The properties of plastering mortar after hardening include the compressive strength, bond tensile strength, contraction, permeability resistance and frost resistance. Plastering mortar has requirements for appropriate compressive strength, high bond tensile strength and low contraction, as well as good permeability and frost resistance if used for exterior walls. The compressive strength should try to match the base course materials, as consistent as possible in order to maintain the same deformation. If the strength of base course material is high, such as small-sized concrete block walls; then the compressive strength of plastering mortar should also be high; while if the strength of base course material is low, such as autoclaved aerated concrete block walls; then the compressive strength of plastering mortar should not be too high. If the base course materials have smooth surfaces, or large water absorption and slow absorption rate, then the requirements for the water retention and bond strength of plastering mortar should be high. However, the increase in the bond strength is related to the amount of cementitious materials used. If the amount of cementitious materials used is high, then the bond strength of mortar will be high, and vice versa.
The cementitious materials mentioned here include not only inorganic cementitious materials, such as cement-based and gypsum-based cementitious materials, and even redispersible latex powder can also be counted as organic cementitious materials. The effective method to improve the bond strength between mortar and base course materials is to use the compound of inorganic and organic cementitious materials. Since the organic cementitious materials are high in price, poor in water resistance and great in contraction, the plastering mortar with the use of compounding inorganic and organic cementitious materials receives many limits in practical engineering applications. Taking the plastering mortar on the surface of insulation boards for example, its thickness used is controlled within 3-5mm. And alkaline-resistant fiberglass gridding cloth is also embedded into the plastering layer to ensure no cracking in the plastering layer.
If the layer of plastering mortar is thicker (with a thickness of plastering layer greater than 10mm, such as 20-30mm), then it is technically not feasible to use the plastering mortar compounded by inorganic and organic cementitious materials for all. First, the setting time of this mortar is very long. If a plastering thickness is 8-10mm, then the setting time is generally 8-12h, and there also exists surface dry and inner wet phenomenon, so the water collection and press polish operations cannot be completed in one construction squad. Secondly, this kind of mortar is more viscous, and not easy for leveling and press polish. Moreover, since organic cementitious materials are expensive, all uses could not be afforded economically. Therefore, in the case that the plastering layer is thicker and difficult to bond with mortar, a layer of 2-3mm plastering mortar compounded by inorganic and organic cementitious materials can be used as the interface transition layer, and then ordinary plastering mortar can be applied to ensure strong bonding. We call the transition layer mortar the interface-treating mortar, or concrete interface-treating agent and aerated concrete interface treatment agent. Engineering practice has proved that the use of interface treating agent and ordinary plastering mortar is an effective method to solve the strong bonding between concrete walls or aerated concrete walls and the plastering layer.
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