# Rheological Properties of Lunar Mortars
## 1 Introduction  
On 21 July 1966, the first manned mission landed on the Moon (Apollo 11) [1]. In 2016, the 50 th anniversary of this event was celebrated, which reawakened interest in the Moon and space travel. National Aeronautics and Space Administration (NASA) has revealed plans to resume manned missions to the Moon: mission Artemis III is planned to be the first crewed lunar landing since Apollo 17 in 1972--the crew is to land on the Moon by 2024, and by 2028, it is planned to implement sustainable exploration of the Moon [2, 3, 4]. Earlier, NASA and the European Space Agency announced that they wanted to ensure the possibility of permanent human residence in the so-called habitats on the Moon or Mars before 2040 [2]. Regardless of the time horizon, the implementation of the concept of extraterrestrial habitats requires the development of material and execution concepts of a lunar habitat. In order to implement such an unusual construction project as a base/habitat on the Moon, many factors need to be considered, including advanced economical, technological, and material engineering aspects. The latter two are closely related and limited by the former, because the cost of transporting cargo to the Moon, although it has decreased in recent years, is still calculated in the tens of thousands of dollars. The price for one launch of a new NASA's heavy-lifting vehicle--the Space Launch System, designed for delivering payloads over 25 metric tons to the Moon, is estimated to be over 2 billion dollars [5]; thiscorresponds to ca. USD 75,000/kg of delivery cost [6]. In 2020, NASA announced a tender for a transport service to the Moon (service delivery until 2023) of the VIPER mobile robot, which will search for water (ice) at one of the Moon's poles [4]. Taking into account the \"astronomical\" cost of transporting cargo, including equipment and materials, scientists for decades have been supporting the concept of using locally available resources to build a base/habit, including the so-called lunar regolith--layers of dust and rock debris covering the lunar surface [7; 8; 9]. These plans received greater attention after the American Lunar Crater Observation and Sensing Satellite (LCROSS) detected a significant amount of water (5.6% by mass) in 15 tons of excavated regolith in 2009 [10]. Various research centers are attempting to develop concrete-like building composites that could potentially be made on the Moon. These composites are often called the term \"lunar concrete\" [11] and belong to the wider group of the so-called extraterrestrial construction materials [8]. Most of those assume the use of lunar regolith as aggregate, while in terms of the binder, mainly three groups of concrete can be distinguished: polymer concretes [11; 12], sulfur concretes [13], and ordinary concretes (with ordinary Portland cement, OPC). The advantages and disadvantages of these material solutions in the context of their manufacture and use on the Moon, as well as the effects of the first attempts to prepare them in Earth laboratories (with the partial simulation of lunar conditions), are discussed later in the article. First, the conditions for the performance and operation of concrete on the Moon are specified, which are important assumptions for the design of durable lunar concrete. The following section describes brief characteristics of the lunar regolith based on samples collected on the Moon [1] (landing and sampling sites are shown in Figure 1)--astronauts have collected and delivered to Earth a total of 382 kg of rock and lunar dust [14]. Particular attention was paid to the granulation of regolith because it was important from the point of view of the research part of the work presented in this paper. Figure 1: Moon seen from the Earth’s side with the landing sites of manned and non-manned missions marked (there was no landing on the other, so-called dark side); screenshot from NASA interactive map [14]. The aim of the research was to initially assess the possibility of using regolith as aggregate in cement mortars.