The Role of HPMC in Drymix Mortars

The Role of HPMC in Drymix Mortars

 

in dry mix mortars, the addition of small amounts of HPMC can significantly improve the performance of wet mix mortars. This shows that HPMC is the main admixture affecting the construction performance of mortar.

 

 

1. Water Retention

 

Water retention is an important property of HPMC, the viscosity, dosage, ambient temperature and molecular structure of HPMC have a significant impact on its water retention properties. Under the same conditions, the higher the viscosity of HPMC, the better the water retention; the higher the content, the better the water retention. Generally a small amount of HPMC can greatly improve the water retention rate of mortar. When the content reaches a certain level, the tendency to increase water retention slows down; as the ambient temperature rises, the water retention of HPMC usually decreases, but some modified HPMC also has better water retention under high temperature conditions; HPMC with lower degree of substitution has better water retention properties.

 

 

 

The hydroxyl group on the HPMC molecule and the oxygen atom on the ether bond will join with water molecules to form hydrogen bonds, making free water into bonded water, thus playing a good role in water retention; the mutual diffusion between water molecules and HPMC molecular chains allows water molecules to enter the HPMC macromolecular chain and be subject to strong bonding forces, forming free water and entangling water together to improve the water retention of cement slurry.

 

 

 

HPMC

 

HPMC      

2. Thickening thixotropy

 

HPMC gives the wet mortar excellent viscosity, which can significantly increase the bonding ability of the wet mortar and the base, and improve the anti-sagging property of the mortar. It is widely used in plastering mortars, decorative tile bonding mortars and external wall insulation systems. HPMC's thickening effect can also increase the anti-dispersing ability and uniformity of freshly mixed materials, prevent material delamination, segregation and water secretion, and can be used in fibre concrete, underwater concrete and self-compacting concrete.

 

 

 

The thickening effect of HPMC on cementitious materials comes from the viscosity of the HPMC solution. Under the same conditions, the higher the viscosity of HPMC, the better the viscosity of the modified cementitious material, but too much viscosity will affect the fluidity and workability of the material. Self levelling mortars and self compacting concretes requiring high fluidity require HPMC with very low viscosity. in addition, the thickening effect of HPMC increases the water requirement of the cementitious material and increases the yield of the mortar.

 

 

 

Aqueous solutions of HPMC with high viscosity have high thixotropy, which is a characteristic of HPMC. aqueous solutions of HPMC are usually pseudoplastic and non-thixotropic below their gel temperature, but exhibit Newtonian flow characteristics at low shear rates. Pseudoplasticity increases with increasing HPMC molecular weight or concentration, independent of the type of substituent and degree of substitution. Therefore, 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, structured gels are formed and high thixotropic flow occurs.

 

 

 

For Drymix Mortar

 

For Drymix Mortar

 

HPMCs with high concentrations and low viscosities show thixotropic properties even below the gel temperature. This property is very beneficial in the construction of building mortars to adjust their levelling and sagging properties. It should be noted here that the higher the viscosity of HPMC, the better the water retention, but the higher the viscosity, the higher the relative molecular weight of HPMC and the corresponding decrease in solubility, which will affect the mortar consistency and construction performance.

 

 

3. Air-entraining effect

 

HPMC has an obvious air-entraining effect on freshly mixed cementitious materials; it has both hydrophilic groups (hydroxyl, ether) and hydrophobic groups (methyl, glucose ring). HPMC is a surface active and air-entraining surfactant, and its air-entraining effect will produce a "ball" effect, which will improve the working properties of freshly mixed materials, such as increasing the plasticity and smoothness of the mortar during operation, which is beneficial to the spreading of mortar; it will also increase the yield of mortar and reduce the production cost of mortar; however, it will increase porosity of the hardened material, reducing its mechanical properties such as strength and modulus of elasticity.

 

 

 

HPMC For Drymix Mortar

 

 HPMC For Drymix Mortar

 

HPMC, as a surfactant, also has a wetting or lubricating effect on cement particles and together with the air-entraining effect increases the fluidity of cementitious materials, but its thickening effect reduces fluidity. the effect of HPMC on the fluidity of cementitious materials is a combination of plasticising and thickening effects. Generally speaking, when the HPMC content is very low, it mainly shows a plasticising or water-reducing effect; when the HPMC content is high, the thickening effect of HPMC increases rapidly and its air-entraining effect tends to saturate, thus improving performance. Thickening effect or increase water demand.

 

 

4. Speed reduction

 

HPMC will prolong the setting time of net cement or mortar, slow down the hydration kinetics of cement, help to increase the working time of freshly mixed materials, improve the consistency of mortar and the loss of concrete slump with time, but may also delay the construction schedule.

 

 

 

Based on the above functions, HPMC has other properties in different types of dry mortar products, such as anti-sagging, increased compatibility, adjustment of setting time and increased bond strength.