The MC used for dry powder mortar is required to be powder, with low water content, and the fineness also requires 20%~60% of the particle size to be less than 63um. The fineness affects the solubility of methyl cellulose ether. Coarse MC is usually in the form of granules, 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 powder mortar.
In dry powder mortar, MC is dispersed between aggregates, fine fillers and cement and other cementing materials. Only fine enough powder can avoid methyl cellulose ether agglomeration when mixing with water. When MC is added with water to dissolve the agglomerates, it is very difficult to disperse and dissolve. Coarse MC is not only wasteful, but also reduces the local strength of the mortar. When such a dry powder mortar is applied in a large area, the curing speed of the local dry powder mortar will be significantly reduced, and cracks will appear due to different curing times. For machine sprayed mortar with mechanical construction, due to the short mixing time, the requirement for fineness is higher.
The fineness of MC also affects its water retention. Generally speaking, for methyl cellulose ethers with the same viscosity but different fineness, under 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 used, and the water retention of methyl cellulose ether decreases with the increase of temperature. Applied on hot substrates under high temperature conditions to accelerate the curing of cement and the hardening of dry powder mortar. The decrease of water retention rate leads to the influence of workability and crack resistance, and it becomes critical to reduce the influence of temperature factor under this condition.
Although methyl hydroxyethyl cellulose ether additives are currently considered to be at the forefront of technological development, their dependence on temperature will still lead to weakening of the performance of dry powder mortar. Through some special treatment on MC, such as increasing the degree of etherification, etc., the water retention effect can be maintained at a higher temperature, so that it can provide better performance under harsh conditions.