Efflorescence of Concrete

There are traditional prejudices against the use of slag cement or Portland cement with fly ash addition in reiirforced concretes, however, they have not been proved experimentally [311, 352], On the contrary, a quite different behaviour of these materials was reported presumably this can be the consequence of reduced permeability of concrete and the resulting lower diffusion (see Table 6.9). However, apphcation of higher content and higher class cement has a positive influence on the dnrabihty of reinforcement. 

The efflorescence on the surface of concrete have no practical importance from the durability point of view however, they affect badly the appearance of external decorative fagades of concrete buildings and other structures. They are particularly noticeable when the colored or white cement plasters are employed. From the aesthetic reasons the remedies for minimizing this phenomenon are needed. In the case of concrete pavers, very often colored, the efflorescence are particnlarly undesired. The producers of pavers are famiUar with this problem and look for the new remedial treatments to avoid it. 

The efflorescence is not a phenomenon noticeable only in the case of concrete or mortar, it means occnrring in cement paste. It is also known in the case of ceramics, especially bricks and roof tiles, which are not bonded with cement mortar. Efflorescence relates generally to the porous materials in which the transport of mass, primarily of liquids, can occur. The presence of compounds easily water soluble in this porous material is a factor promoting this process. The mechanism of efflores- 

On the surface of concrete, finishing plaster or the other building materials the processes generated by the impact of gaseous air components or rain component occirr. Apart from the corrosion phenomena due to the acid rains, the carbonation of calcimn hydroxide influenced by CO2, is of highest importance. This process refers especially to concrete, mortar and cement plasters. 

Transport of liquid from the internal part of concrete to the surface is necessary to initiate efflorescence and hence the capillary phenomena play a decisive role. Diffusion of ions from dissolved compotmds can also occtrr through the absorption of external water, for example from rain. In concrete being in contact with water the so-called capillary action occtrrs it means water penetration to mesopores, tmder the action of water surface tension. This question is discttssed in Chap. 5. 

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