Mobilization and loss of elements from roofing tiles

Deposition, leaching and chemical transformation are processes that affect roofing tile and roof runoff water. Leaching experiments, with artificial rainwater in the laboratory, showed the presence of Na+, K+, Mg2+, Ca2+, Cl-, NO3-, SO42-, with a ratio of Ca2+ and SO42- suggesting gypsum dissolution. X-ray fluorescence (XRF) of the exposed roof tile showed depletion such as Mg, Al, Si, P, Ti and K at the surface of the tile and an enrichment of Fe and Mn which hinted at a process akin to laterite formation. However, calcium appeared to be enriched at the surface as gypsum (confirmed by X-ray diffraction) and to a lesser extent calcite, which is characteristic of deposits on building surfaces in cities.