Sandstone been one off the softest stones made up of sedimentary rock and quartz. Pore space been large allows for more water to flow in its natural state.
Laying sandstone slabs with a traditional bed of mortar slows the permeability down and capturing water in the stone. This water begins to incourage algae, lichens, liverworts and moss to start to grow.
Sealing can be categorized into two main groups coatings or impregnators.
Coatings create a barrier on top of the stone and impregnators soaking into the grain.