Importance of HPMC for cement-based materials
HPMC, also known as hydroxypropyl methylcellulose, is the same product, and the two names will alternate in the following articles. In recent years, with the continuous progress of cellulose ether production technology and the excellent characteristics of cellulose ether hpmc itself, hydroxypropyl methyl cellulose has been widely used in the construction industry.
Hydroxypropyl methylcellulose manufacturers focused on the improvement of cellulose ether hpmc on the cohesive properties of cement-based materials.
The bleeding phenomenon of mortar is similar to that of concrete, which will cause serious aggregate settlement, resulting in an increase in the water-cement ratio of the top layer, which will cause the top layer to undergo a large plastic shrinkage or even crack in the early stage. relatively weak. When the dosage is above 0.5%, there is basically no bleeding phenomenon. This is because when the cellulose ether hpmc is incorporated into the mortar, the cellulose ether hpmc has a film-forming and network structure, as well as the adsorption of hydroxyl groups on the long chain of macromolecules, so that the cement in the mortar and the mixing water form flocculation, ensuring the mortar. the stable structure of the body. After adding hydroxypropyl methylcellulose to the mortar, many independent tiny bubbles will be formed. These air bubbles will be evenly distributed in the mortar and hinder the deposition of aggregates. This technical performance of hydroxypropyl methylcellulose has a great influence on cement-based materials, and is often used to formulate new cement-based composite materials such as dry powder mortar and polymer mortar, so that it has good water retention and plasticity retention.
The setting time of concrete is mainly related to the setting time of cement, and the effect of aggregate is not large, so the setting time of mortar can be used to study the effect of cellulose ether hpmc on the setting time of underwater non-dispersed concrete mixture. The time is affected by the water-cement ratio and the mortar ratio. Therefore, in order to evaluate the influence of the cellulose ether hpmc on the setting time of the mortar, the water-cement ratio and the mortar ratio of the mortar should be fixed.
The experimental reaction shows that the addition of cellulose ether hpmc has a significant retarding effect on the mortar mixture, and with the increase of the content of cellulose ether hpmc, the setting time of the mortar prolongs successively. Mortar formed underwater will take longer to set than mortar formed in air. When measured in water, the setting time of the mortar mixed with cellulose ether hpmc was 6-18 h delayed at the initial setting and 6-22 h at the final setting compared with the blank specimen. Therefore, cellulose ether hpmc should be used in combination with early strength agent.
Cellulose ether hpmc is a high molecular polymer with a macromolecular linear structure with hydroxyl groups on the functional groups, which can form hydrogen bonds with the mixing water molecules and increase the viscosity of the mixing water. The long molecular chains of hydroxypropyl methylcellulose will attract each other, so that the cellulose ether hpmc molecules are entangled with each other to form a network structure, which wraps the cement and mixing water. Because hydroxypropyl methylcellulose forms a film-like network structure and wraps the cement, it can effectively prevent the volatilization of water in the mortar and hinder or slow down the hydration speed of the cement.
water requirement for mortar
When the content of cellulose ether hpmc is small, it has a great influence on the water demand of the mortar. Under the condition of keeping the expansion degree of the fresh mortar basically the same, the content of cellulose ether hpmc and the water demand of the mortar changed linearly within a certain period of time, and the water demand of the mortar first decreased and then increased significantly. When the content of cellulose ether hpmc is less than 0.025%, with the increase of the content, the water demand of the mortar decreases under the condition of the same degree of expansion, which shows that the cellulose ether hpmc has a water-reducing effect on the mortar when the content of cellulose ether hpmc is small. The air-entraining effect of the plain ether hpmc makes the mortar have a large number of tiny independent air bubbles, which play a lubricating role and improve the fluidity of the mortar. When the content is greater than 0.025%, the water demand of the mortar increases with the increase of the content. This is because the network structure of the cellulose ether hpmc is further complete, and the flocculation gap on the long molecular chain is shortened, which has the characteristics of attraction and cohesion. effect, reducing the fluidity of the mortar body. So in the case of basically the same degree of expansion, the slurry shows an increase in water demand.
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