How to prevent white marble from yellowing? (I)

Among the numerous marbles, white marble is undoubtedly one of the most widely used stones. White marble is used in high-end and luxurious walls, floors, background walls, stairs, table panels, and other places, and some people play with white marble, such as clothes, light bulbs, fruit trays, loudspeakers, and even toilets.

The simple elegance, noble temperament, tenderness, and delicacy of white marble are really fascinating, and the decorative effect is even better than that. It is simply wonderful. Its mild color, white snow, pure and beautiful like snowflakes, let many designers make full use of and show.

But at the same time, it is also the most beloved and feared variety. However, some users regret that white marble is sentimental, fragile, and difficult to serve in terms of anti-pollution. In particular, the fact that white marble is prone to yellowing makes many users who love white marble stay away from it. If we want to solve the quality problems such as the yellowing of white marble, we must analyze the internal and external factors, thoroughly understand the nature of marble and the root causes of quality problems, and then we can solve the fundamental problems. So what is the cause of the yellowing of white marble? How can white marble keep its original appearance? This article will talk about this knowledge.

1、 Structure and composition of stone

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 grain of white marble, some distributed on the surface of the board, and some concentrated in the grain. The oxidation of iron minerals in white marble is considered to be the main reason for the yellowing of marble. Except for the grain part of most marble, the design and color of other parts are relatively simple and uniform. When iron-containing minerals randomly dispersed in marble are oxidized to form yellow or brown hydrated iron oxides, defects and aesthetic stains on the surface of the slab are formed, which is particularly prominent for white marble. The iron in white marble mainly exists in the form of ferrous sulfide, iron carbonate, and magnesium iron silicate, which are dispersed in the marble matrix or enriched in the marble veins. When these bivalent iron ore substances on the stone surface are oxidized to ferric iron by oxygen in the air and combined with water to form the hydrated ferric hydroxide, rust, commonly known as yellowing, is formed. If no protective measures are taken, the yellowing of the marble will occur 8-12 months after the marble is placed.

It is interesting to note that the yellowing phenomenon does not occur in marble placed near heat sources, such as hot water pipes, which indicates that the yellowing phenomenon of white marble is closely related to water content. However, contact with water does not necessarily lead to the yellowing of white marble. The thermal shock test of white marble shows that stone samples are immersed in 15-25 ℃ deionized water for 6h, and then the samples are baked in an oven at 100-110 ℃ for 18h. After 20 such cycles, it was found that only the sample with rich iron on the surface showed slight yellowing, and the degree of yellowing was far lower than that of natural yellowing. This experimental phenomenon shows that due to the very low conductivity of deionized water, the redox reaction of ferric iron is oxidized to ferric iron and proceeds very slowly. Therefore, although the sample has been immersed in water for a long time and recycled many times, there is no obvious yellowing phenomenon. The test results show that the yellowing phenomenon of white marble is the result of oxidation and hydration acting on the bivalent iron ore material on the stone surface, and both conditions are indispensable.

Unlike soaking in ionic water, white marble soaking in an alkaline solution very easy to causes yellowing. Place the same stone sample in alkaline water at 15-25 ℃ for 6h, and then place the sample in an oven at 50-60 ℃ for 18h. After 20 such cycles, the test found that all the white marble selected for the experiment had yellowed, and Karala’s white marble was the most serious. The alkaline solution is easy to make white marble yellow, because in an alkaline medium, the redox 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 ferric hydroxide formed by the oxidation of divalent iron on the stone surface to ferric iron by oxygen in the air and its combination with water is extremely low. Therefore, yellowing pollution on the board caused by iron is not easy to be removed. The second type of yellowing pollution on the white marble surface comes from the organic substances inside the marble. These organic substances exist in the formation process of the marble, and their structures are similar to the saprophytes. These water-soluble organic substances existing in the interior of marble often show brown-yellow when reacting with alkaline water. Due to the existence of such organic matter in the marble, when the marble floor or wallboard without waterproof treatment on the back of the board is installed by wet pasting, the alkaline water of the cement slurry seeps through the marble and reacts with the organic matter in the marble to generate colored products. When these colored products are brought to the stone surface by water, the second kind of pollution on the stone surface is formed. Even during the use of stone after installation, due to climate reasons, whenever water seeps out of the stone surface after penetrating the cement concrete, colored organic pollutants will always be brought to the stone surface.

2、 External causes

The above two types of pollution of white marble come from the internal composition and structure of stone and occur under the influence of external conditions. In addition to the two types of pollution related to the composition and structure of stone, external pollution also includes the following aspects:

1. Yellowing caused by wear With the wear of the polished marble surface, pollutants are brought into the pores of the marble surface with walking, and these pollutants make the white marble surface yellow.

2. The most common cause of yellowing caused by incorrect care is to wet the floor with a dirty mop. The pollutants brought by the dirty mop accumulate in the marble, which makes the marble surface polluted and yellow.

3. Yellowing caused by waxing When the marble surface is polished with wax, the thick wax layer turns yellow due to its own aging.

4. Yellowing caused by crystal surface treatment The steel wool used for crystal surface treatment remains in the micropores of the polished marble plate with tiny fine powder, which becomes the root of yellowing. Especially when the marble is treated with a crystal surface in a wet state, yellowing is easy to occur.

5. Yellowing caused by the installation process Yellowing caused by the installation is easy to be ignored, but generally exists. Before the installation of the marble floor or wall, the so-called six-side waterproof treatment is carried out for the stone specification plate: waterproof glue on the bottom side, the four sides, and the front side are treated with a permeable waterproof agent. If such six side waterproof treatment is done very well, the quality of the selected waterproof material is also guaranteed. In theory, there should be no water-related yellowing. However, the fact is that even before the installation, the six waterproof surfaces of the stone slab are considered to be well done, and the yellowing phenomenon still occurs from time to time after the installation.

6. The yellowing of stone materials related to installation is mainly caused by two aspects: one is that the original waterproof treatment of one side is damaged due to the need for installation size and on-site cutting; Another possible situation is that the construction environment is dirty and messy, and the cement slurry falls on the board surface during the construction process. In order to prevent the falling cement slurry from drying up on the board surface, wipe the fallen cement slurry with a wet towel. At this time, it is very likely to wipe strong alkaline cement water into some hidden cracks on the board surface, because even if the board surface has been treated with permeable waterproof treatment, however, it is impossible for tiny waterproof agent molecules to form a protective film across the crack to shield the crack.

Therefore, the coverage of waterproof agents at the crack is very limited. During installation, use the ash knife to scrape the side of the specification plate with cement paste back and forth, which will also damage the waterproof treatment. If the cement water is accidentally brought into the cracks of the board when wiping the board, the board will turn yellow sooner or later rather than later.

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