clear silicone will make a mess. Check with your local Hardware store, there are products made especially for this kind of repair. Masonry patch and repair, it's in a caulking tube. If the cracks are large though, you will have to tuck point the cracks. Tuck pointing is filling the cracks with mortar mix. Tuck pointing is usually not necessary unless the mortar is actually falling out or has been weathered so badly that erosion has washed most of the mortar out of the joints.
Tom in KY, the repair stuff comes in a few different colors.