The first section of rust yellow
Rust yellow is a common issue that occurs on stone surfaces, often due to the presence of iron-bearing minerals or improper handling during installation. It can be categorized into three main causes: natural oxidation of unstable iron minerals within the stone, contamination from improper processing techniques, and pollution from metal fixtures used during installation. Let’s explore each in detail.
Original rust yellow arises from the presence of iron-containing minerals such as iron carbonate, iron sulfide, and ferrite within the stone. These minerals are more stable when they are in the form of ferric oxide, but iron sulfide is particularly unstable and prone to oxidation. Over time, this oxidation leads to the formation of rust-like stains on the surface. Additionally, biotite, though more stable than iron sulfide, can also oxidize under prolonged exposure to water and acid, leading to similar discoloration. As the rust forms, it may expand and cause flaking or small pits on the stone’s surface.
Rust yellow caused by improper stone processing often happens during cutting or polishing. For example, if steel sand remains on the surface after using a wire saw, it can react with moisture and air over time, causing rust. Similarly, if acidic cleaners are used during the polishing process, they can accelerate the oxidation of iron-bearing minerals.
Stone hanging accessories can also contribute to rust yellow. If the metal fasteners used during installation rust, they can transfer their oxidation to the stone surface. To prevent this, it’s important to use high-quality, corrosion-resistant materials and ensure proper sealing of joints.
Preventive measures include maintaining the stone before installation to prevent water penetration, avoiding acidic cleaning products, selecting stainless steel hangers, and applying sealants after installation to protect against moisture and pollutants.
The second section: Baihua (White Efflorescence)
Baihua, commonly known as white efflorescence, appears as a white powdery substance on stone surfaces or in joints. It typically occurs in areas exposed to moisture, such as outdoor steps, wall joints, or flower beds. This phenomenon is caused by alkaline substances like calcium hydroxide in cement mortar dissolving and migrating to the surface. When the water evaporates, these substances crystallize, forming a white residue.
There are two types of Baihua: primary and secondary. Primary Baihua occurs when excess water from the cement mix dissolves the alkaline compounds directly. Secondary Baihua develops when rainwater or groundwater leaches the alkaline materials from the surrounding area and deposits them on the stone.
To prevent Baihua, it’s essential to properly seal the stone before installation, use high-quality joint materials, reduce the water-cement ratio in mortar, and avoid using mineral-rich water for mixing. Applying waterproofing agents can also help prevent the migration of alkalis and reduce the likelihood of white spots.
The third section: Water Stains
Water stains refer to dull, wet-looking patches on the surface of the stone. They can occur due to water absorption through the porous structure of the stone. In many cases, this is temporary and will dry out naturally. However, if the stain is caused by improper construction methods—such as excessive water in cement mortar or poor curing—it can become permanent.
When cement contains unreacted calcium oxide, it can dissolve in water and migrate to the stone surface. There, it reacts with carbon dioxide in the air to form calcium carbonate, which retains moisture and creates a dull, wet appearance. This type of damage can affect the stone’s aesthetics and even lead to structural issues over time.
To prevent water stains, it’s crucial to maintain the stone with proper sealants, control the water content in cement mixes, and use high-quality curing agents. Applying waterproof coatings on both the front and back of the stone can significantly reduce the risk of moisture penetration and subsequent staining.
The fourth section: Staining
Stones are highly porous and susceptible to staining from various sources, including tea, coffee, ink, oil, and even construction residues like cement dust or adhesive. These substances can penetrate deep into the stone if not cleaned promptly, making removal difficult.
For example, silicone-based sealants used in joints can sometimes leave behind dark, banded stains along the seams, especially if they are not fully cured. This is due to the low molecular weight components of the silicone sealer seeping into the stone.
To avoid staining, it’s important to apply a high-quality sealer before installation, especially in high-traffic or high-pollution areas. During installation, care should be taken to avoid contact with contaminants like betel nut juice or oil. If staining does occur, immediate cleaning is essential to prevent the stain from setting in.
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