High Water Retention Agent Construction Grade HPMC Used in Drymix MortarJune 7th 2023
HPMC is a non-ionic hydroxypropyl methyl cellulose. It is the most commonly used water retaining agent in cement/mortar applications. It is less expensive than other types of cellulose ethers and is widely used in dry mortars.
Role of HPMC
Thickening effect: HPMC cellulose can thicken fresh mortar and has a certain wet viscosity to prevent segregation.
Water retention: Water retention is also one of the most important properties of mortar, it helps to keep the free water in mortar. It leaves more time for the hydration of the cementing material after the construction of mortar.
Outgassing effect: HPMC cellulose has outgassing effect, which can introduce uniform fine air bubbles in the mortar and improve the structure of mortar.
For hydroxypropyl methyl cellulose, the higher the viscosity, the better the water retention performance. Viscosity is an important parameter for the performance of HPMC cellulose. Different HPMC cellulose manufacturers use different methods and instruments to determine the viscosity of HPMC cellulose.
In terms of particle size, the finer the particle, the better the water retention. When large particles of cellulose ether come in contact with water, its surface immediately dissolves and forms a gel coating material that prevents further penetration of water molecules. Sometimes the long time stirring cannot make the HPMC cellulose evenly dispersed and dissolved, forming turbid flocculent solution or lumps. This will greatly affect the water retention performance of cellulose ether.
HPMC cellulose requires good quality of dry mortar, low water content, 20% ~ 60% particle size should be less than 63um. fineness affects the solubility of hydroxypropyl methyl cellulose ether. Coarse HPMC cellulose is usually granular, easily soluble in water without clumping, but dissolves very slowly and is not suitable for dry mortar. In dry mortars, HPMC cellulose is dispersed in cementitious materials such as aggregates, fine-grained fillers and cement. Only sufficient fines can avoid clumping of methyl cellulose ether with water.
If it is not fine enough, it not only wastes mortar, but also reduces the local strength of mortar. In such large-scale dry mortar construction, if the local dry mortar has significantly lower maintenance performance, cracking will occur due to different maintenance time. The use of shotcrete mortar in mechanical construction requires a finer powder due to the shorter mixing time.
The water retention effect of cellulose ether decreases with the increase of temperature. The decrease in water retention affects construction and cracking resistance. In this case, the effect of temperature reduction becomes particularly critical. Methyl hydroxypropyl cellulose ether additives are considered to be at the forefront of current technological developments. Despite the increased amount of methyl hydroxypropyl cellulose, its structural and crack resistance effects are still unsatisfactory.
The HPMC cellulose content in mortar should not be too high, otherwise it will increase the water demand, sticky mud, prolong the setting time and eventually affect the construction. Different mortar products use different viscosity of HPMC cellulose, please do not use high viscosity HPMC cellulose at will.