Ceramic, Glass, and Concrete Construction Materials
In 1986, the National Commission on Space proposed that "...NASA specifically include vigorous development of the technologies for robotic and teleoperated production of shielding, building materials, and other products from locally available raw materials." Since only physical processing is required, and not a chemical transformation, it is likely that this technology will be among the first to be practiced at a lunar outpost.
When humans return to the moon-this time to stay-we need to develop self sufficiency The material necessary to regenerate our life support systems, produce propellants, protect ourselves from the radiation of outer space, produce structures, and perhaps even provide clean, non polluting energy to Earth, is already present in the lunar soil. We can choose to use them or ignore them. The goals and justification of the Space Exploration Initiative have been made clear by the President, the National Space Council, and NASA. The use of Indigenous Space Resources can extend our reach and lower the cost of our next step into the universe.
The Apollo astronauts only stayed on the Moon for a few days, so they did not need massive shielding from radiation. People staying for any length of time must be protected, however, not only from solar flares but also from galactic cosmic radiation. Advanced lunar habitats could be constructed underground in tunnels, giving natural protection.
Early habitats, on the other hand, may be prefabricated Space Station Freedom-type modules or inflatable structures brought from Earth. One way to provide shielding for these habitats is to simply pile a large amount of regolith on them. This might be the first use of lunar material at an outpost. A thickness of several meters would be required for long stay times, so the mass savings of using local material rather than bringing it from Earth is obvious.
Burying the modules under a pile of soil has some drawbacks, however. It places constraints on the design of habitat modules and airlocks and may limit operations near the habitat. The soil could slide off, so it would have to be stabilized with supports or retaining walls. A thick layer of soil would make it difficult to add on rooms or to connect new wires, antennae, or pipes as the outpost was improved. Putting soil in sand bags would solve some of these problems but not all of them.
An improvement over loose soil would be a radiation protection system constructed of cast basalt or sintered blocks made from regolith. These can provide much denser radiation protection compared to loose soil; therefore, the shield can be less thick. These blocks could be made in an automated block factory, formed into interlocking shapes, and moved and stacked by robotic arms. If access to the habitat wall is needed, the robotic arm could as easily remove the blocks. Later, when modules are added, the blocks can be removed easily and reused to accommodate expansion. Cast basalt has been used on Earth for years in many areas where hard, chemically resistant material is required. Much of the science and technology needed for lunar processing is already available in industry today.
The production of pressed and fused (sintered) bricks for use in a variety of applications was studied by Battelle-Columbus in 1988. Their design resulted in a 6-step process for the production of more than 200,000 bricks per year using only currently available ceramics processing technology. This is sufficient to cover six lunar habitat modules, as currently envisioned, with two meters of densified and stabilized regolith as shielding.
The first step after mining the raw feedstock is to screen it to remove the coarse stones which would interfere with the mechanical handling involved in the production process. Next, the fine material would be loaded into a mold and pressed to compact the regolith. Heating to 1100°ree; C for a pre determined time is the key to allowing the compacted mass to achieve its strength. Controlled cooling allows the now solid bricks to retain this strength rather than fracture from internal stress. Finally, there must be a way of robotically removing and storing these bricks for use at the outpost.
Estimates for the mass of such a plant run from 25 to 40 metric tons. Its power requirement is about 375 kW. Both estimates depend on the desired strength of the bricks as well as the thermal properties of the starting material. More recently, other groups have suggested the use of microwaves to lower the power requirement and increase the heating rate. If feasible, this would decrease the processing time and thus greatly reduce the plant mass. Even without this improvement, such a plant would pay for itself by returning its own mass in 3 to 4 days of operation.
Such blocks might also be useful for paving the launch and landing area to prevent engine exhaust from sandblasting the outpost. Simple walls made from these blocks can provide shade for outside equipment such as thermal radiators and protect them from some of the direct sunlight. Similarly, simple garages made of blocks can provide thermal protection during the cold lunar night for critical equipment such as rovers. In these and other ways, the ability to produce simple, low-tech construction material may significantly improve outpost operation.
Another possible use of cast basalt blocks might be as a storage mass for thermal energy. By simply heating these blocks using solar concentrators during the day, and then withdrawing this energy at night, heat engines might become competitive with other forms of power generation. Because of the long lunar night, power storage devices brought from Earth would be massive. By using materials indigenous to the Moon, even with an inefficient technology, a more efficient system (defined as kilowatts per imported kilogram) based on thermal energy storage may be possible. This is a case where the true product is a service, namely energy storage, rather than a one-for-one replacement.
Other related products include glass blocks, glass windows, fibers for reinforcement, and even fiberglass mats and fabrics which can be produced by melting and processing soil. In the extremely dry lunar environment, these glass products may rival metals as strong construction materials. In the longer term, the production of cement and concrete is also under consideration. The manufacture of these materials presents the opportunity to expand the inhabitable volume of the outpost with minimal support from Earth.