Original articleHydrophobic and hydrophilic SiO2-based hybrids in the protection of sandstone for anti-salt damage
Graphical abstract
Introduction
Salt damage of stone monuments is considered as the most serious rock breakdown process under various environments [1], [2], [3]. It has been proved that this damage is closely related to water movement and salt transport/re-crystallization, which depends on specific microclimates [4], [5], [6]. Salts crystallization from a supersaturated solution and crystals growth within the porous materials are undoubtedly capably to produce local pressure that may increase the empty spaces between sand grains [4], [5], and finally to cause the damage of salt-loaded stone monuments outdoor [6], [7]. Therefore, in recent years, much attention has been paid to the protection of stone monuments against salt damage by using functional materials [8], [9], [10], [11], [12]. However, before protecting the stone monuments by functional materials, it is required to understand the damage behavior caused by water and salts.
Actually, many researchers have proved that most of the damaging salts are highly soluble one, due to them easily transportable through the porous material with water movement [2], [3], [4], [5], companied with dissolving and crystallizing with variations in the relative humidity, such as NaCl and Na2SO4 [13], [14]. Normally, NaCl grows favorably into the isometric crystals until the continuous granular crust (named as efflorescence) is formed on stone surface. But with the supply of NaCl solution and the repeating dissolution and crystallization cycles, the formation of crystals may also occur within the pores even if initially microclimate conditions favors efflorescence of sub-florescence [15], [16]. When the crystals extensively fill the stone-grain pores, the pressure exerted by crystallization against the pore walls can lead to stone disruption [7], [17], [18]. Another very important salt is Na2SO4, which is regarded as the most damaging salt in many cases [19], [20], [21] due to its strong response to temperature and relative humidity (RH). The formation of cumulate crystals of thenardite (Na2SO4), mirabilite (Na2SO4·10H2O) and their transition to each other under different conditions are believed to be the main source of damage [22], [23], [24], [25]. Mirabilite (Na2SO4·10H2O) and thenardite (Na2SO4) would precipitate as both efflorescence on the stone surface and sub-efflorescence inside sandstone depending on the microclimate in the pore structure of stones [14], [24].
On the other hand, the pore structure of the stone also plays an important role. Indeed, it not only affects the mechanical strength and therefore affects the ability to resist crystallization pressure of salt, but also determines the rates of water and vapor transport as well as the total porosity in which salts may accumulate [15]. Among the differently structured stones, the porous sandstones are generally considered to be the most susceptible to decay [4], [6], [26], because their inner porous structure is beneficial for water circulation and salt crystals growth. This will facilitate the repeated occurrence of evaporation-crystallization [17], [18], deliquescence-crystallization and hydration-dehydration [24], all of which aggravate the salt damage on stones. However, this kind of porous-structured nature of sandstone could also provide with the possibility of absorbing functional materials for protective treatments aimed at enhancing its resistance to salt damage. Therefore, the protective treatment on decayed sandstone by functional materials is recognized as the effective method to against the salt damage.
Unfortunately, up to now, it is still a challenge to select the feasible and reliable functional materials for the protection of sandstone. Considering SiO2 is the main component of sandstone, the SiO2-based protective material should gain good compatibility between protective material and sandstones [12], [13], [14]. But the SiO2-based protective material is normally produced as inorganic-organic hybrids [10], [11], [12] to give the special function and improve the adhesive strength between protective agent and stone grains. In this case, it has not demonstrated whether the hydrophobic SiO2-based hybrid or the hydrophilic SiO2-based hybrid can improve the protection of sandstone from salt damage. In order to answer this question, our lab has prepared the hydrophobic and hydrophilic SiO2-based hybrid for the protection of sandstone [27], [28]. The hydrophobic SiO2-based hybrid SiO2-g-PMMA-b-PDFHM is obtained by silica surface-initiating atom transfer radical polymerization of methylmethacrylate (MMA) and dodecafluoroheptyl methacrylate (PDFHM) [27], and the hydrophilic SiO2-based hybrid SiO2-g-O(Me2Si)nOH is obtained by mixing the sol of tetraethoxysilane (TEOS) and the hydrolyzed dimethoxydimethylsilane (Me2Si(OMe)2). To further understand the contribution of hydrophobic effect on the anti-salt damage, the updated type of nano SiO2 (1–3 nm), denoted as polyhedral oligomeric silsesquioxane (POSS) initiated hybrid ap-POSS-PMMA-b-PDFHM [28] is also used for protection and evaluation.
This study explores the salt-damage behavior on the porous sandstone from Dafosi Grotto in China (a famous cultural heritage named by UNESCO), which is composed of quartz, feldspar and rock fragment). After the stone samples are protected by hydrophobic SiO2-based hybrid of SiO2-g-PMMA-b-PDFHM and hydrophilic SiO2-based hybrid SiO2-g-O(Me2Si)nOH, then they are evaluated during NaCl, Na2SO4 and NaCl-Na2SO4 salt-loaded hydrothermal aging (SLHA) cycles. A POSS-based hybrid ap-POSS-PMMA-b-PDFHM is used for further evidence of the hydrophobic effect. The anti-salt damage behaviors on sandstones protected by hydrophobic or hydrophilic hybrids are evaluated by the micrograph of salt in capillary tube, aggregated micelles of protective solution, static contact angle, adhesive strength, water vapor diffusivity, water absorption, pore size distribution, ultrasonic hardness, alteration index (AI) and alteration velocity (AV), and QCM-D measurement for surface water absorption. It is believed that these results could contribute much to the future protection of stone monuments.
Section snippets
Research aims
There are three main aims in this research. The first aim is to understand the anti-salt damage behaviors of sandstone protected by hydrophobic and hydrophilic SiO2-based hybrids in NaCl, Na2SO4 and NaCl-Na2SO4 salt-loaded hygrothermal aging cycles through the salts natures and the properties of protective materials. The second aim is to evaluate the effect of hydrophobic and hydrophilic SiO2-based hybrids on anti-salt damage of sandstone by finding an exterior-to-interior salt-damage behavior
Materials
The hydrophobic and hydrophilic hybrids prepared in our lab for the protection of ancient sandstone were used in this research [27], [28]. Their chemical structure and wettability are shown in Fig. 1.
Hydrophobic SiO2-based hybrid: SiO2-g-PMMA-b-PDFHM (S1) with Mw of (11700)n g/mol and > 98% purity was synthesized by silica surface-initiated polymerization of methylmethacrylate (MMA) and dodecafluoroheptyl methacrylate (PDFHM, C11H8O2F12) in sequence as the mass ratios of SiO2-Br/MMA/12FMA =
Effect of salt nature on salt-damage of un-protected sandstone
Although much attention has been given to the investigation of stone damage caused by both NaCl and Na2SO4 salts [19], [20], [21], up to now, the influence of salt nature on surface and inner of sandstone decay is still unclear. Here, the different responses of un-protected sandstone to NaCl, Na2SO4 and NaCl-Na2SO4 SLHA cycles are compared to explain the effect of salt nature on sandstone decay. The salt-damage appearance and mass-loss during SLHA cycles are showed in Fig. 4. It reveals that
Conclusion
The resistance to salt damage of Dafosi Grotto sandstones was evaluated after the application of hydrophobic or hydrophilic SiO2-based hybrids. These treatments were applied by submersing the samples in the solution, thus leading to a full treatment of their external surfaces. The salt resistance tests involved submersion in salt solutions, so they were strongly influenced by the ability of these treatments to prevent the ingress of salt solutions.
The outcome of these tests was further
Acknowledgements
This work has been financially supported by the National Natural Science Foundation of China (NSFC Grants 51873173, 51573145, 51373133), and the National Basic Research Program of China (973 Program, No. 2012CB720904). The authors also wish to express their gratitude for the MOE Key Laboratory for Non-equilibrium Condensed Matter and Quantum Engineering of Xi’an Jiaotong University.
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