Static Yield Stress of Cement-based Grouting Material under Different Rheological Modes

NAN Xueli, CHEN Hao1,, LI Rongyang1,, JI Jianrui1,, WANG Yi1,, TANG Weibin

(1. School of Material Science and Engineering, Lanzhou University of Technology, Lanzhou 730050, China; 2. State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals, Lanzhou University of Technology, Lanzhou 730050, China; 3. Gansu Province Transportat Planning, Survey and Design Institute Co.,Ltd, Lanzhou 730000, China)

Abstract: To quantitatively estimate the workability of cement-based grouting material, from the perspective of rheology, the result of the static yield stress evaluated using the rate-controlled and stresscontrolled modes, respectively, was compared using the Rheowin rheometer. Also, the correlation of workability and solid concentration of slurry with static yield stress was studied. Results show that the static yield stress of cement-based grouting slurry relates to the established slurry structure, and is the shear stress corresponding to the transformation of elastics to plastics; In rate-controlled mode, the static yield stress of the slurry is related to the shear rate. The higher the shear rate, the greater the yield stress of the slurry. For the stress-controlled mode, the result is more accurate and suitable for testing static yield stress under different water-cement ratios.Since the water-cement ratio has a good correspondence with the static yield stress and the static yield stress has a good correspondence with the slump flow of the slurry, the static yield stress is the minimum stress to be overcome when the slurry begins to flow and it reflects the yield behavior and structural stability of the cement.

Key words: shear mode; stress-controlled mode; static yield stress; solid concentration

1 Introduction

Cement-based materials are the most important materials in all kinds of buildings[1]. Cement-based grouting material is mainly composed of cement, water and admixture, and is used rapidly to repair concrete structures[2]. Good workability is essential in the construction performance. The workability of the slurry is related to its rheological properties, and its rheological parameters such as static yield stress, dynamic yield stress, and plastic viscosity are important indicators to judge the workability of slurry.

Dynamic yield stress is the stress necessary to terminate the flow, after the slurry is sheared and the uniform microstructure is destroyed. It is achieved by fitting the flow curve through different rheological models. The static yield stress is the minimum stress that the slurry should overcome when it flows and is related to the microstructure of the slurry. Examples include the Van der Waals force, the electrostatic force between the cement particles or flocculation structure,and the bonding effect of the hydration products on the cement particles[3,4]. Xiao[5]and Mahoutet al[6]concluded that the static yield stress accounts for the real-time structural strength of the slurry. The greater the static yield stress of the slurry, the stronger the structural strength. Koehleret al[7]concluded that the static yield stress represents the minimum stress needed to destroy the flocculation structure of the fresh cement paste at a lower loading rate. Lowked[8]and Yuanet al[9]concluded that the static yield stress characterizes the thixotropy of fresh concrete structures. Because the static yield stress test does not damage the slurry structure, most scholars use a fixed shear rate to test the static yield stress of the slurry. However, the shear rate used by different scholars is different[9-14], and some determine the static yield stress using the stress relaxation and creep behavior of the slurry[4,15,16]. Although test methods differ, they all reduce the error caused by factors such as particle migration, uneven settlement, and container reasons. Therefore, the static yield stress of slurry more accurately reflects the establishment and failure of the slurry structure and better characterizes the yield behavior of the paste[17]. Therefore, it is theoretically and practically significant to test the static yield stress of cement-based grouting slurry.

This study presents a test for the static yield stress of cement-based grouting slurry through stress-controlled and rate-controlled modes. The test accuracy and the applicability of the two methods are compared,and a test method that can adapt to the static yield stress of the slurry under different water-cement ratio states is given. The method is verified by establishing the water-cement ratio and the correlation between the workability of the slurry and the static yield stress. Finally, the study identifies the role and significance of the static yield stress.

2 Experimental

2.1 Materials

The cement used in this study is 42.5-grade sulphoaluminate cement and is produced by Gansu Jinlan Company. The physical and chemical characteristics are given in Table 1. The water is tap water.

2.2 Experimental program

The effects of different rheological test modes on the static yield stress of fresh cement paste were studied using the water-cement ratio of 0.27, and 0.5. Adjusting the water-cement ratio to namely, 0.3, 0.35, 0.4,0.45, and 0.55, the relationship between the water-cement ratio and the static yield stress was studied.

Pour the cement and water of different water-cement ratios into the mixer. Stir slowly for two minutes,stop for 0.5 min, and stir quickly for two minutes. Then pour into a container of diameter 63 mm and a height of 100 mm. The rotor was inserted into the slurry slowly. After 30 s, the static yield stress test was performed using the Rheowin rheometer produced by the Thermo Fisher Company.

2.3 Rheological test modes

The static yield stress of fresh cement slurry was tested by rate-controlled and stress-controlled modes.The two test modes are described as follows.

Rate-controlled mode: The constant shear rates were set at 0.01, 0.1, 0.5, 1, 10, 20, 50, 100, and 150 s-1, respectively. Apply 30 s shear to the slurry while taking 100 points. It varies linearly until the yield stress is reached. After the yield stress is reached, the structure of the slurry is destroyed and flow occurs, while the shear stress drops. At this moment, the shear stress value is the static yield stress.

Stress-controlled mode: Set the initial stress range from 0 to 2 000 Pa. Then, a slowly increasing stress is applied to the slurry, and the deformation increases slowly and linearly at the elastic stage before the yield stress is reached. When the shear stress equals the yield stress, the slurry rapidly deforms. Then, the logarithm of stress and deformation is plotted, with tangent lines for the elastic and plastic stages drawn, respectively.The intersection point of the tangent lines is the static yield stress point. According to the static yield stress point, the shear stress test range is narrowed and the above steps are repeated three times.

3 Results and discussion

3.1 Effect of rate-controlled mode on the static yield stress of cement-based grouting material

The shear rate for elastoplastic fluids is zero when the shear stress is lower than the yield stress.When the shear stress exceeds the yield stress, the cement paste begins to flow. Fig.1 shows the variation of the shear stress of cement slurry with time under the rate-controlled mode at a constant shear rate. The figure shows that when the shear rate is 0.01 s-1, the shear stress of the slurry increases slowly and linearly with time, and there is no obvious yield point, and the slurry does not yield. When the water-cement ratio is 0.5, as shown in Fig.1(a), as the shear time increases,the shear stress of the slurry first increases rapidly to a certain value, and then decreases and remains stable.As the shear rate increases, the yield stress value of the slurry gradually increases, and the time to reach the yield stress value gradually decreases. The reason for this phenomenon is that the faster the shear rate is, the faster the slurry structure is destroyed, and at a highershear rate, the grid structure formed by overlapping the particle structures in the slurry under the action of Van der Waals force and electrostatic gravity will produce greater resistance on the rotor, and thus greater shear stress. When the water-cement ratio is 0.27, as shown in Fig.1(b), it can be found that the rheological curve trend of the slurry under different shear rates is equal to the water-cement ratio is 0.5. However, as the shear rate increases, the yield stress value of the slurry does not increase, and there is no definite regularity. However, because the water-cement ratio is too low, the dispersion of the cement is uneven and the flocculation and agglomeration are serious when no water-reducing agent is added, and the internal viscosity is large. Some bubbles and voids are not dense, which has a greater impact on the rheological results; However, this test mode method has certain limitations.

Table 1 Physical and chemical characteristics of sulphoaluminate cement

Fig.1 The curve of slurry shear stress under constant shear rate

3.2 Effect of stress-controlled mode on the static yield stress of cement-based grouting material

Fig.2 Deformation curve with stress

From the stress and deformation logarithmic curve in Fig.2, the slurry shows limited shear deformation under the action of shear stress in the elastic stage,which increases linearly. The flocculation structure of the slurry is not destroyed, and the slurry structure is restored when the stress is removed. When the shear stress exceeds the yield value of 900 Pa, the deformation increases rapidly. From the plastic stage, the flocculation structure of the slurry and the cemented structure of the hydration product are destroyed at this time.This method employs the logarithm of the deformation as the ordinate to intuitively indicate the state change of the slurry from static to yield, and the shear rate does not affect the slurry structure.

3.3 Comparison of the two modes

Fig.3 Yield stress comparison of cement paste under different test modes

According to Fig.3, in the rate-controlled mode,the larger the water-cement ratio, the more uniform the cement particle dispersion, and the more noticeable the static yield stress of the slurry changes with the shear rate. When the water-cement ratio is low, due to the large slurry viscosity, uneven dispersion, and the existence of voids in the container, the phenomenon of non-compact voids has a greater impact on the static yield stress of the slurry, and there is no certain regularity, and the reproducibility of the test results is poor.In the stress control mode, the static yield stress of different water-cement ratios is significantly lower than in the strain control mode, and the test results under different water-cement ratios have better reproducibility.

In the rate-controlled mode, the static yield stress of the same water-cement ratio and different shear rates are different. If the shear rate is too small, the yield stress of the slurry cannot be measured. The larger the shear stress, the larger the static yield stress. Therefore,to achieve different water-cement ratio slurry, different shear rates need to be selected to test their static yield stress. Some literature compared the influence of different shear rates on the yield stress of the slurry[13].They believed that much shear stress will cause particles to re-aggregate, increase the friction of the shear layer, and then increase the shear stress of the slurry.Shear rate 0.1 s-1accurately measures the yield stress of cement paste. Therefore, the subsequent static yield stress is tested in this way[14]. But this shear rate is only for cement paste when the water-cement ratio is 0.45,and it is not necessarily applicable to other water-cement ratios. Some literature relies on the time to reach the yield point to select the appropriate shear rate for testing the static yield stress, but these methods have certain limitations. Therefore, the stress-controlled mode is more suitable for testing the static yield stress of cement-based grouting material.

3.4 Effect of water-cement ratio on static yield stress

Table 2 shows the effect of the water-cement ratio on the yield stress of cement paste. The table shows that as the water-cement ratio increases, the yield stress of cement pasted gradually decreases. When the water-cement ratio increases from 0.27 to 0.35, the yield stress decreases rapidly. The reason for this phenomenon is that as the free water inside the slurry increases,the adsorption and friction effects between cement particles are reduced under the lubrication of free water.Especially, when the water-cement ratio increases from 0.57 to 0.63, the static yield stress of the slurry changes very little, free water further increases, and the solid-phase volume concentration in the slurry decreases below the minimum concentration at which the slurry starts to appear as a flocculated structure. The dispersion is uniform, and the yield stress of the water-cement ratio will not change if it continues to increase.It also quantitatively proves the theory of Roussel[18]that only when the slurry reaches a certain solid-phase volume concentration, the flocculation structure will appear. Below this solid-phase volume concentration,the yield stress of the slurry changes slightly.

Rosquoët[19]and Nguyen[20]et alestablished the volume fraction of solid-phase in cement paste and rheological parameters to explain the different rheological behavior of fresh cement paste and predict the rheological behavior of cement paste. Therefore, to verify the adaptability of the stress control mode to the test of the rheological properties of the slurry under different water-cement ratios, and to predict the rheological behavior of the cement slurry under different water-cement ratios, Eq.(1) and Eq.(2) are used to establish the water-cement ratio and solid concentration. The relationship between water-cement ratio and static yield stress is derived. The results are shown in Table 2, and the fitting results are shown in Fig.4.

whereτ0is the static yield stress, ϕ is the solid concentration,ρwis the water density,ρcis the cement density,cis the cement mass,wis the water mass,ris the water-cement ratio,aandbare the constant.

Fig.4 Fitting results of water-cement ratio and static yield stress

Table 2 The relationship between water-cement ratio and static yield stress

Fig.4 indicates that the fitting formula of the water-cement ratio and static yield stress can predict the rheological behavior of cement paste under different water-cement ratios. It also quantitatively proves that the static yield stress of the cement paste relates to the flocculation of particles in the paste. The smaller the solid-phase volume fraction, the less the internal flocculation structure of the paste, and the lower the static yield stress of the paste, respectively. Furthermore, It proves that this shear mode is appropriate for testing the static yield stress of cement pastes with various water-cement ratios.

3.5 The correlation between static yield stress and slump flow

Fig.5 Correlation between static yield stress and slump flow

Fig.5 shows that the static yield stress of cement mortar and pure slurry have a good correlation with the slump flow. The larger the slump flow, the lower the static yield stress. Alternately, it shows the static yield stress quantitatively evaluates the workability of the slurry. Both slump flow and static yield stress are related to the effect of Van der Waals force and the electrostatic force between the flocculation structure of the slurry and the connection of cement hydration products. Additionally, it shows that the static yield stress accurately reflects the yield behavior of the cement paste and reflects the structural stability of the paste.

4 Conclusions

a) The static yield stress of cement-based grouting slurry relates to the structural strength of the slurry and is the shear stress corresponding to the transition from elasticity to plasticity. The rate-controlled and stress-controlled modes can test the static yield stress of the slurry;

b) In the rate-controlled mode, the static yield stress of the slurry relates to the shear rate. When the shear rate is large, the static yield stress of the slurry is large. Also, if the shear rate is too low, the slurry will not yield;

c) In the stress-controlled mode, the measured static yield stress is more accurate and reproducible.Also, the water-cement ratio has a better correlation with the static yield stress; Therefore, the stress-controlled mode is more suitable for testing the static yield stress of cement paste under different water-cement ratios;

d) The static yield stress has a good correlation with the slump flow of the slurry, indicating that the static yield stress better reflects the yield behavior of the slurry.