In recent years, with the continuous development of external wall thermal insulation technology, the continuous progress of hydroxypropyl methyl cellulose production technology, and the excellent characteristics of hydroxypropyl methyl cellulose HPMC itself, hydroxypropyl methyl cellulose HPMC has been widely used in the construction industry.
setting time
The setting time of concrete is mainly related to the setting time of cement, and the aggregate has little influence. Therefore, the setting time of mortar can be used to replace the research on the influence of HPMC on the setting time of underwater non dispersive concrete mixture. Since the setting time of mortar is affected by the water cement ratio and cement sand ratio, in order to evaluate the influence of HPMC on the setting time of mortar, it is necessary to fix the water cement ratio and cement sand ratio of mortar.
The experimental reaction shows that the addition of HPMC has a retarding effect on the mortar mixture, and the setting time of mortar increases with the increase of the amount of cellulose ether HPMC. With the same amount of HPMC, the setting time of mortar formed under water is longer than that formed in air. When measured in water, the setting time of mortar mixed with HPMC is 6~18h later in initial setting and 6~22h later in final setting than that of blank specimen. Therefore, HPMC should be used together with early strength agent.
HPMC is a polymer with macromolecular linear structure, with hydroxyl groups on the functional groups, which can form hydrogen bonds with mixing water molecules to increase the viscosity of mixing water. The long molecular chains of HPMC will attract each other, making HPMC molecules intertwine to form a network structure, and wrapping cement and mixing water. As HPMC forms a network structure similar to a film and wraps the cement, it can effectively prevent the volatilization of water in the mortar and hinder or slow down the cement hydration rate.
Bleeding
The bleeding phenomenon of mortar is similar to that of concrete, which will cause serious settlement of aggregates, increase the water cement ratio of the top layer slurry, cause the top layer slurry to have large plastic shrinkage, or even crack in the early stage, and the strength of the slurry surface is relatively weak.
When the dosage is more than 0.5%, there is basically no bleeding. This is because when HPMC is mixed into mortar, HPMC has film-forming and reticular structure, as well as the adsorption of hydroxyl on the long chain of macromolecule, which makes the cement and mixing water in mortar form flocculent, ensuring the stable structure of mortar. When HPMC is added to mortar, many independent tiny bubbles will be formed. These bubbles will be evenly distributed in the mortar and hinder the deposition of aggregates. This technical performance of HPMC has a great impact on cement-based materials, and is often used to prepare new cement-based composites such as dry mortar and polymer mortar, so that they have good water and plastic retention.
Water demand of mortar
When the amount of HPMC is very small, it has a great influence on the water demand of mortar. Under the condition that the expansion of fresh mortar is basically the same, the amount of HPMC and the water demand of mortar change linearly in a certain period of time, and the water demand of mortar decreases first and then increases. When the HPMC content is less than 0.025%, with the increase of the HPMC content, the water demand of the mortar decreases under the same expansion degree, which shows that the smaller the HPMC content, the water reducing effect of the mortar. The air entraining effect of HPMC makes the mortar have a large number of small independent bubbles, which play a role in lubrication and improve the fluidity of the mortar. When the dosage is greater than 0.025%, the water demand of mortar increases with the increase of the dosage, which is due to the further integrity of the network structure of HPMC, the shortening of the gap between the flocs on the long molecular chain, the attraction and cohesion, and the reduction of the fluidity of the mortar. Therefore, when the expansion degree is basically the same, the slurry shows an increase in water demand.
Post time: Nov-25-2022