Sintered regolith falls into the category of ceramic materials. Sintering is the process most common to ceramics. When bricks are made from clay on Earth first the bricks are heated long enough and hot enough to drive out the water. Then the heating is increased to cause partial melting or vitrification. This is the sintering process. The unmelted particles provide a stable shape and size during the process which involves some shrinkage and a decrease in porosity.
On Luna water for mixing the various particles would be very expensive and recycling the water would never be 100% effective. So it is hoped that mixing of dry powders will provide satisfactory bricks. There is the example of mixing dry powders for powder metallurgy, which is a sintering process performed on Earth. More will be known about the quality of bricks possible from lunar materials when the research can be done on Luna. A possibility to consider is the sintering of bricks in an oxygen atmosphere as opposed to in a vacuum.
Hybrid Microwave Sintering
Research conducted by NASA Johnson Space Center and Lockheed Engineering & Sciences Co. has indicated that the lunar simulant MLS-1 can be successfully sintered using a combination of both microwave and radiant heating.
The crucible was surrounded by silicon carbide bricks, which converted part of the 2.45GHz microwave energy to heat. This radiant heating kept the outside of the regolith hot while microwaves heated the inside at the same temperature of 980 degrees Celsius for 35 minutes. Microwave power was then slowly reduced over several hours. The result was an evenly heated and cooled sintered block with no fractures capable of withstanding up to approximately 1,100 psi.
Oxygen Retrieval and Magnetic Handling
Hybrid microwave sintering in a flowing hydrogen atmosphere resulted in the reduction of iron oxide to produce iron in the sample and water which escaped as vapor. This additional process provides a means of oxygen retrieval as well as increasing the iron content so that bricks can be lifted using a magnet.
- Construction Materials for Planetary Outposts: A Review - LPI .. PDF
- Lunar Regolith Simulant Materials: Recommendations for Standardization, Production and Usage - NASA MSFC .. PDF
- Sintering of Lunar and Asteroidal Material - PERMANENT
- Structural Design of a Lunar Habitat - Journal of Aerospace Engineering, July 2006, pg 137 .. PDF
- Workshop on using In-Situ resources for construction of planetary outposts - LPI .. PDF
- Sintering Bricks on the Moon - NASA JSC
- Vacuum Pyrolysis and Related ISRU Techniques - NASA GSFC .. PDF