Hydroxypropyl methyl cellulose 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. It can be dissolved in water to form a solution or dispersion. Experiments show that when the amount of naphthalene-based superplasticizer increases, the incorporation of water-reducing agent will reduce the dispersion resistance of fresh cement mortar. This is because the naphthalene-based superplasticizer is a surface active agent. When the water reducer is added to the mortar, the water reducer is aligned 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 is dismantled, and the water wrapped in the structure is released, which will cause a part of the cement to be lost. At the same time, it was found that with the increase of HPMC content, the dispersion resistance of fresh cement mortar became better and better.
Strength characteristics of concrete:
HPMC underwater non-dispersible concrete admixture is applied in the bridge foundation engineering of expressway, and the design strength grade is C25. After basic test, the cement dosage is 400kg, the compound mixed with silica fume is 25kg/m3, the optimum dosage of HPMC is 0.6% of the cement dosage, the water-cement ratio is 0.42, the sand ratio is 40%, and the output of naphthalene-based superplasticizer is 8% of the cement content, 28 days of concrete specimens in air, the average strength of 42.6MPa, 28 days of underwater pouring concrete with a drop height of 60mm in water, the average strength of 36.4MPa, the strength ratio of concrete in water to that in air It is 84.8%, the effect is quite significant.
1. The addition of HPMC has an obvious retarding effect on the mortar mixture. With the increase of the HPMC content, the setting time of the mortar is successively extended. Under the same HPMC content, the mortar formed underwater is better than that in the air. The setting time of molding is longer. This feature is beneficial for underwater concrete pumping.
2. The fresh cement mortar mixed with hydroxypropyl methylcellulose has good cohesive properties and almost no bleeding.
3, HPMC content and water demand of mortar show a decrease first and then a significant increase.
4. The incorporation of water-reducing agent improves the problem of increasing water demand of mortar, but it must be reasonably controlled, otherwise the underwater dispersion resistance of fresh cement mortar will sometimes be reduced.
5. There is little difference in structure between the cement paste specimens mixed with HPMC and the blank specimens, and the structure and compactness of cement paste specimens poured in water and in the air are not much different. The specimens formed under water for 28 days are slightly loose. The main reason is that the addition of HPMC greatly reduces the loss and dispersion of cement during pouring in water, but at the same time it also reduces the compactness of the cement stone. In the project, under the condition of ensuring the non-dispersion effect under water, the content of HPMC should be reduced as much as possible.
6. Mixing of HPMC underwater non-dispersible concrete admixture and controlling the mixing amount is beneficial to the strength. The pilot project shows that the strength ratio of concrete formed in water to that in air is 84.8%, and the effect is more significant.