Desulfurization gypsum is the flue gas produced by the combustion of sulfur-containing fuels (coal, petroleum), industrial solid waste produced during the desulfurization purification process, and the hemihydrate gypsum (chemical formula CaSO4· 0.5H2O), the performance is comparable to that of natural building gypsum. Therefore, there are more and more researches and applications of using desulfurized gypsum instead of natural gypsum to produce self-leveling materials. Organic polymer admixtures such as water reducing agent, water retaining agent and retarder are essential functional components in the composition of self-leveling mortar materials. The interaction and mechanism of the two with cementitious materials are issues worthy of attention one. Due to the characteristics of the formation process, the fineness of desulfurized gypsum is small (the particle size is mainly distributed between 40 and 60 μm), and the powder gradation is unreasonable, so the rheological properties of desulfurized gypsum are poor, and the mortar slurry prepared by it is often easier Segregation, stratification and bleeding occur. Cellulose ether is the most commonly used admixture in mortar, and its combined use with water reducing agent is an important guarantee to realize the comprehensive performance of desulfurized gypsum-based self-leveling materials such as construction performance and later mechanical and durability performance.
In this paper, the fluidity value is used as the control index (spreading degree 145 mm±5 mm), focusing on the impact of the content of cellulose ether and molecular weight (viscosity value) on the water consumption of desulfurized gypsum-based self-leveling materials, the loss of fluidity over time, and the coagulation The law of influence of basic properties such as time and early mechanical properties; at the same time, test the law of influence of cellulose ether on the heat release and heat release rate of desulfurized gypsum hydration, analyze its influence on the hydration process of desulfurized gypsum, and initially discuss this type of admixture Compatibility with desulfurization gypsum gelling system.
1. Raw materials and test methods
1.1 Raw materials
Gypsum powder: desulfurized gypsum powder produced by a company in Tangshan, the main mineral composition is hemihydrate gypsum, its chemical composition is shown in Table 1, and its physical properties are shown in Table 2.
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Admixtures include: cellulose ether (hydroxypropyl methylcellulose, HPMC for short); superplasticizer WR; defoamer B-1; EVA redispersible latex powder S-05, all of which are commercially available.
Aggregate: natural river sand, self-made fine sand sieved through a 0.6 mm sieve.
1.2 Test method
Fixed desulfurization gypsum: sand: water = 1:0.5:0.45, appropriate amount of other admixtures, fluidity as the control index (expansion 145 mm ± 5 mm), by adjusting the water consumption, respectively mixed with cementitious materials (desulfurization gypsum + Cement) 0, 0.5‰, 1.0‰, 2.0‰, 3.0‰ cellulose ether (HPMC-20,000); further fix the dosage of cellulose ether to 1‰, choose HPMC-20,000, HPMC-40,000 , HPMC-75,000, and HPMC-100,000 hydroxypropyl methylcellulose ethers with different molecular weights (corresponding numbers are H2, H4, H7.5, and H10 respectively), to study the dosage and molecular weight (viscosity value) of cellulose ether The impact of the changes on the properties of gypsum-based self-leveling mortar, and the influence of the two on the fluidity, setting time and early mechanical properties of the desulfurized gypsum self-leveling mortar mixture are discussed. The specific test method is carried out in accordance with the requirements of GB/T 17669.3-1999 “Determination of Mechanical Properties of Building Gypsum”.
The heat of hydration test is carried out using a blank sample of desulfurized gypsum and samples with cellulose ether content of 0.5‰ and 3‰, respectively, and the instrument used is a TA-AIR type heat of hydration tester.
2. Results and analysis
2.1 Effect of cellulose ether content on the basic properties of mortar
With the increase of the content, the workability and cohesion of the mortar are significantly improved, the loss of fluidity over time is significantly reduced, and the construction performance is more excellent, and the hardened mortar has no delamination phenomenon, and the surface smoothness, smoothness and Aesthetics have been greatly improved. At the same time, the water consumption of the mortar to achieve the same fluidity increased significantly. At 5‰, the water consumption increased by 102%, and the final setting time was prolonged by 100 min, which was 2.5 times that of the blank sample. The early mechanical properties of mortar decreased significantly with the increase of the content of cellulose ether. When the content of cellulose ether was 5‰, the 24 h flexural strength and compressive strength decreased to 18.75% and 11.29% of the blank sample respectively. The compressive strength is 39.47% and 23.45% of the blank sample respectively. It is worth noting that with the increase of the amount of water-retaining agent, the bulk density of mortar also decreased significantly, from 2069 kg/m3 at 0 to 1747 kg/m3 at 5‰, a decrease of 15.56%. The density of the mortar decreases and the porosity increases, which is one of the reasons for the obvious decrease in the mechanical properties of the mortar.
Cellulose ether is a non-ionic polymer. The hydroxyl groups on the cellulose ether chain and the oxygen atoms on the ether bond can combine with water molecules to form hydrogen bonds, turning free water into bound water, thereby playing a role in water retention. Macroscopically It is manifested as an increase in the cohesiveness of the slurry [5]. The increase in slurry viscosity will not only increase the water consumption, but also the dissolved cellulose ether will be adsorbed on the surface of gypsum particles, hindering the hydration reaction and prolonging the setting time; during the stirring process, a large number of air bubbles will also be introduced. Voids will form as the mortar hardens, eventually reducing the strength of the mortar. Comprehensively considering the unilateral water consumption of mortar mixture, construction performance, setting time and mechanical properties, and later durability, etc., the content of cellulose ether in desulfurized gypsum-based self-leveling mortar should not exceed 1‰.
2.2 The effect of molecular weight of cellulose ether on the performance of mortar
Usually, the higher the viscosity and the finer the fineness of cellulose ether, the better the water retention and increase the bonding strength. performance will be negatively affected. Therefore, the influence of cellulose ethers of different molecular weights on the basic properties of gypsum-based self-leveling mortar materials was further tested. The water demand of the mortar increased to a certain extent, but had no obvious effect on the setting time and fluidity. At the same time, the flexural and compressive strengths of mortar in different states showed a downward trend, but the decline was far less than the influence of cellulose ether content on the mechanical properties. In summary, the increase in the molecular weight of cellulose ether has no obvious effect on the performance of mortar mixtures. Considering the convenience of construction, low-viscosity and small-molecular-weight cellulose ether should be selected as desulfurized gypsum-based self-leveling materials.
2.3 Effect of cellulose ether on the heat of hydration of desulfurized gypsum
With the increase of the content of cellulose ether, the exothermic peak of hydration of desulfurized gypsum gradually decreased, and the time of the peak position was slightly delayed, while the exothermic heat of hydration decreased, but not obviously. This shows that cellulose ether can delay the hydration rate and hydration degree of desulfurized gypsum to a certain extent, so the dosage should not be too large, and should be controlled within 1‰. It can be seen that the colloidal film formed after cellulose ether meets water is adsorbed on the surface of desulfurized gypsum particles, which reduces the hydration rate of gypsum before 2 h. At the same time, its unique water retention and thickening effects delay the evaporation of slurry water and Dissipation is beneficial to the further hydration of desulfurized gypsum in the later stage. To sum up, when the appropriate dosage is controlled, cellulose ether has limited influence on the hydration rate and hydration degree of desulfurized gypsum itself. At the same time, the increase of cellulose ether content and molecular weight will significantly increase the viscosity of the slurry and show excellent water retention performance. In order to ensure the fluidity of the desulfurized gypsum self-leveling mortar, the water consumption will increase significantly, which is due to the prolonged setting time of the mortar. The main reason for the decline in mechanical properties.
3. Conclusion
(1) When fluidity is used as the control index, with the increase of cellulose ether content, the setting time of desulfurized gypsum-based self-leveling mortar is significantly prolonged, and the mechanical properties are significantly reduced; compared with the content, the molecular weight of cellulose ether The increase has little effect on the above properties of mortar. Considering comprehensively, cellulose ether should be selected with a small molecular weight (viscosity value lower than 20 000 Pa·s), and the dosage should be controlled within 1‰ of the cementitious material.
(2) The test results of hydration heat of desulfurized gypsum show that within the scope of this test, cellulose ether has limited influence on the hydration rate and hydration process of desulfurized gypsum. The increase in water consumption and the decrease in bulk density are the main reasons for the decrease in the mechanical properties of desulfurized gypsum-based mortar.
Post time: May-08-2023