Why does stone return alkali?

I. pan alkali phenomenon

During the installation of wet pasted natural stone wall, there will be patches like "watermarks". With the hardening and drying of the pasted mortar, "watermarks" will be slightly reduced, or even some disappear. They appear in the plates in isolation and dispersion, the indoor degree is not serious, and the appearance is not affected much. However, with the passage of time, especially when the external wall repeatedly encounters rain or wet weather, the water invades from the slab joint, wall root and other parts, the water spots of natural stone gradually become larger, and they are connected into pieces at the slab joint, the partial deepening of the plate, dim luster, and white crystals are separated from the slab joint, which will not fade for a long time, seriously affecting the appearance. This phenomenon is called pan alkali phenomenon.

II. Cause analysis

1. The crystal of natural stone is relatively coarse, and there are many capillary tubes that cannot be seen by naked eyes. The fineness of granite is 0.5-1.5%, and the fineness of marble is 0.5-2.0%. Its impermeability is not as good as that of ordinary cement mortar. The water absorption of granite is 0.2-1.7%, which is relatively low. Water can still be transferred from the capillary immersion surface in the stone to the other side. This characteristic of natural stone and the existence of pores provide a channel for the infiltration and precipitation of water, alkali, salt and other substances in the bonding material and the formation of Pan alkali.

2. The bonding material produces substances containing alkali, salt and other components. It is mainly that Ca (OH) 2 (calcium hydroxide) is separated out from the inlaid mortar and follows the excess mixing water. The more mixing water is, the more Ca (OH) 2 will move to the mortar surface. After the water evaporates, Ca (OH) 2 will accumulate in the plate. Others, such as the addition of sodium Na + in cement, Na +, Mg2 +, K +, Ca2 +, C1 -, SO42 -, c032 - in clay brick soil, when dissolved in water, will penetrate into the capillary pores of stone, forming "White China" and other phenomena. Alkali, salt and other components produced by bonding materials are the direct source of the materials penetrating into the pores of stone materials to produce efflorescence.

3. Infiltration of water. Because the joint of the outer wall is pointed with cement fine sand mortar, the waterproof effect is poor; the surface water (or damp) invades the stone plate along the wall or mortar layer; the stone is sprinkled with too much water during installation, so that the water invades the stone plate, and dissolves into Ca (oh) 2 and other salt substances into the stone capillary to form a pan alkali. It can be seen that water is the solvent and carrier of Pan alkali substance. The mechanism of pantothene.

III. governance measures

Once the natural stone wall appears the phenomenon of efflorescence, because the soluble alkali (or salt) material has penetrated into the stone along the pores (the one that seeps out of the stone slab surface can be removed), it is difficult to remove, so it should be focused on prevention, and only the following remedies can be made after the occurrence of efflorescence.

1. Waterproof treatment shall be carried out for walls, slab joints, slab surfaces, etc. as soon as possible to prevent the water from continuing to invade, so that the pantothene is no longer expanded.

2. It can use the stone pan alkali cleaning agent on the market. The pan alkali cleaning agent is a colorless and translucent liquid made of non-ionic surfactants and solvents, which has certain effect on the surface pan alkali cleaning of some natural stones. However, before use, it is necessary to make small sample test block to test the effect and decide whether to use.