The lunar soil is 40% oxygen. Lunar cement could be made to block off and enclose a lava tube. This would make it hundreds to thousands of times easier to make safe and large habitats on the moon.
Lava tubes could be enclosed and pressurized to support facilities such as greenhouses and maintenance enclosures. Astronauts could also use inflatable tents inside of lava tubes and caves while being protected from temperatures, radiation and meteorites.
They are working to simulate Moon and Mars lava tube habitats in a lava tube in Iceland.