Experimental research on hygroscopic behaviour of porous specimens contaminated with salts

摘要

Salt crystallisation is one of the main causes of masonry decay in monuments. The salt content in a wall is a very important parameter that should be known in order to give a reliable diagnosis on the causes of damage; if a restoration work and a surface treatment are necessary, the salt content has to be known. In fact the presence of salts in the wall can not only reduce the efficiency of a treatment but also form a potential risk for the occurrence of a damaging salt crystallisation process. Different methods and instruments are available to detect salt content and type of salt present, but most of them are quite expensive and time consuming. For this reason, other methods, cheaper and quicker, are preferably adopted. In building practice often the hygroscopic moisture content of powder samples is measured in order to have an indication of the salt content. This method is cheap and simple and, in case of material contaminated with a single salt, gives reliable results on the quantity of salts present because of the linear relation between HMC and salt content. However, in case a mix of salts is involved, as usual in reality, the relation between the HMC and the salt content is not clear yet and the HMC measurement can give only an indication and not a quantitative value of the salt content. This paper presents the experimental results of a research carried out on the hygroscopic behaviour of sodium salts introduced in a typical clay-brick either as single salts or as a mix. The obtained results point out a clear linear relation between salt content and hygroscopic moisture content. This proportionality is verified for (brick) specimens contaminated with pure salts as well as with salt mixtures. Salt mixtures appeared to correspond with hygroscopic moisture contents that are higher than the one calculated according to the amount of single salts present in the mixture. As expected beforehand no difference was found between the hygroscopic behaviour of solid brick specimen and brick powder, only a longer time to reach equilibrium in case of brick powder. This paper suggests in the case of the presence of single salts to use the HMC measurement instead of more complicate and expensive analyses (as for example in laboratory experiments). In building practice, the measurement of the HMC can help in selecting the most interesting spots on which to focus the research avoiding extensive campaigns of sampling and analyses. Besides, HMC measurements performed at different RH, can give suggestions about the type of salt present.