Thickening is an important modification effect of cellulose ether on cement-based materials. The effects of cellulose ether content, viscometer rotation speed and temperature on the viscosity change of cellulose ether modified cement based plaster were studied. The results show that the viscosity of cement based plaster increases continuously with the increase of cellulose ether content, and the viscosity of cellulose ether solution and cement based plaster has a “composite superposition effect”; the pseudoplasticity of cellulose ether modified cement based plaster is lower than that of pure cement based plaster, and the viscosity The lower the rotation speed of the instrument, or the lower the viscosity of cellulose ether modified cement based plaster, or the lower the content of cellulose ether, the more obvious the pseudoplasticity of cellulose ether modified cement based plaster; With the combined effect of hydration, the viscosity of cellulose ether modified cement based plaster will increase or decrease. Different types of cellulose ether have different changes in the viscosity of the modified cement based plaster.
Key words: cellulose ether; cement based plaster; viscosity
0、Preface
Cellulose ethers are often used as water retention agents and thickeners for cement-based materials. According to different substituents, cellulose ethers used in cement-based materials generally include methyl cellulose (MC), hydroxyethyl cellulose (HEC), hydroxyethyl methyl cellulose Ether (Hydroxyethyl methyl cellulose, HEMC) and hydroxypropyl methyl cellulose (Hydroxypropyl methyl cellulose, HPMC), among which HPMC and HEMC are the most commonly used.
Thickening is an important modification effect of cellulose ether on cement-based materials. Cellulose ether can endow the wet mortar with excellent viscosity, significantly increase the bonding ability between the wet mortar and the base layer, and improve the anti-sag performance of the mortar. It can also increase the homogeneity and anti-dispersion ability of freshly mixed cement-based materials, and prevent the delamination, segregation and bleeding of mortar and concrete.
The thickening effect of cellulose ether on cement-based materials can be quantitatively evaluated by the rheological model of cement-based materials. Cement-based materials are usually regarded as Bingham fluid, that is, when the applied shear stress r is less than the yield stress r0, the material remains in its original shape and does not flow; when the shear stress r exceeds the yield stress r0, the object undergoes flow deformation, and the shear stress The stress r has a linear relationship with the strain rate y, that is, r=r0+f·y, where f is the plastic viscosity. Cellulose ethers generally increase the yield stress and plastic viscosity of cement-based materials, however, lower dosages lead to lower yield stress and plastic viscosity, mainly due to the air-entraining effect of cellulose ethers. Patural’s research shows that the molecular weight of cellulose ether increases, the yield stress of cement based plaster decreases, and the consistency increases.
The viscosity of cement based plaster is an important index to evaluate the thickening effect of cellulose ether on cement-based materials. Some literatures have explored the viscosity change law of cellulose ether solution, but there is still a lack of relevant research on the effect of cellulose ether on the viscosity change of cement based plaster. At the same time, according to the different types of substituents, there are many types of cellulose ethers. The impact of different types and viscosities of cellulose ethers on the change of cement based plaster viscosity is also a very concerned issue in the use of cellulose ethers. This work uses a rotational viscometer to study the viscosity changes of cellulose ether modified cement slurries of different types and viscosities under different poly-ash ratios, rotational speeds and temperatures.
1. Experiment
1.1 Raw materials
(1) Cellulose ether. Six kinds of cellulose ethers commonly used in my country were selected, including 1 kind of MC, 1 kind of HEC, 2 kinds of HPMC and 2 kinds of HEMC, among which the viscosities of 2 kinds of HPMC and 2 kinds of HEMC were obviously different. The viscosity of cellulose ether was tested by NDJ-1B rotational viscometer (Shanghai Changji Company), the concentration of the test solution was 1.0% or 2.0%, the temperature was 20°C, and the rotational speed was 12r/min.
(2) Cement. Ordinary Portland cement produced by Wuhan Huaxin Cement Co., Ltd. has a specification of P·O 42.5 (GB 175-2007).
1.2 Viscosity measurement method of cellulose ether solution
Take a cellulose ether sample of specified quality and add it to a 250mL glass beaker, then add 250g of hot water at about 90°C; stir fully with a glass rod to make the cellulose ether form a uniform dispersion system in the hot water, and at the same time put the beaker Cool in the air. When the solution starts to generate viscosity and will not precipitate again, stop stirring immediately; when the solution is cooled in the air until the color is uniform, put the beaker in a constant temperature water bath, and keep the temperature to the specified temperature. The error is ± 0.1°C; after 2h (calculated from the contact time of cellulose ether with hot water), measure the temperature of the center of the solution with a thermometer. Production) rotor inserted into the solution to the specified depth, after standing for 5min, measure its viscosity.
1.3 Viscosity measurement of cellulose ether modified cement based plaster
Before the experiment, keep all the raw materials at the specified temperature, weigh the specified mass of cellulose ether and cement, mix them thoroughly, and add tap water at the specified temperature into a 250mL glass beaker with a water-cement ratio of 0.65; then add the dry powder into the beaker and wait for 3 minutes Stir thoroughly with a glass rod for 300 times, insert the rotor of a rotational viscometer (NDJ-1B type, produced by Shanghai Changji Geological Instrument Co., Ltd.) into the solution to a specified depth, and measure its viscosity after standing for 2 minutes. In order to avoid the influence of cement hydration heat on the viscosity test of cement based plaster as much as possible, the viscosity of cellulose ether modified cement based plaster must be tested when the cement is in contact with water for 5 minutes.
2. Results and analysis
2.1 Effect of cellulose ether content
The amount of cellulose ether here refers to the mass ratio of cellulose ether to cement, that is, the polyash ratio. From the influence of P2, E2 and H1 three kinds of cellulose ethers on the viscosity change of cement based plaster at different dosages (0.1%, 0.3%, 0.6% and 0.9%), it can be seen that after adding cellulose ether, the viscosity of cement based plaster Viscosity increases; as the amount of cellulose ether increases, the viscosity of cement based plaster increases continuously, and the range of increase in viscosity of cement based plaster also becomes larger.
When the water-cement ratio is 0.65 and the cellulose ether content is 0.6%, considering the water consumed by the initial hydration of cement, the concentration of cellulose ether relative to water is about 1%. When the concentration is 1%, the P2, E2 and H1 aqueous solutions The viscosities are 4990mPa·S, 5070mPa·S and 5250mPa·s respectively; when the water-cement ratio is 0.65, the viscosity of pure cement based plaster is 836 mPa·S. However, the viscosities of P2, E2 and H1 three cellulose ether modified cement slurries are 13800mPa·S, 12900mPa·S and 12700mPa·s respectively. Obviously, the viscosity of cellulose ether modified cement based plaster is not the viscosity of cellulose ether solution and The simple addition of the viscosity of pure cement based plaster is significantly greater than the sum of the two viscosities, that is, the viscosity of the cellulose ether solution and the viscosity of the cement based plaster have a “composite superposition effect”. The viscosity of cellulose ether solution comes from the strong hydrophilicity of hydroxyl groups 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 based plaster comes from the network formed between cement hydration products structure. Since the polymer and cement hydration products often form an interpenetrating network structure, in the cellulose ether modified cement based plaster, the three-dimensional network structure of cellulose ether and the network structure of cement hydration products are intertwined, and the cellulose ether molecules Adsorption with cement hydration products together produces a “composite superposition effect”, which significantly increases the overall viscosity of cement based plaster; since one cellulose ether molecule can interweave with multiple cellulose ether molecules and cement hydration products, Therefore, with the increase of cellulose ether content, the density of the network structure increases more than the increase of cellulose ether molecules, and the viscosity of cement based plaster increases continuously; in addition, the rapid hydration of cement needs to react part of the water. , which is equivalent to increasing the concentration of cellulose ether, which is also a reason for the significant increase in the viscosity of cement based plaster.
Since cellulose ether and cement based plaster have a “composite superposition effect” in viscosity, under the same cellulose ether content and water-cement ratio conditions, the viscosity of cellulose ether modified cement based plaster with obvious difference when the concentration is 2% The viscosity difference is not big, for example, the viscosities of P2 and E2 are 48000mPa·s and 36700mPa·s respectively in the aqueous solution with concentration of 2%. S, the difference is not obvious; the viscosities of E1 and E2 in 2% aqueous solution are 12300mPa·S and 36700mPa·s respectively, the difference is very large, but the viscosities of their modified cement paste are 9800mPa·S and 12900mPa respectively ·S, the difference has been greatly reduced, so when choosing cellulose ether in engineering, it is not necessary to pursue excessively high cellulose ether viscosity. Moreover, in practical engineering applications, the concentration of cellulose ether relative to water is usually relatively low. For example, in ordinary plastering mortar, the water-cement ratio is usually about 0.65, and the content of cellulose ether is 0.2% to 0.6%. The concentration of water is between 0.3% and 1%.
It can also be seen from the test results that different types of cellulose ethers have different effects on the viscosity of cement based plaster. When the concentration is 1%, the viscosities of P2, E2 and H1 three kinds of cellulose ether aqueous solutions are 4990mPa·s, 5070mPa·S and 5250mPa·S respectively, the viscosity of H1 solution is the highest, but the viscosity of P2, E2 and H1 three kinds of cellulose ether The viscosities of the ether-modified cement slurries were 13800mPa·S, 12900mPa·S and 12700mPa·S respectively, and the viscosity of the H1 modified cement slurries was the lowest. This is because cellulose ethers usually have the effect of delaying cement hydration. Among the three kinds of cellulose ethers, HEC, HPMC and HEMC, HEC has the strongest ability to delay cement hydration. Therefore, in the H1 modified cement based plaster, due to the slower cement hydration, the network structure of cement hydration products develops slower, and the viscosity is the lowest.
2.2 Effect of rotation rate
From the influence of the rotation speed of the viscometer on the viscosity of pure cement based plaster and cellulose ether modified cement based plaster, it can be seen that as the rotation speed increases, the viscosity of cellulose ether modified cement based plaster and pure cement based plaster decreases to varying degrees , that is, they all have the property of shear thinning and belong to pseudoplastic fluid. The smaller the rotation rate, the greater the decrease of the viscosity of all cement based plaster with the rotation rate, that is, the more obvious the pseudoplasticity of the cement based plaster. With the increase of the rotation rate, the curve of viscosity decrease of the cement based plaster gradually becomes flatter, and the pseudoplasticity weakens. Compared with pure cement based plaster, the pseudoplasticity of cellulose ether modified cement based plaster is weaker, that is to say, the incorporation of cellulose ether reduces the pseudoplasticity of cement based plaster.
From the influence of rotation speed on the viscosity of cement based plaster under different cellulose ether types and viscosities, it can be known that cement based plaster modified with different cellulose ethers has different pseudoplastic strength, and the smaller the viscosity of cellulose ether, the higher the viscosity of the modified cement based plaster. The more obvious the pseudoplasticity of the cement based plaster is; the pseudoplasticity of the modified cement based plaster has no obvious difference with different types of cellulose ethers with similar viscosities. From P2, E2 and H1 three kinds of cellulose ether modified cement based plaster in different dosage (0.1%, 0.3%, 0.6% and 0.9%), the influence of rotation speed on the viscosity can be known, P2, E2 and H1 three kinds of fiber The cement slurries modified with plain ether have the same test results: when the amount of cellulose ether is different, their pseudoplasticity is different. The smaller the amount of cellulose ether, the stronger the pseudoplasticity of the modified cement based plaster.
After the cement is in contact with water, the cement particles on the surface are rapidly hydrated, and the hydration products (especially C-S-H gel) form an agglomeration structure. When there is a directional shear force in the solution, the agglomeration structure will open, so that along the direction of the shear force The directional flow resistance is reduced, thereby exhibiting the property of shear thinning. Cellulose ether is a kind of macromolecule with an asymmetric structure. When the solution is still, the cellulose ether molecules can have various orientations. When there is a directional shear force in the solution, the long chain of the molecule will turn and go along. The direction of the shear force is reduced, resulting in a decrease in flow resistance, and also exhibits the property of shear thinning. Compared with cement hydration products, cellulose ether molecules are more flexible and have a certain buffering capacity for shear force. Therefore, compared with pure cement based plaster, the pseudoplasticity of cellulose ether modified cement based plaster is weaker, and, as the viscosity or content of cellulose ether increases, the buffering effect of cellulose ether molecules on shear force is more obvious. Plasticity becomes weak.
2.3 The influence of temperature
From the effect of temperature changes (20°C, 27°C and 35°C) on the viscosity of cellulose ether modified cement based plaster, it can be seen that when the content of cellulose ether is 0.6%, as the temperature increases, the pure cement based plaster and M1 The viscosity of the modified cement based plaster increased, and the viscosity of other cellulose ether modified cement based plaster decreased, but the decrease was not large, and the viscosity of the H1 modified cement based plaster decreased the most. As far as the E2 modified cement based plaster is concerned, when the polyash ratio is 0.6%, the viscosity of the cement based plaster decreases with the increase of temperature, and when the polyash ratio is 0.3%, the viscosity of the cement based plaster increases with the increase of temperature.
Generally speaking, due to the decrease of the intermolecular interaction force, the viscosity of the fluid will decrease with the increase of temperature, which is the case for cellulose ether solution. However, as the temperature rises, and the contact time between cement and water increases, the speed of cement hydration will be significantly accelerated, and the degree of hydration will increase, so the viscosity of pure cement based plaster will increase instead.
In cellulose ether modified cement based plaster, cellulose ether will be adsorbed to the surface of cement hydration products, thereby inhibiting cement hydration, but different types and amounts of cellulose ethers have different abilities to inhibit cement hydration, MC (such as M1 ) has a weak ability to inhibit cement hydration, and as the temperature increases, the hydration rate of the cement based plaster is still faster, so as the temperature increases, the viscosity It is generally increased; HEC, HPMC and HEMC can significantly inhibit cement hydration, as the temperature increases, the hydration rate of cement based plaster is slower, so as the temperature increases, HEC, HPMC and HEMC modified cement The viscosity of based plaster (0.6% polyash ratio) is generally reduced, and because the ability of HEC to delay cement hydration is greater than that of HPMC and HEMC, the change of cellulose ether in temperature changes (20°C, 27°C and 35°C) The viscosity of H1 modified cement based plaster decreased the most with the increase of temperature. However, cement hydration still exists when the temperature is higher, so the degree of reduction of cellulose ether modified cement based plaster with the increase of temperature is not obvious. As far as the E2 modified cement based plaster is concerned, when the dosage is high (the ash ratio is 0.6%), the effect of inhibiting cement hydration is obvious, and the viscosity decreases with the increase of temperature; when the dosage is low (the ash ratio is 0.3%) ), the effect of inhibiting cement hydration is not obvious, and the viscosity increases with the increase of temperature.
3. Conclusion
(1) With the continuous increase of cellulose ether content, the viscosity and viscosity increase rate of cement based plaster continue to increase. The molecular network structure of cellulose ether and the network structure of cement hydration products are intertwined, and the initial hydration of cement indirectly increases the concentration of cellulose ether, so that the viscosity of cellulose ether solution and cement based plaster has a “composite superposition effect”, that is, cellulose ether The viscosity of the modified cement based plaster is much greater than the sum of their respective viscosities. Compared with HPMC and HEMC modified cement slurries, HEC modified cement slurries have lower viscosity test values due to slower hydration development.
(2) Both cellulose ether modified cement based plaster and pure cement based plaster have the property of shear thinning or pseudoplasticity; the pseudoplasticity of cellulose ether modified cement based plaster is lower than that of pure cement based plaster; the lower the rotation rate, or the cellulose The lower the viscosity of ether-modified cement based plaster, or the lower the content of cellulose ether, the more obvious the pseudoplasticity of cellulose ether-modified cement based plaster.
(3) As the temperature continues to rise, the speed and degree of cement hydration increase, so that the viscosity of pure cement based plaster gradually increases. Due to different types and amounts of cellulose ethers have different abilities to inhibit cement hydration, the viscosity of the modified cement paste varies with temperature.