Water

Unfortunately, Mars appears to have very little water. This means that it will be a treasured resource on the planet. Although there is the possibility of trade with Earth, or more likely mining of the asteroid belt, the available water must be used wisely. I have seen many suggestions for the use of plastic/kevlar domes. I have also seen people suggest Martian concrete be used in colonies. This strikes me as a waste of a pressure resource. Water should not be locked up into construction materials. Nor should it be broken down for use in plastics.

The following elements are available on Mars in plentiful supply:

Silicon
Aluminum
Iron
Calcium
Magnesium
Titanium
Oxygen
Carbon

These elements exist lesser amounts:

Sodium
Sulphur
Chlorine
Potassium
Nickel
Zinc
Argon

The iron on Mars mainly exists as FeO. All that is required to free this iron is the addition of carbon monoxide. As such the steel industry on Mars will flourish. Major alloying materials are also available in plentiful supply. I believe that glass will also be a major building material. Silicon dioxide makes up the majority of the crust. New methods will be required to make the glass ductile to prevent unexpected failures in dome/habitat structures.