15th Southeast Asian Geotechnical Society Conference, 22 to 26 November 2004, Bangkok, Thailand

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Evaluation of Fly Ash-Rock-Cement Admixtures as Engineered Lining Materials of Slake Prone Rocks

I. A. Sadisun Department of Geology, Faculty of Earth Science and Mineral Technology Bandung Institute of Technology, Indonesia sadisun@enggeol.org

H. Shimada, M. Ichinose, K. Matsui

Department of Earth Resources Engineering, Graduate School of Engineering Kyushu University, Japan

Abstract: The utilization of fly ash as resources is not a new concept such as in mining operation as backfilling materials and in build-ing product like concrete, cement, and bricks. Meanwhile, the development of sealing or lining masses made of fly ash has been in-creasingly studied for decades and carried out till now. An effort was made in this study to analyze the suggested lining materials of fly ash, Portland cement and water mixtures in combination with the debris materials of slake-prone argillaceous rocks from susceptibility to slaking, particularly concerning coal mine waste embankments. The satisfying results were achieved from both experimental labora-tory and numerical analyses of the admixtures for stabilizing slake-prone rock embankment slopes.

1 INTRODUCTION

Fly ash is waste material residue that is the by products of the coal combustion process for energy production in thermal power plants. The properties of fly ash vary depending on the minera-logical contents of the mother rock, in which the coal is embed-ded, and the quality is depending on the method of mining and cleaning. Generally, the main mineral composition of fly ash in-cludes SiO2 and Al2O3, which are regarded as pozzolanic addi-tions. There is a small amount of crystalline minerals, i.e. quartz, plagioclase, mica, etc., in the fly ash but there is an abundance of amorphous glass. Accordingly, the fly ash usually has hydration and hardening properties under normal condition. Moreover, the particle size range is from less than 0.1 up to 1 mm, but the ma-jority (90%) is less than 0.1 mm, and thence the addition of fly ash into mixtures can increase the specific surface area per unit weight and produces beneficial aspects on their segregation, rate of settlement, permeability, bleeding and pozzolanic activity. The predominately spherical particle shape of the fly ash also gives an advantage by acting as a lubricant, providing a smooth flow when applied in the admixtures. Another advantage of fly ash is that its low heat properties can effectively reduce the temperature of hy-dration in admixtures containing Portland cement by as much as 30%. Due to these characteristics, fly ash has been receiving in-creasing attention on its utilization.

The utilization of fly ash as resources is not a new concept such as in mining operation as backfilling materials and in build-ing product like concrete, cement, and bricks. Meanwhile, the development of sealing or lining masses made of fly ash has been increasingly studied for decades and carried out till now. In this study, an effort was made to analyze the suggested lining materi-als of fly ash, Portland cement and water mixtures in combination with the debris materials of slake-prone argillaceous rocks from susceptibility to slaking, particularly concerning coal mine waste embankments. Some fundamental testing of admixtures was also carried out, such as rheological and mechanical properties.

Moreover, a numerical method was applied to analyzing the slope stability when the admixtures were implemented as lining materi-als to the coal mine waste embankment.

2 TESTING METHODS AND ADMIXTURE PROPORTION

Among the man-made pozzolans, fly ash is probably the most successful component used in admixtures. When fly ash is added to Portland cement, the same kinds of oxides as those of the ce-ment are being added. In this case, fly ash acts, in part, as a fine aggregate and, in part, as a binding component. Then, there is no substantial alteration of fly ash properties from its original com-ponent (Davis, 1949; Joshi & Roauer, 1973).

From these points of view, in order to clarify to what degree the fly ash content affects the properties of admixture, five differ-ent combinations of fly ash, Portland cement and water were con-sidered in rheological and mechanical analyses. The amount of each component of admixtures is listed in Table 1. Table 1. Mix proportion of admixtures used in rheological and mechanical analyses.

Specimen# Cement (wt %) Fly Ash (wt %) Water (wt %) CFW1 10 0 5 CFW2 8 2 5 CFW3 5 5 5 CFW4 3 7 5 CFW5 1 9 5

Viscosity, bleedings, and fluidability were used to test the rheology of admixtures. The apparent viscosity of each sample was measured by using a Brookfield viscometer in order to clar-ify the effect of fly ash on the flow properties of the admixtures. Then, in order to evaluate the stability of the admixtures, a bleed-ing test was carried out using a 1,000 cm3 laboratory jar. Mean-

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while, fluidability analysis was performed using injection test with regard to know the seepage/dispersion performance of the admixtures.

Cylindrical molds 50 mm in diameter and 100 mm long were prepared to produce the admixture specimens for determining mechanical and slaking properties. The first stage of the testing was to make molded specimens. In the second stage, these molds were exposed to dry conditions for two days. After that, the specimens were removed from the molds and placed into water curing boxes for 3, 7, 14 and 28 days. Uniaxial compressive strength and Brazilian tensile strength tests were performed on the specimens after the described curing period. Each type of strength test was run on 5 to 10 specimens and the test results were averaged to obtain the mean value of their properties.

The molded specimens were also analyzed for their slaking characteristics by means of slaking index tests. In this test, the admixtures was also added by the debris materials of slake-prone argillaceous rocks obtaining from TE claystone aggregates of less than 10 mm in size (Table 2). The test samples consist of six ad-mixture lumps having a mass between 100 and 150 grams. Each piece of the test sample is then placed in a separate beaker and oven-dried to a constant mass at 105°C. After cooling at room temperature, the water is then carefully poured into the beakers so that the samples are covered by at least 10 mm of water. After 12-16 hours of immersion, the samples in each beaker are washed with distilled water on a 2 mm standard sieve. The material re-tained on the sieve is then put back into the beaker, decanted and oven-dried to a constant mass. The percent of loosened sample to an initial oven dried mass is calculated and recorded as the slak-ing index value (Is) for that cycle. Table 2. Mix proportion of admixtures used in slaking analysis.

Specimen# Cement Fly Ash Rock Water (wt %) (wt %) (wt %) (wt %) CFR1 2 48 50 3.5 CFR2 4 46 50 3.5 CFR3 6 44 50 3.5 CFR4 8 42 50 3.5 CFR5 10 40 50 3.5

3 RESULTS AND DISCUSSIONS

3.1 Rheological Properties

The results of the apparent viscosity measurement of each admix-ture are shown in Table 3. It can be clearly recognized that the viscosity of the admixture decreases when the substitute ratio of fly ash for Portland cement increases. Table 3. Apparent viscosity of the admixtures.

Specimen# CFW1 CFW2 CFW3 CFW4 CFW5 App. Viscosity (MPa.s) 1373 1130 838 733 496

Figure 1 shows the relationship between the apparent shear rate and shear stress of the samples. This figure represents that the admixtures can be classified as Bingham fluids. It can be also seen that the increasing of substitute ratio of fly ash to Portland cement decreases yield stress and plastic viscosity. The yield stress is the value of the shear stress when the value of the shear rate is 0, and the plastic viscosity is the gradient of the curve. The

yield stress and the plastic viscosity are the rheological parame-ters for Bingham fluid, and the rheological behavior is highly in-fluenced by these two parameters. Therefore, it is clearly under-stood that rheological properties of the admixture are changed by the substitution of fly ash for Portland cement.

Meanwhile, the result of bleeding test is represented by Fig. 2 which showing relationship between the elapsed time and the height of the interface between the clear water and denser sus-pension. This figure expresses the influence of the fly ash content on the stability of admixtures. It was clear that the stability of admixture increase as the substitute ratio of fly ash for Portland cement increases.

Fig. 1. The relationship between the apparent shear rate and shear stress.

Fig. 2. Relationship between the elapsed time and the height of interface between the clear water and denser suspension.

Fig. 3. Relationship between the elapsed time after the injection and the discharge of the injected material.

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(a) Portland Cement

(b) Fly Ash Cement

Fig. 4. Relationship between the elapsed time and the water head inside the cell.

In term of fluidability characteristic, Fig. 3 shows the rela-

tionship between the elapsed time after the injection and the dis-charge of the injected material. It can be recognized that the dis-charge decreases with time gradually in the case of the fly ash cement injection, but the tendency is more drastic in the case of the Portland cement. The relationship between the elapsed time and the water head inside the cell cement injection are shown in Fig. 4 (a) and (b). The water head at the elapsed time of minute 0 shown in this figure is one under the steady condition during the permeability test. In the case of the fly ash cement injection, though the water head is kept constant from the start of injection to about minute 100, the decline tendency of the water head is seen thereafter. On the other hand, it is clear that it declines rap-idly after 10 minutes has elapsed in the case of the Portland ce-ment injection.

From this fluidability test, it can be considered that the mechanism of the void clogging was dominant. On the other hand, cement particles became easy to filter from clogging of the voids within the sample, and it is shown that the clogging mod-els, such as the complete blocking model (Hermans & Bredée, 1936), are occurred where the cement particles accumulate on the surface of the sample. Hence, it is clear that fly ash cement has advantages for the injection material into voids rather than only Portland cement from the fluidability points of view.

Fig. 5. The relation between the curing time and the mechanical properties.

3.2 Mechanical and Slaking Properties

The relationship between curing time and mechanical properties of the admixtures are presented in Fig. 5. This figure shows that, in general, the increase of curing time is followed by the increase of mechanical properties. Besides, the larger the substitution of fly ash for Portland cement is, the longer the values of their me-chanical properties approach their expected maximum values. On the other hand, the increase of fly ash substitution for Portland cement decreases the mechanical properties. However, in the case when the substitute ratio of fly ash for Portland cement is 20%, it seems that its maximum values of mechanical properties are close to that of the only Portland cement mixture. Therefore, it is pos-sible to decrease the cement content without deteriorating me-chanical properties by the substitution of fly ash for Portland ce-ment within adequate levels.

Figure 6 shows the results of the slaking index test for some coal-bearing rock samples studied. The slaking indices tend to increase with an increase in the cycle. The test results indicate that the OM siltstone is comparatively characterized by an low to medium slaking characteristic with a range Is of 6.7 % to 15.2 %; the TE mudstone possessed a medium slaking behavior having a maximum Is of 20.3 %; while, the TE claystone has a high slak-ing characteristic with a range Is of 25.1 % to 31.1 %, which is always less durable than OM siltstone and TE mudstones.

Figure 7 shows the typical results of various slaking indexes obtained for the admixtures with various content of cement-fly ash. In natural condition, TE claystone used in this study had a medium to high slaking characteristic. It can be obviously identi-

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fied that there is a significant gain in the slaking characteristic even only with a small addition of cement, and the gain clearly depends on the cement content. Moreover, the use of fly ash seems significant to reduce the content of cement in admix-tures. Stavridakis & Hatzigogos (1999) suggested that slake du-rability value of 55% might provide useful criterion for success-ful stabilization. On the basis of the relationship between slake durability and slaking indexes suggested by Sadisun et al. (2002) such a value has an equivalent value of slaking index of about 10%. A slaking value of 10% or less was obtained when the rock materials were added with 8 wt. % cement and 42 wt. % fly ash. Thereupon, this would satisfy for the economical reason of using a small amount of cement, and would suggest that the slaking in-dex value of 10% might provide as a minimal requirement for satisfactory cover or lining material.

4 NUMERICAL ANALYSIS OF STABILIZED SLOPE

In relation to the coal mining operation, where the samples used in this study were collected, the use of lining technique can be implemented at the embankment of coal waste materials in order to prevent the rock from slaking. Figure 8 illustrates a conceptual model of the waste rock dump utilizing a lining technique as a slaking control structure. This model was deriving from the modification of dump construction specifications suggested by KPC (2000).

0

20

40

60

80

100

0 1 2 3 4 5 6Number of Cycle

Slak

ing

Inde

x (%

)

OM-SlT E-MdT E-Cl

Fig. 6. Relationship between the number of cycle and the slaking index.

Fig. 7. Cement content versus the slaking index.

Numerical analysis of finite element method (FEM) was then used for determining the stability condition of the model. This method was selected for its advantages that can deal with a com-plex incremental staging of waste rock dumping process. The analysis was performed with the Phase2 finite element package (Rocscience, 2003) for the stress ratios (K0) 1. It has been ini-tially sought the best boundary condition by gradually increasing the dimension of the model as shown in Fig. 9.

In this numerical analysis, it has been optimize the stage of dumping processes into four stages. Undrained analysis was car-ried out in term of total stress. The rock material was modeled as an isotropic medium, characterized by a linearly elastic behavior until yield criteria. The values of rocks and mixtures parameters employed in this analysis are summarized in Table 4.

The principal stress distribution (σ1, σ3), the safety factor, yield point (shear, tension) and the induced displacements devel-oped were analyzed. An example of computed displacement paths of the model are illustrated in Fig. 10. It can be recognized that the zones where the consolidation is likely to occur, with maximal displacement occur close to mid-height of total em-bankment.

The results of numerical safety factor analysis of the em-bankment as a result of four stages with and without lining mate-rials are shown in Fig. 11. In both figures, it can be carefully rec-ognized the difference in the extent of stability zone after lining material enforced. Compare with the embankment without lining material, the safety factor increase and displacements have com-paratively reduced. Therefore, it can be obvious to infer based on the above computations that the lining structure can significantly strengthened to stability condition of the embankment.

With regard to the factor of safety, the results of the analysis show that the factor of safety (SF) was determined to be greater that 1.3, so slope failure is not expected for the dumping slope.

Fig. 8. Waste rock embankment construction with a lining tech-nique.

Fig. 9. Dimensions of the finite element models used.

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Fig. 10. Computed displacement path of the model.

Fig. 11. Computed safety factor of the model: a) without (left) and b) with covering materials (right).

Table 4. Geomechanical parameters for each studied material as determined by RocLab (Rocscience, 2003).

Material Type UCS (MPa)

Young’s Modulus (MPa)

Poisson’s Ratio

Unit Weight (kN/m3)

Cohesion (MPa)

Friction Angle (deg)

Base Materials 1.068 282 0.30 21.7 0.019 23.8 Slake-prone rocks 0.752 154 0.32 19.4 0.009 12.6

Slake-prone and poten-tially acid forming rocks 0.820 184 0.31 20.1 0.014 15.4

Admixtures* 4.275 2921 0.24 26.5 0.088 41.1 * obtained from the CFR5 admixture specimen.

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5 CONCLUSIONS

In this study, an effort was made to analyze the suggested lining materials of fly ash, Portland cement and water mixtures in com-bination with the debris materials of slake-prone rocks from their fundamental behaviors and their susceptibility to slaking, particu-larly concerning to the coal mine waste dumps or embankments. The results show that the use of fly ash has some benefits, such as improving the rheological and slaking properties as well as de-creasing the use of cement without deteriorating mechanical properties. Moreover, the results obtained from slope stability analysis using a numerical method of FEM, applied to analyzing the slake-prone rock slope stability when the mixtures were im-plemented as lining materials to the coal mine waste embankment, inferred that the lining structure can significantly strengthened to stability condition of the embankment.

ACKNOWLEDGMENTS

The authors would like to express their gratitude to the Manage-ment of PT Bukit Asam for providing coal-bearing rock samples. Any opinion expressed in this paper are those of authors them-selves and not necessary those of the mines.

REFERENCES

Davis, R.E. 1949. A review of pozzolanic materials and their use in cement concrete, ASTM 99: 3-15.

Joshi, R.C. & Rosauer, E.A. 1973. Pozzolanic activity in syn-thetic ashes, American Ceramic Society Bulletin 52: 456-463.

Hermans, P.H. & Bredée, H.L. 1936. Principals of the mathe-matical treatment of constant-pressure filtration, Journal of Society of Chemical Industry, 55: 1T.

KPC 2000. Rehabilitation Specifications, Version 2, Environ-mental Department of PT Kaltim Prima Coal (KPC), Indone-sia, 52 pp.

Rocscience 2003. Phase2 v. 5.038 & RocLab v. 1.007. Roc-science Inc., Toronto.

Sadisun, I. A., Shimada, H., Ichinose, M. & Matsui, K. 2002. Improved Procedures for Evaluating Physical Deterioration of Argillaceous Rocks. Proceedings of the 2nd International Conference on New Development in Rock Mechanics and Rock Engineering, Shengyang, P. R. China: 36-39.

Stavridakis, E. I. & Hatzigogos, T. N. 1999. Influence of Liquid Limit and Slaking on Cement Stabilized Clayey Admixtures, Geotechnical and Geological Engineering 17: 145-154.