YUAN Hai－bin(袁海濱)，YIWei－xin(易偉欣)，YU Hong－yi(于弘奕)

1 Institute of Materials Science and Engineering，Henan Polytechnic University，Jiaozuo 454000，China

2 Institute of Resources and Environment，Henan Polytechnic University，Jiaozuo 454000，China

Preparation of Fine Cement Slurries by Wet－Ground Using a Pneumatic Colloid Mill

YUAN Hai－bin(袁海濱)1*，YIWei－xin(易偉欣)2，YU Hong－yi(于弘奕)1

1 Institute of Materials Science and Engineering，Henan Polytechnic University，Jiaozuo 454000，China

2 Institute of Resources and Environment，Henan Polytechnic University，Jiaozuo 454000，China

This study aim s to investigate the preparation of fine cement slurries by wet-ground using a pneumatic colloid Mill．A pneumatic colloid Mill was designed and produced．Furthermore，ordinary Portland cement slurries were ground using the pneumatic colloid Mill．Moreover，the fineness of ground cement slurries was studied．The particle sizes of ground cement slurries with various cumulated percent were all better than those of cement slurries before grinding．W hen water was used as the dispersant，the best average diameter of cement slurrieswasobtained by grinding for 10，and 15 m in for cement slurries with water/cement ratio of 1∶1，and 0．9∶1，respectively．W hen ethanol was used as the dispersant，the particle sizes of all cumulated percent decreased gradually with the increasing grinding time，and the particle sizes of cumulated percent of D97 decreased rapid ly with the increasing grinding time．The grinding effect of big particle sizes of cement slurries is better than that of small particle sizes．

fine cement slurry;wet-ground;pneumatic colloid mill; particle size distribution;fineness

Introduction

Coal plays an important role in economic and social development.Nowadays，coal resources are less and less，and the shallow coal resources in the old coal m ining areas have been nearly exhausted，so the deep coal resources have to be mined.Deep coal mines are under the complex m ining environment conditions of high ground pressure.For the action of ground pressure and long－term service，fissures are generated in the mineworkings in coal m ine.The fractures cause the strength of m ineworkings decreasing seriously［1－5］.Ordinary Portland cement is by far the most common type of cement in general use around the world.Since ordinary Portland cement has many advantages of high strength，high durability，low cost，and so on，it has been not only w idely used in construction，communication，and water conservancy，but also used in coalm ine.It has been used in coalmine to make sure the safe work by repairing the fissures，roadways supporting，m ine reinforcing，sealing off coal mine gas extraction holes，etc.［6－9］For the large particle sizes of ordinary Portland cement，it can not repair the m icro－fissures of concrete engineering well.Fine and ultrafine cementhave been produced to repair themicro－fissures of concrete engineering for their fine particle size in several countries since 1980's［10－12］.Ultrafine cementwas also called“microfine cement”.According to EN 12715，“microfine cement”should have D95＜20μm.Fine and ultrafine cement can be produced by two ways of drygrinding and wet－grinding［13－15］.Colloid mill ismainly used in food industry to grind and homogenize food with low hardness［16－17］.Huang et al．introduced a wet grinding technology to prepare fine cementslurries using colloidmill［15］. Furthermore，they reported that cost of preparation of fine cement slurries by wet－grinding was lower than that of drygrinding.Although it is safe to prepare fine cement slurries using colloidmill driven bymotor on ground，it is fatal to cause explosive accident for the spark of motor underground in coal mine.In this study，we try to assemble a colloidMill driven by a pneumaticmotorwhich can be used safely underground in coal mine.And the fineness of cement slurries prepared by wetgrinding using the pneumatic colloid Mill is also studied.

1 Experimental

Figure 1 shows the sketch of a pneumatic colloidMill，and the parameters of the pneumatic colloid Mill are shown in Table 1.The pneumatic colloid mill is mainly composed of three parts:motive part，actuator，and control valve.Motive part of pneumaticmotor is connected with high pressure source by high pressure rubber tube，and pneumatic motor is driven by high pressure air.Actuator includes fixed plate and rotative plate. Rotative plate is driven by the shaft of pneumatic motor，and cement slurries areMilled between fixed plate and rotative plate. Control valve is used to change flow ing direction of cement slurries.

Fig.1 Sketch of a pneumatic colloid Mill

*Correspondence should be addressed to YUAN Hai－bin，E－mail:yuanhb@hpu.edu.cn

Table 1 Parameters of the pneumatic colloid mill

No.425 common Portland cement was the raw material. Water or ethanol was used as the dispersant.Firstly，2 kg cement slurries were prepared by mixing of proper cement and dispersant in a cement slurry mixer.Secondly，cement slurries were poured into the hopper of the pneumatic colloid mill.And then，cement slurries were ground for certain time.Finally，ground cement slurries were discharged from the pneumatic colloid mill.

Laser diffraction techniques were used to determ ine the particle size distribution of each cement slurry sample by a laser diffraction particle size analyser typed Rise－2008 of Jinan Rise Science and Technology Co.，Ltd.，China.

2 Results and Discussion

2．1 W ater used as the dispersant

Table 2 shows the particle size distribution of cement slurries with water/cement ratio of 1∶1 before and after grinding.It can be observed that the particle sizes of ground cement slurrieswith various cumulated percentare allbetter than those of cement slurries before grinding.It indicates that the particle size of cement slurries become fine after grinding，and the pneumatic colloid mill can be used to prepare fine cement slurries.Usually，the average diameter of cement slurries becomes better as the grinding time increasing.In contrast，the average diameter of cement slurries grinding for 10 m in is the most fine than those of grinding for 5，15，and 20 min.When grinding time is more than 10 min，the average diameter of cement slurries increases as the grinding time increases.

Table 2 The particle size distribution of cement slurries with water/cement ratio of 1∶1 before and after grinding

The particle size distribution of cement slurrieswith water/ cement ratio of 0.9∶1 before and after grinding is shown in Table 3.It can be also observed that the particle sizes of ground cement slurrieswith various cumulated percentare allbetter than those of cement slurries before grinding，and the most fine average diameter of cement slurries is obtained by grinding for 15 m in.The average diameter of cementslurries increasesas the grinding time increasing when grinding time is more than 15 m in.The similar phenomenon was reported in the medium diameter of wet－ground fine cement with water/cement ratio of 0.6∶1 grinding formore than 4 min［15］.It is well known that the hydration reactions will be generated between water and cementwhen they aremixed［18－19］.For cementwith big particle size，the specific area is small，and activity is bad.So it reacts with water slow ly.For cement with small particle size，the specific area is large，activity is good，and it reacts with water quickly.When cement slurries are milled，the particle size of cement decreases，the specific area and activity increase，and hydration reactions are generated easily.It was supposed that the reason of the average diameter of cementslurries increasesas the grinding time increasing is ow ing to the hydration reactions.

Table 3 The particle size distribution of cement slurries with water/cement ratio of 0.9∶1 before and after grinding

2．2 Ethanol used as the dispersant

In order to study the grinding properties of the pneumatic colloid mill in detail，dispersant was changed from water to ethanol to avoid the effect of hydration reactions.And the relationship of particle size of certain cumulated percent and grinding time of cement slurries with various ethanol/cement ratios was also studied.Figure 2 shows the relationship of particle size of certain cumulated percent and grinding time of cement slurries with ethanol/cement ratio of 0.9∶1.It can be seen that the particle sizes of all cumulated percent decrease gradually with the increasing grinding time.The gradient of the change of the particle sizes becomesmild with the decreasing of cumulated percent，and the gradient of cumulated percent of D97 is the steepest.The gradient of the change of the particle sizesmeans the speed of fineness of cement slurries is different for various particle sizes.It shows that the particle sizes of cumulated percent of D97 decrease fast with the increasing grinding timewhile the particle sizesof others cumulated percent decrease slow ly with the increasing grinding time.

Fig.2 Relationship of particle size of certain cumulated percent and grinding time of cement slurries with ethanol/cement ratio of 0.9∶1

The relationship of particle size of certain cumulated percent and grinding time of cement slurries with ethanol/ cement ratio of 0.8∶1 is shown in Fig.3.Similar results can also be found that the particle sizes of all cumulated percent decrease gradually with the increasing grinding time.The gradient of the change of the particle sizes becomesmild with the decrease of cumulated percent，and the gradient of cumulated percent of D97 is the steepest.

Fig.3 Relationship of particle size of certain cumulated percent and grinding time of cement slurries with ethanol/cement ratio of 0.8∶1

In order to study the effect of thickness of cement slurries on grinding，thick cement slurrieswere ground.Figure 4 shows the relationship of particle size of certain cumulated percent and grinding time of cement slurries with ethanol/cement ratio of 0.4∶1.In general，the particle sizes of all cumulated percent decrease gradually with the increasing grinding time，the gradient of the change of the particle sizes becomesmild with the decrease of cumulated percent，and the gradient of cumulated percent of D97 is the steepest.But，it is not so regular like the thin cementslurrieswith ethanol/cement ratio of 0.9∶1 or 0.8∶1.It is supposed that the free distance of particles of thick cement slurries is shorter than those of thin cement slurries，and the particles of cement slurries are not ground sufficiently.

Fig.4 Relationship of particle size of certain cumulated percent and grinding time of cement slurries with ethanol/cement ratio of 0.4∶1

The grinding process of cement slurries was analyzed to explain the above mentioned results.The grinding process of cement slurries is very complex.When cement slurries pass through the gap of fixed plate and high－speed rotative plate，cement particles are crushed mainly by the shear action，friction，impact，and so on.Kinetic energy of cement particles can be calculated by the kinetic energy formula as shown below.

In the kinetic energy formula，kinetic energy(E)of cement particles is in direct proportion to its mass(M)for cement particleswith the same velocity(V).The particle sizes of cumulated percent of D97 are larger than those of low cumulated percent，namely，they have larger volume andmass. Therefore，particles of cumulated percent of D97 have larger kinetic energy.And shear action，friction，impact are more serious.So，grinding effect of cement slurries with larger particle sizes is better than that of with small particle sizes.

3 Conclusions

In summary，a colloidMill driven by apneumaticmotor has been assembled，which could be used safely underground in coal mine.And the fineness of cement slurries prepared by wetground using the pneumatic colloidMill has also been studied to test the grinding effect.Nomatter water or ethanolwas used as the dispersant，the particle sizes of cement slurries become fine after grinding，and it indicates that the pneumatic colloid mill can be used to prepare fine cement slurries.When ethanol was used as the dispersant，it can avoid the effect of hydration reactions，and the particle sizes of all cumulated percent decrease gradually with the increasing grinding time.The particle sizes of cumulated percentof D97 decrease fastwith the increasing grinding time while the particle sizes of others cumulated percent decrease slow ly.The grinding effects of big particle sizes of cement slurries are better than those of small particle sizes.

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TB321

A

1672－5220(2015)04－0594－03

date:2014－11－06

s:National Natural Science Foundation of China(No.51272068);Natural Science Foundation of Henan Province，China(No. 2010A430010);Foundation of Henan Key Discipline Open Laboratory of M ining Engineering Materials，China(No.MEM 12－21)