# Rheological Properties of Lunar Mortars
## 1 Introduction
### Lunar Regolith Simulants  
A lunar regolith simulant is any material made from natural or synthetic earth or meteoric components to replicate one or more of the physical or chemical properties of the lunar regolith. It is impossible to produce a model of lunar soil on the basis of rocks on Earth that would reflect its physical, mechanical, electromagnetic, thermal properties, as well as the chemical or mineralogical composition. For this reason, all models and analogs of lunar regolith are produced to simulate one or two of the aforementioned characteristics necessary for research and experimentation. All stimulants are produced by mechanical rock crushing. In order to better reflect the particle size distribution and shape of the particles, the rocks are crushed by impacts. The granulometric composition of most simulants differs from the lunar regolith. They mainly correspond to coarser fractions since obtaining very fine particles is associated with a difficult and expensive technology. Basalt rocks or anorthosite with an admixture of olivine and pyroxene are usually used as the main components of lunar regolith analogs. In order to map agglutinates, glass is added to the mixture of rocks [16]. Over the years, several different lunar regolith stimulants were developed, including several variants of Johnson Space Center Number One (JSC-1), two lunar highlands simulants, namely, the Lunar Highlands Type (NU-LHT) series and Olivine Bytownite (OB-1), as well as OPRL2N Standard Representative Lunar MareSimulant, Standard Representative Lunar Highland Simulant, Minnesota Lunar Simulant 1 (MLS-1), Chenobi, etc. [26].  
Polish scientists from the Space Research Center of the Polish Academy of Sciences, due to the need to use large amounts of lunar regolith simulant for research but also high prices of known lunar regolith simulants resulting from the difficult methods of production, decided to create their own simulant [27]. The aim of the invention was to produce material needed in research on assessment of the reliability of machines and tools working in simulated lunar-like conditions, i.e., in the presence of a large amount of abrasive dust. It was important to map the sharp-edged grains of lunar regolith minerals [28].  
Generally, Polish lunar regolith simulant is a mix of components appearing in Polish geological conditions and/or commercial components produced by local raw mineral producers, including mechanically crushed quartz, granite sand, and grit. As the components were only mixed without additional technological procedures, the new LRS was obtained at a much lower cost and required a shorter production time.  
Despite the simple approach to production, the Polish lunar regolith simulant is characterized by the grain size distribution, color, mechanical strength, cohesion, and an internal friction angle comparable to the Chenobi simulant. In Figure 3, one can compare the particle size distribution plots of Chenobi simulant and Polish lunar regolith simulant (marked AGK-2010). In terms of morphology, as required, it consists of full sharp-edged particles (mechanical fragmentation effect). It is mainly a gray noncohesive dusty powder of bulk density of (1.290-1.300) g/cm\({}^{3}\), an internal friction angle in the range of (36.5-38.5)\({}^{\circ}\), and cohesion in the range of (3.8-4.1) kPa [26; 27].  
The development of the Polish lunar regolith simulant was aimed at testing the KRET penetrator, testing the planetary rovers, promoting research related to space exploration in Poland, etc. [27]. This LRS was also used in the research considering the subject of this paper.