3 Key Roles Of HPMC In Dry Mix Mortars

Cellulose ethers are derivatives of the natural material cellulose and can be used as additives to improve product performance in the building/construction industry. For example, etherified cellulose can be used as a water-retaining agent, thickening agent, binder, and dispersing agent, as well as a protective colloid, stabilizer, suspending agent, emulsifier, and film-forming aid. Cellulose ethers are used to improve the workability of cement mortars while maintaining the water holding capacity and fluidity. HPMC is the most widely-used cellulose ether.

HPMC is used primarily in construction materials like tile adhesives and renders where it is used as a rheology modifier and water retention agent. As an essential additive for building materials, Hydroxypropyl Methylcellulose (HPMC) plays 3 key roles in the dry mix mortars.

1. Water retention

Water retention reflects the workability of cement mortars. In modern building products, HPMC plays an important role, particularly in dry-mix mortars such as wall renders and plasters based on mineral binders including lime and cement. Their main function is to prevent uncontrolled water loss into porous substrates.
The most important property of HPMC is its ability to retain water in building material systems. To obtain good workability, it is necessary to maintain sufficient moisture in the mortar for a long time. Since the water acts as a lubricant and solvent between the inorganic components, the thin-layer mortar can be combed, and the plastering mortar can be spread with a trowel. Walls or tiles prone to water absorption do not require pre-wetting after using cellulose ether-added mortar. Therefore, HPMC can bring a fast and economical construction effect.

2. Thickening and thixotropy

Another important property of HPMC is the ability to increase the viscosity of liquids, so they can be used as thickeners. The thickening effect of cellulose ether depends on the degree of polymerization, the concentration of HPMC, shear rate, temperature, the concentration of dissolved salt, etc. By controlling the content, viscosity, and modification degree of HPMC, the mortar can achieve suitable sag resistance and optimum consistency.
The addition of HPMC affects the water demand of mortar and enhances the thickening effect of mortar, the higher the addition, the higher the water consumption. HPMC is used for powdered building materials, dissolves quickly in cold water, and provides a suitable consistency. Secondly, HPMC of the same viscosity grade always exhibits the same rheological properties as long as the concentration and temperature remain constant. When the temperature is increased, a structural gel is formed and high thixotropic flow occurs. This property greatly improves the anti-sagging performance of construction mortar.

3. Retarding

HPMC imparts beneficial properties to the mortar and reduces the early hydration of cement, releases heat, and delays the setting time of cement. It has been found that the higher the content of HPMC in the cement-based system, the stronger the hydration delay effect. The setting time of mortar increases significantly with increasing the amount of HPMC. Moreover, HPMC can delay not only the setting but also the hardening process of the cement mortar system. However, this effect is diminished in cold regions; that is to say, it is especially suitable for hot areas for HPMC with a good retarding effect.
In addition to water retention, thickening, and slow setting, HPMC can also improve construction performance. Good water retention can make cement hydration more comprehensive, thus improving the bonding strength of mortar, and also prolonging the opening time of fresh mortars. We can control the open time of mortar by changing the amount of cellulose ether.

In conclusion, HPMC is an essential additive in many applications of dry-mixed mortar products. If cellulose ether is not added, the thin layer of fresh mortar will become dry quickly, so that the cement cannot hydrate properly, and the mortar cannot harden and obtain good bonding performance with the substrate.