Heather asked: "I have a 75 year old brick semi. I've noticed that on one of the pillars there's white powder. I'm wondering what it is and how to prevent it?"
I assume that her pillars are on her front porch. We took some brick and soaked it in salt water. When it dried, the salt was all over the outside. We sat a second brick in some salt water and you can see how the white line forms about an inch or so above the water line.
The white power that Heather sees is called Efflorescence, and in fact it is just salt. There is salt naturally in all mortar and concrete. When the masonry gets saturated with excess water, that salt dissolves and moves to the evaporation point with the water. Then the water evaporates off, leaving the salt behind. It can actually be cleaned off easily with muriatic acid and a bristle brush. How can you prevent it?
The key is to not allow water to saturate the bottom of the brick pillar. The white ring on the bottom of pillars is usually associated with a bad slope to the porch and having water puddle at the bottom of the pillar. If you can find a way to drain the water away, even cutting a small gully in the concrete porch, the problem shouldn't come back.