By measuring the mechanical properties, water retention rate, setting time and heat of hydration of cellulose ether with different viscosities in different dosages of cement paste, and using SEM to analyze the hydration products, the effect of cellulose ether on the performance of cement paste was studied. law of influence. The results show that the addition of cellulose ether can delay cement hydration, delay cement hardening and setting, reduce hydration heat release, prolong the appearance time of hydration temperature peak, and the retarding effect increases with the increase of dosage and viscosity. Cellulose ether can increase the water retention rate of mortar, and can improve the water retention of mortar with thin-layer structure, but when the content exceeds 0.6%, the increase in water retention effect is not significant; the content and viscosity are the factors that determine the cellulose modified cement slurry. In the application of cellulose ether modified mortar, the dosage and viscosity should be mainly considered.
Key words: cellulose ether; dosage; retardation; water retention
Construction mortar is one of the necessary building materials for construction projects. In recent years, with the large-scale application of wall insulation materials and the improvement of anti-crack and anti-seepage requirements for external walls, higher requirements have been put forward for the crack resistance, bonding performance and construction performance of mortar. Due to the shortcomings of large drying shrinkage, poor impermeability, and low tensile bond strength, traditional mortar often cannot meet the construction requirements, or cause problems such as falling off of decorative materials. Such as plastering mortar, because the mortar loses water too quickly, the setting and hardening time is shortened, and problems such as cracking and hollowing occur during large-scale construction, which seriously affect the quality of the project. The traditional mortar loses water too fast and the cement hydration is insufficient, resulting in a short opening time of the cement mortar, which is the key to affecting the performance of the mortar.
Cellulose ether has a good thickening and water retention effect, and has been widely used in the field of mortar, and has become an indispensable admixture to improve the water retention of mortar and provide construction performance, effectively alleviating the construction and later use of traditional mortar. The problem of water loss in the medium. Cellulose used in mortar usually includes methyl cellulose ether (MC), hydroxypropyl methyl cellulose (HPMC), hydroxyethyl methyl cellulose ether (HEMC), hydroxyethyl cellulose ether (HEC), etc. Among them, HPMC and HEMC are the most widely used.
This paper mainly studies the effect of cellulose ether on the workability (water retention rate, water loss and setting time), mechanical properties (compressive strength and tensile bond strength), hydration law and microstructure of cement paste. It provides support for the properties of cellulose ether modified cement paste and provides reference for the application of cellulose ether modified mortar.
1. Experiment
1.1 Raw materials
Cement: Ordinary Portland cement (P.O 42.5) cement produced by Wuhan Yadong Cement Company, with a specific surface area of 3500 cm²/g.
Cellulose ether: commercially available hydroxypropyl methylcellulose ether (MC-5, MC-10, MC-20, viscosities of 50,000 Pa·S, 100000 Pa·S, 200000 Pa·S, respectively).
1.2 Method
Mechanical properties: In the process of sample preparation, the dosage of cellulose ether is 0.0%~1.0% of the cement mass, and the water-cement ratio is 0.4. Before adding water and stirring, mix the cellulose ether and cement evenly. A cement paste with a sample size of 40 x 40 x 40 was used for testing.
Setting time: The measurement method is carried out according to GB/T 1346-2001 “Cement Standard Consistency Water Consumption, Setting Time, Stability Test Method”.
Water retention: The test of the water retention of cement paste refers to the standard DIN 18555 “Testing method for inorganic cementitious material mortar”.
Heat of hydration: The experiment adopts the TAM Air microcalorimeter of TA Instrument Company of the United States, and the water-cement ratio is 0.5.
Hydration product: Stir water and cellulose ether evenly, then prepare cement slurry, start timing, take samples at different time points, stop hydration with absolute ethanol for testing, and the water-cement ratio is 0.5.
2. Results and discussion
2.1 Mechanical properties
From the influence of cellulose ether content on strength, it can be seen that with the increase of MC-10 cellulose ether content, the strengths of 3d, 7d and 28d all decrease; cellulose ether reduces the strength of 28d more significantly. From the influence of cellulose ether viscosity on strength, it can be seen that whether it is cellulose ether with a viscosity of 50,000 or 100,000 or 200,000, the strength of 3d, 7d, and 28d will decrease. It can also be seen that cellulose ether Viscosity has no significant effect on strength.
2.2 Setting time
From the effect of the content of 100,000 viscosity cellulose ether on the setting time, it can be seen that with the increase of the content of MC-10, both the initial setting time and the final setting time increase. When the content is 1%, the initial setting time It reached 510min, and the final setting time reached 850min. Compared with the blank, the initial setting time was prolonged by 210min, and the final setting time was prolonged by 470min.
From the influence of cellulose ether viscosity on setting time, it can be seen that whether it is MC-5, MC-10 or MC-20, it can delay the setting of cement, but compared with the three cellulose ethers, the initial setting time and final setting The time prolongs with the increase of viscosity. This is because cellulose ether can be adsorbed on the surface of cement particles, thereby preventing water from contacting with cement particles, thereby delaying cement hydration. The greater the viscosity of cellulose ether, the thicker the adsorption layer on the surface of cement particles, and the more significant the retarding effect.
2.3 Water retention rate
From the influence law of cellulose ether content on water retention rate, it can be seen that with the increase of the content, the water retention rate of mortar increases, and when the cellulose ether content is greater than 0.6%, the water retention rate is stable in the region. However, comparing the three cellulose ethers, there are differences in the influence of viscosity on the water retention rate. Under the same dosage, the relationship between the water retention rate is: MC-5≤MC-10≤MC-20.
2.4 Heat of hydration
From the effect of cellulose ether type and content on heat of hydration, it can be seen that with the increase of MC-10 content, the exothermic heat of hydration decreases gradually, and the time of hydration temperature peak shifts later; The heat of hydration also had a great influence. With the increase of viscosity, the heat of hydration decreased significantly, and the peak of hydration temperature shifted significantly later. It shows that cellulose ether can delay cement hydration, and its retarding effect is related to the content and viscosity of cellulose ether, which is consistent with the analysis result of setting time.
2.5 Analysis of hydration products
From the SEM analysis of the 1d hydration product, it can be seen that when 0.2% MC-10 cellulose ether is added, a large amount of unhydrated clinker and ettringite with better crystallization can be seen. %, the ettringite crystals are significantly reduced, which shows that cellulose ether can delay the hydration of cement and the formation of hydration products at the same time. By comparing the three kinds of cellulose ethers, it can be found that MC-5 can make the crystallization of ettringite in the hydration products more regular, and the crystallization of ettringite is more regular. related to the thickness of the layer.
3. Conclusion
a. The addition of cellulose ether will delay the hydration of cement, delay the hardening and setting of cement, reduce the heat release of hydration, and prolong the appearance time of hydration temperature peak. With the increase of dosage and viscosity, the retarding effect will increase.
b. Cellulose ether can increase the water retention rate of mortar, and can improve the water retention of mortar with thin-layer structure. Its water retention is related to the dosage and viscosity. When the dosage exceeds 0.6%, the water retention effect does not increase significantly.