Continued about HPMC in mortar from previous article The relationship betweenHPMC and mortar quality(2)
For particle size, the finer the particle, the better the water retention. After the large particles of cellulose ether come into contact with water, the surface immediately dissolves to form a gel. Which wraps the material to prevent water molecules from penetrating. And sometimes it is not evenly dispersed and dissolved after long time stirring, forming a turbid flocculent solution or agglomerate . It greatly affects the water retention effect of its cellulose ether. And solubility is one of the factors to choose cellulose ether.
Fineness is also an important performance index of methyl cellulose ether.
The MC used for dry mortar is required to be powder with low water content, and the fineness also requires 20% to 60% of the particle size to be less than 63um. The fineness affects the solubility of methyl cellulose ether. The coarser MC is usually granular, and it is easy to dissolve in water without agglomeration. But the dissolution rate is very slow, so it is not suitable for use in dry mortar.
In the dry mortar, MC is dispersed between the aggregates, fine fillers and cement and other cementing materials. Only a sufficiently fine powder can avoid the agglomeration of methyl cellulose ether when mixing with water. When MC is dissolved and agglomerated with water, it is difficult to disperse and dissolve.
The coarser MC is not only wasteful, but also reduces the local strength of the mortar. When such dry mortar is applied in a large area, the curing speed of the local dry mortar is significantly reduced. And cracks due to different curing times occur. For sprayed mortar that uses mechanical construction, the fineness is higher due to the shorter mixing time.
The fineness of MC also has a certain effect on its water retention. Generally speaking, for methyl cellulose ethers with the same viscosity but different fineness. In the case of the same addition amount, the finer the finer the better the water retention effect.
The water retention of MC is also related to the temperature of use
The water retention of methyl cellulose ether decreases with the increase of temperature. However, in actual material applications, dry mortars are often applied to hot substrates under high temperature (above 40 degrees) conditions in many environments. Such as external wall putty plastering under the sun in summer, which often accelerates the curing of cement and the hardening of dry mortar. The decrease in water retention rate has led to the obvious feeling that both workability and crack resistance are affected. Under such conditions, reducing the influence of temperature factors becomes particularly critical.
Although the methyl hydroxyethyl cellulose ether additive is currently considered to be at the forefront of technological development. Its dependence on temperature will still lead to the weakening of the performance of dry mortar. Despite increasing the amount of methyl hydroxyethyl cellulose (summer formula), the workability and crack resistance still cannot meet the needs of use. Through some special treatments to MC, such as increasing the degree of etherification.It can maintain its water retention effect at higher temperatures and make it perform better under harsh conditions.
The articles about HPMC in mortar are end.