Causes and prevention of efflorescence in wet-attached natural stone wall

Causes and prevention of efflorescence in wet-attached natural stone wall

　　Natural stone is gradually recognized by architects and is widely used in the interior and exterior of buildings for its natural solemnity, brilliant colors, good weathering stability, abrasion resistance, acid and alkali resistance, etc.

Natural stone is gradually recognized by architects and widely used in building interior and exterior wall decoration for its natural dignity, gorgeous color, good weather resistance stability, abrasion resistance, acid and alkali resistance, etc. Highlight the artistic effect of the building. The installation and construction of natural stone include dry hanging method and wet pasting method. Among them, the wet pasting method has the advantages of simple construction and lower cost, but it is also easier to produce some common quality problems. The prominent point is the phenomenon of efflorescence on the surface of the stone. The "watermark" on the wall after alkali is spotty, it is ineffective to remove it, and it is difficult to remove it, which greatly destroys the effect of architectural decoration. Some iconic public buildings and plaza buildings built in Guangzhou in recent years have efflorescence and white-sagging (white beard) phenomena. Some are even serious, causing irreparable losses and regrets to this building. For this reason, it is of great significance to analyze the causes of efflorescence and take corresponding preventive measures.

　　 One, efflorescence phenomenon

During the installation of wet-pasted natural stone walls, the stone slabs will appear like "watermarks". As the mortar hardens and dries, the "watermarks" will shrink slightly, or even disappear. They appear in isolation and scattered. In the plate, the indoor degree is not serious and the appearance is not affected much. However, as time goes by, especially when the exterior wall is repeatedly exposed to rain or wet weather, water invades from the slab joints, wall roots and other parts, the water spots of the natural stone gradually become larger and connect into pieces in the slab joints, and the plates are partially deepened and dull. , The white crystals are precipitated concurrently in the plate seam, which will not fade for many years, which seriously affects the appearance. This phenomenon is called efflorescence.

　　 2. Cause analysis

1. Natural stone crystals are relatively coarse, and there are many capillaries that are invisible to the naked eye. Granite has a fine porosity of 0.5 to 1.5%, and marble has a fine porosity of 0.5 to 2.0%. Its impermeability is not as good as ordinary cement mortar and granite. The rate of 0.2 to 1.7% is low, and water can still be transmitted to the other side through the capillary immersion surface in the stone. This characteristic of natural stone and the existence of pores provide channels for the penetration and precipitation of water, alkali, salt and other substances in the bonding material to form efflorescence.

　　 2. The bonding material produces ingredients containing alkali, salt and other ingredients. The main reason is that Ca(OH)2 (calcium hydroxide) precipitates out of the mortar and follows the excess mixing water, which invades the plate along the pores of the stone. The more mixing water, the more Ca(OH)2 moves to the surface of the mortar. After the water evaporates, Ca(OH)2 will be stored in the plate.

Others, such as the addition of sodium and Na+ additives to the cement, the clay brick soil contains Na+, Mg2+, K+, Ca2+, C1-, So42-, C032-, etc., which will dissolve in water and penetrate into the pores of the stone. , The formation of "Baihua" and other phenomena.

　　The alkali, salt and other constituent substances produced by the bonding material are the direct source of substances that penetrate the pores of the stone to produce efflorescence.

　　 3. Penetration of water. Because the external wall joints are jointed with cement fine mortar, the waterproof effect is poor; ground water (or moisture) penetrates the stone slab along the wall or mortar layer; excessive water is sprayed on the stone during installation, so that water invades the stone slab. And dissolve Ca(OH)2 and other salt substances into the capillary of the stone to form efflorescence. It can be seen that water is the solvent and carrier of efflorescence substances.

　　 Three, preventive measures

　　 In view of the causes of efflorescence, measures can be taken to prevent it from cutting off the pathway of efflorescence, reducing Ca(OH)2, salts and other products, and reducing water intrusion.

　　 (1) Preparation before construction

　　 1. In the design, consider eliminating efflorescence, and design as dry hanging form as much as possible; consider the waterproof treatment of the structure; choose stone slabs with water absorption and other physical properties that meet the requirements.

　　 2. Before construction, we must fully consider the various construction process links that may occur efflorescence, and take preventive measures in advance. If you are not sure, you should first make a model.

　　3. The relevant materials should be inspected before use. Not only the appearance and size are required to be qualified, but also the physical performance indicators are also qualified.

　　(2) Use anti-alkali back coating agent

　　 1. Before the stone slab is installed, apply special treatment agent on the back and side of the stone. The solvent will penetrate into the stone and block the capillary, preventing water, Ca(OH)2, salt and other substances from entering, cutting off the path of efflorescence. Without back-coating treatment, efflorescence is inevitable, and the adhesion of the back-coated stone will not be affected.

　　 2. Brush resin glue on the bottom of the stone slab, and then paste chemical fiber mesh cloth to form a tensile and waterproof layer, but you must not forget to paint the side.

　　(3) Reduce the production of Ca(OH)2, salt and other substances

　　 1. The cement mortar used for mounting should be mixed with reducing agent to reduce the precipitation of Ca(OH)2. The consistency of the paste method mortar should be 6-8cm, and the grouting mortar consistency should be 8-12cm.

　　2. Cement-based commercial adhesives (dry mixtures) can be used for outdoor mounting. It has good water retention and can greatly reduce cement bleeding. Indoor veneer can be glued with stone chemical adhesive.

　　(4) Prevent water intrusion

　　 1. Do not spray large amounts of water on the stone and wall before operation.

　　 2. Moisture-proof layer should be installed under the ground wall roots. If the outer wall of the water room such as toilet and bathroom is decorated with stone, the inner wall should be treated with anti-seepage treatment.

　　3. In order to prevent intrusion from the board seam, the wall panel block must be inlaid away from the seam, and the seam width should not be less than 5mm, and use the special silicon chamber weathering sealant for stone to seal.

　　4. Set up rain canopy for outdoor construction, and handle the joints between the periphery of the door and window frames and the outer wall to prevent rainwater from leaking into the wall.

　　 5. After finishing the installation, spray organic silicon waterproofing agent or other colorless surface protection agent on the outdoor stone material.

　　 Fourth, governance measures

Once the efflorescence phenomenon occurs on the natural stone wall surface, the soluble alkali (or salt) substances have penetrated into the stone along the pores (the exuded stone surface can be removed), and it is difficult to remove it. Therefore, we should focus on prevention. The following remedies can be made.

　　 1. Waterproof the walls, board seams, and board surfaces as soon as possible to prevent the intrusion of moisture and prevent the expansion of efflorescence.

2. The efflorescence cleaning agent on the market can be used. The cleaning agent is a colorless and translucent liquid made of non-ionic surfactants and solvents. It has a certain effect on the cleaning of some natural stone surfaces. . But before use, be sure to make a sample test block to test the effect and decide whether to use it.

In short, the prevention of efflorescence from wet pasted natural stone walls is the most important thing. As long as the root cause is found, and the design, materials, and construction are strictly checked and scientifically prevented, the occurrence of efflorescence can be completely avoided, and Natural stone, an ancient building material, regains modern architecture.