Thickening is an important modification effect of cellulose ether on cement-based materials. The effects of cellulose ether dosage, viscosity meter rotation speed, and temperature on the viscosity changes of cellulose ether modified cement slurry were studied. The results showed that as the content of cellulose ether increased, the viscosity of the cement slurry continued to increase, and the viscosity of the cellulose ether solution and cement slurry had a “composite superposition effect”; The pseudoplasticity of cellulose ether modified cement slurry is lower than that of pure water slurry. The lower the rotation speed of the viscometer, the lower the viscosity of cellulose ether modified cement slurry, or the lower the content of cellulose ether, the more obvious the pseudoplasticity of cellulose ether modified cement slurry; As the temperature increases, the viscosity of cellulose ether modified cement slurry will increase or decrease due to the combined effect of temperature and cement hydration. The viscosity variation of modified cement slurry varies depending on the type of cellulose ether.
Cellulose ether is commonly used as a water retaining agent and thickener for cement-based materials. According to the different substituents, cellulose ethers commonly used in cement-based materials include methyl cellulose ether, hydroxyethyl cellulose ether, hydroxyethyl methyl cellulose ether, and hydroxypropyl methyl cellulose ether. Among them, HPMC and HEMC are the most commonly used applications. Thickening effect is an important modification effect of cellulose ether on cement-based materials. Cellulose ether can endow wet mortar with excellent viscosity, significantly increase the bonding ability between wet mortar and base layer, and improve the anti sagging performance of mortar. It can also increase the uniformity and anti dispersion ability of freshly mixed cement-based materials, preventing delamination, segregation, and bleeding of mortar and concrete. The thickening effect of cellulose ether on cement-based materials can be quantitatively evaluated through the rheological model of cement-based materials. Research has shown that as the molecular weight of cellulose ether increases, the yield stress of cement slurry decreases and the viscosity increases. The viscosity of cement slurry is an important indicator for evaluating the thickening effect of cellulose ether on cement-based materials. Some literature has explored the viscosity change law of cellulose ether solutions, but there is still a lack of relevant research on the influence of cellulose ether on the viscosity change of cement slurry. The impact of different types and viscosities of cellulose ethers on the viscosity of cement slurry is also a very concerned issue during the use of cellulose ethers.
1. Measurement method for viscosity of cellulose ether solution
Take a cellulose ether sample of the specified quality, add it to a 250mL glass beaker, and then add 250g of hot water at around 90 ℃; Stir thoroughly with a glass rod to form a uniform dispersion system of cellulose ether in hot water, while cooling the beaker in air. Stop stirring immediately when the solution begins to develop viscosity and no longer precipitates; When the solution is cooled to a uniform color in air, place the beaker in a constant temperature water bath and keep it at the specified temperature with an error of ± 0.1 ℃; After 2 hours (calculated from the time the cellulose ether comes into contact with hot water), measure the temperature at the center of the solution with a thermometer. If the specified temperature has been reached, insert the rotating viscosity rotor into the solution to the specified depth, let it stand for 5 minutes, and then measure its viscosity.
2. Viscosity measurement of cellulose ether modified cement slurry
Before the experiment, all raw materials should be kept at the specified temperature. A specified amount of cellulose ether and cement should be weighed and thoroughly mixed. Then, tap water at the specified temperature should be added to a 250mL glass beaker at a water cement ratio of 0.65; Add the dry powder to the beaker again, stir thoroughly 300 times with a glass rod within 3 minutes, insert the rotor of the rotary viscometer into the solution to the specified depth, let it stand for 2 minutes, and then measure its viscosity. In order to minimize the impact of cement hydration heat on the viscosity testing of cement slurry, the viscosity of cellulose ether modified cement slurry must be tested after 5 minutes of contact between cement and water.
3. Results and Analysis
The influence of cellulose ether content refers to the mass ratio of cellulose ether to cement, namely the fly ash ratio. This reflects the influence of three types of cellulose ethers, P2, E2, and H1, on the viscosity changes of cement slurry at different dosages (0.1%, 0.3%, 0.6%, and 0.9%). The test temperature is 20 ℃ and the rotation rate of the rotary viscometer is 12r/min. After adding cellulose ether, the viscosity of the cement slurry increases; As the content of cellulose ether increases, the viscosity of the cement slurry continues to increase, and the magnitude of the increase in viscosity of the cement slurry also increases. When the water cement ratio is 0.65 and the cellulose ether content is 0.6%, considering the initial water consumption for cement hydration, the concentration of cellulose ether relative to water is about 1%. Obviously, the viscosity of cellulose ether modified cement slurry is not simply the sum of the viscosity of cellulose ether solution and pure water slurry, but significantly greater than the sum of their viscosities, that is, the viscosity of cellulose ether solution and the viscosity of cement slurry have a “composite superposition effect”. The viscosity of cellulose ether solution comes from the strong hydrophilicity of hydroxyl and ether bonds in cellulose ether molecules and the three-dimensional network structure formed by cellulose ether molecules in the solution; The viscosity of pure cement slurry comes from the network structure formed between cement hydration products. Due to the interpenetrating network structure often formed between polymers and cement hydration products, in cellulose ether modified cement slurry, the three-dimensional network structure of cellulose ether and the network structure of cement hydration products interweave, and the adsorption between cellulose ether molecules and cement hydration products jointly produce a “composite stacking effect”, resulting in a significant increase in the overall viscosity of the cement slurry; Due to the fact that one cellulose ether molecule can interweave with multiple cellulose ether molecules and cement hydration products, as the content of cellulose ether increases, the degree of increase in network structure density is higher than the degree of increase in cellulose ether molecules, and the degree of increase in cement slurry viscosity continues to increase; In addition, the rapid hydration of cement requires the reaction of a portion of water, which increases the concentration of cellulose ether, which is also one of the reasons for the significant increase in viscosity of cement slurry.
(1) With the continuous increase of cellulose ether content, the viscosity and viscosity increase of cement slurry continue to increase. The interweaving of cellulose ether molecular network structure and cement hydration product network structure, as well as the indirect increase in cellulose ether concentration during the initial hydration of cement, result in a “composite superposition effect” in the viscosity of cellulose ether solution and cement slurry, that is, the viscosity of cellulose ether modified cement slurry is much greater than the sum of their respective viscosities. Compared with HPMC and HEMC modified cement slurry, HEC modified cement slurry has slower hydration development and lower viscosity test values.
(2) Both cellulose ether modified cement slurry and pure water slurry have shear thinning properties or pseudoplasticity; The pseudoplasticity of cellulose ether modified cement slurry is lower than that of pure water slurry; The lower the rotation rate, or the lower the viscosity of cellulose ether modified cement slurry, or the lower the dosage of cellulose ether, the more obvious the pseudoplasticity of cellulose ether modified cement slurry.
(3) As the temperature continues to rise, the speed and degree of cement hydration increase, resulting in a gradual increase in the viscosity of pure water slurry. Due to the varying abilities of different types and dosages of cellulose ether to inhibit cement hydration, there are differences in the trend of viscosity variation of modified cement slurry with increasing temperature.
