The meme comes also from the fact that Fern is fast, precise and strong enough that she only needs basic spells to beat almost anyone.
So when given a choice, she didn't need anything for combat.
And writing this, you have to remember that Fern isn't a combat oriented mage of past generations

For the concrete related part of the discussion, my guess is that the reference was to Roman era concrete that still functions properly in some case 2000 years later. A self healing and flexible yet strong concrete that works well in a wide range of temperatures gradients and also from water/salt water to dry places.