WEATHERING RATES OF MARBLE IN LABORATORY AND OUTDOOR CONDITIONS

In the modern urban atmosphere SO2 and NO2 attack calcite (CaCO3) in m arble exposed at rain-sheltered surfaces creating largely gypsum (CaSO 4 . 2H(2)O) crusts that eventually exfoliate. In combination with CO2 these gases erode the marble at unsheltered surfaces. We report the de velopment of mathematical models to predict the rate of growth of crus t and the rate of surface recession. To determine the rate of growth o f crust the kinetic rate constant, diffusion rate, and the order of re action were determined by the application of the shrinking-core model applied to data generated in laboratory experiments. Based on these pa rameters and average ambient levels of 10 parts per billion (ppb) SO2 and 25 ppb NO2 in Louisville, Ky., the rate of crust formation for thi s metro area was calculated to be 1.8 mu m in the first year. However, the rate of recession was modeled from data obtained by exposing marb le slabs to rainfalls. A surface recession of 15 mu m/yr was calculate d. The models predicted well the rate of growth of crust observed at s everal sites in Louisville and the predicted surface recession compare d well with values reported in the literature.