# Regolith-Rich PEEK Composite Bricks: Steps Towards Space-Ready Lunar Construction Materials
## 4 Discussion on the Material Properties
### Effects of Vacuum Processing and Temperature Testings  
To evaluate the effects of vacuum processing and different testing temperatures, the decision is made to fix the processing parameters as follows: compaction pressure at 7.50 MPa and binder weight percentage at 10 wt%. As will be discussed in more detail in Section 4.3, these values already result in components that, processed in air and tested at ambient temperature, exhibit mechanical properties suitable for construction. The compression strength (16.3 MPa) exceeds that of industrial floor bricks and is comparable to that of the building bricks used in Earth applications, while the bending strength (7.4 MPa) falls within the range of steel fiber-reinforced concrete and high-strength concrete. Figures 8 and 9 present the results of the second campaign, following a similar approach to the first one. In each graph, a red line highlights the reference value for the corresponding quantity (strength or modulus) measured at ambient temperature (20 \({}^{\circ}\)C) on the samples cured in air. Figure 5: Experimental results: compression strength and modulus per DoE run. The effects of vacuum processing and conditioning temperature can be discussed separately by analyzing the graphs with different focus areas. The performance of the samples cured in vacuum and tested at room temperature is significantly superior to that of the corresponding samples cured in air. This improvement is evident across all the mechanical properties, with an increase of +66% in bending strength, +125% in bending modulus, +33% in compression strength, and +53% in compression modulus. The results indicate that the effect of vacuum processing is more pronounced on the moduli than on the strengths. Notably, the effect on the bending properties is approximately double that on the compression properties. Since bending involves a combination of both compression and tensile forces, this disparity implies that vacuum processing has a dramatic impact on the material's tensile properties. This suggests that vacuum curing likely improves particle cohesion and reduces internal voids, resulting in a material that is not only stiffer but also better able to withstand tensile stresses. Figure 6: Bending strength and modulus: main effects plots for means. Figure 7: Compression strength and modulus: main effects plots for means. The conditioning temperature has a distinct impact on both bending and compression properties, with varying effects on the strength and modulus. For the bending strength and modulus, there is a clear monotonic trend: lower conditioning temperatures enhance both properties, while higher temperatures degrade them. Specifically, at \(-40\)\({}^{\circ}\)C, the bending strength and modulus show improvements of \(+15\%\) and \(+7\%\), respectively, compared to the room temperature reference, while at \(+130\)\({}^{\circ}\)C, the variations are \(-15\%\) and \(-13\%\). However, these temperature-induced changes are secondary compared to the impact of vacuum processing. Even in the worst-case scenario, the bending properties of vacuum-cured samples remain superior to those of air-cured specimens tested at room temperature. In contrast, the trend for compression strength differs from that of bending strength. At \(-40\)\({}^{\circ}\)C, compression strength decreases by \(-9\%\) compared to the room temperature reference, while at \(+130\)\({}^{\circ}\)C, it increases by \(2\%\). The compression modulus behaves differently, reaching its peak at room temperature, with decreases observed at both \(-40\)\({}^{\circ}\)C (\(-21\%\)) and \(+130\)\({}^{\circ}\)C (\(-29\%\)). Again, despite these variations, the compression properties of vacuum-cured samples still outperform those of air-cured specimens tested at room temperature, underscoring the dominant effect of vacuum processing over temperature conditioning.