# Rheological Properties of Lunar Mortars
## 1 Introduction
### Material Characteristics of the Lunar Regolith  
Regolith is the upper dust-like layer of lunar ground, with a thickness of around 4-15 m [15]. In terms of morphology, the lunar regolith consists of spherical and oblong grain dust, angular rock particles, and dendritic slag grains [16]. Already in the 1970s, it was stated that the particle size distribution (PSD) of the Moon soil is the basis for its classification [17]. The size of individual particles of lunar regolith delivered to Earth generally varies from few microns to hundreds of millimeters. Samples provided in subsequent Apollo missions (Apollo 11, 12, 14, and 15) enabled the development of some average particle size characteristics of the fine fraction of lunar regolith, pointing d\({}_{50}\) (the size point below which 50% of the tested material is contained, i.e., median) with values in the range of 42-130 \(\upmu\)m (see Table 1).  
A more recent paper (published by NASA in 2007) [18] shows that the particle size distribution generally follows the log-normal curve with the mean values in the range of 45-100 \(\upmu\)m. \"Dust\" fraction of lunar regolith includes particles smaller than 10 \(\upmu\)m, of which about 90% (by weight) are particles smaller than 1 \(\upmu\)m [19], although some particles can be even as small as 10 nm [18]. Particles with dimensions larger than 0.25 mm (250 \(\upmu\)m) constitute 10% (by weight) of regolith [20]. Moreover, lunar regolith contains morphological forms that do not exist on Earth--these are spherical lunar chondrules (with dimensions from a few microns to 0.5 mm) formed as a result of meteorite falls and the sudden melting of lunar rocks) [19]. The fragmentation of the lunar regolith is the result of mechanical impacts but also thermal stress caused by high daily temperature differences and erosion caused, among others, by ionizing radiation of the solar wind and galactic cosmic radiation [21]. Figure 2 shows the averaged chemical composition of regolith and the graining curve and an example of an SEM micrograph [20] of lunar dust.  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Mission** & **Number of Samples** & **d\({}_{20}\), mm** & **d\({}_{50}\), mm** & **d\({}_{50}\), mm** \\ \hline Apollo 11 & 13 & 0.016–0.030 & 0.048–0.105 & 0.163–0.720 \\ Apollo 12 & 55 & 0.013–0.033 & 0.042–0.094 & 0.167–0.440 \\ Apollo 14 & 24 & 0.013–0.036 & 0.044–0.130 & 0.061–0.200 \\ Apollo 15 & 19 & 0.014–0.019 & 0.051–0.108 & 0.180–0.400 \\ Altogether & 111 & 0.013–0.036 & 0.042–0.130 & 0.163–0.720 \\ \hline \end{tabular}
\end{table}
Table 1: Lunar soil grain distribution based on the results obtained after missions Apollo 11, 12, 14, and 15 [17].  
Lunar regolith is characterized by very strong adhesion to various surfaces. This is a negative property in the context of functioning on the surface of the Moon. Regolith adheres to the surface of astronauts' suits and to the surface of machines. Adhesion in combination with the high abrasiveness of regolith can lead to damage to machinery mechanisms, research equipment, including optics. However, in the context of adhesion of binder to aggregate in cementitious materials, this feature seems to be very important and desirable.