The Relationship Between Cellulose’s Water-Holding Capacity and Its Operating Temperature
Release Time:
2021-12-14
Source:
The greater the dosage of cellulose ether in mortar, the better the water retention and the higher the viscosity, resulting in superior water-retention performance.
With regard to particle size, the finer the particles, the better the water-retention performance. When large-particle cellulose ethers come into contact with water, their surfaces dissolve immediately, forming a gel that encapsulates the material and prevents further water penetration; in some cases, even prolonged stirring fails to achieve uniform dispersion and dissolution, resulting in a turbid, flocculent suspension or clumping. This significantly impairs the water-retention capability of the cellulose ether, making solubility one of the key criteria for selecting a cellulose ether. Particle fineness is also an important performance indicator for methyl cellulose ethers. For dry-mix mortars, the cellulose is required to be in powder form, with low moisture content, and a particle-size distribution such that 20%–60% of the particles are smaller than 63 µm. Fineness directly affects the solubility of methyl cellulose ether. Coarser cellulose is typically granular; it dissolves readily in water without clumping, but its dissolution rate is very slow, making it unsuitable for use in dry-mix mortars. In dry-mix mortars, cellulose is dispersed among aggregates, fine fillers, and cementitious binders; only sufficiently fine powder can prevent clumping of the methyl cellulose ether during mixing with water. Once the cellulose has clumped upon hydration, it becomes extremely difficult to disperse and dissolve uniformly. Coarser cellulose not only leads to material waste but also reduces the local strength of the mortar. When such dry-mix mortar is applied over large areas, this manifests as a markedly slower setting rate in localized regions, resulting in cracking due to differential curing times. For sprayed mortars applied by mechanical equipment, where mixing time is shorter, the requirements for particle fineness are even more stringent.
The fineness of cellulose also has a certain impact on its water-retention performance. Generally speaking, for methyl cellulose ethers with the same viscosity but different fineness, the finer the particle size, the better the water-retention effect at the same dosage.
The water-retention properties of cellulose are also temperature-dependent; for methyl cellulose ethers, water retention decreases as temperature rises. In practical applications, however, dry-mix mortars are often applied under high-temperature conditions (above 40°C) to hot substrates—for example, exterior wall putty and plastering in summer when exposed to direct sunlight. Such conditions typically accelerate cement setting and the hardening of the dry-mix mortar. The resulting decline in water retention markedly impairs both workability and crack resistance, making it especially critical to mitigate the adverse effects of temperature in these circumstances. Although methyl hydroxyethyl cellulose ether additives are currently regarded as state-of-the-art, their temperature sensitivity still leads to a deterioration in dry-mix mortar performance. Even increasing the dosage of methyl hydroxyethyl cellulose—particularly in summer formulations—often fails to adequately meet the requirements for workability and crack resistance. By subjecting cellulose to specific modifications, such as raising the degree of etherification, its water-retention performance can be maintained at an optimal level even at elevated temperatures, thereby delivering superior performance under harsh conditions.
