Crumbling walls and sandy mortar is usually caused two ways. In the old clay tile blocks of the old homes built before about 1930, a lime based putty mortar was used. Today, Portland cement, which has a stronger binding agent, is used. In the old lime based mortar water or acid leaches the lime out and eventually the mortar must be replaced. This mortar starts to break down after forty or fifty years (based on conditions of moisture, temperature, humidity, etc.). It will become soft and “sandy” and break down to light pressure.
To test the integrity of your mortar joints, poke them with an ice pick. If the mortar seems sandy and falls out easily, the joints have deteriorated to the point where you have to do something. Moisture will continue to work its way into the brick or block and if you don’t correct the problem will eventually cause more deterioration.
If there is water coming through above ground walls or basement walls, once the source of the water has been eliminated, the mortar joints can be repointed, covered with stucco, or touched up and painted. The least expensive repair is to repoint. Repointing is highly labor intensive. Remove any material that crumbles easily, don’t hammer hard enough to crack the brick. Clean out the joints with a hose. Put the mortar on a flat board. Fill the joints with a small trowel. After the mortar has stiffened slightly, run the trowel or a shaping tool (a jointer) along the new joints to make their profile match that of the old ones.
If concrete blocks are laid and cemented when it is much below freezing, they may not cure properly, as the water which is in the cement will not be able to fully evaporate through capillary action at the surface because the capillary channels froze first at the surface preventing it’s movement, and the remainder will freeze in place preventing the cement from attaining proper chemical bonding and thus reducing its strength. This will result in diminished strength for the wall. Occasionally basement walls will cave in when the earth is replaced against them, as construction, in cold weather, is completed. This is because the bonding took place during freezing temperatures. The problem may not show up immediately upon completion of construction and will therefore shift to the homeowner sometimes years down the road with a crumbling wall. You may want to see http://www.basementwaterproofinginfo.com for a more detailed explanation on various types of mortar problems and possible solutions.