Effect of hydroxyethyl methylcellulose on the cement mortar

The influence of factors such as the viscosity change of hydroxyethyl methylcellulose (HEMC), whether it is modified or not, and the content change on the yield stress and plastic viscosity of fresh cement mortar was studied. For unmodified HEMC, the higher the viscosity, the lower the yield stress and plastic viscosity of the mortar; the influence of the viscosity change of modified HEMC on the rheological properties of mortar is weakened; no matter whether it is modified or not, the higher the viscosity of HEMC, the lower the The retardation effect of the yield stress and plastic viscosity development of mortar is more obvious. When the content of HEMC is greater than 0.3%, the yield stress and plastic viscosity of the mortar increase with the increase of the content; when the content of HEMC is large, the yield stress of the mortar decreases with time, and the range of plastic viscosity increases with time.

Key words: hydroxyethyl methylcellulose, fresh mortar, rheological properties, yield stress, plastic viscosity

I. Introduction

With the development of mortar construction technology, more and more attention has been paid to mechanized construction. Long-distance vertical transportation puts forward new requirements for pumped mortar: good fluidity must be maintained throughout the pumping process. This needs to study the influencing factors and restrictive conditions of mortar fluidity, and the common method is to observe the rheological parameters of mortar.

The rheological properties of mortar mainly depend on the nature and amount of raw materials. Cellulose ether is an admixture widely used in industrial mortar, which has a great influence on the rheological properties of mortar, so scholars at home and abroad have conducted some research on it. In summary, the following conclusions can be drawn: an increase in the amount of cellulose ether will lead to an increase in the initial torque of the mortar, but after a period of stirring, the flow resistance of the mortar will decrease instead (1); when the initial fluidity is basically the same, the fluidity of the mortar will be lost first. increased after decreasing (2); the yield strength and plastic viscosity of mortar showed a trend of decreasing first and then increasing, and cellulose ether promoted the destruction of mortar structure and prolonged the time from destruction to reconstruction (3); Ether and thickened powder have higher viscosity and stability etc. (4). However, the above studies still have shortcomings:

The measurement standards and procedures of different scholars are not uniform, and the test results cannot be compared accurately; the testing range of the instrument is limited, and the rheological parameters of the measured mortar have a small range of variation, which is not widely representative; there is a lack of comparative tests on cellulose ethers with different viscosities; There are many influencing factors, and the repeatability is not good. In recent years, the appearance of the Viskomat XL mortar rheometer has provided great convenience for the accurate determination of the rheological properties of mortar. It has the advantages of high automatic control level, large capacity, wide test range, and test results more in line with actual conditions. In this paper, based on the use of this type of instrument, the research results of existing scholars are synthesized, and the test program is formulated to study the effect of different types and viscosities of hydroxyethyl methylcellulose (HEMC) on the rheology of mortar in a larger dosage range. performance impact.

2. Rheological model of fresh cement mortar

Since rheology was introduced into cement and concrete science, a large number of studies have shown that fresh concrete and mortar can be regarded as Bingham fluid, and Banfill further elaborated the feasibility of using Bingham model to describe the rheological properties of mortar (5). In the rheological equation τ=τ0+μγ of the Bingham model, τ is the shear stress, τ0 is the yield stress, μ is the plastic viscosity, and γ is the shear rate. Among them, τ0 and μ are the two most important parameters: τ0 is the minimum shear stress that can make the cement mortar flow, and only when τ>τ0 acts on the mortar, the mortar can flow; μ reflects the viscous resistance when the mortar flows The larger the μ, the slower the mortar flows [3]. In the case where both τ0 and μ are unknown, the shear stress must be measured at at least two different shear rates before it can be calculated (6).

In a given mortar rheometer, the N-T curve obtained by setting the blade rotation rate N and measuring the torque T generated by the shear resistance of the mortar can also be used to calculate another equation T=g+ that conforms to the Bingham model The two parameters g and h of Nh. g is proportional to the yield stress τ0, h is proportional to the plastic viscosity μ, and τ0 = (K/G)g, μ = ( l / G ) h , where G is a constant related to the instrument, and K can be passed through the known flow It is obtained by correcting the fluid whose characteristics change with the shear rate[7]. For the sake of convenience, this paper directly discusses g and h, and uses the changing law of g and h to reflect the changing law of the yield stress and plastic viscosity of mortar.

3. Test

3.1 Raw materials

3.2 sand

Quartz sand: coarse sand is 20-40 mesh, medium sand is 40-70 mesh, fine sand is 70-100 mesh, and the three are mixed in a ratio of 2:2:1.

3.3 Cellulose ether

Hydroxyethyl methylcellulose HEMC20 (viscosity 20000 mPa s), HEMC25 (viscosity 25000 mPa s), HEMC40 (viscosity 40000 mPa s), and HEMC45 (viscosity 45000 mPa s), of which HEMC25 And HEMC45 is a modified cellulose ether.

3.4 Mixing water

tap water.

3.5 Test plan

The lime-sand ratio is 1:2.5, the water consumption is fixed at 60% of the cement consumption, and the HEMC content is 0-1.2% of the cement consumption.

First mix the accurately weighed cement, HEMC and quartz sand evenly, then add the mixing water according to GB/T17671-1999 and stir, and then use the Viskomat XL mortar rheometer to test. The test procedure is: the speed is rapidly increased from 0 to 80rpm at 0~5min, 60rpm at 5~7min, 40rpm at 7~9min, 20rpm at 9~11min, 10rpm at 11~13min, and 5rpm at 13~15min , 15~30min, the speed is 0rpm, and then cycle once every 30min according to the above procedure, and the total test time is 120min.

4. Results and discussion

4.1 Effect of HEMC viscosity change on the rheological properties of cement mortar

(The amount of HEMC is 0.5% of the cement mass), correspondingly reflecting the variation law of the yield stress and plastic viscosity of the mortar. It can be seen that although the viscosity of HEMC40 is higher than that of HEMC20, the yield stress and plastic viscosity of mortar mixed with HEMC40 are lower than those of mortar mixed with HEMC20; although the viscosity of HEMC45 is 80% higher than that of HEMC25, the yield stress of mortar is slightly lower, and the plastic viscosity is between After 90 minutes there was an increase. This is because the higher the viscosity of cellulose ether, the slower the dissolution rate, and the longer it takes for the mortar prepared with it to reach the final viscosity [8]. In addition, at the same moment in the test, the bulk density of the mortar mixed with HEMC40 was lower than that of the mortar mixed with HEMC20, and that of the mortar mixed with HEMC45 was lower than that of the mortar mixed with HEMC25, indicating that HEMC40 and HEMC45 introduced more air bubbles, and the air bubbles in the mortar have ” “Ball” effect, which also reduces the mortar flow resistance.

After adding HEMC40, the yield stress of mortar was in equilibrium after 60 minutes, and the plastic viscosity increased; after adding HEMC20, the yield stress of mortar reached equilibrium after 30 minutes, and the plastic viscosity increased. It shows that HEMC40 has a greater retarding effect on the development of mortar yield stress and plastic viscosity than HEMC20, and takes longer to reach the final viscosity.

The yield stress of the mortar mixed with HEMC45 decreased from 0 to 120 minutes, and the plastic viscosity increased after 90 minutes; while the yield stress of the mortar mixed with HEMC25 increased after 90 minutes, and the plastic viscosity increased after 60 minutes. It shows that HEMC45 has a greater retarding effect on the development of mortar yield stress and plastic viscosity than HEMC25, and the time required to reach the final viscosity is also longer.

4.2 The effect of HEMC content on the yield stress of cement mortar

During the test, the factors affecting the yield stress of mortar are: mortar delamination and bleeding, structure damage by stirring, formation of hydration products, reduction of free moisture in mortar, and retarding effect of cellulose ether. For the retarding effect of cellulose ether, the more generally accepted view is to explain it by the adsorption of admixtures.

It can be seen that when HEMC40 is added and its content is less than 0.3%, the yield stress of mortar decreases gradually with the increase of HEMC40 content; when the content of HEMC40 is greater than 0.3%, the mortar yield stress gradually increases. Due to bleeding and delamination of the mortar without cellulose ether, there is not enough cement paste between the aggregates to lubricate, resulting in an increase in the yield stress and difficulty in flowing. Proper addition of cellulose ether can effectively improve the mortar delamination phenomenon, and the introduced air bubbles are equivalent to tiny “balls”, which can reduce the yield stress of the mortar and make it easy to flow. As the content of cellulose ether increases, its fixed moisture content also increases gradually. When the content of cellulose ether exceeds a certain value, the influence of the reduction of free moisture begins to play a leading role, and the yield stress of mortar increases gradually.

When the amount of HEMC40 is less than 0.3%, the yield stress of the mortar decreases gradually within 0-120min, which is mainly related to the increasingly serious delamination of the mortar, because there is a certain distance between the blade and the bottom of the instrument, and the aggregate after delamination sinking to the bottom, the upper resistance becomes smaller; when the HEMC40 content is 0.3%, the mortar will hardly delaminate, the adsorption of cellulose ether is limited, the hydration is dominant, and the yield stress has a certain increase; the HEMC40 content is When the content of cellulose ether is 0.5%-0.7%, the adsorption of cellulose ether increases gradually, the hydration rate decreases, and the development trend of the yield stress of mortar begins to change; On the surface, the rate of hydration is lower and the yield stress of the mortar decreases with time.

4.3 Effect of HEMC content on the plastic viscosity of cement mortar

It can be seen that after adding HEMC40, the plastic viscosity of mortar increases gradually with the increase of HEMC40 content. This is because cellulose ether has a thickening effect, which can increase the viscosity of the liquid, and the greater the dosage, the greater the viscosity of the mortar. The reason why the plastic viscosity of the mortar decreases after adding 0.1% HEMC40 is also due to the “ball” effect of the introduction of air bubbles, and the reduction of bleeding and delamination of the mortar.

The plastic viscosity of ordinary mortar without adding cellulose ether gradually decreases with time, which is also related to the lower density of the upper part caused by the layering of the mortar; when the content of HEMC40 is 0.1%-0.5%, the mortar structure is relatively uniform, and the mortar structure is relatively uniform after 30 minutes. The plastic viscosity does not change much. At this time, it mainly reflects the viscosity effect of cellulose ether itself; after the content of HEMC40 is greater than 0.7%, the plastic viscosity of mortar increases gradually with the increase of time, because the viscosity of mortar is also related to that of cellulose ether. The viscosity of the cellulose ether solution increases gradually within a period of time after the start of mixing. The greater the dosage, the more significant the effect of increasing with time.

V. Conclusion

Factors such as the viscosity change of HEMC, whether it is modified or not, and the change of the dosage will significantly affect the rheological properties of the mortar, which can be reflected by the two parameters of yield stress and plastic viscosity.

For unmodified HEMC, the greater the viscosity, the lower the yield stress and plastic viscosity of the mortar within 0-120min; the influence of the viscosity change of modified HEMC on the rheological properties of mortar is weaker than that of unmodified HEMC; no matter the modification Whether it is permanent or not, the greater the viscosity of HEMC, the more significant the delaying effect on the development of mortar yield stress and plastic viscosity.

When adding HEMC40 with a viscosity of 40000mPa·s and its content is greater than 0.3%, the yield stress of the mortar increases gradually; when the content exceeds 0.9%, the yield stress of the mortar begins to show a trend of gradually decreasing with time; The plastic viscosity increases with the increase of HEMC40 content. When the content is greater than 0.7%, the plastic viscosity of mortar begins to show a trend of increasing gradually with time.