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 further explore the mechanism of action between HPMC and cement-based materials, this paper focuses on the improvement effect of HPMC on the cohesive properties of cement-based materials.
clotting time
The setting time of concrete is mainly related to the setting time of cement, and the aggregate has little influence, so the setting time of mortar can be used instead to study the influence of HPMC on the setting time of underwater non-dispersible concrete mixture, because the setting time of mortar is affected by water 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 ratio of mortar.
According to the experiment, the addition of HPMC has a significant retarding effect on the mortar mixture, and the setting time of the mortar prolongs successively with the increase of the HPMC content. Under the same HPMC content, the underwater molded mortar is faster than the mortar formed in the air. The setting time of medium molding is longer. When measured in water, compared with the blank specimen, the setting time of the mortar mixed with HPMC is delayed by 6-18 hours for initial setting and 6-22 hours for final setting. Therefore, HPMC should be used in combination with accelerators.
HPMC is a high-molecular polymer with a macromolecular linear structure and a hydroxyl group on the functional group, which can form hydrogen bonds with the mixing water molecules and increase the viscosity of the mixing water. The long molecular chains of HPMC will attract each other, making the HPMC molecules entangle with each other to form a network structure, wrapping cement and mixing water. Since HPMC forms a network structure similar to a film and wraps the cement, it will effectively prevent the volatilization of water in the mortar, and hinder or slow down the hydration rate of the cement.
Bleeding
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 of slurry, causing a large plastic shrinkage of the top layer of slurry in the early stage, and even cracking, 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 the mortar, HPMC has a film-forming and network structure, and the adsorption of hydroxyl groups on the long chain of macromolecules makes the cement and mixing water in the mortar form a flocculation, ensuring the stable structure of the mortar. After adding HPMC to the mortar, many independent tiny air bubbles will be formed. These air bubbles will be evenly distributed in the mortar and hinder the deposition of aggregate. The technical performance of HPMC has a great influence on cement-based materials, and it is often used to prepare new cement-based composite materials such as dry powder mortar and polymer mortar, so that it has good water retention and plastic retention.
Mortar water demand
When the amount of HPMC is small, it has a great influence on the water demand of the mortar. In the case of keeping the expansion degree of the fresh mortar basically the same, the HPMC content and the water demand of the mortar change in a linear relationship within a certain period of time, and the water demand of the mortar first decreases and then increases obviously. When the amount of HPMC is less than 0.025%, with the increase of the amount, the water demand of mortar decreases under the same expansion degree, which shows that when the amount 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 air bubbles in the mortar, and these air bubbles act as a lubricant to improve the fluidity of the mortar. When the dosage is greater than 0.025%, the water demand of the mortar increases with the increase of the dosage. This is because the network structure of HPMC is further complete, and the gap between the flocs on the long molecular chain is shortened, which has the effect of attraction and cohesion, and reduces the fluidity of the mortar. Therefore, under the condition that the degree of expansion is basically the same, the slurry shows an increase in water demand.
01. Dispersion resistance test:
Anti-dispersion is an important technical index to measure the quality of anti-dispersion agent. HPMC is a water-soluble polymer compound, also known as water-soluble resin or water-soluble polymer. It increases the consistency of the mixture by increasing the viscosity of the mixing water. It is a hydrophilic polymer material that can dissolve in water to form a solution. or dispersion.
Experiments show that when the amount of naphthalene-based high-efficiency superplasticizer increases, the addition of superplasticizer will reduce the dispersion resistance of freshly mixed cement mortar. This is because the naphthalene-based high-efficiency water reducer is a surfactant. When the water reducer is added to the mortar, the water reducer will be oriented on the surface of the cement particles to make the surface of the cement particles have the same charge. This electric repulsion makes the cement particles form The flocculation structure of the cement is dismantled, and the water wrapped in the structure is released, which will cause the loss of part of the cement. At the same time, it is found that with the increase of HPMC content, the dispersion resistance of fresh cement mortar is getting better and better.
02. Strength characteristics of concrete:
In a pilot foundation project, HPMC underwater non-dispersible concrete admixture was applied, and the design strength grade was C25. According to the basic test, the amount of cement is 400kg, the compounded silica fume is 25kg/m3, the optimal amount of HPMC is 0.6% of the cement amount, the water-cement ratio is 0.42, the sand rate is 40%, and the output of naphthalene-based high-efficiency water reducer is The amount of cement is 8%, the average 28d strength of the concrete specimen in the air is 42.6MPa, the 28d average strength of the underwater concrete with a drop height of 60mm is 36.4MPa, and the strength ratio of the water-formed concrete to the air-formed concrete is 84.8 %, the effect is more significant.
03. Experiments show:
(1) The addition of HPMC has an obvious retarding effect on the mortar mixture. With the increase of HPMC content, the setting time of mortar is extended successively. Under the same HPMC content, the mortar formed under water is faster than that formed in air. The setting time of medium molding is longer. This feature is beneficial for underwater concrete pumping.
(2) The freshly mixed cement mortar mixed with hydroxypropyl methylcellulose has good cohesive properties and almost no bleeding.
(3) The amount of HPMC and the water demand of mortar decreased first and then increased obviously.
(4) The incorporation of water reducing agent improves the problem of increased water demand for mortar, but its dosage must be controlled reasonably, otherwise the underwater dispersion resistance of freshly mixed cement mortar will sometimes be reduced.
(5) There is little difference in the structure between the cement paste specimen mixed with HPMC and the blank specimen, and there is little difference in the structure and density of the cement paste specimen poured in water and in air. The specimen formed under water for 28 days is slightly crisp. The main reason is that the addition of HPMC greatly reduces the loss and dispersion of cement when pouring in water, but also reduces the compactness of cement stone. In the project, under the condition of ensuring the effect of non-dispersion under water, the dosage of HPMC should be reduced as much as possible.
(6) Adding HPMC underwater non-dispersible concrete admixture, controlling the dosage is beneficial to the strength. The pilot project shows that the strength ratio of water-formed concrete and air-formed concrete is 84.8%, and the effect is relatively significant.
Post time: May-06-2023