Hydroxypropyl methylcellulose (HPMC for short) has excellent thickening properties and can be used as an excellent anti-dispersing agent for concrete. 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.
1. Anti-dispersion test
Anti-dispersion is an important technical indicator to measure the quality of anti-dispersant. HPMC is a water-soluble polymer compound, also known as a water-soluble resin or a 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 be dissolved in water to form a solution. or dispersion.
It can be seen from the experiment that when the content of naphthalene-based superplasticizer increases, the addition of superplasticizer will reduce the dispersion resistance of freshly mixed cement mortar. This is because naphthalene-based superplasticizers are surfactants. When the water-reducing agent is added to the mortar, the water-reducing agent is oriented on the surface of the cement particles, so that the surface of the cement particles has the same electric charge. This electric repulsion makes the flocculation structure formed by the cement particles to be disintegrated, and the water wrapped in the structure is released, will cause a part of the loss of cement. At the same time, it was found that with the increase of HPMC content, the dispersion resistance of freshly mixed cement mortar became better and better.
2.Strength characteristics of concrete
(1) The addition of HPMC has an obvious retarding effect on the mortar mixture. With the increase of the content of HPMC, the setting time of the mortar is prolonged successively. Under the condition of the same HPMC content, the setting time of the mortar formed under water is longer than that of the mortar formed in the air.
(2) This feature has good cohesive properties for fresh cement mortar mixed with HPMC, which is favorable for underwater concrete pumping, and has almost no bleeding.
(3) The content of HPMC and the water demand of mortar first decreased and then increased significantly.
(4) The addition of water reducing agent improves the problem of increasing water demand of mortar, but its content must be reasonably controlled, otherwise the underwater resistance of freshly mixed cement mortar will sometimes be reduced.
(5) There is little difference in structure between the cement paste specimens mixed with hydroxypropyl methylcellulose and the blank specimens, and the differences in the structure and compactness of the cement paste specimens poured in water and in the air are not big. The specimens molded under water for 28 days were slightly crispy. 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 non-dispersion effect under water, the dosage of HPMC should be reduced as much as possible.
(6) Adding HPMC underwater non-dispersible concrete admixture, and controlling the dosage is beneficial to the strength. The pilot project shows that the strength ratio of concrete formed in water to that formed in air is 84.8%, and the effect is relatively significant.