I. Pan-alkali phenomenon
In the process of installing wet-bonded natural stone walls, patches similar to "watermarks" will appear on the slate. With the hardening and drying of plastering mortar, "watermark" will be slightly reduced or even disappeared, and it will appear in the plate in an isolated and dispersed way. However, with the passage of time, especially in outdoor rainy or humid weather, water invades from the joint of the plate or the surface of the stone, and the water spots of natural stone will gradually become larger and become one piece at the joint of the plate.
Second, the reason analysis
1. The crystals of natural stones are rough and there are many capillaries invisible to the naked eye. The porosity of granite is 0.5 ~ 1.5%, and that of marble is 0.5 ~ 2.0%. Its impermeability is not as good as ordinary cement mortar, and the water absorption of granite is 0.2 ~ 1.7%, so water can still pass through the stone. This characteristic of natural stone and the existence of capillary holes provide a channel for the infiltration and precipitation of water, alkali, salt and other substances in cementitious materials and the formation of pan-alkali.
2. The main component of stone is calcium carbonate, accounting for more than 58%. When affected by sulfur dioxide and moisture in the atmosphere, its surface layer will undergo chemical changes and be transformed into gypsum, which will quickly lose its luster and become loose and porous.
3. Bonding materials produce substances containing alkali, salt and other components. Ca(OH)2 (calcium hydroxide) is mainly precipitated from mortar, and it follows the excess mixed water and invades the board along the capillary of stone. The more mixing water, the more Ca(OH)2 moves to the mortar surface, and after the water evaporates, Ca(OH)2 will accumulate in the board.
4. Water infiltration, groundwater (or moisture) invades the slate along the stone surface and cracks, and dissolves into salts such as Ca(OH)2 and enters the stone capillaries to form pantothenic alkali. It can be seen that water is the solvent and carrier of pantothenic acid.
Third, preventive measures.
In view of the causes of pan-alkali phenomenon, we can take corresponding measures to prevent it from three aspects: cutting off the pan-alkali pathway, reducing the products such as Ca(OH)2 and salts, and reducing the invasion of water.
(1) Preparation before construction
1. In the design, it is considered to eliminate the pan-alkali, and try to design it as a dry hanging form; Consider the waterproof treatment of the structure; Choose stone plates whose physical properties meet the requirements, such as water absorption.
2. Before construction, fully consider all construction process links that may cause alkali flooding, and take preventive measures in advance. If you are not sure, make a model first.
3. Before use, relevant materials should be inspected, which requires not only the appearance and size to be qualified, but also its physical performance indicators to be qualified.
(B) the use of alkali-resistant back coating agent
1. Before installing the slate, the back and side of the stone are coated with a special treatment agent. This solvent will penetrate into the stone and block the capillaries, making it impossible for water, Ca(OH)2, salt and other substances to invade, thus cutting off the way of pan-alkali phenomenon. Without the back coating treatment, flashing is inevitable, and the adhesion of stone treated with back coating is not affected.
2. Brush resin glue on the bottom of the slate, and then stick chemical fiber mesh cloth to form a tensile waterproof layer, but don't forget to brush it on the side.
(3) Reduce the generation of Ca(OH)2, salt and other substances.
1. Cement-based commercial adhesive (dry-mixed) can be used for outdoor pavement with good water retention, which can greatly reduce cement coagulation and bleeding.
Prevent the invasion of water
1. Don't spray water on stones and walls before operation. After proper watering, wait for the stone to dry before paving.
2. After paving, outdoor stone can be sprayed with silicone waterproofing agent or other colorless topcoat.
Control and handling of materials
1, protect the interface of stone.
Protect the back and * * * small side of the stone, and brush with protective agent, which will penetrate into the surface of the stone and form a closed barrier to prevent the precipitation of water and alkali in the mortar. The specific method is: clean its five interfaces first, and then brush the anti-seepage sealant twice. When painting, the coating surface should be dry and painted evenly. After painting, it should be dried in the shade. You can brush for the second time after the first time, and the direction of the brush should be vertical.
2. Surface protection
For marble or outdoor rough granite, surface protectants are used to seal stone structures.
Gap between, so as to achieve the purpose of waterproof, pollution prevention and erosion prevention. Its construction method is the same as above (products include SG-l protectant from the United States, a protective coating for silicone developed by silicone Research Office of Institute of Chemistry of Chinese Academy of Sciences and China Cultural Relics Research Office, and Baoshijie SG-l produced by Beijing Wanbo Vanke Technology Development Co., Ltd., which can be selected according to specific conditions).
3, cement related indicators control
The sulfur trioxide content shall not exceed 3.5% and the alkali content shall not exceed 0.6%.
Verb (abbreviation of verb) governance measures
Once the natural stone pavement is submerged by alkali, it is difficult to remove it because the soluble alkali (or salt) substances have infiltrated into the stone along the capillary (which can be removed from the slate surface), so we should focus on prevention. After the flood, we can only make the following remedial measures.
1. Fully waterproof the board surface as soon as possible, so as to prevent water from continuing to invade, so that the pantothenic alkali does not swell.
2. You can use the stone pan-alkaline cleaning agent on the market, which is a colorless and translucent liquid made of nonionic surfactant and solvent, and has certain effect on pan-alkaline cleaning of some natural stone surfaces. But before using it, you must make a small sample to test the effect and decide whether to use it.
The main reason for returning alkali is-don't empty stones when facing, but be sure to fill mortar.
Another method is to use special stone protectant for pretreatment before paving. For example: France Likang stone protectant. Combined with the previous methods, the phenomenon of flooding can be effectively prevented. Of course, considering the cost, it is generally only suitable for stone treatment of waterscape.
For the ground pavement, the problem of salt flooding can be avoided by generally using hard cement mortar.
I encountered the above-mentioned flashing phenomenon of light-colored natural stone in engineering practice, and Party A was not satisfied with the paving effect, so all of them were redone, and the loss was as high as 6,543.8+10,000. At first, because the stone manufacturer provided and implemented the painting work of stone protective agent, the construction period was tight, and the protective agent was good when paving. Within one month after the completion, a large area of watermark was found, and the stone looked black. After cleaning, singeing and other means, it still can't be eliminated. In desperation, the new pavement has used a better protective agent, and the dry mortar has been used for three months, and there is still no pollution. It can be seen that the light-colored stone pavement must use a good stone protective agent and operate in strict accordance with the instructions of the protective agent, which can avoid this phenomenon. If dry hard ash is added with 10% waterproof powder, it should be more sure.
There are two forming conditions for the phenomenon of flooding salt.
Namely chemical conditions and physical conditions,
Chemical conditions: there are water-soluble substances in stone and paste, mostly potassium, sodium and magnesium salts.
It may also be an acid substance in rainfall,
The former forms white massive salt,
The latter is mostly calcium carbonate.
Physical conditions: water evaporation, salt liquid crystallization,
According to these two conditions to prevent flooding,
Is the root of solving the problem,
Waterproofing of the bottom layer can only prevent overflow caused by capillary phenomenon,
I suggest that this problem can be easily solved by maintaining good drainage at the base and surface of the pavement.
Because the water content of the substrate is not saturated,
No upward capillary phenomenon is generated,
It won't rain for long,
Even acid rain doesn't have much impact. (Remember: the surface and foundation are well drained)
According to my experience, if the project funds permit, the dry hanging method can reduce the occurrence of pan-alkali.
Or used in major landscape nodes, such as the facing of landscape walls or falling into the water.
There is another way;
Adding washing powder into mortar can improve it. In addition, the seams of the board are sealed with marble glue, which can be used for a long time, but it still won't work after a long time.
When protecting the stone, the water in the stone itself must be dried and then protected. When paving stone, don't empty your drum. After the construction is completed, the phenomenon of alkali flooding can be basically solved by cracking with waterproof slit opener.
Stone is made of alkali instead of salt.
In the construction, it is related to cement. During construction, brush 107 glue on the back of stone, and then use good national standard cement. The smaller the cement particles, the better. After the construction is completed, you can keep healthy all the time.
Oxalic acid can be used for washing, and it is best to make an isolation coating on the back of the stone! Safest! And the choice of stone is very important, but the density of similar colors is very different!
Yes, it is made of stone protective agent, and when paving, a layer of white cement is coated on the bottom of the stone and waterproof agent is added.
Generally speaking, the groundwater level is high and the soil salt content is high, such as Dongying saline-alkali land. Our practice is that the binding layer must be ready and not empty. It is best to use green water or tap water for bonding layer mortar. The simple way for us to check the empty drum is to see if it is pan-alkaline.