The walls of my Basement are Rock with mortar fillings. The mortar is letting go, and is like sand. What can I use to seal this and fill the wall with mortar. I tried to fill the gaps with hyd. Cement, but it falls out due to the sand, it sifts constantly. Redoing the basement with blocks is not an option. Need help.

You need to etch with muratic acic (dilute hydrochloric) or a powder that you mix with water from Drylok otherwise NOTHING will stick. You may also have funky mold like stuff called effervesence and the above will remove that too.

Use adequate ventillation, gloves and goggles.

You can repair the walls with Rockite if you have holes. It's an expansion cement.

Moisten the walls before painting. Drylok paint works nicely.

Washing the stone with water,like from a hose,so you get a little pressure will loosen the 'sandy' mortar from the openings.

Using concrete bonder mixed in with your new mortar will not only help in tooling it in(it makes the mortar stick to the trowels a little better) but it will adhere to the old stone/mortar quite well.

As KISS described, if you have white effervescence(lime) in the old mortar,using chemicals might be needed.Lime Away or CLR diluted to MFG. specs might be safer than harsher chemicals,especially indoors.

Scrubbing the joints with a wire brush while using the chemicals will remove the build-up, but it tends to burn holes in your clothes... wear old stuff,and layer it.

I apply the new mortar with a flat concrete trowel (a 2 1/2" X 12") and a flat mortar trowel. When the material begins to set, I wipe it with a large sponge(wet) to even out the surface,it also fills the imperfections that trowels leave.

Hope this helps,

KBC