Abundant element

Ground

The most common elements on the earths surface are the ones found in rock forming minerals: Most contain silicon as the main ingredient.

Wikipedia: the abundance of elements in earths crust

Atmosphere

The elements easiest to attain are not equal to the most abundant ones. E.g. Nitrogen is easier to get than Silicon or Titan since it can be drawn from the atmosphere. Given a bit of patience carbon can be drawn directly from the atmosphere too. Carbon already an infrastructure for delivery too: gas pipes.

Ocean

Salts in the sea (alkali metals and halogenides) are a huge reservoir of potential building material but unfortunately with view exceptions like

• Periclase MgO

• Fluorite CaF2,
• Sellait MgF2)
• Magnesium Diboride MgB2 (superconductor)
• Calcium Hexaboride CaB6 (barely dissolvable but irritating)

Simple compounds of those elements make no good building materials. They tend to dissolve in water (that was the reason why they where in there to begin with) and are brittle. More complex compounds are often more water stable but also rather weak.