Soluble salts and efflorescence in structural clay products: a scheme to predict the risk of efflorescence

The correlations between phase composition of efflorescence (XRD) and chemical composition of soluble salts (ICP-OES) were studied over a wide range of Italian structural clay products. The most frequent types of efflorescence are those with calcium carbonate, followed by those with calcium sulphates or an association of Ca carbonate + Ca sulphates. Minor diffusion have the efflorescence with alkaline sulphates or magnesian compounds. The risk thresholds, expressed as the maximum quantity of soluble element beyond which a medium/strong degree of efflorescence may occur, were defined for each type of efflorescent salts. These thresholds correspond to the limits of sodium and potassium established by the British and German standards as well as the European draft standard. However, it would seem appropriate to lower the limit of acceptability for magnesium and introduce a limit also for calcium.