Hydroxypropyl methylcellulose has an important role in mortar, and that is water retention. Mortars with poor water retention are prone to segregation and water loss during transport. When laid on the surface of porous materials, most of the water is easily absorbed and the properties of the mortar are altered. This affects the normal hardening of the mortar and the bond between the block materials, thus reducing the strength of the masonry.
The water retention of mortar is indicated by the degree of delamination.
The mortar stratification is tested as follows: the fresh mortar mix is loaded into a stratification tester with an internal diameter of 15 cm, an upper section height of 20 cm, a lower section height of 10 cm and a bottom row, and the degree of sinkage is measured. The mortar is then left to stand for 30 minutes, the top 20 cm of mortar is removed, and the remaining 10 cm of mortar is remixed and the degree of sinkage is measured and other values. The degree of delamination of the mortar is then easily calculated.
A mortar with good water retention has a small delamination, generally requiring a negative delamination of 1-2 cm. Mortars larger than 2 cm do not retain water well and are prone to segregation. Mortars with a degree of delamination close to zero have high water retention and no delamination above or below. However, this phenomenon is often caused by the excessive amount of cementitious material or the fine sand. The mortar is therefore susceptible to shrinkage cracks. And it should not be used as a plastering mortar in particular.
I. The need for water retention
The water retention of mortar refers to the ability of mortar to retain all water. Mortar with poor water retention is prone to water secretion during transport and storage. This means that the water floats on top, the sand and cement sink below. And they must be re-mixed before use.
All types of substrates that require mortar for construction have a certain degree of water absorption. If the mortar has poor water retention. It will be absorbed during the mortar application process as soon as the ready-mixed mortar comes into contact with the block or the base. At the same time the mortar surface evaporates water towards the atmosphere. Resulting in insufficient water in the mortar due to water loss. This affects the further hydration of the cement and at the same time the normal development of the mortar strength. This leads to low strength, especially the interface strength between the mortar hardening body and the base. Causing the mortar to crack and fall off. With a good water retention mortar. The cement hydrates more fully, the strength develops normally. And it adheres well to the substrate.
Ready-mixed mortar is usually masonry between water-absorbing blocks or applied to the substrate, which together with the substrate forms a whole.
And the effects of poor mortar water retention on the quality of the project are:
1. Due to excessive water loss in the mortar thus affecting the normal setting and hardening of the mortar, reducing the bond between the mortar and the material surface. Not only does not facilitate the construction operation, but also makes the strength of the masonry decreased, thus making the quality of the project greatly reduced.
2. If the mortar retains water poorly, the water is easily absorbed by the brick. Make the mortar too dry and thick, unevenly coated, in the implementation of the project, not only affects the progress, but also makes the wall surface easy to cause cracking due to dry shrinkage;.
Therefore, increasing the water retention of the mortar not only facilitates construction, but also increases the strength.
Why should mortar have hydroxypropyl methylcellulose added？ (2)