each other due to external forces, or simply due to the effect of weight.It is evaluated as the ratio between the magnitude of the applied shear stress and the speed of deformation or shear rate (in other words the speed of application of an effort).Therefore, the rheological behavior is a fundamental parameter to guarantee adequate performance of the geopolymeric compounds, both in the mixed state in the act of 3D extrusion (or in the partially hardened state) and in the hardened and "aged" state, literally aged. completed constructions.Cementitious and geopolymeric compounds are generally classified as pseudoplastic non-Newtonian fluids, or “shear thinners”.In other words, the deformation rate depends in a non-linear way on the applied shear stress and the viscosity decreases as the deformation rate increases.In the short term, the viscosity determines the effective workability of the mixture and its 3D extrusion.In the long term, however, the viscosity of the solid state is responsible for much greater deformations than the elastic ones.It is therefore evident that both of these aspects must be carefully evaluated in the design phase.Appropriate viscosity values ​​of the compounds in the mixed state are of primary importance to allow printing procedures, while the viscous deformations of the solid state in the long term represent a "secondary" phenomenon, albeit not to be overlooked.The state of the art on research in the field of geopolymers therefore highlights the following aspects:At the same time, "atypical" results compared to common cementitious materials show that for lower values ​​of the load application speed and only in compounds in the mixed state, the increase in temperature produces an increase in viscosity, since the reactions of geopolymerization.Workability under these conditions drastically decreases, affecting the extrusion of the compounds.Known the essential properties of geopolymers, we shift our attention to the 3D printing process, the real protagonist.3D printing technologies have assumed particular importance in the space field as they will allow the construction of future “extraterrestrial” habitats using resources collected in situ.One of the most accredited solutions therefore sees the use of pneumatic structures covered by a geopolymer shell with single or double curvature, printed using special rovers equipped with extruding robotic arms.Alternatively, the geopolymer protection could be composed of a succession of discrete elements, also 3D printed and subsequently arranged to form a protective layer.Research in the field of geopolymers is making big steps forward, driven by considerable interest also in the "terrestrial" field, and will probably represent the future of Lunar constructions.As mentioned in the previous paragraphs, the challenges of geopolymer lunar construction engineering are many and concern in particular the environmental conditions of geopolymerization, the viscous characteristics of the mixtures, strongly conditioned by the temperature in the initial stages, and the characteristics of the raw materials available in situ. .The super-plasticising additives are therefore of great importance, able to confer adequate viscosity and workability values ​​of the extruded mixture.Recent research has shown the efficiency of human urea as an additive for geopolymeric compounds, capable of significantly reducing the consumption of water for making mixtures and at the same time guaranteeing excellent mechanical characteristics.In conclusion, during the writing of this article a 3D extruded geopolymer sample was created for AstronautiNEWS, using Metakaolin 750 and an alkaline solution based on sodium hydroxide.The sample was also supplemented with spirulina to provide self-healing capabilities.3D printing showed a very good tendency for the compound to hold its shape and, in addition, no fractures of any kind occurred during geopolymerization or shrinkage of the mixture.In the following photos the sample made at the Superforma laboratory in Milan:Geopolymer prototype made using a 3D printer at the Superforma laboratory in Milan o.Credit: Manuel De Luca - ISAAResearch is certainly on the right track and geopolymers currently represent a very important field of application.The technologies made using these compounds will allow a safe stay on the lunar surface and, not least, can actively contribute to the reduction of CO₂ emissions in the earth's atmosphere if used as a future building material on our planet.But that's another story ...© 2006-2022 ISAA Association - LicenseThe launch date of the third Chinese human mission in space has been announced, it will take place ...The press agencies yesterday released the news of the dismissal of the President of the Space Agency ...To speak is the management of Energia, adding that from 2009 to 2015 Russia ...Tags: ESAMoon explorationNon-Newtonian fluidspseudoplastic fluidsgyopolymersmounar habitatStructural engineeringLunamooncreteNASAregolithereologiasvolcanic slagsalkaline solutions3D printerpneumatic structuresTransHabCivil Engineer, Scholar of Applied Geology and Underground Engineering.Training as a Private Pilot.Active in the field of research in Structural Engineering, with particular reference to rheological phenomena.(C) Association 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