Mortar mix crumbles like dry dirt

How old was that mortar mix and how was it stored? If moisture was allowed to get near it in storage, that is likely to cause it to fail.

Since you "didn't want to make an extra trip to Home Depot," can we safely assume that you used a bag of mortar mix you already had at home? An open bag?

I have lots of stuff laying around in storage so its properly old but it was a full bag. The mixture was nice and loose so it didn't seem like much moisture gotten in but don't know for sure.

That'd be your problem -- old mortar. It doesn't last long, and it doesn't take much moisture to ruin it.

Old material, "struck" in the bag. Still looks dry and powdery in the bag, but the particles of portland cement have lost their chemical activity from slow moisture intrusion over time.

If you want to use old cement products, test them first before committing costly labor.

The mix may have also dried out on the wall before it was completely cured.

Don Young

Even brand new type N mortar is not as strong as type S. It is only intended to bond bricks together and not to stand on it's own. Type S has a lot more portland in it and sets up harder.

replying to gfretwell, Nancy wrote: Yes, I think that's what I need.