Marble consists of the mineral calcite (CaCO3) with porosity usually below 0.5%, so it is not susceptible to damage from freezing or salt crystallization, but the stone is relatively soluble in acids. We are developing a method to deposit a thin layer of hydroxyapatite (HAP) on the surface of calcite, because HAP dissolves about 10,000 times more slowly than calcite. The idea is to expose calcite to an aqueous solution of diammonium hydrogen phosphate (DAP):
Calcite + DAP → HAP + volatiles
10CaCO3 + 5(NH4)2HPO4 → Ca10(PO4,CO3)6(OH,CO3)2 + 5(NH4)2CO3 + 3CO2 + 2H2O
The HAP may include some carbonate groups substituting for phosphate and hydroxyl groups, but the solubility and dissolution rate are still far less than for calcite. The additional products (ammonium carbonate, carbon dioxide, and water) are volatile, so there are no extra phases deposited on the stone.
The thesis project of Sonia Naidu is to prepare dense protective layers of HAP on marble, and she has now demonstrated significant enhancement in the acid resistance of the stone. In addition, the reaction has proved to be very effective as a consolidant for limestone, and as a strong binder for creating artificial stone from marble powder.
Marble dissolves in acid
This video shows how Carrara marble reacts to acid (at 1.0 N, which is ~10000 times more aggressive than the worst acid rain!). Both hydrochloric (HCl) and nitric (HNO3) acids react with marble (CaCO3) and release CO2. You can see it bubbling through the acid as it runs down the stone.
In contrast, sulfuric acid reacts with marble to form gypsum (CaSO4), which forms a protective barrier on the surface of the stone, so that no visible damage occurs. Unfortunately, gypsum is also slightly soluble, so it doesn’t really protect monuments in the long run. Nevertheless, marble is less rapidly attacked by acid rain containing SO2 than NOx.