Improvement effect of hydroxypropyl methyl cellulose on cement-based materials
In recent years, with the continuous development of external wall 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. In order to deeply explore the mechanism of hydroxypropyl methylcellulose HPMC and cement-based materials, this paper focuses on the improvement effect of HPMC on the cohesive properties of cement-based materials.
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. Because the setting time of mortar is affected by water The influence of mortar ratio and mortar-sand ratio. Therefore, in order to evaluate the influence of HPMC on the setting time of mortar, it is necessary to fix the water-cement ratio and mortar-sand ratio of the mortar.
In the experimental reaction, the addition of HPMC has a retarding effect on the mortar mixture, and with the increase of the cellulose ether HPMC content, the setting time of the mortar is successively prolonged. Under the same HPMC content, the mortar formed under water is better than that of the mortar. The setting time of molding in the air is longer. When measured in water, the setting time of the mortar mixed with HPMC is 6~18 hours later than that of the blank sample, and the final setting delay is 6~22 hours. Therefore, HPMC should be used in combination with an early-strength agent.
HPMC is a high-molecular 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 will 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.
The bleeding phenomenon of mortar and concrete is similar, which will cause serious aggregate settlement, resulting in an increase in the water-cement ratio of the top layer slurry, causing great plastic shrinkage and even cracking of the top layer slurry in the early stage, and the strength of the surface layer of the slurry Relatively weak.
When the dosage is above 0.5%, there is basically no bleeding phenomenon. 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 the mixing water in the mortar form a flocculation, ensuring the stable structure of the mortar. After adding HPMC 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 formulate 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 the HPMC content is small, it has a great influence on the water demand of the mortar. Under the condition of keeping the expansion degree of the freshly mixed 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 increases. When the content of HPMC is less than 0.025%, with the increase of the content, the water demand of the mortar will decrease under the same degree of expansion. This shows that when the content of HPMC is small, it has a water-reducing effect on the mortar, and HPMC has an air-entraining effect. There are a large number of tiny independent bubbles in the mortar, these bubbles play a lubricating effect, so that the fluidity of the mortar can be improved. When the content of the mortar 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 HPMC is further complete, and the gap between the flocculation groups on the long molecular chain is shortened, which has the effect of attracting and cohesive, reducing The fluidity of the mortar body. Therefore, when the degree of expansion is basically the same, the slurry shows an increase in water demand.