Immovable silicate cultural relics, such as grottoes, murals and earthen sites, are very important members of historical and cultural heritages [1–5]. They record the traces of human historic activities and are of major value to scientific research and cultural education [6]. However, due to factors such as natural erosion and environmental changes, the current conservation state of these silicate cultural relics is not optimistic. Salt efflorescence are the most widespread diseases in grottoes, murals and earthen sites, which are directly or indirectly associated with the transport of soluble salts [7–9]. Damages occur when soluble salts are transported to the surface of silicate cultural relics for crystallization by capillary action and diffusion [10]. Two of the most abundant and destructive salts found in silicate cultural relics are sodium sulfate and sodium chloride, and the destructive effects of salt efflorescence are mainly attributed to the stress generated during salt crystallization [11]. If proper actions are not employed timely, soluble salts will continuously migrate upward with groundwater. Coupled with changes in temperature and humidity of the external environment, the salt solution residing in the pores of cultural relics will constantly undergo crystallization-dissolution process, ultimately resulting in the destruction of cultural relics [12–15].
In recent years, there have been many studies for the prevention and protection of salt efflorescence. Crystallization modifiers (inhibitors or promoters) are one of the approaches, which aims to reduce the crystallization pressure or to promote salt crystallization occurring at the surface rather than in the pores of the cultural relics [16–18]. Ferrocyanide, citrate, organophosphorus compounds and borax are the most studied crystallization modifiers in the field of heritage conservation [19–24]. However, the method is not aways feasible and effective. Crystallization modifiers are typically effective only for specific types of salts, and environmental problems associated with modifiers are inconclusive [25, 26]. In addition, the crystallization modifiers will lead to a significant increase in concentration of soluble salts in silicate cultural relics, which arouses serious concerns.
Several studies have shown that blocking the pathway of salt solution migration from the interior of relics to the surface layers, such as constructing a so-called water-salt barrier in between, can an effective solution to control salt efflorescence in heritage [27–30]. Similar technique is currently applied in the prevention of salinization in agricultural land [31, 32]. The principle of the technology is by reducing the suction of the matrix in the soil water transport layer. Generally, the larger the particle size used, the more durable its water-salt barrier effect [33]. This technique has been demonstrated to be effective to some extent, and it has the potential to destroy silicate cultural relics.
In immovable heritages, it is often observed that salt efflorescence is not homogeneously distributed. Some positions always show worse or much more obvious salt efflorescence than other positions. Paper pulp can be applied on the salty spots to remove some salt [34]. This is a quite common conservation practice used by many conservators. The inhomogeneous salty distribution is obviously due to the inhomogeneous water absorbing ability at different spots. The question is if we can control the water absorbing ability of different spot in cultural heritages, can we control the spots where salt efflorescence occurs?
In this work, a novel method and technique for salt efflorescence prevention and protection in silicate cultural relics using high water-absorbing materials (HWAMs) is proposed. It is primarily based on the assumption that more water and salt will transport to the spots containing HWAMs, where eventually salt efflorescence occurs. Two HWAMs, one organic polymer and one inorganic silicate material with significantly different water absorbing ability, are applied. The effect of two HWAMs in directional induction of water and salt migration in sand are investigated, and the feasibility of this method for salt efflorescence prevention and protection in silicate cultural relics is preliminarily evaluated.