Limesone is a calcium carbonate and when rainwater mixes with carbon dioxide in the atmosphere it becomes weak carbonic acid.  Calcium carbonate is soluble in this acid and thus causing the process of carbonation. 

As limsetone is permeable due to its mainy joints, water is able to enter the joints, cauing it to get bigger as limestone solution takes place and the dissolved limestone is removed. This results in larger gaps in between the limestone blocks.