Hydrophobic and hydrophilic SiO₂-based hybrids in the protection of sandstone for anti-salt damage

Highlights

• Anti-salt damage of Dafosi Grotto sandstone protected by hydrophobic and hydrophilic SiO₂-based hybrids are evaluated.

• Hydrophobic SiO₂-based hybrid is found nearly no salt-damage.

• Hydrophilic SiO₂-based hybrid induce exterior-to-interior salt-damage behavior.

• The hydrophobic-protective effect is also confirmed by hydrophobic POSS-based hybrid.

• SiO₂-based hybrid is much better protection than POSS-based hybrid.

Abstract

The anti-salt damage of sandstone protected by hydrophobic and hydrophilic SiO₂-based hybrids is evaluated in NaCl, Na₂SO₄ and NaCl-Na₂SO₄ salt-loaded hydrothermal aging (SLHA) cycles. Although both hydrophobic and hydrophilic SiO₂-based hybrids could prevent the sandstone from salt damage through improving the matrix strength, the hydrophobic hybrid performs much better protection than hydrophilic one. The sandstone protected by hydrophobic SiO₂-based hybrid shows nearly no salt-damage, which is attributed to its excellent water repellence, high adhesive strength and good compatibility with sandstone matrix. However, the hydrophilic SiO₂-based hybrid tends to induce an exterior-to-interior salt-damage behaviour due to the frequent circulation movement of water with salt to result in the formation of surface efflorescence and interior sub-efflorescence in the protected sandstone. Furthermore, the hydrophobic-protective effect is also confirmed by another alternative hydrophobic POSS-based hybrid to offer stronger protection in anti-salt damage than that protected by hydrophilic hybrid. Nevertheless, there lies the difference, the hydrophobic SiO₂-based hybrid penetrates into the inner pores of the sandstone and develops a strong cohesion with sand-grain through the formation of Si-O bonds, but the hydrophobic POSS-based hybrid protects the sandstone with a weaker physical interaction between hybrid and sand-grains resulting in a fractured damage. Therefore, SiO₂-based hybrid is superior to POSS-based hybrid in promoting the anti-salt ability of sandstone. It is believed that these results could contribute much to the future protection of stone monuments by different hybrids.

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.