The Importance of Water Retention of HPMC
April 24th 2024
In recent years, with the continuous development of external wall insulation technology, the continuous progress of cellulose production technology, and the excellent characteristics of HPMC itself, HPMC has been widely used in the construction industry. In order to understand the mechanism of HPMC and cement-based materials in depth, this article introduces the improvement effect of HPMC on the cohesive properties of cement-based materials.
Setting time
The setting time of concrete is mainly related to the setting time of cement, and the influence of aggregates is not significant. Therefore, the setting time of mortar can be used instead to study the effect of HPMC on the setting time of underwater non-dispersible concrete mixtures. Due to the setting time of mortar is affected by the water-cement ratio and mortar-sand ratio, in order to evaluate the impact of HPMC on the setting time of the mortar, it is necessary to fix the water-cement ratio and the mortar-sand ratio of the mortar.
According to the experiment, the addition of HPMC has an obvious retarding effect on the mortar mixture, and with the increase of the HPMC content, the setting time of the mortar is successively prolonged. Under the same HPMC content, the mortar is formed under water. The mortar has a longer setting time than when it is formed in the air. When measured in water, the setting time of the mortar mixed with HPMC is 6~18 h after initial setting and 6~22 h after final setting compared with the mortar without HPMC. Therefore, HPMC should be used in combination with an early-strength agent.
HPMC is a macromolecular polymer with a linear structure of macromolecules, with hydroxyl groups on the functional groups, which can form hydrogen bonds with the mixing water molecules to increase the viscosity of the mixing water. The long molecular chains of HPMC attract each other, causing the HPMC molecules to entangle with each other to form a network structure, which wraps the cement and mixing water. Because HPMC forms a film-like network structure and wraps the cement, it will effectively prevent the volatilization of moisture in the mortar and hinder or slow down the hydration rate of the cement.
Property of slurry separate from water
The property of slurry separating from water of mortar and concrete is similar, which will cause serious settlement of aggregates and increase the water-cement ratio of the top layer slurry, which will cause the top layer slurry to undergo large plastic shrinkage or even cracking in the early stage, and the strength of the surface layer of the slurry relatively weak.
How to make mortar
When the dosage is above 0.5%, there is basically no phenomenon of slurry separate from water. This is because when HPMC is mixed into mortar, HPMC has a film-forming and network structure, as well as the adsorption of hydroxyl groups on the long chain of macromolecules, which makes the cement and mixing water in the mortar form a flocculent state, ensuring the mortar body stable structure. When HPMC is added to the mortar, many independent tiny bubbles will be formed. These bubbles will be evenly distributed in the mortar, hindering the deposition of aggregates. This technical performance of HPMC has a great influence on cement-based materials, and is often used to prepare new cement-based composite materials such as dry mortar and polymer mortar to make it have good water retention and plasticity retention.
Water demand of mortar
When HPMC content is small, it has a great influence on the water demand of mortar. Under the condition of keeping the expansion degree of the fresh mortar basically the same, the HPMC content and the water demand of the mortar change linearly within a certain period of time, and the water demand of the mortar first decreases and then significantly increases. HPMC is being mixed.
Tags:
