HPMC (Hydroxypropyl Methyl Cellulose) is a polymer material commonly used in building materials, especially in cement-based materials and gypsum-based materials. It has good water solubility, adhesion, water retention and thickening properties, so it is widely used in mortar, putty powder, tile adhesive and other materials.
1. Causes of shrinkage and cracking of building materials
During the hardening process, building materials often shrink in volume due to water evaporation, chemical reactions and changes in external environmental factors, leading to stress concentration and crack formation. The main types of shrinkage include:
Plastic shrinkage: When the cement-based material has not yet hardened, the volume shrinks due to the rapid evaporation of water.
Dry shrinkage: After the material hardens, it is exposed to the air for a long time, and the water evaporates slowly, resulting in volume shrinkage.
Temperature shrinkage: Volume change caused by temperature changes, especially in an environment with a large temperature difference between day and night.
Autogenous shrinkage: During the cement hydration process, the internal volume shrinks due to the consumption of water by the hydration reaction.
These shrinkages often lead to stress accumulation inside the material, eventually causing microcracks or cracks, which affect the durability and aesthetics of the building structure. In order to avoid this phenomenon, additives are usually needed to improve the performance of the material, and HPMC is one of them.
2. Mechanism of action of HPMC
HPMC plays an important role in reducing the shrinkage and cracking of building materials, which is mainly achieved through the following mechanisms:
Water retention: HPMC has a strong water retention capacity and can form a water retention film in mortar or putty powder to slow down the evaporation rate of water. Since the rapid evaporation of water inside the material will cause plastic shrinkage, the water retention effect of HPMC can effectively reduce the early shrinkage phenomenon, keep the water in the material sufficient, thereby promoting the full hydration reaction of cement and reducing shrinkage cracks caused by water loss during the drying process. In addition, HPMC can improve the performance of the material under wet and dry conditions and reduce cracking caused by water loss.
Thickening and strengthening effect: HPMC is a thickener that can effectively increase the consistency and viscosity of mortar and enhance the overall adhesion of the material. During the construction process, if the material is too thin, it is easy to delaminate or sag, resulting in uneven surface or even cracks. By using HPMC, the mortar can maintain appropriate viscosity, enhance the strength and surface density of the material after construction, and reduce the possibility of cracking. In addition, HPMC can also enhance the shear resistance of the material and improve its crack resistance.
Improve the flexibility of the material: HPMC molecules can play a certain role in enhancing flexibility in cement-based materials or gypsum-based materials, so that the material has better tensile and bending resistance after curing. Since building materials are usually subjected to tensile or bending stress under ambient temperature changes and loads, after adding HPMC, the flexibility of the material increases, which can better absorb external stress and avoid brittle cracking.
Control the cement hydration reaction rate: In cement-based materials, the speed of the hydration reaction rate directly affects the performance of the material. If the hydration reaction is too rapid, the stress inside the material cannot be released in time, resulting in cracks. HPMC can appropriately slow down the rate of hydration reaction through its water retention and protective film formation, prevent cement from losing water too quickly in the early stage, and thus avoid the phenomenon of spontaneous shrinkage and cracking during the hardening process of the material.
Improve construction performance: HPMC can improve the construction performance of building materials, mainly manifested in its good fluidity, water retention and lubricity, increase the uniformity of materials, and reduce cracks caused by improper construction. It can make mortar, putty powder, etc. easier to spread and level during construction, reduce the void ratio of materials, improve the overall density and strength of materials, and reduce the risk of local cracking caused by uneven construction.
3. Application of HPMC in specific building materials
Tile adhesive: HPMC can greatly improve the anti-slip performance of tile adhesive, ensure that tiles can be evenly attached to the substrate during installation, and reduce shedding or cracking caused by uneven stress or shrinkage. In addition, the thickening and water retention effects of HPMC also enable tile adhesive to maintain a longer open time after construction, improve construction efficiency, and reduce cracks caused by uneven curing.
Putty powder: In putty powder, the water retention property of HPMC can prevent the putty from losing water too quickly during the drying process, and reduce the shrinkage and cracking caused by water loss. At the same time, the thickening effect of HPMC can improve the construction performance of putty, making it easier to apply evenly on the wall, and reducing surface cracks caused by uneven application.
Mortar: Adding HPMC to mortar can effectively improve its working performance, make the mortar smoother during construction, reduce segregation and stratification, and thus improve the uniformity and adhesion of the mortar. At the same time, the water retention effect of HPMC can make the water evaporate more slowly during the hardening process of the mortar, avoiding shrinkage and cracking caused by early water loss.
4. Precautions for the use of HPMC
Dosage control: The amount of HPMC added has a direct impact on its effect, and it usually needs to be adjusted according to the material ratio and specific application scenarios. Excessive HPMC will cause the material to have too high a consistency, affecting construction performance; while insufficient HPMC will not be able to play the role of water retention and thickening as it should.
Use with other additives: HPMC is usually used in combination with other chemical additives (such as water reducers, air entraining agents, plasticizers, etc.) to achieve better results. When using, it is necessary to consider the interaction of different additives to avoid mutual influence on the performance of the materials.
As an important building additive, HPMC has a significant effect in reducing the shrinkage and cracking of building materials. It effectively reduces cracks caused by water loss and stress concentration by improving the water retention, thickening, flexibility of the material and improving the cement hydration reaction rate. Reasonable use of HPMC can not only improve the construction performance of the material, but also extend the service life of the building structure and reduce the cost of later maintenance. With the continuous advancement of building material technology, the application of HPMC in the construction field will be more extensive and in-depth.
Post time: Sep-21-2024