HAO Meimei, LI Xiaolong＊, WANG Xinling, ZHONG Yanhui,ZHANG Bei, WANG Fuming, ZHANG Yulong＊

(1. School of Water Conservancy Engineering, Zhengzhou University, Zhengzhou 450001, China; 2. School of Civil Engineering, Zhengzhou University, Zhengzhou 450001, China)

Abstract: The viscosity evolution for different temperatures was studied experimentally. A timevarying viscosity model was derived and the influence of the initial temperature on gel time was analyzed.The experimental results show that the viscosity of polymer grout increases exponentially with time. It can be divided into two phases. Before gelation, the viscosity variable quantity is very small. At the gel point, there is a sudden increase in viscosity. The initial viscosity and gel time decrease with the increasing initial temperature within a certain range, The study contributes to deepening understanding of the rheological properties of polymer grout, which can provide some references for polymer grouting construction.

Key words: polymer grout; viscosity; initial temperature; gel time

1 Introduction

Grouting technique, an important means of antiseepage and reinforcement for rock mass, has been widely applied to engineering fields, such as water conservancy, transportation and mining. The success of the grouting project depends on an appropriate selection of the grout material and the reasonable design of the grouting procedure. As one of the most popular solution grout, polymer grout has been used for nearly 50 years. It attracts wide attention around the world and develops rapidly due to its lightweight,fast reaction, high expansion rate, impermeability, and durability[1-9].

A critical parameter of rheological properties,viscosity is needed to determine the grout diffusion and its relation to the injection period. Saeidiet almade an attempt to simulate the flow of grout with different viscosity in fracture network[10]. The results show that the penetration distance and flow rate decrease with the increase of the grout viscosity. Erikssonet al[11]repeatedly carried out rheological measurements of the cement-based grouts to ascertain the most suitable grouting mixture. Zhuet al[12]found that the difference in the grouting pressure between cementsodium silicate grout and cement grout can result from their different viscosity. Funehaget al[13]presented a conceptual model of grout spread where viscosity is an important factor concerning the penetration length.Kimet al[14]reported that the time-varying behaviour of the viscosity of cement grout apparently affects grouting performance. The higher grout viscosity can lead to less penetration length and grout inflow rate.It has been proved that the desired penetration length,desired gel time and good sealing of the fractures can be achieved by controlling the grout viscosity.Therefore, the viscosity characteristics of grout have always been focused on by scholars. Rosquoëtet al[15]studied experimentally the rheology of cement grout with varyingw/cratio. Then the effect of grout composition on the viscosity and yield stress of two natural hydraulic lime was further investigated by Baltazar[16]. Rahmanet al[17]used the ultrasound velocity profiling combined with the pressure difference method to measure the rheological properties of commonly used cement grouts during hydration.Sogaardet al[18]described the influences of the particle size, Ph and temperature on the gelling and gel strength development of silica sol as grouting material. Zhanget al[19]discussed the viscosity changes of the grout with different C:S ratios over the grouting time and radial distance. Chenet al[20]investigated the influences of phase composition, ground granulated blast furnace slag content and curing temperature on the rheological behaviour of the medium cement. Tanget al[21]studied the rheological and mechanical properties of highstrength anchorage grouting materials to optimize the mix proportion.

The aforementioned studies have shown that an important key to improving grouting technology is to understand the evolution of grout viscosity. However,there are few research reports on the viscosity characteristics of polymer grouting materials.Therefore, this study aimed at the investigation of viscosity characteristics of two-component polymer grout with self-expansion trait commonly used in engineering. The evolution of polymer viscosity with time was measured after mixing two components, and the influence of the initial temperature on the viscosity and gel time of polymer grout was analyzed.

2 Experimental

2.1 Materials

The polymer grout used for the experiments contained two components, called A and B. which were produced by Wanhua Energysav Science & Thchnology Group Co., Ltd. Component A (Grade 9802A) was polymethylene polyphenylene isocyanate (PAPI).Component B (Grade 9802B) was a blend of polyether polyol, phosphotriester, physical blowing agent, amine catalyst and other additives. The weight percentage of each ingredient with respect to the polyol mixture is shown in Table 1. The molecular weight distributions of two prepolymer were measured by Waters2414 Liquid Chromatograph. The results are shown in Table 2.

The polymer was formed by mixing component A and B with a ratio of 1:1. After mixing the two components, the chemical reaction started immediately.It could be represented by two primary reactions:gelling reaction and blowing reaction. During the gelling process, isocyanate reacted with polyol to produce polyurethane, as shown in Eq.(1). The viscosity and temperature of the polyurethane increased because the gelling process was highly exothermic. The blowing reaction involved the reaction of isocyanate with water to finally form urea and carbon dioxide,which is given in Eq.(2). The polyurethane foam was expanded by the generation of the carbon dioxide gas into the nucleated bubbles and ended up with the cellular structure[22]:

Table 1 Ingredient of component B

Table 2 Molecular weight distributions of two prepolymers

The foaming process began with the bubble nucleation, during which the initial nuclei inducing the foam expansion generated. The foaming process produces a great deal of heat, which leads to the decreased solubility of the physical blowing agent as well as its evaporation[23]. The gas bubbles within the foam continued expanding with the physical blowing agent vaporized and the carbon dioxide produced,manifesting as the increase of macroscopic volume and gradual decrease of polyurethane density[24]. The process proceeded until gelation occurred. At this point,the polymer became a semi-solid material with high viscosity. During curing process blocks of foam rested.

The polymer grout has advantages of fast reaction,high expansion rate, impermeability and durability,showing excellent comprehensive performance. The grout is mainly used in the medium such as soil and water-bearing fracture[25]. The immersion tests of the cured polymer in the water showed that the materials are safe and friendly to the environment[26].

After mixing components A with B, the morphologic change of polymer grout has undergone four phases[27]. In the initial phase, the polymer is a viscous fluid. After stirring well, lots of bubble nuclei emerge in the liquid. As the chemical reaction progresses, the polymer expands rapidly because of the generation and expansion of more bubbles. Finally, the polymer solidifies with a certain shape and strength.

The flow of most polymer materials does not obey Newton’s law, meaning that the shear rate is not linear relationship with shear stress. At present, there is no definite rheological equation that can reflect the nature of polymer. According to the Ostwald and Dewaele, the rheological equation of polymer is given as:

where,Kis the consistency coefficient, which depends on the fluid nature.nis flow index, and its value indicates the deviate degree of the fluid from Newtonian fluid. For expanding grout, there isn＞ 1.τis the shear stress and du/dyis the shear rate.

Eq.(3) is rewritten into the form of Newton’s law as follows:

with

where, ηais called the apparent viscosity, which is usually obtained by experimental method.

2.2 Experimental procedures

A digital rotational viscosimeter SNB-2 was used to measure the viscosity of polymer grout during the expansion process and a thermocouple thermometer to measure the temperature change of polymer grout.The digital rotational viscosimeter drove the rotor to move at a constant speed by the operation of the motor. The viscous resistance of the rotor in the liquid to be tested was detected, processed into viscosity and then displayed on the liquid crystal screen. The measurement range was 1 mPa·s-6000000 mPa·s,which could accommodate the drastic change in viscosity if any. Different combinations of the rotor and rotate speed corresponded to different full-scale values.The measurement accuracy was ±2% of the full-scale value. The error was ±1% and the repeat error was±0.5%.

The test procedure is as follows: firstly the appropriate rotor and rotate speed were chosen to obtain enough reliable data points. when the viscosity of polymer grout reached to 10 Pa·s, it represented the loss of fluidity. Therefore, the full-scale value was set to 10 Pa·s, the rotor 4# was selected and the corresponding rotate speed was 60 r/min. 100 mL of each of the components were taken for the test. At the start of each experiment, the temperature of the two components was adjusted to a specific value (20 ℃/25 ℃/30 ℃/35℃) in an incubator. Then component A was mixed with component B. The mixture was stirred for 10 seconds and placed at a position where the liquid level was consistent with the groove of a rotor. At this time, the viscosity started being recorded until it reached nearly 10 Pa·s. Once a group of test finished, the viscosimeter was turned off and the rotor was pulled out quickly so as to clean the semi-solid polymer.

3 Results and discussion

The viscosity of components A and B at normal temperature is shown in Fig.1. It can be seen that the viscosity of the two prepolymer materials is constant.The viscosity of component A is 0.476 Pa·s. That of component B is 0.179 Pa·s, which is lower than that of component A.

Tables 3-6 show the change of polymer viscosity with time at different initial temperatures. It can be seen that the time-varying behaviour of viscosity is similar.Taking Table 6 as an example, the polymer viscosity increases with time. It reaches close to 10 Pa·s within 30 s. Before 27 s, the viscosity variable quantity is very small. In the next 3 s, the viscosity increases sharply to 9.652 Pa·s.

The change of polymer viscosity with time at different initial temperatures is shown in Fig.2. As seen,the viscosity variation can be divided into two stages.The polymer viscosity does not change much before gelation, so it can be seen as a constant in this stage.After the gel point, the viscosity increases rapidly and the fluidity of polymer reduces quickly. The inflexion point of the curve is considered to be the gel point.Fig.3 shows that the gel time gradually decreases with the increase of the preheating temperature.

Fig.1 Viscosity of components A and B at normal temperature

Table 3 Change of polymer viscosity with time at 20 ℃

Table 4 Change of polymer viscosity with time at 25 ℃

Table 5 Change of polymer viscosity with time at 30 ℃

Table 6 Change of polymer viscosity with time at 35 ℃

Table 7 Coefficients of the viscosity evolution model

The fitting analysis was conducted to obtain a model of viscosity evolution with time. The results indicate that polymer viscosity increases exponentially with time. The function of the fitted curve is derived as:

where, ηais the viscosity,tis the time,A1andt1are the cofficients, and η is the initial viscosity. The coefficients at different initial temperatures are listed in Table 7. As seen the correlation coefficients between fitting formulas and the measured data are more than 0.97441, which demonstrates the model can describe the evolution of the polymer viscosity properly.

Fig.4 shows the initial viscosity η as a function of the initial temperature. As the initial temperature increases, the initial viscosity of the polymer grout decreases exponentially.

Fig.2 Change of polymer viscosity with time at different initial temperatures

Fig.3 Change of gel time with the preheating temperature

Fig.4 Initial viscosity as a function of the initial temperature

Fig.5 shows the viscosity evolution of cement grout for differentw/cratios. The curves in Fig.5(a)and Fig.5(b) were measured by Ruan[28]and Rahmanet al[17], respectively. Due to different measurement methods and fitting methods, some differences exist between the two results for the viscosity of cement grout. However, it is still obvious that the viscosity characteristics of expanding polymer grout are greatly different from the cement grout. Firstly, the viscosity increase of the cement grout is gentle overall while the polymer viscosity is characterized by a sudden increase at the gel point. Secondly, the gel time of the cement grout is much longer than the polymer grout.At last, the viscosity of the cement grout decreases asw/cincreases, and it is very low at aw/cratio of 1.The temperature makes little influence on the viscosity of cement grout, but it has a great influence on the polymer viscosity. In general, the polymer grout has an individual feature in viscosity compared with the traditional cement grout.

Fig.5 Time-dependent behaviours of cement grout for different w/c ratios

4 Conclusions

Viscosity is a critical parameter that should be considered in the grouting design. In this study, the viscosity tests for a expanding polymer grout were conducted. A viscosity evolution model of the polymer grout was derived, and the influence of the initial temperature of grout on the initial viscosity and gel time was analyzed.

The morphologic change of polymer grout comprises four phases: mixing the components,bubble nucleation, grout expansion and curing. The viscosity of two prepolymer materials before mixing remains stable with different values. After mixing the two components, the viscosity of the polymer grout increases exponentially with time. The initial viscosity decreases exponentially with the increasing initial temperature. Before gelation, the viscosity variable quantity is very small. At the gel point, there is a sudden increase in viscosity. The gel time of polymer grout reduces with the increase of the initial temperature within a certain range. Compared to the viscosity of cement grout, the polymer viscosity is characterized by a sudden increase at the gel point. Besides, the gel time of polymer grout is much shorter than cement grout.Last but not least, the temperature makes little influence on the viscosity of cement grout, but it greatly affects the polymer viscosity evolution. In short, the viscosity characteristics of expanding polymer grout are greatly different from traditional cement grout, which should be taken into full consideration in the design and control of polymer grouting.

In practical engineering, the viscosity and gel time of polymer grout could be designed by adjusting the initial temperature, making the grout fit for the grouting condition. The research on viscosity characteristics of polymer could provide support and guidance for engineering application of polymer grouting.