ملات های مقتدر برای نمک زدایی سطح Sacrificial mortars for surface desalination

1. Introduction One of the most prominent causes of built heritage decay is the presence of salts in the materials [1,2]. Such salts may crystallise on the surface of (saline efflorescence) or inside (sub- and crypto-efflorescence) materials, to the detriment of the value of the heritage asset. The effect of the former is essentially aesthetic, whereas the latter affects dimensional stability. When the crystallisation pressure exerted by a salt exceeds the mechanical strength of a material, the latter cracks, scales, flakes or crumbles. Salts also hamper asset restoration and conservation, for they limit the effectiveness of consolidation and water-repellency treatments [3]. Hence the importance of establishing their presence in buildings, determining their origin, assessing their implications for decay and defining the most suitable elimination technique. Such techniques depend on the location of the salts in the walls, their chemical and mineralogical composition and the microclimate prevailing in the area surrounding the materials affected [4]. The petrophysical properties of the material housing salts must also be established, for factors such as porosity and pore size distribution affect salt concentration [5–8]. Some salts undergo mineralogical transformation when the relative humidity (RH) and temperature (T) of the surrounds vary. That, in turn, often induces volume change and the concomitant damage to the material [9– 12]. Prior to any intervention, the superficial salts must be eliminated from the areas involved and their presence inside the walls must be lowered with the techniques presently available [13]. The origin of salts varies depending on their location and determines the most suitable desalinating technique. If the salts are found in depth, the origin may be attributed to human activity, certain types of mortars, underground water or the material itself. The desalination methods most commonly used in such cases are poultices, sacrificial mortars and immersion or electrochemical techniques. If the salts are located near or on the surface, their source may be, in addition to the above, air pollution or microorganisms; in such cases they are eliminated primarily with mechanical, chemical or laser methods. Despite the variety of methods in place to extract soluble salts [14–17], they are often scantly effective or inviable. None serves for all manner of materials, the results are questionable [18] and none is able to wholly eliminate salts deep within walls [19]. Very few studies have been conducted on the effectiveness of salt reduction in situ due primarily to the difficulty of assessing the findings, for the samples that would be needed cannot always be collected for analysis in light of the heritage value of the element to be conserved. To assess salt extraction efficiency, the pre- and posttreatment salt concentration must be determined. Efficacy is not the same on the substrate surface as at greater depths [19]. The environmental conditions (RH, T) that prevent salt mobility must also be established and the source of the salt eliminated, for otherwise it will re-impregnate the area to be protected [20].