# Rheological Properties of Lunar Mortars
## 4 Results and Discussion  
### Characterization of Lunar Regolith Simulant (LRS) and Quartz Powder (QP)  
Figure 4 presents the particle size distribution plots (the relative frequency and the cumulative relative frequency of the particle size distribution) of lunar regolith simulant, LRS and quartz powder, QP, and the micrograph of the LRS. The values of statistical parameters describing the fillers' particle size distributions, as well as specific surface area (calculated from the distribution, making an assumption about the spherical shape of the particles) and density determined in a Le Chateliere volumonometer are given in Table 4.  
The simple micrograph confirmed that the tested lunar regolith simulant is characterized by sharp-edged particles, as assumed during production, which enables a good simulation of the mixing of the components of the lunar micro-mortars and mortars. Already at this stage, it can be inferred that in relation to round and smooth grains of sand, the presence of regolith simulant would affect the consistency of mixes negatively. Water demand, i.e., the need for water to obtain a specific consistency of a mix, depends primarily on the size of the grains but is also influenced by the roughness and shape of the grains.  
Taking into account the important role of the smallest fractions that determine the specific surface area (the finer the aggregate, the larger the total surface area) and the resulting water demand ratio, the focus was primarily on the finer fractions of the tested material. The results of the test performed on the laser analyzer showed that 16% of the LRS particles are smaller than 10 \(\upmu\)m. The real lunar regolith consists of about 90% of particles larger than 10 \(\upmu\)m [16]; therefore, it was confirmed that the simulating material is in agreement with the simulated lunar soil for particles < 10 \(\upmu\)m. The median of LRS PDS is 37 \(\upmu\)m, which is also in line with the real lunar regolith data (40 \(\upmu\)m-130 \(\upmu\)m) [16; 17]. Additional analysis showed that the average size of RLS particles does not exceed 55 \(\upmu\)m, while the largest registered size of the LRS is 300 \(\upmu\)m. Despite the last result, it is worth mentioning that 90% of total lunar regolith simulants are not larger than 133 \(\upmu\)m (less than half of the size of the biggest registered particle).  
As for the density, taking into account that the tested material was a simulant obtained by grinding rocks rich in quartz (silica, granites from Polish deposits, which are mainly made of plagioclase, quartz, biotite, and hornblende [32]), it is not surprising that the experimentally obtained value of density of the LRS simulant (2.641 g/cm\({}^{3}\)) is close to the value typical for the pure quartz (2.650 g/cm\({}^{3}\)). The coefficient of variation for this test was very low (CV = 0.14%), which confirms that the lunar regolith simulant is very homogeneous in terms of density (see Table 4).  
Among the quartz powders initially tested in terms of particle size distribution, the powder that turned out to be the most similar to the lunar regolith simulant discussed earlier was the one whose PSD plots were also included in Figure 4 above, and the statistical parameters describing the distribution are presented in Table 4. The quartz powder, however, is a bit finer than LRS. It contains particles smaller than 1 \(\upmu\)m, the median is 21.5 \(\upmu\)m (42% smaller than the median of LRS), average size and mode are about 30 \(\upmu\)m,and maximal size slightly exceeds 150 \(\upmu\)m. All these results indicate that the quartz powder has a much larger (1.77\(\times\) larger) specific surface area than the LRS, which makes LRS a more advantageous mortar micro-filler than the quartz powder described above.