foundation mortar turning to sand!

My house is 100 + years old and when I bought it 4 months ago the inspector said it was very strong and solid. I am having some work done to correct a sinking kitchen floor and the contractor discovered a problem with the foundation bricks in a few places in the basement under the kitchen. The mortar between some of the bricks is turning to sand and spilling out! Is there a way to stop this progression and repair it myself?... some kind of compound that I can "squeeze in" between the bricks, something akin to that wood stuff you can squeeze into rotting wood to firm it up again? So far everything is still very solid, all the bricks are firmly in place, but there is clearly quite a bit of missing mortar between some of them. Has anyone encountered this, or does anyone know a solution? Thank you!