Protective or damage promoting effect of calcium carbonate layers on the surface of cement based materials in aqueous environments

Cement based materials permanently exposed to aggressive aqueous environments are subject to chemical changes affecting their durability. However, this holds also for tap water that is considered to be not aggressive to cementitious materials, although in that case a formation of covering layers of CaCO3 on the alkaline surfaces is commonly supposed to provide protection against reactive transport processes. Thus, investigations of the structural and chemical properties of the material/water interface were carried out in laboratory experiments and case studies to elucidate the consequences of surface reactions for the durability of cement based materials exposed to tap water. Focused Ion Beam investigations revealed that a protective effect of a CaCO3 covering layer depends on its structural properties, which are in turn affected by the hydro-chemical conditions during crystallization. Surface precipitation of CaCO3 can trigger further chemical degradation, if the required calcium is supplied by the pore solution of the material.