Concrete is a well known material on Earth, and would be useful for building settlement facilities such as houses and infrastructure elements. It has excellent characteristics for protection against radiation and small meteorites. Possibly, concrete can be made in situ on Mars, using local resources. Concrete is usually a mixture of water, Portland cement, sand and stone in various proportions. Portland Cement ingredients may not exist on Mars, hence other types of binders have been explored over time. The density of concrete depends on the mix and the density of the aggregate, but is usually about 2400 kg/m3. Concrete has an embodied energy of about 1,1 to 2 MJ/kg. Steel Rebar is usually required and increases the embodied energy of the material.
Concrete has the great advantage that it can be cast in place into practically any shape. It is much cheaper than steel, although is is also much less tough. It provides much better radiation protection than steel.
Concrete preparation follows certain ratios for the main ingredients.: Cement, Sand and Aggregate. (M) stands for mix and the number is the compressive strength in MPa (N/m2) after 28 days of curing. Water is usually mixed in at about 0,45 per volume of the cement. There proportions are for volumes, nor for masses.
|Grades of Concrete||Ratios of Concrete mix design(Cement:Sand:Aggregate)|
|M15||1:2:4 (most common mix)|
Cement is the binder in concrete. There are many possible materials for this function. On Earth the most common binders are lime and calcium silicate, than produce hydrate when mixed with water, hence the name hydraulic cements.
Hydraulic (Portland) cement
There seems to be plenty of water on Mars, but hydraulic cement also requires calcium, silicon oxide (sand) and aluminum oxide. It is unclear whether these substances can be found on Mars in a form that allows a simple production process.
Sorel cement is magnesium based cement, a non hydraulic cement made from a mixture of magnesium oxide and magnesium brine. It has poor water resistance but excellent sheer strength.
The ultimate strength and tensile strength was found to be best at a mixing ratio of 50% sulfur and 50% JSC Mars-1A regolith simulant sieved to a maximum particle size of 1 mm. The concrete was found to have a compression strength of > 50 MPa, a flexural strength of 1.75 MPa, and a splitting tensile strength of 3.9 MPa.
Utilizing sulfur-regolith concrete is possible on Mars, but not the Moon. On the moon, the concrete mass would be gradually lost due to sublimation of sulfur in vacuum, and the large temperature swings between lunar day and night which compromise the structure. Sulfur-regolith concrete is stable under Martian conditions and would not experience a loss in mass due to sublimation.
Mars is considered a sulfur-rich planet, but it in unclear where sulfur may be and if it is present in a form suitable for the production of sulfur concrete.
An important disadvantage of sulfur cement is that is is not fireproof. The sulfur used as binder can melt at higher temperatures, and may emit large amounts of smoke.
Sand and stone
Sand and stone are common on Mars. Most types of volcanic rock are compatible with concrete.
The stability of concrete structures can be increased significantly by glass fibers or reinforcing steel. Concrete is almost always reinforced with rebar, steel bars that give it strength in tension. Composite basalt fiber rebar is available on Earth, but problems with installation may reduce its adoption. The binder required for the composite rebar may be expensive, in embodied energy terms.
- Journal of Aerospace Engineering - Analysis of Lunar-Habitat Structure Using Waterless Concrete and Tension Glass Fibers
- https://arxiv.org/pdf/1512.05461.pdf "A Novel Material for In Situ Construction on Mars: Experiments and Numerical Simulations." Lin Wan et al. 2016