Moon Miner
  

Lunar Cement

Cement Defined

In "The Case for Mars" Dr. Zubrin states that lime, CaO, and sand can be mixed together, some gypsum (calcium sulfate) added, and the result is Portland cement. Basically that's true, but there's a little more to it.

From Linus Pauling's "General Chemistry" (Dover:1988) we read that: "Portland cement is an aluminosilicate powder that sets to a solid mass on treatment with water...Portland cement before treatment with water consists of a mixture of calcium silicates, mainly Ca2SiO4 and Ca3SiO5, and calcium aluminate, Ca3Al2O6. When treated with water the calcium aluminate hydrolyzes, forming calcium hydroxide and aluminum hydroxide, and these substances react further with the calcium silicates to produce calcium aluminosilicates, in the form of intermeshed crystals.

Ordinary mortar for laying bricks is made by mixing sand with slaked lime [CaOH]. This mortar slowly becomes hard through reaction with carbon dioxide in the air, forming calcium carbonate. A stronger mortar is made by mixing sand with Portland cement. The amount of cement needed for a construction job is greatly reduced by mixing sand and crushed stone or gravel with the cement, forming the material called concrete. Concrete is a very valuable building material. It does not require carbon dioxide from the air in order to harden, and it will set under water and in very large masses."

Making Lunar Cement & Concrete

On the Moon we will get a mixture of silica, SiO2, and calcium sulfate, CaSO4, after we leach magnetically beneficiated regolith that has had the iron and ilmenite (FeTiO3 which also contains the titanium) removed, with sulfuric acid to extract aluminum sulfate and magnesium sulfate and some trace metals. Sulfur is fairly abundant in regolith at 500 ppm or more and will be obtained during volatile harvesting. Most of the sulfur and sulfuric acid will be recycled. The CaSO4 can be calcined, or heated to high temperatures (~1500 C.) in solar furnaces, to get lime, CaO. This can be mixed with aluminous Highland regolith in the right proportions and heated in solar furnaces to make small glassy marbles called "clinker" which are then ground in a ball mill. This powder is then mixed with CaSO4 about 5% and ground some more to make cement.

Acid leaching puts precious water at risk and requires complex apparatus to leach and recycle everything, but it's the only way to get CaSO4 on the Moon. Calcining this to get lime might not be the best way to make the actual cement. Anorthite, CaAl2Si2O8, which composes 75% to 98% of highland regolith based on analysis of Apollo samples can be roasted at 2000 C. in the vacuum to decompose the mineral and drive off the SiO2 component leaving calcium aluminate, CaAl2O4, behind. Calcium aluminate makes good cement. Calcium sulfate is added to retard setting time.

On Earth limestone and clay are mixed together and heated to 1500 C. to get clinker. The limestone breaks down into lime and the clay which is basically just feldspar or plagioclase like the Highland regolith react to make the mixture of calcium silicates and calcium aluminate. Water will come from polar ice, hydrogen from volatile harvesting combined with oxygen, and imported hydrogen combined with lunar oxygen. Since water is only 1/9 hydrogen importation could be practical. Concrete made by mixing cement with lunar regolith and gravel will be very useful for construction within pressurized lava tubes someday. It cannot be used out-vac because the precious water will just evaporate into the vacuum and the chemical reactions that make the cement harden will not occur. Concrete items could be cast inside pressurized modules then moved outside and cement board for walls inside of modules could be made. It might even be possible to spray concrete inside of inflated modules to make habitat.

Concrete can be made from 1 part cement, 2 parts sand (raw regolith that has been sized by screening and sieving), and 3 parts gravel. Thus, a meager amount of cement makes six times as much concrete.

                                          Pressurized structure for making molds and pouring concrete

Sulfur Cement

Sulfur might also be used in place of water to make sulfur cement. This is made simply by mixing molten sulfur with sand. No lime is required. Sulfur cement can be mixed with gravel to make concrete. Sulfur can be obtained by roasting large tonnages of regolith at 900 to 1200 C. It has one drawback--it will melt in the heat of lunar day. Large foil heat shields will be required and we could cast sulfur cement structures in the shadow, perhaps with contour crafting gantries, to make structures that would then be buried with several feet of regolith for thermal insulation and radiation shielding for habitat stationed within the structures.

The View Toward the Future

On Mars, ordinary mortar will react with the CO2 in the martian atmosphere to harden and bond bricks made from martian regolith. We can also mine gypsum, CaSO4*H2O, on Mars. On the Moon we will need to use mortar made from cement and sand which does not require CO2 to harden to bond bricks made of sintered basalt perhaps in lava tubes. We will not want mortar to absorb CO2 from the air of lava tube habitations because carbon is scarce on the Moon and we need it for supporting crops.

Leaching aluminous Highland regolith with sulfuric acid will be a job for more advanced Moon communities when we can manufacture the necessary equipment on the Moon and have water from polar ice mining and water made by combining hydrogen from solar wind implanted volatiles mining with LUNOX in abundance. We will need quite a bit of industry there to do the job. The original "seed packages" of robotic devices will grow and grow using lunar materials to make more equipment until we are ready to seal, pressurize and inhabit lava tubes by building towns for several thousand people with concrete, bricks, plaster, glass and metal mainly iron. There will be no intrusion by ground water on the Moon to erode or rust our structures. These underground towns will be more homey than the metal and inflatable plastic bases we build in the early days of lunar industrialization. They will have gardens and farm sections illuminated by light piped in from the surface during the long day and super efficient microwave sulfur lamps that mimic the spectrum of the Sun without the UV and IR by night.

The sulfuric acid leaching tanks and related equipment will be made with an alloy of abundant lunar iron and about 15% silicon instead of stainless steel. We will also get plenty of silica for glass and calcium sulfate which is dry plaster. Plaster can be wetted and applied between two layers of glass fiber cloth and allowed to harden to make a wallboard that resists moisture and mildew. We will not need precious paper for wallboard.

Iron, titanium and sintered or cast basalt can be obtained early on the Moon. It may be possible to decompose regolith with intense solar heat at 10,000 degrees or with lasers to break down the soil into its constituent elements including oxygen which are separated electrostatically or by condensation at various temperatures. This will result in Ox, Si, Fe, Ti, Al, Mg and other elements but it will not directly produce the useful compounds SiO2 and CaSO4 or CaAl2O4. It requires extreme temperatures. Lasers would demand vast amounts of energy. Sulfuric acid leaching does not require either. I have suggested solar silicothermal and carbothermal reduction of Al2O3 and MgO obtained by solar calcining of the sulfates resulting from sulfuric acid leaching. It may seem simpler to decompose these oxides at super high temperatures (vapor pyrolysis) and separate the metals electrostatically or by condensation at differing temperatures. Whether or not large scale vapor pyrolysis and separation is practical remains to be seen. Sulfuric acid leaching is a common industrial process that should be possible on the Moon without any hindrance. Reacting pure silicon and calcium from large vapor pyrolysis devices with oxygen to get silica and lime will require intense combustion at high temperatures and pressures, so I favor the sulfuric acid leach approach. Water formed during H2SO4 leach and oxides of sulfur released during calcining of sulfates at comparatively low temperatures versus vapor pyrolysis* will be recombined to recycle sulfuric acid. Fresh sulfur must be harvested to replace leakage and sulfur that goes into plaster, wallboard and the CaSO4 used as a cement hardening retardant.

Pure silica glass will replace Pyrex and some silica will be blended with Al2O3, MgO, CaO and available Na2O to lower its melting point. These oxides will be plentiful as they will be derived by solar calcining sulfates formed during H2SO4 leach. Direct application of solar heat will be possible for 14 days in a row without elaborate electrical equipment. Work will shut down in the cold of night when solar energy is unavailable and temperature stress on cold equipment is preferrably avoided. Work by night would only be possible with extensive solar energy farms and a power grid on the Moon or nuclear powerplants which will be even far more advanced activities requiring some exotic metals not abundant on the Moon.

* Sulfuric acid leaching can be done at "room temperature" or at ambient temperatures of the lunar day. Sulfuric acid boils at 340 C. at 1 ATM and water at 100 C. MgSO4 decomposes to MgO at only 850C and Al2(SO4)3 decomposes to Al2O3 at 1040 C. releasing oxides of sulfur in the process. MgO can be reduced with silicon at 1200 to 1800 C and Al2O3 can be reduced with carbon at 2000 to 2300 C. and at lower temperatures with electrolysis. These temperatures are far lower than the 10,000 degrees K. of vapor pyrolysis!