WHITE

MARBLE

Yellowing of White StoneWhat is the reason?How to prevent it?

Among numerous marbles, white marble is undoubtedly one of the most widely used stones, and the decorative effect is unparalleled. The gentle color tone, snowy white, pure and beautiful like snowflakes, allows many designers to fully utilize and showcase.
But at the same time, it is also the most beloved and feared variety. However, the affectation, fragility, and difficulty in serving white marble in terms of anti fouling have made some users regret it, especially the fact that white marble is prone to yellowing, which has made many users who love white marble stay away.

To solve quality problems such as white and yellow hair, we need to analyze internal and external factors, thoroughly understand the properties of marble, and the root causes of quality problems, in order to solve the fundamental problems.
So what is the reason for the yellowing of white marble? How can white marble maintain its original appearance and effect? I’ll give you some advice!

PART-01
The structure and composition of stone itself

The Scanning electron microscope study on the surface of the most common white marble found that there were iron ores with particle sizes ranging from 2 μ m to several hundred μ m on the surface and texture of white marble, some distributed on the surface of the board, and some concentrated in the texture.
The oxidation of iron minerals contained in white marble is believed to be the main reason for the yellowing of marble. Due to the fact that most marble, except for the textured parts, has a relatively uniform pattern, when randomly dispersed iron minerals in the marble are oxidized to form yellow or brown hydrated iron oxides, defects and aesthetic stains on the board surface are formed, which is particularly prominent for white marble.
The iron in white marble mainly exists in Iron(II) sulfide, iron carbonate and magnesium iron silicate, and these iron minerals are dispersed in the marble matrix or enriched in the marble veins.
When these bivalent iron ores on the stone surface are oxidized to ferric iron by oxygen in the air and combined with water to form hydrated Iron(III) oxide-hydroxide, rust, commonly known as yellowing, is formed. If no protective measures are taken, this phenomenon of marble yellowing will occur within 8-12 months after marble placement.

An interesting phenomenon is that when placed in a heat source, such as marble next to a hot water pipe, this yellowing phenomenon does not occur, indicating that the yellowing phenomenon of white marble is closely related to water content. However, contact with water does not necessarily lead to yellowing of white marble.
The Thermal shock test on white marble shows that the stone sample is immersed in 15-25 ℃ deionized water for 6h, and then the sample block is baked in an oven at 100-110 ℃ for 18h. After 20 such cycles, it was found that only the iron rich sample on the surface showed a slight yellowing phenomenon, and the degree of yellowing was much lower than the natural yellowing phenomenon.
This experimental phenomenon shows that due to the very low conductivity of deionized water, the Redox of ferric iron being oxidized to ferric iron proceeds very slowly. So, although the sample was immersed in water for a long time and subjected to multiple cycles, there was no obvious yellowing phenomenon. The results of this experiment indicate that the yellowing phenomenon of white marble is the result of the combined action of oxidation and hydration on the surface of divalent iron ore on the stone, and both conditions are indispensable. Unlike soaking in ionized water, soaking white marble in an alkaline solution can easily cause yellowing.
Soak the same stone sample in alkaline water at 15-25 ℃ for 6 hours, and then bake the sample in an oven at 50-60 ℃ for 18 hours. After 20 such cycles, the test found that all the white marble selected for the experiment had yellowing, and Carrara white marble was the most serious.
Alkaline solution tends to make white marble yellow, because in alkaline medium, the oxidation Reduction potential potential of ferric iron oxidized to ferric iron is reduced, and it is easy to be oxidized to ferric iron by oxygen in the air. The combination of ferric iron and water forms yellow pollution on the board surface.

The solubility of hydrated Iron(III) oxide-hydroxide formed by the oxidation of divalent iron on the stone surface to trivalent iron by oxygen in the air and its combination with water is extremely low. Therefore, the yellowing pollution caused by iron on the board surface is not easily removed. The second type of yellowing pollution on the white marble surface comes from the organic matter inside the marble, which exists in the formation process of the marble and has a structure similar to that of decaying plants.
These water-soluble organic compounds present inside marble often react with alkaline water in a brownish yellow color. Due to the presence of organic matter inside marble, marble floors or wall panels that have not undergone waterproofing treatment on the back of the panels during wet installation, the alkaline water of the cement slurry seeps through the marble and reacts with the organic matter inside the marble, generating colored products. When these colored products are carried by water to the stone surface, they form the second type of pollution on the stone surface.

Even during the use period after the installation of the stone, due to climate reasons, whenever water penetrates through the cement concrete and then seeps out of the stone surface, it will always bring colored organic pollutants to the stone surface.

PART-02
External reasons

The above two types of pollution in white marble come from the internal composition and structure of the stone, and occur under the influence of external conditions. In addition to these two types of pollution related to stone components and structures, external pollution also includes the following aspects:
1. Yellowing caused by wear and tear
As the polished surface of marble wears away, dirt is carried into the fine pores on the marble surface, causing the white marble surface to turn yellow.
2. Yellowing caused by incorrect care
The most common incorrect care is to wet mop the floor with a dirty mop. The dirt brought by dirty mops accumulates inside the marble, causing the surface of the marble to become yellow due to pollution.
3. Yellowing caused by waxing
When polishing marble surfaces with commonly used wax, the thick wax layer turns yellow due to its own aging.
4. Yellowing caused by crystal surface treatment
The steel wire wool used for crystal surface treatment remains in the micro pores of the polished marble board with tiny fine powder, becoming the root cause of yellowing, especially when the marble is treated with crystal surface in a damp state, yellowing is prone to occur.

5. Yellowing caused by installation process
The yellowing caused by installation is easily overlooked, but it is common.
Before installing marble floors or walls, although the so-called six sided waterproofing treatment is applied to the stone specification board: the bottom surface is waterproof with adhesive backing, and the four sides and front surface are treated with permeable waterproofing agent.
If this six sided waterproofing treatment is done very well, the quality of the selected waterproofing material is also guaranteed, and theoretically, there should be no further yellowing related to water. But the fact is that even the six sided waterproofing of the stone board surface before installation is considered to be well done, and the phenomenon of yellowing of the board surface after installation still occurs from time to time.
6. Installation related stone turning yellow
The phenomenon is mainly caused by two aspects. Firstly, due to the need for installation dimensions and on-site cutting, the original waterproof treatment on a certain side was damaged; Another possible scenario is that the construction environment is dirty and disorderly, and cement slurry falls onto the board surface during the construction process. To prevent the fallen cement slurry from drying up on the board surface, use a wet towel to wipe the fallen cement slurry. At this time, it is highly possible to apply strong alkaline cement water to some hidden cracks on the board surface, because even if the board surface has been treated with permeable waterproofing, But small waterproof agent molecules cannot form a protective film that spans the crack to shield and protect it.

So the coverage of waterproof agent at the crack is very limited. When installing, scraping back and forth with a gray knife to remove the side of the specification board with cement slurry can also damage the waterproof treatment already done. If cement water is accidentally brought into the cracks of the board surface when wiping it, then the yellowing of the board surface is a matter of time rather than whether it will turn yellow.

PART-03
Prevention of yellowing of white marble

To prevent quality issues such as yellowing of white marble, the following points should be taken into account during processing and installation to ensure that white marble perfectly presents the beauty of decoration.
Prevention during processing
1. Selection of raw materials: Check around 6 sides of the stone to see if there are any stones, cracks, yellow ribs, etc. to determine whether to choose;

2. Cutting of waste materials: The water source for cutting waste materials should be tap water as much as possible, and it is not advisable to use circulating water after sedimentation to prevent pollution; Remember to thoroughly rinse the large board with clean water after it comes out to prevent rust stains on machinery, equipment, and blades, and to eliminate all sources of pollution to prevent rust stains from occurring

3. Waterproof scraper for large board
4. Large board cutting: After cutting the stone, it must be dried again, usually for 48-72 hours; After drying, comprehensive oil protection should be applied again, and full immersion should be used for protection;
5. Board layout

6. Finished board packaging: White stone should not come into direct contact with wooden packaging frames (or iron packaging frames), and should be completely covered with white packaging film for isolation and protection to prevent pollution sources from entering the interior of the stone;
7. On site paving: All dry hanging parts must be made of 304 stainless steel and fixed with special AB adhesive. The cut and grooved parts of the dry hanging must be coated with oily protective fluid twice; After laying the stone, it should be dried and ventilated for protection, and should not come into contact with any chromosomes to prevent pollution;

8. Texture beautification effect processing
9. Stone polishing: After each polishing, it is necessary to air dry and brush with water-based protective fluid once, with a drying time of 6-8 hours; Key point 2: Use 0.2mm thick cutting thin sheets for stone seam opening, especially for repeatedly absorbing dust from the gaps;
10. Subsequent maintenance disclosure

Prevention during installation process
1. Due to the internal composition of marble
The yellowing caused by structure occurs through external conditions. This external condition is the oxygen in water and air. So, as long as water is effectively prevented from passing through the interior of the stone, the yellowing caused by the composition and structure of the stone will be effectively suppressed.
2. The effective way to prevent water from passing through the interior of the stone is to waterproof the back of the board before installation.
The early waterproofing treatment for the back of stone slabs was applied with siloxane/silane penetrating waterproofing agent. The fact shows that applying this waterproof agent to the back of the board cannot effectively suppress the occurrence of stone lesions, including yellowing. Permeable waterproofing agents have certain limits.
No matter how good the waterproof agent is, its waterproof performance will gradually decline due to the influence of the environment. This is why after the first application of waterproof agent, it is necessary to reapply it at regular intervals to repair the deterioration of its waterproof performance.
For this reason, permeable waterproofing agents are only applicable to boards that can be repainted at any time, such as the front of the floor or the wall facing outward.

When permeable waterproofing agents are used at the bottom of the board, the waterproofing function of the waterproofing agent cannot be monitored. Even if there are signs that the waterproof agent’s function has declined, we cannot repaint and repair it, and can only let it deteriorate and ultimately lose its waterproof function completely. This is why many projects still use this floor installation process to avoid the main cause of stone damage.
As a result, a kind of waterproof back adhesive for stone materials, which uses cement as the base material and is matched with high molecular polymer lotion, came into being. This kind of waterproof back adhesive is called two-component waterproof back adhesive.
One component is cement, with other inorganic fillers and a small amount of polymer additives, and the other component is organic polymer lotion. This two component waterproof adhesive must be mixed on-site, and workers lacking professional knowledge often do not realize the importance of the two component ratio and do not strictly mix according to the specified ratio. Therefore, quality cannot be fully guaranteed.

In addition, this type of waterproof backing has many brands and uneven quality. If the polymer type in the lotion is improperly selected or the solid content is too low, it is likely that it will not play a waterproof role. This two component waterproof adhesive is not suitable for white marble with high iron content or marble with more hidden cracks, because when applying this waterproof adhesive, strong alkaline cement water will immediately enter the cracks or pores of the stone and seep out from the board surface, and the yellowing phenomenon of the board surface may occur quickly.
3. The yellowing of white marble caused by external factors can be prevented through reasonable use and maintenance
For example, placing a fiber carpet at the entrance to remove sand or other hard particles from the sole of the shoe before entering the marble paving area, reducing the wear of foreign hard particles on the stone surface.
The mop used for cleaning the floor should be kept clean, and the water used for mopping the floor must be clean. After mopping the floor, wipe the water on the ground clean, and use dry wiping as much as possible with less wet mopping. If conditions permit, it is best to use marble cleaning solution for maintenance. Waxing the floor has been proven to be a more harmful than beneficial method for maintaining marble panels.
Firstly, the gloss of the ground polished with wax is not maintained for a long time. On the other hand, most of the wax has a blocking effect on the ventilation pores of the stone, which hinders the normal gas channel of the marble that adjusts with the climate.
Finally, when the accumulated wax layer turns yellow due to aging and needs to be removed, a wax remover must be used, as the use of wax removers can seriously affect indoor air quality. Crystal surface treatment is a commonly used polishing treatment for marble floors. Choosing the correct crystal surface agent can obtain a dense and bright film on the treated stone surface, which does not affect the breathability of the stone.
However, during crystal surface treatment, if steel wire wool is used for polishing, the scattered fine powder of steel wire wool must be cleaned with a vacuum cleaner. Any residual fine powder of wire wool on the ground may become a source of pollution for yellowing of the board surface.
4. Personnel engaged in stone installation must have a certain professional foundation, understand the reasons for stone yellowing, possess the necessary skills for stone installation, maintain a clean and tidy construction environment, and avoid installation causing stone yellowing.

Scientifically care for stones, avoiding the introduction of iron that can cause yellowing during cleaning water or floor polishing processes. Use high-quality permeable waterproofing agents to regularly protect the floor and reduce the time and opportunities for water to come into contact with stone.
Therefore, effective prevention of yellowing of white marble should be a comprehensive measure. In the absence of effective waterproofing treatment for cement floors or walls, the treatment of the back of stone slabs is particularly important. Choosing a waterproof backing adhesive with guaranteed quality for waterproofing the board back is the most important step among all measures.

PART-04
External reasons

Once the phenomenon of yellowing on the white marble surface is discovered, the first step is to identify the cause of the yellowing. Before shifting the suspicion of yellowing to rust, make sure that yellowing is not caused by aging wax or other external factors. After excluding factors other than iron, start preparing for rust removal.
At present, stone care stores have various brands of iron and yellow removing agents. Although no product has disclosed the basic chemical components, most of the iron and yellow removing agents on the market are acidic. Due to the poor acid resistance of marble, many acidic rust removers not only remove rust but also have significant erosion on the board surface. Excessive erosion of the marble board surface will cause permanent damage to the board surface.
So, when using rust remover, remember to apply several layers of toilet paper to the surface of the stone before pouring the rust remover onto the paper. After waiting for about two hours, the yellow will gradually decrease, lasting for several times, and then the yellow can be removed.
Professional stone care personnel generally use weakly acidic or even neutral cleaning agents to minimize damage to the stone.
For stubborn rust spots, a paste method is used, which involves mixing rust remover and diatomaceous earth with water to form a paste. Then, the paste is applied to the rust spot, covered with plastic film and sealed with a non drying adhesive tape around it. After 24 hours, the dry paste is removed. After removal, the dissolved iron is collected clean with a specialized cleaning agent.
This final step is important because during the rust removal process, when iron ions diffuse to the surface of the board, if these diffusion iron ions are not collected thoroughly, secondary yellowing will occur quickly and the area will be wider. Of course, you can also manually remove yellowing at home (as shown in the following picture)

Considering that rust removal on stone panels is a professional task, it is not recommended for unprofessional owners or caregivers to purchase unfamiliar rust removal agents to remove yellow or rust spots from the stones, in order to avoid permanent damage to the stones caused by improper use.

/MARBLE/
Appreciation of White Marble Cases