How to Improve the Water Retention of Hydroxypropyl Methyl Cellulose

Hydroxypropyl Methyl Cellulose (HPMC) is the most commonly used additive in dry mortar. Cellulose ether plays a crucial role in dry mortar, where dissolved cellulose ether ensures the even distribution of cementitious materials in the system through its surfactant action. Acting as a protective colloid, cellulose ether ‘wraps’ solid particles, forming a lubricating film on their outer surface, enhancing the stability of the mortar system and improving the flow and smoothness during the mixing and construction processes.

Hydroxypropyl Methyl Cellulose (HPMC) possesses excellent water retention properties, preventing premature evaporation or absorption of water in wet mortar. This ensures thorough cement hydration, ultimately guaranteeing the mechanical performance of the mortar. This is particularly beneficial for thin-layer mortar, water-absorbing substrates, or mortar construction under high-temperature and dry conditions. The water-retaining effect of HPMC can alter traditional construction processes, improving construction progress, such as allowing plastering on water-absorbing substrates without the need for pre-wetting.

The water retention performance of Hydroxypropyl Methyl Cellulose (HPMC) is influenced significantly by its viscosity, dosage, environmental temperature, and molecular structure. Under the same conditions, higher viscosity cellulose ethers exhibit better water retention. Increased dosage also improves water retention, and even small amounts of cellulose ether can greatly increase the water retention of mortar. As the dosage reaches a certain level, the increase in water retention tends to plateau. Higher environmental temperatures generally decrease the water retention of cellulose ethers, although some modified cellulose ethers exhibit good water retention even at high temperatures. Cellulose ethers with lower substitution degrees show better water retention performance.

The hydroxyl groups on the molecular structure of Hydroxypropyl Methyl Cellulose (HPMC) and the oxygen atoms on the ether bonds form hydrogen bonds with water molecules, converting free water into bound water, thereby providing effective water retention. The diffusion of water molecules between cellulose ether chains allows them to enter the interior of large cellulose ether chains, experiencing strong constraints, leading to the formation of free water and entangled water, thereby enhancing the water retention of the mortar. Cellulose ether improves the rheological properties, porous network structure, and osmotic pressure of freshly mixed cement slurry, while the film-forming properties of cellulose ether hinder water diffusion.

However, current cellulose ethers face challenges due to their less-than-ideal water retention properties, resulting in poor mortar adhesion, suboptimal construction performance, and common issues such as cracking, hollowing, and detachment after construction.

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