Concrete Production and Construction

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A type of cement called Magnesium Oxychloride cement has shown promise for construction on Mars. It is much stronger than Portland cement, it is impermeable and unlike Portland cement, it can cure in the atmospheric conditions found on Mars.

Characteristics

Magnesium oxychloride cement (MOS), also called Sorel cement, is a type of hydraulic cement made from a mixture of reactive magnesium oxide, magnesium chloride and water. It was discovered by French civil engineer Stanislas Sorel in 1867 and has had limited application in grindstones, tiles and billiard balls. It has not been used in structural applications due to poor water resistance.

Chemical properties

MOS is a mixture of magnesium oxide, magnesium chloride and water in a stoichiometric ratio of 5:1:13, although this can be varied according to the application and temperature. The mixture forms microscopic needle like crystals in different phases depending on the temperature, with the phase being denoted by the ratio of magnesium to magnesium chloride. At temperatures below 10C, phase 3 dominates, while phase 5 dominates at higher temperatures. Below 10C, it will not normally solidify and will form crystals in suspension (why?).

Physical properties

MOC without aggregate has shown compressive strength approaching 20000 PSI.

Production

Raw Materials

Magnesium Chloride and Sulfate

Magnesium Sulfate has been directly detected on Mars and could be easily extracted from soil or salt deposits.

The magnesium Chloride could be extracted from some of the salt deposits that have been detected on Mars by processes similar to those used on Earth.

Magnesium Oxide

The reactive magnesium oxide can be produced from several sources. brucite (magnesium hydroxide) or magnesite (magnesium carbonate) can be heated and broken down to magnesium oxide directly with either water or carbon dioxide as a by-product. It can also be produced by pyrohydrolysis of magnesium chloride, with hydrochloric acid as a by-product.

Processing

Magnesium Chloride and Sulfate

These salts can be separated from saline deposits by simple evaporative processes requiring little energy input, as commonly done here on Earth.

Magnesium Oxide

Depending on the raw materials available, a variety of options are available for producing the reactive magnesium oxide. With brucite and magnesite, direct calcination (thermal decomposition) will work. Using magnesium chloride, some form of hydrolysis is needed to convert it to magnesium oxide.

Using brucite, heating to 300C will produce reactive magnesium oxide with water as a by-product. This can be accomplished by a simple solar concentrator or eklectric heating, depending on power generating options available. This temperature is low enough that waste heat could be used from other processes.

With magnesite as a raw material, heating to around 600C will produce magnesium oxide with carbon dioxide as a by-product. This requires much greater energy input than with brucite, and the higher temperature precludes the use of most waste heat. Using brucite or magnesite would require finding high grade deposits, as well as a great deal of quarrying and preprocessing.

Magnesium oxide can also be produced from magnesium chloride through some variation of the Aman process, in which brine is fed into a spray roaster and hydrolyzed to magnesium oxide at around 600C. The energy input is much greater than the two previous processes due to heating a great deal of water, but logistically it would be much simpler. It requires only one raw material for both major components of the cement, and handling of the raw materials could potentially be much simpler.

If large quantities of alkali salts are found, the magnesium hydroxide could be precipitated out of a magnesium chloride solution and calcined with a fairly low energy input as described previously. Adding an alkali salt raises the pH of the solution, causing hydroxide ions to bind with magnesium ions to form insoluble magnesium hydroxide.