What is the reason of 'stone-cancer'?

'Stone cancer' refers to the degradation and discoloration of monuments built from marble and limestone (such as the Taj Mahal). This is caused primarily by acid rain containing sulfuric and nitric acids, which react chemically with calcium carbonate:
$${\text{CaCO}}_3+{\text{H}}_2{\text{SO}}_4\to {\text{CaSO}}_4+{\text{H}}_2\text{O}+{\text{CO}}_2$$