What are the hard skills of HPMC in cement mortar?
In premixed mortar, only a little HPMC can significantly improve the performance of wet mortar, which shows that HPMC is a major additive affecting the construction performance of mortar.
Selection of different varieties, different viscosity, different particle size, different degree of viscosity and addition of hydroxypropyl methyl cellulose have different effects on the performance of dry mortar. At present, many masonry and plastering mortars have poor water retention performance. Slightly standing for a few minutes will lead to the separation of water slurry. So it is very important to add HPMC in cement mortar.
Next, let’s see in detail what hard skills HPMC has in cement mortar.
Hydroxypropyl methyl cellulose-water retention is an important property of methyl hydroxypropyl methyl cellulose, and it is also concerned by many domestic dry mortar manufacturers, especially those in southern regions with high temperature.
In the production of building materials, especially dry mortar, HPMC plays an irreplaceable role, especially in the production of special mortar (modified mortar), it is an indispensable and important part.
The viscosity, content, ambient temperature and molecular structure of HPMC have great influence on its water retention. Under the same conditions, the higher the viscosity of HPMC, the better the water retention; the higher the content of HPMC, the better the water retention. Often, HPMC with little amount of HPMC can greatly improve the water retention of mortar. When the amount of HPMC reaches a certain level, the increasing trend of water retention becomes slower; when the ambient temperature rises, the water retention of cellulose ether usually decreases, but some modified hydroxypropyl ether can greatly improve the water retention of mortar. Methyl cellulose also has better water retention at high temperature, and hydroxypropyl methyl cellulose with lower degree of substitution has better water retention.
Oxygen atoms on hydroxyl and ether bonds of hydroxypropyl methyl cellulose molecules can associate with water molecules to form hydrogen bonds, which make free water become bound water, thus playing a good role in water retention. The interaction between water molecules and hydroxypropyl methyl cellulose molecular chains makes water molecules enter into the internal chain of hydroxypropyl methyl cellulose macromolecules and are strongly constrained, thus forming self-contained molecules. The water retention of cement slurry is improved by water and winding water. Hydroxypropyl methyl cellulose improves the rheological properties, porous network structure and osmotic pressure of fresh cement slurry, or the film forming property of hydroxypropyl methyl cellulose hinders the diffusion of water.
Hydroxypropyl methylcellulose-Thixotropy and Thixotropy
Hydroxypropyl methyl cellulose (HPMC) has been widely used in plastering mortar, brick bonding mortar and external thermal insulation system of exterior wall. The thickening effect of HPMC can also increase the anti-dispersion ability and homogeneity of fresh mix materials, prevent material delamination, segregation and bleeding, and can be used in fiber reinforced concrete, underwater concrete and self-compacting concrete.
The thickening effect of hydroxypropyl methyl cellulose on cement-based materials comes from the viscosity of hydroxypropyl methyl cellulose solution. Under the same conditions, the higher the viscosity of HPMC, the better the viscosity of the modified cement-based materials. However, if the viscosity is too high, the fluidity and operability of the materials will be affected (e.g., viscous plastering knife). For self-leveling mortar and self-compacting concrete which require high fluidity, the viscosity of HPMC is very low. In addition, the thickening effect of HPMC will also increase the water demand of cement-based materials and the output of mortar.
Hydroxypropyl methyl cellulose (HPMC) aqueous solution with high viscosity has high thixotropy, which is also a major characteristic of HPMC. The aqueous solution of methyl cellulose is usually pseudoplastic and non thixotropic, below the gel temperature, but it exhibits Newtonian flow at low shear rate. Pseudoplasticity increases with the increase of molecular weight or concentration of HPMC, and has nothing to do with the type and degree of substitution. Therefore, hydroxypropyl methyl cellulose of the same viscosity grade, whether MC, HPMC or HEMC, always exhibits the same rheological properties as long as the concentration and temperature remain constant. When the temperature is increased, the structure gels are formed and highly thixotropic flow occurs.
Hydroxypropyl methyl cellulose with high concentration and low viscosity exhibits thixotropy even below the gel temperature. This property is of great benefit to the adjustment of leveling and hanging of construction mortar. It should be pointed out that the higher the viscosity of HPMC, the better the water retention, but the higher the viscosity, the higher the relative molecular weight of HPMC, the lower the solubility, which has a negative impact on the mortar concentration and construction performance.
Hydroxypropyl methyl cellulose-air entrainment
Hydroxypropyl methyl cellulose (HPMC) has obvious air-entraining effect on fresh cement-based materials. Hydroxypropyl methyl cellulose (HPMC) has hydrophilic groups (hydroxyl, ether group) and hydrophobic groups (methyl, glucose ring). It is a surfactant with surface activity and air entrainment effect. The air entrainment of HPMC can produce «ball» effect, which can improve the performance of fresh mix materials, such as increasing the plasticity and smoothness of mortar during operation, which is beneficial to the spreading of mortar, increasing the output of mortar and reducing the production cost of mortar, but increasing the porosity of hardened materials, reducing their strength and elastic modulus.
As a surfactant, hydroxypropyl methyl cellulose (HPMC) also has wetting or lubricating effect on cement particles, which together with its air entrainment effect increases the fluidity of cement-based materials, but its thickening effect decreases the fluidity. The effect of HPMC on the fluidity of cement-based materials is a combination of plasticizing and thickening effects. Generally speaking, when the content of hydroxypropyl methyl cellulose is very low, the main performance is plasticization or water reduction; when the content of hydroxypropyl methyl cellulose is high, the thickening effect of hydroxypropyl methyl cellulose increases rapidly, and its air entrainment tends to be saturated, so the performance is thickening or increasing water demand.
Hydroxypropyl methyl cellulose-retarding effect
Hydroxypropyl methyl cellulose (HPMC) can prolong the setting time of cement paste or mortar and delay the hydration kinetics of cement, which is conducive to improving the operation time of fresh mix materials, improving the consistency of mortar and the durable loss of concrete slump, but it may also delay the construction progress.