This is the normal deterioration of the porous limestone from local quarries that have been used in our region. Water seeps into the porous stone face, expands when it freezes and pops the stone face off the block. This is called “spalling” and it is how Mother Nature erodes mountains, even when we build our dwellings out of these limestone mountains. The long term goal is to effectively seal the stone to minimize the water penetration into the blocks, which will lessen the deterioration over time.
The other issue is what to do with the pocked stone blocks with missing “chunks.” This is a tuck pointing “art,” similar to faux finishing paint that restores the appearance of the stone face. The tuck pointer mixes a sand & cement mortar mixture with the (very artful) appropriate color. This material is applied to the exterior of the stone to craft and model the stone face until it looks like the original stone block. This is a repair and not a restoration.
True restoration of the stone face is performed by actually replacing the stone block or epoxy gluing another stone face onto the front of the stone block so that the face of block is truly stone. The time, care and cost of historic restoration are substantially more than a repair.
The next secret is to effectively seal the stone and the sealing needs to be done about every 10 years. The sealing protects the stone by limiting the amount of water soaking into the block. Sealing is typically done by a professional because the cleaning is critical to not “blow off” more spalled chunks from the stone faces with a pressure washer and the proper cleaner. Also, it is important to select the appropriate sealing material according to the stone or brick and to apply it correctly. A great example of this problem, dialogue and solution is the work being considered for cleaning the St. Louis City Hall limestone façade.