Water-retention requirements for cellulose in plastering mortar
Release Time:
2022-04-06
Source:
Water-retention requirements for cellulose in plastering mortar
The Necessity of Water Retention
All types of substrates that require mortar application exhibit a certain degree of water absorption. When the substrate absorbs water from the mortar, the workability of the mortar deteriorates; in severe cases, the cementitious materials in the mortar may fail to hydrate fully, resulting in reduced strength—particularly the interfacial bond strength between the hardened mortar and the substrate—and leading to cracking and spalling of the mortar. By contrast, if the plastering mortar possesses appropriate water-retention properties, it can not only effectively enhance its workability but also minimize the rate at which the substrate draws moisture from the mortar, thereby ensuring complete hydration of the cement.
Problems with Traditional Water-Conservation Methods
The traditional approach is to sprinkle water directly onto the substrate; however, this method cannot ensure uniform and adequate moistening of the substrate. The ideal hydration process for cement mortar on the substrate involves the hydration products of cement gradually penetrating into the substrate while the substrate simultaneously absorbs moisture, thereby establishing an effective “keyed interlock” between the mortar and the substrate and achieving the required bond strength. By contrast, direct surface watering of the substrate can lead to substantial variability in the amount of water absorbed, depending on ambient temperature, timing of watering, and degree of uniformity. When the substrate absorbs too little water, it continues to draw moisture from the mortar, depleting the available water before cement hydration has commenced and thus impeding both the hydration reaction and the penetration of hydration products into the substrate. Conversely, if the substrate absorbs excessive water, the migration of moisture from the mortar into the substrate slows down, and a water-rich layer may even form at the interface between the mortar and the substrate, again compromising bond strength. Therefore, the conventional practice of simply watering the substrate not only fails to effectively address the problem of high water absorption in wall substrates but also undermines the bond between the mortar and the substrate, leading to hollowing and drying cracks.
Requirements for Water Retention of Plastering Mortar
Due to variations in region and ambient temperature and humidity conditions, the target water-retention rate for plastering mortar products is provided for reference only.
① High-Absorbency Base Plaster Mortar
High-water-absorption substrates, exemplified by autoclaved aerated concrete, including various lightweight partition panels and masonry blocks, are characterized by substantial water absorption and prolonged absorption duration. For plastering mortars applied to such substrates, the water-retention capacity of hydroxypropyl methylcellulose shall be no less than 93%, and the mortar’s water-retention rate shall be no less than 88%.
② Low-water-absorption base plastering mortar
Low-water-absorption substrates, exemplified by cast-in-place concrete and including polystyrene boards used for external thermal insulation, exhibit relatively low water absorption. For plastering mortars applied to such substrates, the water-retention capacity of hydroxypropyl methylcellulose shall be no less than 93%, and the mortar’s water-retention rate shall be no less than 88%.
③ Thin-layer plastering mortar
Thin-layer plastering refers to plastering work in which the thickness of a single plaster layer is between 3 and 8 mm. Due to the thinness of the plaster layer, such applications are prone to rapid moisture loss, which can adversely affect workability and strength. For mortars used in this type of plastering, the water-retention rate shall be no less than 99%.
④ Thick-layer plastering mortar
Thick-layer plastering refers to plastering work in which the thickness of a single plaster layer ranges from 8 to 20 mm. Due to the relatively thick plaster layer, moisture tends to be retained more effectively; hydroxypropyl methylcellulose exhibits a water-retention capacity of no less than 93%, and the water-retention rate of the corresponding mortar-based plastering mortar should be no less than 88%.
⑤ Water-resistant putty
Water-resistant putty, as an ultra-thin plastering material, is typically applied at a thickness of 1–2 mm. Such materials require exceptionally high water-retention properties to ensure workability and bond strength. For putty materials, the water-retention rate should be no less than 99%; moreover, the water-retention rate for exterior-wall putty should be higher than that for interior-wall putty.
