Though this question is, well, questionable, posted in a cabinetmaker’s forum, beach sand is a subject I know a little about. It doesn’t work for mortar because all the particles are rounded over from being tumbled around instead of sharp. That makes for a mix that slumps easily when it is placed, and it doesn’t hold up over time. Sharper particles (#2 mason’s sand) hook onto one another and make a much stronger mix.

Outside of shoring up the walls by excavating and pouring concrete up against them – which some people have done – there’s really not much you can do at this point.

You can tuckpoint the failing joints with new mortar from time to time. But other than that, or shoring up with concrete from the outside as mentioned above, rebuilding the walls entirely is the only other option. On the other hand, you could just leave things as they are and see how long the walls stand on with no other work…

Zolton